LYME DISEASE: A COMPREHENSIVE REVIEW

Natascha M.D. Israël

ABSTRACT

Lyme disease is currently the most common vector-borne disease in the United States,

with about 30,000 reported cases every year (CDC 2015a). The disease is caused by a bacterial

infection of Borrelia burgdorferi, which was first identified by Dr. Willy Burgdorfer in 1981

after a series of unexplained cases of rheumatoid arthritis in children living in Lyme, Connecticut

during the 1970’s (Burgdorfer et al. 1982). The bacterium is spread via the bite of infected

Ixodes ticks and can infect a wide variety of hosts. Lyme disease is predicted to spread with

climate change and has caused numerous scientific debates pertaining to its diagnosis, treatment,

and severity. This review will bring together studies on this disease to empower the American

public with the required knowledge to understand the inner workings of this disease and how we

can best protect and defend ourselves from its sly pathogen.

BACKGROUND

Lyme disease pathogen: Borrelia burgdorferi

Borrelia burgdorferi sensu lato, which consists of 18 subspecies (Table 1), is a spirochete

and obligate parasite. Only three of the 18 subspecies cause Lyme disease, namely B.

burgdorferi sensu stricto (from now on referred to as Borrelia burgdorferi), B. garinii, and B.

afzelii. Europe has all three of these subspecies, while North America currently only has B.

burgdorferi. There is a possibility, however, that B. garinii may be making its move over the

ocean via migratory seabirds in the near future (Smith et al. 2006).

Margos et al (2008) found that B. burgdorferi originated in Europe rather than in North

America as previously proposed (Marti et al. 1997). The bacteria, however, have been present in

North America since about one million years ago (Hoen et al. 2009). This indicates a prehistoric

spread of B. burgdorferi in both North America and Europe, long before the emergence of

modern Lyme disease (1970’s). Additionally, the spread of B. burgdorferi within North America

went in an east-west direction, beginning in the northeast (Hoen et al. 2009). Potential

explanations for how the bacterium got to the northeast in the first place include migrating birds,

as B. burgdorferi has been found in birds from both continents (Ginsbert et al. 2005; Comstedt et

al. 2006). It is currently most abundant in the northeast and upper Midwest states (Figure 1;

Table 2).

During European colonization and industrial development in North America,

deforestation and unregulated hunting (especially on deer) led to a drastic decrease in the

bacterium’s primary vector (Ixodes scapularis; Halls 1984). As the geographic range and

abundance of both vectors and hosts decreased, so did B. burgdorferi’s population size. This led

to a decrease in the spirochete’s genetic diversity due to genetic drift, which is especially

effective on smaller population sizes (Qiu et al. 2002; Kliman et al. 2008). Researchers believe

that some of their genetic diversity has been regained, however, since the reforestation of much

of the eastern US during the mid-20th century (Spielman et al. 1985). Reforestation led to a

massive increase in the geographical range of I. scapularis, which also coincides with the

modern emergence of Lyme disease.

There is still relatively little genetic heterogeneity in B. burgdorferi’s genome compared

to other Borrelia species/strains (Qiu et al. 2002). The diversity that does exist generally arises

due to mutation, recombination, and natural selection. Since B. burgdorferi is such a slow-

growing bacterium, however, their mutation rate is very low (Hoen et al. 2009). Genes often

used to differentiate between different Borrelia species include the outer surface protein (osp)

encoding loci (Bunikis et al. 2004; Girard et al. 2009; Margos et al. 2011). The ospA gene aids in

tick-spirochete interactions while ospC is important for spirochete-host interactions (please see

vector section; Schwan et al. 1995; Pal et al. 2000; Schwan and Piesman, 2000; Pal et al. 2004).

Different ospA genes among Borrelia species may be a result of differential adaptation to

genetically different ticks. For example, B. burgdorferi produces different OspA proteins than

Borrelia bissettii, a Borrelia strain from Colorado (Postic et al. 1998; Postic et al. 2007). The

main vector for B. bissettii is Ixodes spinipalpis, which is genetically different from I. scapularis

(Burkot et al. 2001). Similarly, infecting different hosts requires the expression of slightly

different OspC proteins (Brisson and Dykhuizen, 2004). For example, B. afzelii is specialized on

rodent hosts and is killed by the complement system of birds, whereas B. garinii is specialized on

avian hosts and is killed by the complement system of rodents (Kurtenbach et al. 1998, 2002).

Lyme disease vector: Ixodes ticks

The bacterium is spread via the bite of infected Ixodes ticks, making it a vector-borne

disease. In North America, the Ixodes ticks that are known to carry B. burgdorferi are Ixodes

scapularis (black-legged tick, deer tick) and Ixodes pacificus (western black-legged tick) (CDC

2015b). Their typical life cycle involves four stages: egg, six-legged larvae, eight-legged

nymphs, and adults (Figure 2). The eggs are laid by spring and hatch into larvae in the summer.

After their first blood meal, these larvae molt into nymphs during the spring season. Nymphs

then molt into adults following their second blood meal in early fall. Adults typically find mates

on popular hosts, such as white-tailed deer (Odocoileus virginianus), which results in the female

ticks laying over a thousand eggs before she dies. There is no vertical transmission of B.

burgdorferi in ticks, so all eggs are free from infection. People are especially advised to be

cautious of ticks during nymph and adult season, as most of these life stages are infected with the

pathogen. Nymph season typically goes from late spring to early summer, while adult season

spans from late fall to early spring.

These Ixodes ticks are believed to have coevolved with the bacterium due to the intricate

relationship they have pertaining the bacterium’s outer surface proteins (Osp). The OspA protein

is synthesized when the bacterium enters the tick via its blood meal, which some researchers

believe mediates their adherence to the tick’s gut (Pal et al. 2000). When the infected tick

becomes engorged on its next host, B. burgdorferi multiply within the tick’s gut and levels of

spirochetal OspA go down while OspC concentrations go up (Schwan and Piesman, 2000). This

occurs via the downregulation of the ospA gene and upregulation of the ospC gene due to

temperature changes in the tick’s gut (Schwan et al. 1995). The tick’s gut temperature increases

from 24ºC to 34-37ºC as the tick sucks in the host’s warm blood, which triggers this genetic

switch. The OspC protein is believed to help the spirochete invade the tick’s salivary glands so

that they can be transferred into the next host together with the tick’s saliva (Gilmore and

Piesman, 2000; Pal et al. 2004). It may also aid in colonizing tissues of its new host as B.

burgdorferi continues expressing OspC within the mammalian/avian host (Montgomery et al.

1996; Pohl-Koppe et al. 2001). These Osp proteins are specialized towards the Ixodes tick family

and will not function in other tick species. This shows how B. burgdorferi has coevolved with its

Ixodes tick vector by expressing OspA when it needs to adhere to the tick’s gut (host-vector

transmission) and expressing OspC when it needs to move from the tick to the next host (vector-

host transmission).

Lyme disease reservoirs and hosts

Borrelia burgdorferi can survive in a wide variety of hosts. However, some hosts are

more competent for pathogen transmission than others. The white-footed mouse (Peromyscus

leucopus) is known to be one of the most competent reservoirs for B. burgdorferi (Levine et al.

1985; Donahue et al. 1987; Ostfeld et al. 1996; CFSPH 2011). Other possible reservoir species

include short-tailed shrews (Blarina brevicauda), eastern chipmunks (Tamias striatus), the deer

mouse (Peromyscus maniculaus), the brush mouse (Peromyscus boylii), and the western gray

squirrel (Sciurus griseus) (CFSPH 2011). However, Mather et al (1989) estimates that one white-

footed mouse infects as many nymphal ticks as 12 chipmunks or 221 voles.

The white-footed mouse has a broad distribution that ranges from southern Canada to

Central America. Population densities may reach as high as 15 individuals/acre and they serve as

an important prey species for several predators, including owls, weasels, and snakes (Marsh and

Howard 1990). The mouse plays an important role in the life cycle of the vector of Lyme

disease, the Ixodes tick, as most larvae generally feed on small mammals, with the white-footed

mouse being the species most commonly parasitized (Main et al. 1982; Bosler et al. 1984;

Anderson et al. 1987; Lane et al. 1991). Most white-footed mice are infected with B. burgdorferi,

which then results in all larvae feeding on them becoming infected with the spirochete as well

(Mather et al. 1989). Any host the tick feeds on can infect it with B. burgdorferi, although the

infection most commonly stems from their first blood meal as larvae (Ostfeld et al. 1995).

The incidence rate of B. burgdorferi infection among white-footed mice is high,

suggesting that the pathogen spreads quickly within a population of white-footed mice. Bunikis

et al (2004) found that the incidence of B. burgdorferi in a natural population of white-footed

mice in Connecticut was 0.2 cases/mouse/week. A study in Maryland found an incidence rate of

6-11.5 cases/1,000 mouse-days, pointing out that rates were 10 times greater during periods of

high exposure to nymphal I. scapularis ticks (nymph season) (Hofmeister et al. 1999).

Interestingly, the incidence rate reported by Hofmeister et al (1999) was 3 times lower than the

rate found by Bunikis et al (2004). This may be because Maryland has a lower endemicity of B.

burgdorferi than Connecticut (CDC 1997). The high incidence rate of infection with B.

burgdorferi in the Connecticut study is unusual for human zoonotic diseases with enzootic

transmission cycles. The fact that nearly all mice became infected during their study suggests

that it is more typical of an epizootic outbreak. In that way, the spread of Lyme disease among

white-footed mice is similar to that of plague (Kartman and Hudson, 1971).

Similar to their incidence rates, the pathogen’s prevalence among white-footed mice is

also high. Anderson et al (1987) found that the prevalence of B. burgdorferi in white-footed mice

was about 75% in mice collected between June and August, while it was about 33% during

December-March months. This is probably tightly linked with nymph season, which occurs in

late spring/early summer. Mather et al (1989) found that the prevalence of infection in white-

footed mice in Massachusetts was 90%, which was a lot higher than the prevalence in chipmunks

(Tamia striatus; 75%) and meadow voles (Microtus pennsylvanicus; 5.5%). Another study done

in Wisconsin found that the prevalence of B. burgdorferi in white-footed mice was 88%

(Anderson et al. 1987). Overall, prevalence of B. burgdorferi is relatively high in white-footed

mice, especially during peak nymph season.

While some researchers found that white-footed mice do mount an antibody response to

the bacterium, consisting of both IgG and IgM (Schwan et al. 1989), others found that the

majority of B. burgdorferi proteins do not elicit an antibody response during infection (Barbour

et al. 2008). Donahue et al (1987) found that white-footed mice can remain infective for at least

200 days after a single infective tick bite. As these mice experience constant feeding by Ixodes

ticks due to their similar wooded habitat requirements, most of them have a persistent spirochetal

infection (Magnarelli et al. 1988). This long-term infection can lead to the rodent’s high

infectivity of the tick larvae. Potential reasons for their persistent infections of B. burgdorferi

include the bacterium’s active immune suppression and immune evasion.

Immune suppression

There are several ways B. burgdorferi has managed to suppress their host’s immune

system. First, the spirochete is able to avoid pathogen opsonization by recruiting the host’s

complement inhibitory factors onto its own cell surface (Embers et al. 2004). These inhibitory

factors inactivate the C3b pathway, which is normally responsible for pathogen opsonization and

cell apoptosis via the formation of C3 convertase (Merle et al. 2015). This ultimately results in

phagocytes not destroying the bacterium. Another way in which B. burgdorferi suppresses the

host’s immune system is via the suppression of inflammatory cytokines. The spirochete produces

interleukin-10 (IL-10), which is an anti-inflammatory cytokine that downregulates the

production and function of inflammatory cytokines (Giambartolomei et al. 1998; Murphy et al.

2000). This results in the bacterium mitigating the host’s early inflammatory response to

infection. Lastly, B. burgdorferi also seems to release soluble antigens, which leads to the

formation of immune complexes (antigen/antibody aggregates). This strategy seems to be

responsible for the spirochete’s effective prevention of opsonization when in vivo (Coyle et al.

1990; Schutzer et al. 1990). It can also lead to false seronegativity in patients who actually are in

fact infected with B. burgdorferi.

The Ixodes tick also produces proteins that are useful for B. burgdorferi. First, the

previously discussed spirochetal OspA protein attaches to a tick receptor within Ixodes known as

TROSPA (Pal et al. 2004). These TROSPA receptors help the bacterium adhere to the tick’s gut.

Interestingly, the gene that expresses TROSPA (TROSPA) is upregulated directly following

spirochetal infection but downregulated when the tick becomes engorged on its next host.

Researchers believe the increase in TROSPA will aid in vector colonization while the TROSPA

decrease will help the bacterium move from the vector to its next host. Another helpful Ixodes

protein is Salp15, a protein found in the tick’s saliva. Salp15 is injected into the

mammalian/avian host together with the tick’s saliva (and spirochetes) and is responsible for

inhibiting the host’s CD4+ T cell activation (Anguita et al. 2002). It also lowers the host’s

production of interleukin-2 (IL-2). Other effects of Ixodes tick saliva on the host’s immune

system include the inhibition of the complement cascade (Valenzuela et al. 2000), impairment of

natural killer (NK) cell function (Kopecky and Kuthejlova, 1998), and the reduction of

interferon-gamma (Schoeler et al. 1999). This immunosuppression of the host via the tick’s

saliva allows for a more efficient transmission of B. burgdorferi into the host (Zeidner et al.

1996).

Immune evasion

Borrelia burgdorferi can also evade the host’s immune system. One way the spirochete

accomplishes this is via phase and antigenic variation. Some bacteria employ gene switching to

create a diverse array of surface glycoproteins. A diverse surface-exposed lipoprotein (VlsE) is

created by B. burgdorferi via genetic recombination involving a linear plasmid containing 15

non-expressed, or silent, vls genetic cassette sequences and the central domain of the expressed

VlsE cassette region (Zhang et al. 1997). This can lead to the production of millions of antigenic

variants in mammalian hosts, such as the white-footed mouse. Other possible strategies for

increasing VlsE diversity include mutation and selective (on-off) expression of genes encoding

antigenic proteins (Embers et al. 2004). While the host’s antibodies may evolve to recognize

some of these antigenic variants, others will remain unnoticed by the host’s immune system.

Physical seclusion is another strategy employed by B. burgdorferi to evade the host’s immune

system. Following the localized phase of infection, the spirochete typically spreads into its

preferred organs: eyes, central nervous system, and joints. These areas contain extracellular

fluids that do not circulate though the conventional lymphatics, making the bacterium less

accessible to the host’s immune cells/molecules (Janeway and Travers 1996). Additionally, B.

burgdorferi can also hide itself either inside a host cell or within a cyst membrane. Cells

commonly invaded include endothelial cells, fibroblasts, macrophages, Kupffer cells, and

synovial cells (Ma et al. 1991; Klempner et al. 1993; Montgomery et al. 1993; Girschick et al.

1996; Sambri et al. 1996). Cyst formation typically occurs in response to nutritional stress or the

presence of β-lactum antibiotics (Murgia et al. 2002). Once conditions become favorable again,

the spirochetes will emerge from their cells or cysts (Gruntar et al. 2001).

Lyme disease in humans

Symptoms

Symptoms of Lyme disease vary among individuals. Early stages of Lyme disease, which

usually occurs 3 to 30 days after the tick bite, include fever, chills, headache, fatigue, muscle and

joint aches, swollen lymph nodes, and a distinct rash known as erythema migrans or bull’s eye

rash (CDC 2016a). This rash occurs in 70-80% of infected people and is the only unambiguous

symptom of Lyme disease. Later stages occur days to months after the tick bite and can include

severe headaches and neck stiffness, neurological issues, arthritis, and cardiovascular issues.

Some common neurological symptoms are Bell’s palsy, inflammation of the brain and spinal

cord, nerve pain, numbness or tingling in the hands and/or feet, and problems with short-term

memory. Cardiovascular problems include heart palpitations and an irregular heartbeat (CDC

2016a).

Risk factors

A principal risk factor for contracting Lyme disease is exposure to wooded areas. This

may be due to recreational activities (hiking, camping), outdoor occupations (landscaping,

forestry), or residential proximity to a wooded area (especially in suburban/rural areas) (Halsey

and Abramson, 2000). For example, outdoor workers are significantly more likely to contract

Lyme disease than people who work primarily indoors (Schwartz and Goldstein, 1990). Behavior

of the individual is therefore tightly linked to contracting Lyme disease. Interestingly, enhancing

people’s knowledge of Lyme disease does not lead to significantly greater precautionary

behaviors (Shadick et al. 1997). All people are thought to be equally susceptible to Lyme

disease, although some studies found that certain age ranges have the highest reported rates of

Lyme disease: children from 2 to 15 and adults from 30 to 55 (Dennis 1998). Additionally, the

reported incidence for men is generally higher than that of women, especially for individuals in

the 5 to 19 and 60+ year age ranges (Orloski et al. 2000). These findings may be due to increased

exposure of these individuals to Lyme-infected ticks, decreased use of protective measures

against ticks, behavior associated with these age ranges and sex, or a result of reporting bias.

Parasitism heterogeneity among the hosts of B. burgdorferi sensu lato also affects an

individual’s risk of contracting Lyme disease. For example, avian hosts are competent hosts for

some Borrelia strains/species, but not for others. The same applies to rodents. Borrelia

burgdorferi sensu stricto, however, can invade both avian and rodent hosts (Kurtenbach et al.

2002). This host selectivity among Borrelia strains is due to the bacterium’s genetics, which is

responsible for expressing specific outer surface proteins (Osp) that can suppress/evade the

host’s complement pathway (Brisson and Dykhuizen, 2004). A similar situation exists for

infecting ticks, as different Borrelia strains infect different Ixodid tick species (Burkot et al.

2001). The bacterium’s genetics therefore plays a big role in parasitism heterogeneity.

Seasonality can also affect the heterogeneity seen in Lyme disease. Hatched larvae

typically feed in the summer and early fall months, nymphs feed in late spring and early summer,

and adults mainly feed in late fall and early spring (CDC 2015b). Larvae only feed on small-

sized hosts (ex. white-footed mice), while adults typically feed on larger hosts (ex. humans).

Nymphs can feed on both small and large hosts. Most states warn people that the deer tick

nymph season typically starts early June, peaks in early July, and ends in August. Due to the

nymph’s small size, they play a larger role in Lyme-transmission to humans than adults. For

example, Piesman et al (1987) found that the risk of contracting Lyme disease in Massachusetts

is greatest in late spring/early summer, especially May and July (nymph season). The adult tick

season usually peaks at the end of October, but are more easily detected due to their larger size.

They can still play a big role in transmitting Lyme disease to our pets, especially long-haired

pets, since they are a lot less visible when buried under their hairy coat.

Lastly, abiotic factors can influence one’s risk of contracting Lyme disease by affecting

the overall parasitism of ticks. Although adult ticks are present from fall to spring, they are

typically less active during freezing temperatures. There are two different threshold temperatures

for Ixodid ticks: uncoordinated activity threshold temperature and activity threshold temperature

(Clark 1995). The first is the temperature below which the tick is no longer able to seek a host in

a coordinated manner, while the latter refers to the temperature at which all tick activity ceases.

Clark (1995) found that the average uncoordinated activity threshold temperature was 9.2 ±

4.1°C for females and 11.2 ± 3.4°C for males. The mean activity threshold temperature, on the

other hand, was 6.2 ± 3.6°C for females and 8.5 ± 3.0°C for males. Ticks are therefore a lot less

active during the cold winter months, thereby reducing the overall risk of contracting Lyme

disease during this time. Another interesting abiotic limitation for ticks is elevation, as ticks tend

to avoid higher elevations. Bunnell et al (2003), for example, found no I. scapularis ticks above

530 meters (1,739 feet) in elevation. Humidity is also important, as ticks prefer areas that have

greater moisture (Stafford 1994). This is especially important for larvae, who have a small water

mass and permeable cuticle.

Diagnosing Lyme disease

Diagnosing Lyme disease can be tricky. Most of the diagnostic tests that are currently

used for Lyme disease are not perfect, which is why there are several cases of both false

negatives and false positives every year (Borchers et al. 2015). Doctors generally approach

suspected Lyme disease cases in the following way. First, a patient history is taken in order to

establish a patient’s probable exposure to infected Ixodes ticks. The doctor should consider

whether the areas visited are endemic areas for Lyme disease and if it occurred at an appropriate

time of year (nymph season or adult season for ticks). Next, the doctor will assess the physical

symptoms of the patient. Unless the patient is exhibiting erythema migrans, a doctor cannot

diagnose a patient with Lyme disease using only physical symptoms and patient history as most

symptoms of Lyme disease are very general and could be due to other pathogens (CDC 2011;

Borchers et al. 2015). The erythema migrans, however, is specific to Lyme disease and doctors

often decide not to use serological tests if the patient has both the distinct rash and exposure to

infected ticks.

If the doctor feels a serological test is needed, he/she will usually begin with a sensitive

enzyme immunoassay (EIA), such as the enzyme-linked immunosorbent assay (ELISA)

(Borchers et al. 2015). Some doctors prefer to use an indirect immunofluorescence assay instead

of an EIA, although this is rare. If results from these initial tests are positive or indeterminate, the

blood serum will undergo immunoblotting, such as a western blot. An immunoblot is considered

positive if it meets the CDC’s recommended number and types of IgM and IgG bands (CDC

1995). It should be noted that serological confirmation can be hampered if the patient is tested

before antibodies against the Borrelia species are generated. For example, IgM antibodies are

usually not detectable for the first 1-2 weeks while IgG antibodies may not emerge until 4-6

weeks after the initial infection (Craft et al. 1984; Aguero-Rosenfeld et al. 1993; Berglund et al.

1995; Engstrom et al. 1995; Aguero-Rosenfeld et al. 1996; Glatz et al. 2008; Steere et al. 2008).

Another laboratory test that is sometimes used by doctors is to culture the bacteria (Borchers et

al. 2015). This procedure is not common, however, since culture is expensive and requires

special media and laboratory expertise. Additionally, results from culture are not available for 2-

6 weeks, making it less useful for clinical decision making.

Treating Lyme disease

Lyme disease is routinely treated with antibiotics. The main antibiotics used include β-

lactams (especially cephalosporins), tetracyclines (ex. Doxycycline), and macrolides (though not

as effective as the other two kinds) (Borchers et al. 2015). The antibiotic prescribed depends on

certain patient characteristics, such as age, antibiotic allergies, and pregnancy. There is currently

a big debate on how long the treatment should last. This is due to the large variety present in both

patient immunocompetence and Borrelia strain pathology (Preac Mursic et al. 1996). Several

randomized controlled trials and retrospective studies found that there is no benefit in extending

the duration of Doxycycline treatment from 10 days to 15 or 20 days (Wormser et al. 2003;

Kowalski et al. 2010; Stupica et al. 2012). Similarly, Oksi et al (2007) found that patients who

received both a 3-week course of intravenous ceftriaxone and 100 days of amoxicillin did not

have a significantly improved outcome when compared to patients who only received the 3-week

course of intravenous ceftriaxone. Borchers et al (2015) point out, however, that some patients

may benefit from a longer and/or more aggressive therapy. Doctors have to therefore carefully

assess the risk-benefit ratio for each individual patient.

Dosages depend on the antibiotic prescribed and the patient’s age and symptoms. For

example, adult patients with classic early Lyme symptoms (erythema migrans rash, flu-like

symptoms) are prescribed 2 x 100 mg of doxycycline (for 10-21 days), while children with

similar symptomology receive 2 x 2 mg/kg of doxycycline (also for 10-21 days) (Wormser et al.

2006; Mygland et al. 2010; BIA 2011). Given the same set of symptoms, the dosage for

amoxicillin would be 3 x 500 mg (14-21 days) for adults and 50 mg/kg in 3 divided doses for

children. If the adult patient is exhibiting signs of Lyme neuroborreliosis, however, their

doxycycline dosage would be increased to 2 x 200 mg (for 10-28 days).

As mentioned previously, B. burgdorferi is an expert at hiding itself in the body which

allows it to evade the body’s immune system. This evasion, however, also applies to antibiotics.

Smith et al (2014) argue that antibiotic treatment is only effective in the early stages of Lyme

disease before the bacteria has a chance to hide itself. Once the bacteria is well-established, as

with chronic Lyme disease, common antibiotic treatments lose some or all of their effectiveness

(Smith et al. 2014). Sharma et al. 2015 found that B. burgdorferi does not necessarily become

resistant to antibiotics but instead forms drug-tolerant persister cells. Persister cells are dormant

variants of regular cells that can tolerate the presence of antibiotics. This is another potential

reason for why so many people progress to the later stages of Lyme disease, which are

characterized by symptoms such as arthritis, meningitis, encephalopathy, and carditis (Reznick et

al. 1986; Fallon and Nields, 1994; Puius and Kalish, 2008).

Coinfections

Many people infected with B. burgdorferi are also infected with one or more

coinfections. A study done in Canada found that 60% of people with Lyme disease had at least

one coinfection (Sperling et al. 2012). Similarly, Johnson et al (2014) found that 23.5% of

respondents diagnosed with Lyme disease had at least one coinfection, while 29.8% had two or

more coinfections. The most commonly reported coinfections are Bartonella (28.3%) and

Babesia (32.3%) species, although other possible coinfections include Anaplasma (4.8%),

Ehrlichia (14.5%), Rickettsia spp. (5.6%), Mycoplasma (15.1%), and Tularemia (0.8%)

coinfections. Of these coinfections, researchers are especially interested in studying the

prevalence and effect of Babesia on patients with Lyme disease.

Babesia microti is the etiologic agent of Babesiosis. This parasite has been found to be

abundant among both humans and wildlife. For example, Anderson et al (1986) found that this

protozoan parasite was present in 57% of the white-footed mice sampled. A study done on

Nantucket Island found that 31 of the 39 captured white-footed mice were positive for B. microti,

giving a prevalence of 80% (Healy et al. 1976). It seems that this parasite has no problem co-

existing with B. burgdorferi in the same host as coinfections of these two pathogens are common

(Anderson et al. 1986, 1991; Sperling et al. 2012; Magnarelli et al. 2013; Johnson et al. 2014).

Coleman et al (2005) found that both infections proceed independently of one another suggesting

there is no synergism between them. This would mean that a co-infection is not more pathogenic

than a single infection. Similar results were found by Wang et al (2000), who reported that

coinfections did not worsen the long term outcome with regard to constitutional,

musculoskeletal, and neurological symptoms. Other researchers, however, found that babesial

infections can enhance Lyme disease myocarditis and that B. microti may impair host defense

mechanisms that further supports B. burgdorferi infections in the host (Purvis 1977; De Vos et

al. 1987; Kimsey et al. 1990). Coinfections may therefore lead to a longer duration of illness and

exacerbated symptoms (in people) including myalgia, fatigue, sweats, anorexia, erythema

migrans, and conjunctivitis (Krause et al. 1996; Dos Santos and Kain, 1999). More studies are

therefore needed to fully understand how coinfections between these two pathogens differ from

regular single infections.

Public health perspective

Prevention is one major way in which public health professionals try to protect the public

from Lyme disease. This involves warning people of the risks of tick bites, educating them on

how to detect and properly remove ticks, and providing ways in which to reduce their overall

contact with potentially infected ticks. Warning signs may come in the form of actual road signs

posted along wooded areas or as a public message that may be broadcasted on the radio,

television, etc. The CDC’s website has an extensive webpage on how to best prevent tick bites.

They include measures such as knowing where to expect ticks, using a repellent with DEET (on

skin or clothing) or permethrin (on clothing and gear only), creating tick-safe zones in one’s

yard, and performing daily tick checks (CDC 2016b). They also alert the public of signs and

symptoms to be aware of if one does have a tick attach, such as a fever or rash. All of these

preventive measures can significantly reduce one’s chances of contracting Lyme disease.

Vaccination is another important public health component. There is currently no

approved vaccine for Lyme disease. LYMErix was a Lyme disease vaccine that was

discontinued in 2002. Many people that did receive this vaccine reported several adverse effects,

with the most common ones being arthralgia, myalgia, pain, asthenia, headaches, fever, and rash

(Shen et al. 2011). According to the Vaccine Adverse Event Reporting System (VAERS), 7.4%

of the reported adverse events were classified as serious, which is defined as “one which resulted

in life-threatening illness, hospitalization, prolongation of hospitalization, persistent or

significant disability/incapacity, or death” (Braun and Ellenberg, 1997; Lathrop et al. 2002). A

new Lyme disease vaccine, the novel multivalent OspA vaccine, is currently being developed

(Wressnigg et al. 2013). Although this vaccine looks promising for the prevention of Lyme

disease, more studies are needed to confirm its effectiveness and safety.

Another way in which public health tackles Lyme disease is via improving methods used

for diagnosing and treating Lyme disease. As mentioned previously, diagnosing Lyme disease

can be very tricky. Most of the commonly used diagnostic tests are not perfect, allowing for a

great number of both false negatives and false positives every year (Borchers et al. 2015).

Improved diagnostic tests can then help public health authorities to develop and monitor medical

guidelines designed to reduce the prevalence of Lyme disease and enhance outcomes through

earlier diagnosis and treatment of the disease. Treating Lyme disease, however, is not that

simple. Although most doctors agree that antibiotics should be prescribed to treat Lyme disease,

the type of antibiotic used and the duration of treatment depends on both the patient and Borrelia

strain involved. Borrelia burgdorferi can also hide itself from antibiotics, allowing it to persist

after the patient completes his/her treatment. Public health officials are constantly researching

new ways to both diagnose and treat Lyme disease more effectively in order to reduce the

occurrence of under-diagnosis, over-diagnosis, and chronic stages of Lyme disease.

Since B. burgdorferi relies on both ticks and wildlife reservoirs/hosts, wildlife control

management can be used to further prevent/control Lyme disease among the human population.

Pesticides against Ixodes ticks are commonly used and can be helpful in reducing the risk of

contracting Lyme disease. Targeted spraying of acaricides seems to help lower the tick

concentration in residential areas (Curran et al. 1993; Allain and Patrican, 1995; Schulze et al.

2001). Examples of acaricides currently used include permethrin, deltamithrin, carbaryl, and

chlorpyrifos. Schulze et al (2005) found that a single application of granular deltamethrin

reduced the tick population by 100% for up to 12 weeks. However, the chance of ticks

developing resistance to acaricides increases with overuse of insecticides. Rhipicephalus

(Boophilus) microplus ticks, more commonly known as cattle ticks, in the United States are

showing resistance against both organophospates and permethrin (Kunz and Kemp, 1994; Miller

et al. 2007). The Boophilus tick is in the same family as I. scapularis (Family Ixodidae),

suggesting that this resistance may also develop in the main vector of Lyme disease in North

America. A main difference between these two tick species, however, is that Boophilus is a

single-host tick while I. scapularis is a multi-host tick (three hosts to be exact). These multi-host

ticks typically do not develop resistance to insecticides as quickly as ticks who depend on just

one host (Kunz and Kemp, 1994). It should be noted, however, that multi-host ticks have already

developed resistance to earlier acaricides, including OP-carbamate, toxaphene, and lindane

acaricides (Kunz and Kemp, 1994). Therefore, resistance against the newer acaricides will

probably develop over time.

Another method for controlling ticks and Lyme disease is to reduce the tick abundance on

small mammals such as the white-footed mouse. One way of accomplishing this is via the use of

“tick tubes.” These easy-to-make tubes are hollow pipes stuffed with pesticide-treated cotton

balls, which burrowing animals, such as the white-footed mouse, use in their burrows. By

exposing ticks feeding on these rodents to pesticides, we may be able to stop these infected ticks

from spreading the disease. Another way to expose white-footed mice and other rodents to these

pesticides is by luring them into small boxes that brush them with tick-killing chemicals. Dolan

et al (2004) found that bait boxes are an effective delivery method of acaricides onto white-

footed mice, as both nymphal and larval tick infestations on white-footed mice reduced by 68

and 84%, respectively. An alternative to the chemical control of ticks on mice/rodents is the use

of microscopic roundworms that infect and kill ticks, especially adult female ticks. Symbiotic

bacteria living within the guts of these nematodes (genus Xenorhabdus) are released within the

tick once the worms get inside the tick, which then liquefy the tick’s tissues (Suszkiw 1998).

This is beneficial for the microscopic nematode as these worms then feed on the tick’s liquefied

tissues, followed by mating and the generation of thousands of offspring. However, more

research is needed to verify both the effectiveness and safety of this control method.

Another way in which wildlife control management could help prevent the spread of

Lyme disease to the human population is via the control of our deer populations. White-tailed

deer are an important host for I. scapularis, primarily the adult ticks. Researchers found that

managing deer for tick control and Lyme disease can be quite effective. One control strategy is

via deer exclusion/restriction using deer fencing. Stafford III (1993) found that high tensile

electric deer fencing significantly reduced both nymphal and adult I. scapularis numbers.

Similarly, Daniels and Fish (1993) found 84% fewer nymphs inside fenced areas in New York.

The main limiting factor of this strategy, however, is that fencing can only be used to keep deer

out of small areas (ex. around homes) due to installation and maintenance costs. Another

seemingly effective strategy is to treat the deer themselves with acaricides to kill ticks on the

deer. Results from a long-term study on the effectiveness of this strategy found that blacklegged

ticks were reduced by about 60-70% over 5 years of use (Garnett et al. 2011). They also found

that this reduction in ticks had a significant impact on the incidence of Lyme disease in the

surrounding communities. A similar study done in Maryland found a 96-97% reduction in

nymphal blacklegged ticks when exposing deer to acaricides (Solberg et al. 2003). There are

several potential downfalls associated with this strategy. The “4-poster” feeding devices

commonly used are not approved in all states. Most states require permits from state wildlife

authorities before these devices can be installed. This strategy is also costly and requires a great

deal of time and expertise from personnel. A third strategy is deer reduction. This is

accomplished via regulated traditional hunting, controlled hunts, or sharpshooters. It seems that

reducing deer densities to less than 20 deer/mile2 can significantly reduce tick bite risk, while

densities around 8 deer/mile2 can interrupt the enzootic cycle of Lyme disease and the

transmission of B. burgdorferi to both wildlife and humans (Rand et al. 2003; Kilpatrick et al.

2014; Stafford III and Williams, 2014). Problems with this strategy include community

acceptance of lethal deer management strategies (Kilpatrick and LaBonte 2003; Kilpatrick et al.

2007). This strategy is also more difficult to perform in areas that are densely populated by

people.

Using models for Lyme disease

Modelling can be extremely helpful when trying to understand how a disease system

works. The SIR model is a basic model that breaks up a population into three groups:

susceptible, infected, and recovered/removed. There are several assumptions associated with this

model, including a closed population, constant rates (transmission, removal rates), and a well-

mixed population. This model is often too simplistic (and unrealistic) for wildlife diseases, such

as Lyme disease (Lloyd-Smith et al. 2005). Another tool often used in epidemiology is the basic

reproductive number (R0). This value is defined as the expected number of secondary cases

produced by a single infection in a completely susceptible population. It is useful for evaluating

the risk of the disease spreading (epidemic).

Hosts of B. burgdorferi vary in their competence as reservoir for the spirochete. While

some are very competent (white-footed mouse), others are not (white-tailed deer). When

developing models for a disease system, it is vital that one considers how long the infection lasts

in the host and whether or not the host establishes immunity after it has been infected.

Kurtenbach et al (2006) discussed this when considering SIR model alternatives for Lyme

disease. The white-footed mouse endures long-lived infections and maintains high infectivity. It

is therefore pretty well-explained by the SI model, which only consists of two groups:

susceptible and infected. Once the mouse becomes infected, it pretty much stays infected. Other,

less competent hosts of B. burgdorferi may be explained by SIR if they establish immunity to the

bacterium after infection, or by SIS is they don’t build up an immunity to the bacterium.

Kurtenbach et al (2006) argues that neither of these models truly fit the less competent wildlife

hosts as these individuals never fully recover. Instead, they argue that these individuals become

persistent carriers, but with low efficiency (making them less competent hosts). Potential causes

for this persistence include how the bacterium can suppress and evade the host’s immune system.

A more appropriate model for these hosts would therefore be the susceptible-infected-carrier

(SIC) model. Most physicians currently believe that an antibiotic treatment should remove B.

burgdorferi from a human’s system (Feder et al. 2007), although this is heavily debated

(Miklossy 2012; Berndtson 2013; Stricker and Johnson, 2013). If doctors are correct in assuming

this, then humans, or any individual treated for the infection, would probably follow the SIS

model as re-infection of the bacterium often occurs (Nadelman and Wormser, 2016). If antibiotic

treatments are not effective, however, the SIC model would better fit the human host.

Randolph (1998) proposed a mathematical model for the R0 of tick-borne diseases. She

based this equation off of the equation used for insect-borne infections (Dietz 1980). Alterations

were made to this equation due to the differences between Ixodid ticks and other insects (Table

3). The equation for tick-borne parasites goes as follows: R0=Nf β v−t β t−t βt−v pn F

H (r+h), whereas the

original insect-borne equation was: R0=N a2 βv−i β i−v pn

H (r+h)¿¿. Please see Table 4 for parameter

definitions.

This equation is performed for every life stage of the tick (larva, nymph, adult), which are

then summed up to calculate the total R0 value for that tick. Changes were made to the original

insect-borne pathogen equation for the following reasons. First, ticks only have one blood-meal

per life stage instead of multiple as with other biting insects. The daily biting rate (a2) was

therefore replaced by the probability of the tick feeding on an individual of a particular host

species (f). Next, the transmission coefficients (β) includes β t - t for transmission between ticks

(trans-stadially or transovarially). This term is a bit outdated, however, as researchers are now

finding that this type of B. burgdorferi transmission between ticks does not occur (Rollend et al.

2013). The vector’s reproductive rate (F) is also added in the numerator for the tick-borne

parasite equation. This term is highly dependent on the life stage and sex of the tick. The term “-

ln p,” which stands for the vector’s daily mortality rate, was removed from the denominator. This

is because the term confounds survival periods with development periods. Ticks can live for a

long time (~ 2 years) at the same life stage. Longer survival, however, translates into a slower

pace of transmission.

Some recommended alterations to Randolph’s equation for calculating R0 for Lyme

disease include removing the β t - t term, as researchers now know that this type of transmission

for B. burgdorferi does not occur between ticks. The addition of abiotic factors that can strongly

affect a tick’s survival and host-seeking activity is also strongly recommended (Kurtenbach et al.

2006). For example, photoperiod, temperature, and humidity can greatly influence a tick’s

behavior and distribution (Ogden et al. 2005; Randolph 2004). Including these abiotic factors

will be especially important to assess the potential spread of both Ixodid ticks and B. burgdorferi

as a result of climate change (Ogden et al. 2014).

The future of Lyme disease

Urbanization

Urbanization can significantly reduce biodiversity. This is especially true for species

diversity within the actual urban areas. By reducing the biodiversity of urban wildlife,

urbanization can strongly influence the transmission of several vector-borne diseases such as

Lyme disease. This link is explained by the ‘dilution effect’ (Bradley and Altizer, 2007). In this

process, a low host species richness (low wildlife biodiversity) can increase parasite transmission

if it causes an increased proportional abundance of competent reservoir hosts. In the case of

Lyme disease, this would mean an increased proportional abundance of white-footed mice

compared to other urban wildlife species. In a way, the loss of biodiversity is removing the less

competent hosts, thereby helping the pathogen’s amplification by increasing its chances of

coming into contact with competent hosts (Brunner and Ostfeld, 2008). Nupp and Swihart (1998)

confirmed that white-footed mice tend to reach higher abundances in species-poor communities,

such as urban areas. Additionally, other studies verified that a greater proportional abundance of

white-footed mice is linked with an increased infection prevalence in ticks, mice, and humans

(Allan et al. 2003; LoGiudice et al. 2003). This would suggest that people living in these urban

areas would have a greater risk of contracting Lyme disease than people living in rural forested

areas. However, this depends on whether or not the decrease in urban wildlife biodiversity truly

increases the proportional abundance of this competent reservoir host. For example, if there is no

increase in the proportional abundance of white-footed mice in urban areas, then the risk of

contracting Lyme disease would not be expected to go up.

Most urban areas retain a few small areas of “forest” in the form of city parks.

Researchers from different areas of the world found that Lyme disease may be acquired in these

parks (Pokorny 1990; Guy and Farquhar, 1991; Schwartz et al. 1991; Magnarelli et al. 1995).

Matuschka et al (1996) found that the Norwegian rat (Rattus norvegicus) was a very competent

reservoir host for Lyme disease in city parks located in Magdeburg, Germany. All of the rats

sampled were heavily infested with both larval and nymphal I. scapularis, and they had a

larvae:nymph ratio of 6:1. When assessing the rats’ seropositivity for B. burgdorferi, they found

that all rats produced infected ticks. Similar results were later found for black rats (Rattus rattus;

Matuschka et al. 1997). Rats fulfill several criteria for being competent reservoir hosts for B.

burgdorferi: they are comparatively long-lived (~2 years), they remain persistently competent,

they are frequently parasitized by both nymphal and larval I. scapularis, and they forage

relatively far from their nests. Since rats are a common urban species, these findings may suggest

an increased risk of contracting Lyme disease for people who visit urban parks.

Another side-effect of urbanization is habitat fragmentation. While habitat fragmentation

affects many wildlife species in several different ways, one important result of this process is

increased interspersion of wild and populated areas which leads to an amplified edge-effect.

Jackson et al (2006) found that landscapes with a large percentage of forest-herbaceous edge are

associated with higher Lyme disease rates. This positive correlation was also found by Das et al

(2002). Two important reservoirs of B. burgdorferi, the white-footed mouse and white-tailed

deer, thrive in edge habitats, and the abundance of I. scapularis is highly correlated with the

abundance of these two wildlife species. Edge habitats are therefore expected to harbor more

potentially infected ticks, which leads to an increased risk of Lyme disease for people living near

these habitats. Another important component of habitat fragmentation is the creation of patches.

The size of these remaining forested patches can have serious effects on both wildlife

biodiversity and disease transmission. Allan et al (2003) found that Lyme disease risk is

inversely related to forest patch area. He found that patches that were smaller than 1-2 hectares

had a greater Lyme disease risk due to these areas having higher densities of infected nymphal

ticks. A potential reason for these findings is that small patches contain relatively larger

populations of white-footed mice. Due to this reservoir’s high prevalence and infectivity rate,

most ticks in smaller patches are therefore expected to become infected with B. burgdorferi.

Climate change

Understanding how a disease moves, or will move, over time is vital. Pathogens have

their own unique ways of spreading to new places. Some common examples include

hosts/reservoirs moving the pathogen into new places or changing environmental variables

allowing for a geographic spread of both the pathogen and its hosts, reservoirs, and/or vectors. In

the case of Lyme disease, the pathogen is highly dependent on the presence of its vector and

suitable reservoirs such as the white-footed mouse. Ogden et al (2006) explored how the spatial

distribution of Lyme disease may change due to climate change using an I. scapularis population

model from Ogden et al (2005). They were specifically interested in studying the northern spread

of the disease into Canada as the current northern limit is near the United States/Canada border.

As I. scapularis requires temperatures above 0°C in order to establish in a new location (Ogden

et al. 2004), they mapped annual degree-days > 0°C limits using temperatures projected for the

2020s, 2050s, and 2080s. These temperature projections were generated from two Global

Climate Models (Canadian CGCM2 and UK HadCM3) using two different greenhouse gas

emission scenarios (A2 and B2). While the A2 emission scenario assumes no change in the

current emission trends, the B2 scenario does assume a reduction in greenhouse gas emissions.

The A2 scenario found that I. scapularis establishment would move northwards about 200 km by

2020s and 1,000 km by 2080s. While the A2 and B2 scenarios were pretty similar for the initial

years, there was a noticeable decrease in the tick’s range expansion between 2050s and 2080s

under scenario B2. Ogden et al (2006) also explored how the abundance of I. scapularis would

change over time with climate change. They found that tick abundance almost doubled by the

2020s at the current northern limit under scenario A2. They concluded that the increased spatial

spread combined with the tick’s increased survival at already established locations will lead to a

detectable range expansion of I. scapularis within the next two decades. Lyme disease is

therefore predicted to expand significantly northwards due to climate change within this century.

Overall, this study exposed two important findings: (1) there will be a northwards spatial

movement of I. scapularis due to climate change and (2) there will be an increased abundance of

I. scapularis ticks at existing locations. Together, these discoveries suggest that Lyme disease is

very likely to expand its range in the upcoming years. Canada already has white-footed mice,

which will further ease the spread of Lyme disease into its new northern territory. Although they

found no significant differences between the expected northward spread of I. scapularis between

now and the 2050s under scenarios A2 and B2, they did find that reducing greenhouse gas

emissions (scenario B2) will help reduce its distribution between the 2050s and 2080s. We

should therefore see this as a warning sign that the continued high release of greenhouse gasses

will further aid this pathogen in expanding its range, thereby exposing more individuals to Lyme

disease.

Future studies

Lyme disease already affects around 30,000 people living in the United States each year

and is expected to increase in both its range and abundance over the next few years. It is

therefore vital that we continue studying its pathogen, B. burgdorferi, and ways to better protect

ourselves from infection. Future studies on this disease should focus on improving both our

diagnostic tests and treatment plans in order to lower the incidence of misdiagnosis and chronic

Lyme disease. A new vaccine for humans, wildlife, or both could be beneficial in preventing the

infection. Additionally, more studies should be conducted on coinfections as it is still not clear

whether these can further complicate diagnosis and treatment.

Another interesting topic to study is the competency of human hosts. Competent

reservoirs for B. burgdorferi are those with a high “realized reservoir competence” (Brunner et

al. 2008). This competence is a function of the probability that (1) the host is infected (infection

prevalence) and that (2) the infected host will transmit the infection to the vector (infectivity).

The white-footed mouse is a competent reservoir for B. burgdorferi in North America as the

majority of its population is infected and they are highly infective to feeding ticks (Mather et al.

1989). Although most large mammals are generally less competent reservoirs, such as the white-

tailed deer and mule deer (Odocoileus hemionus), we have not yet closely examined the human

host. It has already been established that infection prevalence is high among the human

population in areas where Lyme disease is endemic. However, no concrete research has yet been

done to determine our infectivity to vectors and other hosts. In order to address these questions,

we could calculate infection prevalence (π) and infectivity (φ) in people according to calculations

developed by Brunner et al (2008). This will require blood samples from a number people (as

many as possible), preferably from several locations that have different Borrelia species/strains.

These blood samples can then be fed into artificial feeders using a silicone membrane proposed

by Andrade et al (2014). Another option is to use humanized mouse models, which will require

us to first develop one specifically designed for B. burgdorferi (Vaughan et al. 2012). Once non-

infected nymphs feed on these blood samples, we will let them molt before screening them for B.

burgdorferi using a direct immunofluorescent assay (LoGiudice et al. 2003). Results from these

tests will be used in the before-mentioned calculations.

This information may be able to tell us whether or not people, who travel more than

ordinary animals, are partly responsible for the recent spread in Lyme disease. Many people are

unaware that they have Lyme disease, which is not helped by the high incidence of false

negatives in serological tests (Schutzer et al. 1990). This allows for the infection to persist in

many people unknowingly, increasing the infection prevalence within the human population.

Additionally, if we can get several Borrelia species/strains involved in the study, we could see

whether our competency as hosts differs among Borrelia species/strains.

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Figure 1. Total number of reported cases of Lyme disease from 2005-2014 in the United States. Data was collected from the CDC (2016c).

Figure 2. Diagram of the life cycle of Ixodes ticks (CDC 2015b).

Table 1. The 18 subspecies of Borrelia burgdorferi sensu lato, along with information about their distribution, host type, Lyme disease transmission, and recommended sources.

Subspecies name Range Host type Lyme disease transmission References1 B. afzelii Europe, Asia Mammalian Yes Canica et al. 19932 B. americana North America Avian No Rudenko et al. 2009b3 B. andersonii North America Mammalian No Marconi et al. 19954 B. bavariensis Europe, Asia Mammalian No Margos et al. 20095 B. bissettii North America, Europe Mammalian No Postic et al. 20076 B. burgdorferi North America, Europe Mammalian, Avian Yes Johnson et al. 19847 B. californiensis Western USA Mammalian No Postic et al. 20078 B. carolinensis Southeast USA Mammalian No Rudenko et al. 2009a9 B. garinii Europe, Asia, Arctic-Antarctic circles Avian Yes Baranton et al. 1992

10 B. japonica Japan Mammalian No Kawabata et al. 1993; Postic et al. 199311 B. kurtenbachii North America, Europe Mammalian No Margos et al. 201012 B. lusitaniae Mediterranean basin Reptilian No Le Fleche et al. 199713 B. sinica China Mammalian No Masuzawa et al. 200114 B. spielmanii Europe Mammalian No Richter et al. 200615 B. tanukii Japan Mammalian No Fukunaga et al. 1996a, 1996b, 1996c16 B. turdi Japan Avian No Fukunaga et al. 1996a, 1996b, 1996c17 B. valaisiana Europe, Japan Avian No Wang et al. 199718 B. yangtze China Mammalian No Chu et al. 2008

Table 2. Total number of reported cases of Lyme disease from 2005-2014 in the United States sorted from highest to lowest. Data was collected from the CDC (2016c).

State Total casesPA 43925NY 37977NJ 31565MA 29774CT 21596WI 16903MD 13243MN 10894NH 8650VA 7547ME 7226DE 6270VT 3199RI 2233IL 1733WV 981IA 966CA 856MI 777FL 633IN 625TX 562OH 499DC 414NC 302SC 236GA 185WA 131ND 107KS 106TN 102AL 98OR 82AZ 80NV 71AK 69KY 68NE 64ID 58MO 57MT 53UT 49NM 22SD 19WY 16LA 15MS 11OK 8CO 3AR 1HI 0

Table 3. Differences between Ixodes ticks (vector of Lyme disease) and other biting insects (Randolph 1998).

Table 4. Definitions for the parameters used in the original insect-borne parasite disease R0 equation and the altered tick-borne parasite disease R0 equation (Randolph 1998).