disease in disguise

8/3/2019 Disease in Disguise

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Disease inDisguise

Could yourhorse's vaguesymptoms besigns of Lymedisease?By Elaine Pascoe, withSandra L. Bushmi ch , DVM

YOUR HORSE IS LAMEon and off-first in onelimb, then another. Even onhis sound days, he's just not himself.He pokes around his paddock like ahorse twice his age, and he pins hisears and acts cranky.Overwork? Hard ground? Per-

haps. But those vague complaintscou ld be signs of Lyme disease,which hits horses people and dogs inmany parts of the country. Lyme dis-ease can have devastating effectsnotjust lameness, but also potential dam-

age to a horse's heart. ner\'ous systemand vision. It has been tracked andstudied for more than 20 years, butit's still tricky to diagnose and often ish?rd to treat.Researchers ?re m.aking progress,though. For an overview, veterinarianand Lyme researcher Sandra Bush-mich of the University o f Conne cti-cut, brings us up to date. She helpsexplain how the insidious diseasetakes hold, how to know if your

horse is at risk and what to d o if youthink he shows signs,


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Ticks Off!About 23,000 Americans contracted Lyme dis-ease in 2002, the latest data ava ilable. If youspend time outdoors in fields and woods, or any-where tha t black-legged ticks are likely to befound, take these steps to pro tect yourself. Routinely use an insect repellent containingDEET (n,n-diethyl-m toluamide) on clothes andexposed skin. Wear long pants and a long-sleeved shirt inlight colors (so dark ticks will be easy to spot).Tucking pants into socks or boo t tops may hetpkeep ticks from reaching your skin. Check for ticks carefully when you've been out-side. Remove any you find. If you've been bitten, cleanse the area wi th anantiseptic. It's generally not necessary to see adoctor unless you develop symptoms, which mayinclude a characteristic "bull's-eye" rash aroundthe bite site, fever, malaise, fatigue, headache andmuscle and join t aches.

More information about human Lyme diseasefrom the Centers for Disease Co ntrol and Preven-tion is at www.cdc.gov/ncidod/dvbid/lyme.

Tick T roubleBlame a tiny tick for thespread of Lyme disease.The disease is caused byspiral-shaped bacteria, Bor-relia burgdorferi (Bb), car-ried by black-legged ticks,mainly deer ticks (in theEast) and Westem black-legged ticks. Not all black-legged ticks carry the dis-ease, but up to half of themmay harbor the bacteria insome areas.

The disease takes itsname from Lyme, Conn.,where it was identified inthe 1970s. Since then, caseshave turned up in nearlyevery state. It's most com-mon in New England, theMid-Atlantic region, the up-per Midwest and parts ofthe Pacific Northwest-places where there are lotsof white-footed mice anddeer, the preferred w ildlife

hosts of ticks. The borders ofthese "hot" Lyme disease areasare slowly e xpanding, Bush-mich says, as deer herds in-crease. Migrating birds likely al-so play a role in the spread, bycarrying infected ticks to newareas. But the disease is stillrare throu gho ut the Plainsstates and the Southwest.

Ticks have th ree life stages:larva, nymph and adult. Theymust consume a blood mealbefore they can molt into thenext stage. Ticks pick up thebacteria as tiny larvae by feed-ing on the blood of infectedmice and other small animals.As pinhead-sized nymphs andsomewhat larger adults, theycan transmit bacteria to theirnext hosts, which might be anypassing mam mal or bird. Th etick generally attaches andfeeds for 24 to 48 hours beforeit transmits the bacteria to itsnew host.

Lyme Disease Incidence County of ResidenceUnited States, 2002

Cases per 1 DO.ODD persons

H H 1 -9.9^ B 10-99.9

In the United States, most Lyme diseaseinfections occur in the Northeast, NorthCentral and West Coast states . Lymedisease is caused by black-legged ticks .They're shown below, left to right, in theiradult (female and male), nymph andlarval stages.


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Adult-Stage black-legged ticks are most likely to feedon horses and other large mammals such as humans anddeer. The adults appear in early fall, spend the winter dor-mant in bmsh and leaf litter and come out again in earlyspring. This makes early spring and fall prime times fo r in-fection. But horses can get Lyme disease any time fromspring through fall, as the ticks are active whenever thetemperature is above 40.Confusing SignsOnce In the new host, the bacteria travel through thebloodstream. Nine times out o f 10, even though a horsemay be infected, he doesn t develop any symptoms ofthedisease. Occasionally, though, the bacteria set up shop invarious body tissues. Depending on where they lodge, thedisease may affect tbe musculoskeletal system, tbe nerv-ous system and, less often, other organs. Here are some ofthe common symptoms: Lameness, especially in large joints (the fetlock or knee,for example, but rarely the fo ot). Lameness that is spo-radic, affects m ultiple sites or shifts fro m limb to lim b ischaracteristic. Behavioral changes, such as unwillingness to w ork anda depressed or crabby attitude. Low energy levels. Generalized stiffness or tenderness. Sore back. Low-grade fever.

Symptoms m ay take several weeks to develop, mayvary wide ly (only some horses run a fever, for exam ple)and may com e and go. Sometimes the symptoms just goon their o wn . But don t ignore tbese signseven wh ensymptoms fade, the bacteria can still be lurking in thehorse and damag ing tissues. Weight loss, chron ically stiffand swollen joints, laminitis, neurological problems andinflamm ation of heart or eye tissues have been lin ked t opersistent Lyme disease.

Early diagnosis is the key to preventing these seriouseffects, but getting that diagnosis is a challenge. Manycausesexcessive work, hard ground, injuries, other dis-easescan prod uce similar signs. Blood tests can helpmake the diagnosis. Tbe most wid ely used tests look forantibodies prod uced as part o f the horse's immu ne re-sponse to the Bb bacteria. (See sidebar at right, this page.)

A positive test doesn't tell you that your horse has thedisease at that moment-only that he has been exposed.Antibodies continue to circulate long after the bacteria aregone, and they can take many months to leave the blood-stream. It may take more than one test, a month or twoapart, to tell if he has an ongoing infection. A nd in areaswhere Lyme disease is prevalent, up to 50 percent of horsesmay test positive, says Dr. Bushmich. Of those, only about 9percent actually get sick.With no surefire test available, veterinarians generallybase the diagnosis on clinical signs and use blood tests to

Testing, TestingVeterinarians can turn to three blood tests for helpin identifying Lyme disease, and a fou rth test shouldbe available soo n. Each test has its u sesand itsdrawbackssays Dr. Sandra Bushmich. She directsthe Diagnostic Testing Service at the ConnecticutVeterinary Medical Diagnostic Lab, which bas beena leader in Lyme disease testing and analyzes sam-ples from all over tbe country.

One test, know n as ELISA (enzyme -linked im-mun osorbe nt assay), looks for antibodies to Lyme-causing Bb bacteria.The horse doesn't prod ucethese an tibod ies for a few w eeks, so if blo od isdrawn soon after infection, a false negative result ispossible. If the test is repeated a few weeks laterand antibo dy levels have gone up , tbat's a prettygood indication of in fection. But at tow an tibodylevels, false positives are also possible, since the testdoesn't discriminate well between antibodies to Bband similar organisms.

A Western b lot test is more sensitive. This testlooks for antibodies produced against specific pro-teins ofthe Bb bacteria. Veterinarians often followup on a positive ELISA w ith this tes t, or order b othtests together, to be sure of identifying antibodiesspecific to the organism.The Snap 3Dx test was deve loped fo r dogs and isbein g used by some vets for horses. Like ELISA, it

looks for a ntibod ies. Its main advantage is an in-stant result; tbe blood sample does not bave to beprocessed in a lab as it is a quick screening test. Itdoesn't give much information a bout the levels andtypes of antibodies foun d.A new PCR (polymerase chain reaction) test de-velope d at UConn looks for Lyme bacteria by check-ing for segments ofthe organism's DNA.The labhopes to make the test commercially available

soon. A positive result in this test is a powerful signof current infection . Dr. Bushmicb says. But a nega-tive result doesn't rule out the disease, as the bacte-ria may be absent in blood while hiding in joints orother tissues.

back up their findings. If you live in an area where Lymedisease occurs and your horse shows Lyme-like symptoms,your veterinarian likely will start witb a thorough exam. Ifthat and a review of your horse's history rule out othercauses (sucb as an injury) for his symptoms and, if bloodtests suggest Lyme disease, then your vet will probably rec-omm end that he be treated for it.


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Vaccinat ionBetter Mousetrap?Could the risk of Lyme disease for hu -mans and horses be cut by vaccinat-ing wildlife? It's possible, according toresearch supported by the NationalInstitutes of Health. Wildlife vaccina-t ion programs, using oral vaccines,have been effective against rabies insome areas.

Black-legged ticks usually pick upLyme bacteria while feeding on miceas larvae, in a four-year study, re-searchers trapped nearly 1,000 white-footed mice at sites in Connect icutforests. Mice at half the sites were g iv-en a vaccine similar to that used fordogs; mice from the other sites re-ceived a placebo.

Where wild mice were vaccinated,fewer deer t icks tested posit ive for

Outlook for RecoveryWhen Lyme disease is recognizedand treated early, the outlook for re-covery i5 good . Most cases respondto antibiotics. Your vet may also sug-gest an anti-inflamma tory such asphenylbutazone to ease discomfort.

Horses usually show improve-ment in the first two to five days. Tobe sure of success, an tibiotic med-ication generally continues for 30days. Follow-up blo od tests track im-provement by showing reduced an-tibody levels.

The three drugs most widely usedto treat Lyme disease are oral doxy-cycline, ceftiofur (Naxcel), which isgiven by intramuscular injection andintravenous tetracycline,

Tetracycline is the most effective,but the intravenous (IV) hookup of-ten means that the horse m ust stayat a clinic.Because that treatment is expen-sive and inco nvenient, many vetsprescribe oral doxycyciine or intra-muscular (IM) ceftiofur and recom-

mend IV tetracyciine for horses thatdon t respond or that relapse after

the bacteria, thus cutting the risk thbacteria could be passed on to othtick hosts. Durland Fish, a professore p i d e m i o l o g y at Yale Univers iSchool of Medicine, led the study wiJean Tsao, who is now at MichigaState University, and microb iologiAlan Barbour of the University of Cafornia at Irvine.

Vaccinat ing mice didn' t cut thrisk as much as the investigators hahoped, though.They discovered thmice are not solely responsible for ifecting the ticks. Many ticks pick uthe bacteria while feeding on othsmal l animals: shrews, chipm unkeven birds. To beeffective, a wildlivacc ination program wo uld targthese animals as welL

treatment. U niversity of Connecticu(UConn) researchers are finishing ua two-year study comparing the ef-fectiveness of ceftiofur and doxycy-ciine in infecte d horses; resultsshould be out later in 2005.

The UC onn study uses horsesthat have been infected throughcontact with wild ticks.In some studies, researchers fol-low a protocol in which they first administer drugs, to suppress experi-

mental subjects' immune systems,and then induce an infection.Insead, the UC onn study has

mo nitored several groups of horses,noting if and w hen individuals be-come infected by w ild ticks, thenplacing the infected horses in treat-ment groups and following theircourse of treatment.Some horses suffer recurrentbouts of Lyme disease, and it s unclear whether they are harboring Bbbacteria between episodes or are reinfected each time. There is also achance that damage in joints or oth-

er tissues will have effects that lingeafter the infection clears up.


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Outwit the TickDogs can be vaccinated against Ly-me disease, but there is currently noreal effort to bring a horse vaccineto market. Drug companies believethat Lyme disease in horses has cre-ated a "boutique market," Dr. Bush-mich says.

Some veterinarians have vaccinat-ed horses using the canine vaccine,and it does prompt the horse's systemto produce antibodies. TTie vaccinecontains a proteincalled OspA-thatis found on the outer surface of theLyme bacteria when they are insideticks, but not when they infect dogs(or horses).When an infected tickfeeds on blood from a vaccinated ani-mal, the tick takes in antibodies thatkill the bacteria in its gut before thenew host can be infected.

Researchers at Cornell found thata vaccine nearly identical to the ca-nine vaccine offered protection toponies not previously exposed to

L y m e . A l t h o u g h no il l ef fects weren o t e d in the study, extensive safetystud ies haven ' t been done in horses.Also , no o n e kn o w s how m u ch pro-tect ion th is vaccine provides forhorses in less-contro l led cond i t ionso r how l ong the pro tect ion lasts. Indogs, vacc ina t ion is on ly abou t 75percent e f fect ive , accord ing to p u b -l ished reports. (A h u m a n L ym e vac-cine w as taken off th e marke t in2 0 0 2 , repo r ted ly d u e to poor sales.There a lso were some compla in ts o farthri t is-l ike side effects.)

In any case, Dr . Bushmich says,it's no t useful to vaccina te once ahorse has been exposed to the dis-easeand many horses in " h o t " ar-eas have been exposed. But if y o ul ive in an area where Lyme diseaseis prevalent, y o u c an p ro tec t you rhorse by m i n i m i z i n g his exposu re tot icks. Here are th ree ways:

1 . Use fl y repe l len ts tha t conta inpe rme th r ins , fo l l ow ing t h e labe l di-

Fly repellents with permethrin as an active ingredient will discourage ticks. Be sure to apply inspring and fall, wliich are prime times for Lyme infec tion.

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Horses in high weeds and brush are easily exposed to ticks. Regular mowing of paddocks andpruning of low branches are preventive measures.

rections. These insecticides are ef-fective against t icks as well as flies,so they wil l discourage t icks fromcl imbing onto your horse and set -t l ing in for a meal . Be sure to applythe repellents in fal l and ear lyspring, when adult t icksare act ive.

2. Check your horsefor t icks daily as patt of athorough g room ing . Because the ticks feed atleast 24 hours beforetransmit t ing the disease,p rompt ly remov ing t hemcuts the r isk of infect ion.Black-legged ticks aret iny, but adults, thosemost l ikely to feed onhorses, are large enoughto f ind and remove.Check carefu l ly aroundth e head, ears, throat-la tch, bel ly and under thetailplaces where t icksare l ikely to hide. I f you f ind at ick, use tweezers to remove it .Grasp it r ight at the skin, where i tsmouthpar t s a re embedded , andpul l gent ly up.

3. M o w grass, cut brus h, andt r im overhanging branches in your

horse s pasture or paddock. Tallgrass and brush harbor t icks; thelit t le pests crawl to the t ips ofleaves and branches and latch ontyour horse as he brushes past. Get ing r id of their lurking places wil l

Check your hot^e for ticks during thorough, dailygrooming; pay special attention to ears, throa tlatcbelly and under the tail.

cut the ir numbers.Wh i le you ' re p ro tec t ing your

horse, take steps to protect your-self from tick bites. (See 'TicksOff !" on page 146.) With care, yoand your horse can avoid th is potent ia l ly devastat ing disease, m


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disease in disguise

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