Lecture 4

Announcements1. Email me if you did NOT receive a

message from me last Friday. 2. BlackBoard kinda works but we will stay

with our own site for now: www.uky.edu/~gcbrown/ent561

3. Will try to get the lecture notes on sooner.4. No quiz on Thursday5. Online Reading:

http://www.crid.or.cr/digitalizacion/pdf/eng/doc14606/doc14606-10.pdf

Epidemic Components

• 1. The etiologic agent = microorganism that can cause disease. – Pathogenicity = its ability to cause disease;– Virulence = the severity of infection, expressed as:

• morbidity = incidence of disease• mortality = death rate of the infection.

• 2. The method of transmission = means by which the agent goes from the “source” to the host. We are only concerned with vectored pathogens here.

• 3. The host – Depends on 3 things:– Accessibility– Susceptibility– Transmissibility

Most studies of arthropod-vectored disease study:

• Time Trends – Four kinds– The secular trend - occurrence of disease over years– The periodic trend – temporary change in a secular trend, – The seasonal trend - seasonal changes in disease occurrence– The epidemic occurrence of disease.

• Place Considerations– where the individual was when disease occurred; – where the individual was when he or she became infected;– where the vector became infected with the etiologic agent.

• Infected Person– Individual characteristics: age, sex, occupation, personal habits,

underlying disease, immunization history, etc.– Social characteristics: socioeconomic status, geographical

distribution, public works & policies, disease rates in other group members

Example: Rocky Mountain Spotted Fever

Etiological Agent – Rickettsia rickettsii

Vector – Dog & wood ticks

Example: Rocky Mountain Spotted Fever

Secular trend of the disease

www.cdc.gov/ncidod/dvrd/rmsf/Epidemiology.htm

Periodic Trend

Example: Rocky Mountain Spotted Fever

Seasonal Trend

Example: Rocky Mountain Spotted Fever

Epidemic Time Period

Example: Rocky Mountain Spotted Fever

Geography Age

Parasite types in Medical Entomology

• Micro vs macroparasites – Micro are microorganisms (viruses, bacteria, protozoa, etc.). Macro are arthropods.

• Ectoparasites vs endoparasites – – Ecto – parasitize from outside the host, infestation

normally remains topical and peripheral– Endoparasites – parasitize from inside the host,

Microparasites can produce systemic infestation. Disease is normally systemic.

• Facultative vs. obligate – – Obligate are totally dependent on host.– Facultative have parasitic and free living forms.

Vectors and bloodfeeding

• Most vectors are blood-feeders:– Normally the pathogen does not benefit the

vector, detrimental effects sometimes occur.– The pathogen receives many benefits

(acqusition, transport, re-insertion).– Principal problems for the pathogen from this

system:• Must overcome ingestion by the vector (next slide)• Must be able to infect a new host from a small

inoculum (slide following)• Totally dependent on the vector-host population

dynamics and interaction.

1. Pathogen ingested

2. Pathogen may multiply here (plague bacteria, Leishmania)

3. Passage through gut wall (filarial worms, Borellia) or multiplication in epithelial layer (rickettsiae, arboviruses), or cysts (malaria)

4. Transport by hemolymph to tissues (some arboviruses multiply here too)

5. Tissue concentration in (a) salivary glands (b) reproductive system

Infection

6. Contaminated mouth (fluids, mouthparts, escape through body wall)

7. Infected feces

8. Infective fluids (e.g. coaxial gland fluids in ticks)

9. Crushing/ingestion of vector

Definitive vs. Intermediate Hosts

Definitive

• Host in which gametocycte union occurs for the parasite.

• AKA – Primary host

Intermediate

• Host in which asexual reproduction occurs for the parasite.

• AKA – Secondary host; Alternate host

Example: Lymphatic Filariasis

• Human – Definitive Host, Mosquito the intermediate host

Opposite Example: Malaria

• Mosquito definitive host, human is the intermediate host

More about hosts• accidental host one that accidentally harbors an organism

that is not ordinarily parasitic in the particular species.• amplifying host one that increases parasite numbers &/or

no. infectious vectors. Typically short-lived infections.• dead-end host the disease cannot be transmitted from the

infected host to another animal.• predilection host the host preferred by a parasite.• reservoir host an animal (or species) that is infected by a

parasite, and which serves as a source of infection for humans or another species.

• transfer host, transport host one that is used until the appropriate definitive host is reached, but is not necessary to complete the life cycle of the parasite.

About Vectors – Host Selection

• ‘phagic’ – feeds successfully; ‘philic’ – attracted to.

• Anthropophagic – feed successfully on humans; Anthropophilic – attacks humans

• Zoophagic – feed on vertebrates other than humans. Two big subsets– Mammalophagic– Ornithophagic

Modes of Transmission

• Two main modes:– VERTICAL – vector to vector in subsequent

life stages or generations– HORIZONTAL – passage of parasites

between vectors and hosts

Vertical Transmission Subtypes

• Transstadial – between subsequent life stages

• Transgeneration – between subsequent generations (several methods)

• Venereal – passage of parasites occurs between male and female during mating. Rare in vectors

Horizontal vs Vertical Transmission

Compare Figs. 3.3 & 3.2 in your text

1. Which are the dead end hosts?

2. Where are the bridge vectors?

3. Where is the amplification cycle?

4. Where is the enzootic transmission cycle?

Interseasonal Maintenance

• Most seasonal disease from arthropod-vectored pathogens in temperate zones require interseasonal maint.

• Principal mechanisms:– Continued transmission – no seasonality, tropical

diseases, disease of parasites that continuously live with hosts

– Infected Vectors – most encephalitis viruses– Infected hosts – Reservoirs go dormant, parasite has a

dormant stage in host.– No interseasonal maintenance – occasional local

extinction• Migratory hosts• Weather fronts

Principles of Disease Vector Management

• Surveillance– Vector, disease prevalence, pathogen burden

• Prevention– Personal– Public

• Control– Vector-based– Pathogen-based (pharmacologic)

Surveillance

• Very good general section on this subject in your text:

pp: 29 – 32

Prevention

• Personal– Exclusion agents (clothing, screening, bed nets, sealing

cracks/crevices, etc.)– Chemical agents (repellents)– Private source controls– Sanitation & Personal Hygiene– Source Avoidance

• Public– Public service announcements/education– Vector breeding site controls/sanitation– Disaster Management PlanningREAD:

http://www.crid.or.cr/digitalizacion/pdf/eng/doc14606/doc14606-10.pdf

A Key Concept in DV Management is Risk

• Most DV Management is Govt. Funded

• They fund efforts in response to perceived risk

• Risk = Cost of a Bad Outcome

TIMES

Probability of that Outcome Occurring

The main problem is there are multiple risks

• Multiple “Bad Outcomes” including– Economic– Environmental– Social– Political

• Each outcome has it’s own cost and probability

• Major Complication: Disease risk is inversely correlated with vector control.