Guard Against Tick-Borne Dis­eases

Learn how to pro­tect your horse from ticks, which can trans­mit se­ri­ous and de­bil­i­tat­ing dis­eases through their bites.

Practical Horseman - - Special Eventing Issue - By Elaine Pas­coe with Linda D. Mit­tel, DVM

Your horse is plainly not him­self. He’s gone from perky to plod­ding in work and he flinches and pins his ears when you groom him. Last week he seemed a lit­tle off in front. That lame­ness improved, but now his hocks seem stiff. What’s go­ing on? A tiny tick could be the cause of his prob­lems. Ticks, blood-feed­ing rel­a­tives of spi­ders and mites, can trans­mit se­ri­ous dis­eases through their bites. In this ar­ti­cle Linda Mit­tel, DVM, se­nior ex­ten­sion as­so­ciate with Cor­nell Univer­sity’s An­i­mal Health Di­ag­nos­tic Cen­ter in Ithaca, New York, helps ex­plain what you need to know. Horses are sus­cep­ti­ble to two se­ri­ous tick-borne dis­eases that are wide­spread in the United States, Dr. Mit­tel says—Lyme disease and equine gran­u­lo­cytic anaplas­mo­sis. We’ll cover both as well as equine piro­plas­mo­sis, which is rare in the U.S.. Are there other tick-borne threats to horses? Maybe, Dr. Mit­tel says. The box on page 50 has an up­date on re­search into that ques­tion be­ing car­ried out at the AHDC. Ticks also trans­mit many ill­nesses (in­clud­ing Lyme) to peo­ple, dogs and other an­i­mals, so they are some­thing to watch out for. These par­a­sites have been around for at least 100 mil­lion years, long enough to have fed on (and per­haps spread disease to) di­nosaurs. You won’t be able to elim­i­nate them from your horse’s en­vi­ron­ment, but the box on page 52 has tips for pro­tect­ing him from their bites.

Black­legged Biters

Many kinds of ticks carry disease, Dr. Mit­tel says, but black­legged ticks pose a se­ri­ous dan­ger to horses be­cause they carry both Lyme disease and equine anaplas­mo­sis. These ticks are wide­spread in the North­east, Mid-At­lantic and Great Lakes re­gions, where they’re com­monly known as deer ticks, but they oc­cur in pock­ets in the South and else­where. The West Coast va­ri­ety is called the Pa­cific (or western) black­legged tick. Black­legged ticks go through three life stages—larva, nymph and adult— and need a blood meal be­fore molt­ing from one stage to the next. They

pick up disease-caus­ing bac­te­ria as lar­vae and nymphs by feed­ing on in­fected mice, and they can pass the bac­te­ria on to their next vic­tim, be it horse or hu­man, dog or deer, or some other mam­mal or bird. The ticks over­win­ter in leaf lit­ter and emerge again in spring or when­ever the temperature is above 40° F. “Sunny days in the mid­dle of win­ter can bring the ticks out to ‘quest,’ or look for a meal,” says Dr. Mit­tel. Be­cause the ticks at­tach tightly when they bite and then stay in place, feed­ing slowly for days, the bac­te­ria have plenty of time to move into the new host’s blood­stream.

Here’s more bad news: “The ticks are ex­pand­ing their range,” Dr. Mit­tel says, po­ten­tially putting more horses (and peo­ple) at risk. The ev­i­dence comes from eco­log­i­cal stud­ies as well as serol­ogy (blood tests) for in­fec­tion with Bor­re­lia burgdor­feri (Bb), the bac­te­ria that cause Lyme disease. “The in­fec­tion preva­lence de­ter­mined by the serol­ogy shows more ex­po­sure to the Lyme or­gan­ism,” she ex­plains.

What’s be­hind the in­crease? Changes in land­scape, cli­mate and wildlife pop­u­la­tions play roles. The ticks can’t travel far on their own; in­stead, they hitch rides on deer and other wildlife hosts. As deer and other hosts in­crease in num­ber and move into new ar­eas, the ticks ride along. They drop off and be­come es­tab­lished wher­ever they find con­di­tions to their lik­ing—for in­stance, for­mer farm­land that’s re­vert­ing to brush and for­est or the shrubby borders of fields and yards. Re­searchers have noted cor­re­la­tions be­tween warm­ing cli­mate trends and the ticks’ spread far­ther north. Warm win­ters are also help­ing them emerge ear­lier in spring and stay ac­tive longer in fall. That trans­lates to greater risk of in­fec­tion.

Lyme Disease

Lyme disease is a se­ri­ous threat to horses with long-term com­pli­ca­tions that can in­clude chronic lame­ness and dam­age to the horse’s ner­vous sys­tem, heart and vi­sion. The disease takes its name from Lyme, Con­necti­cut, where it was first iden­ti­fied in the 1970s. It’s now the most com­mon tick-borne in­fec­tion in the United States—but, Dr. Mit­tel notes, in­fec­tion doesn’t al­ways lead to disease.

“In­fec­tion is the first step, when the ‘germ’ or agent en­ters the body,” she ex­plains. Disease oc­curs when the agent repli­cates in the body, dam­ag­ing tis­sues and pro­duc­ing signs. There are no na­tional sta­tis­tics on the in­ci­dence of Lyme disease in horses, but the in­crease in pos­i­tive blood tests shows more ex­po­sure to the Lyme or­gan­ism. “It would make sense that there is more ac­tual disease, but re­ports are lack­ing,” she says. Sta­tis­tics from the Cen­ters for Disease Con­trol show a steady in­crease in hu­man cases.

Black­legged ticks are tiny and gen­er­ally drop off af­ter feed­ing, so if your horse de­vel­ops Lyme disease you may never see the guilty tick. How will you know and what should you do?

Signs: Vague and vari­able signs are a hall­mark of this disease. They gen­er­ally ap­pear weeks af­ter the bite and may in­clude:

Spo­radic lame­ness, typ­i­cally in­volv­ing large joints, such as the fet­lock, knee, hock or sti­fle, and of­ten af­fect­ing mul­ti­ple sites or mov­ing from one site to an­other Lack of en­ergy Be­hav­ioral changes, such as a sour or de­pressed at­ti­tude Hy­per­sen­si­tiv­ity to touch or other stim­uli Mus­cle sore­ness Low-grade fever Over time, chronic weight loss, leg swelling and other prob­lems.

Most of these signs could be caused by a num­ber of prob­lems—even some­thing as sim­ple as over­work—and that makes di­ag­nos­ing Lyme disease a chal­lenge.

Di­ag­no­sis: The vet­eri­nar­ian will start with a phys­i­cal exam and a thor­ough his­tory to as­sess the horse’s risk of in­fec­tion. Then, if Lyme seems likely, blood tests may turn up ev­i­dence of in­fec­tion. The sim­plest is the C6 SNAP, a quick stall­side test that de­tects the pres­ence of an­ti­bod­ies to Bb bac­te­ria. It pro­vides an in­stant re­sult but doesn’t tell much about the level of an­ti­bod­ies or the stage of the in­fec­tion.

Laboratory ELISA (en­zyme-linked im­munosor­bent as­say) tests mea­sure an­ti­body lev­els with high lev­els (200–300 units and above) sug­gest­ing in­fec­tion. This test doesn’t al­ways dis­crim­i­nate well be­tween an­ti­bod­ies to Bb and those pro­duced in re­sponse to cer­tain re­lated or­gan­isms, though. A Western blot test, which de­tects an­ti­bod­ies pro­duced against cer­tain pro­teins in the Lyme bac­te­ria, is more spe­cific.

While these tests have long been used to di­ag­nose Lyme disease, they share two lim­i­ta­tions. First, be­cause the horse’s im­mune sys­tem can take sev­eral weeks to pro­duce an­ti­bod­ies to a new in­fec­tion, tests run soon af­ter ex­po­sure may yield false neg­a­tive re­sults. Sec­ond, be­cause an­ti­bod­ies can con­tinue to cir­cu­late long af­ter the bac­te­ria are gone, a pos­i­tive test may just in­di­cate past ex­po­sure to the bac­te­ria, not ac­tive disease.

A mul­ti­plex test de­vel­oped at the AHDC helps over­come the sec­ond lim­i­ta­tion. It mea­sures an­ti­body lev­els, like the ELISA, while iden­ti­fy­ing dif­fer­ent an­ti­bod­ies to spe­cific outer-sur­face pro­teins of the bac­te­ria. Bb changes the ex­pres­sion of its outer-sur­face pro­teins depend­ing on its en­vi­ron­ment, much as you might change clothes to suit the weather, so the pres­ence of dif­fer­ent an­ti­bod­ies can help de­ter­mine if a Lyme in­fec­tion is new or old.

For ex­am­ple, dur­ing ini­tial in­fec­tion the bac­te­ria ex­press outer sur­face pro­tein C (OspC). “An­ti­bod­ies to OspC are thought to de­velop about three to five weeks af­ter in­fec­tion and dis­ap­pear within four to five months,” Dr. Mit­tel says. Be­cause OspC an­ti­bod­ies in­di­cate re­cent in­fec­tion, she says some vet­eri­nar­i­ans sug­gest treat­ment based on pos­i­tive test re­sults even if the horse isn’t show­ing signs of ac­tive Lyme disease. The goal is to pre­vent the disease from de­vel­op­ing, although the value of the prac­tice hasn’t been de­ter­mined in horses. “The horse may not de­velop Lyme disease, so treat­ment would be un­nec­es­sary,” Dr. Mit­tel says. How­ever, she adds, “It may make sense to con­sider pre­ven­tive treat­ment if the horse is in an area of proven tick ac­tiv­ity and has been ex­posed.”

Once in­fec­tion is es­tab­lished, an­ti­bod­ies to a dif­fer­ent pro­tein, OspF, ap­pear. “OspF may be present for a very long time—years in some cases—whether the horse is treated or not.” A pos­i­tive test for OspF, then, doesn’t nec­es­sar­ily in­di­cate ac­tive disease that must be treated.

Treat­ment: Most cases of Lyme disease re­spond to tetra­cy­cline an­tibi­otics such as doxy­cy­cline or minocy­cline (both oral) and oxyte­tra­cy­cline (IV). “IV oxyte­tra­cy­cline is known to get the best blood lev­els in most of the body for Lyme disease,” Dr. Mit­tel says. But IV treat­ment of­ten means that the horse must stay at a clinic or have the vet­eri­nar­ian visit daily, which can be ex­pen­sive. “Oral treat­ments do not achieve the same lev­els, but many vet­eri­nar­i­ans use the oral prod­ucts and feel they see an im­prove­ment.” IV treat­ment may be rec­om­mended if the horse doesn’t re­spond to oral med­i­ca­tion or re­lapses af­ter treat­ment.

Although many horses show im­prove­ment within days of start­ing an­tibi­otics, the bac­te­ria won’t be elim­i­nated that fast. Treat­ment gen­er­ally con­tin­ues for 30 days or more.

Out­look: When Lyme disease is rec­og­nized and treated early, the out­look for re­cov­ery is good. Some horses suf­fer re­cur­ring at­tacks of Lyme disease, and it’s not clear whether they are re­in­fected or are har­bor­ing the bac­te­ria be­tween at-

tacks. In any case, the longer the disease goes un­treated, the greater the risk of lin­ger­ing ef­fects and last­ing dam­age to joints and other ar­eas.

Neu­ro­log­i­cal ef­fects of­ten take months to ap­pear, although they have been re­ported any­where from two to 730 days af­ter in­fec­tion. These ef­fects are not com­mon. But, Dr. Mit­tel says, “It is very dif­fi­cult to con­firm a di­ag­no­sis. There­fore, there may be more cases than are re­ported.” The di­ag­no­sis is con­firmed by ex­am­i­na­tion of brain tis­sue af­ter the horse’s death.

Preven­tion: The best way to pro­tect your horse from Lyme disease is to limit his ex­po­sure to the ticks that carry it. There is no ap­proved equine Lyme vac­cine and cur­rently no ef­fort to bring one to mar­ket. A ca­nine vac­cine is avail­able and in high-risk ar­eas some vet­eri­nar­i­ans give it to horses. No re­search shows that it pre­vents Lyme disease in horses, though. And while horses re­spond to the vac­cine by pro­duc­ing an­ti­bod­ies, a Cor­nell study pub­lished last year found that the re­sponse is weak and short-lived.


The bac­te­ria that cause equine gran­u­lo­cytic anaplas­mo­sis, Anaplasma phago­cy­tophila (for­merly Ehrlichia equi), take the same route as the Lyme bac­te­ria— picked up from mice or other small an­i­mals by ticks in the nymph stage and passed on when the ticks seek their next blood meal. As with Lyme, the risk is high­est in the ar­eas and at the times

when the ticks are ac­tive—and as those ar­eas and times in­crease, so do re­ports of the disease.

“More cases of anaplas­mo­sis are be­ing seen in horses,” Dr. Mit­tel says. “We are get­ting pos­i­tives in ar­eas where vet­eri­nar­i­ans have not seen the disease be­fore. Since ticks are of­ten co-in­fected with Bor­re­lia and Anaplasma, it ap­pears that both agents are show­ing up in new ar­eas.” Some­times ticks trans­mit both in the same bite.

Signs: Once in the horse’s blood­stream, the bac­te­ria at­tack white blood cells called gran­u­lo­cytes. Signs typ­i­cally ap­pear a week or two af­ter a tick bite. They in­clude:

Fever, which may be high (103– 104° F) for the first one to three days De­pres­sion and lethargy Loss of ap­petite Re­luc­tance to move Swelling in the lower legs Mus­cle stiff­ness. Older horses may be more se­verely af­fected. Dr. Mit­tel notes that any horse with a high fever and no ap­petite is at risk for se­ri­ous prob­lems, in­clud­ing de­hy­dra­tion and colic, so you should call the vet promptly if you see those signs.

Di­ag­no­sis: If the horse’s his­tory and phys­i­cal signs point to anaplas­mo­sis, blood tests can con­firm the in­fec­tion. A PCR (poly­merase chain re­ac­tion) test de­tects seg­ments of Anaplasma DNA; other tests check for lev­els of an­ti­bod­ies pro­duced to fight the in­fec­tion.

Treat­ment: The tetra­cy­cline an­tibi­otics used to fight Lyme disease are also ef­fec­tive against anaplas­mo­sis without the need for pro­longed treat­ment. “Treat­ment with IV oxyte­tra­cy­line of­ten causes a very quick res­o­lu­tion of clin­i­cal signs—within 24 to 48 hours or even less,” Dr. Mit­tel says. “Many vet­eri­nar­i­ans will treat anaplas­mo­sis for two to three days IV and then switch to one of the oral tetra­cy­clines, such as doxy­cy­cline or minocy­cline, for a longer course of about seven days.”

Out­look: With treat­ment horses gen­er­ally re­cover promptly without lin­ger­ing

ef­fects and they seem to de­velop a nat­u­ral im­mu­nity that pro­tects them from this disease for up to two years.

“Anaplasma has been re­ported to cause ataxia [in­co­or­di­na­tion]. I have had clients with horses that were un­sta­ble on their feet early in the disease, usu­ally the first day,” Dr. Mit­tel says. “In my ex­pe­ri­ence these an­i­mals re­cover with no resid­ual neu­ro­log­i­cal signs.”

Preven­tion: As with Lyme, there’s no vac­cine against this disease. Re­duc­ing ex­po­sure to ticks is the best de­fense.


Equine piro­plas­mo­sis is con­sid­ered a for­eign disease in the United States, but it’s en­demic to Mex­ico and oc­ca­sion­ally turns up north of the bor­der. It’s caused by par­a­sitic pro­to­zoa ( Thei­le­ria equi or Babesia ca­balli) that are har­bored by var­i­ous (mostly trop­i­cal) ticks and spread through their bites. The pro­to­zoa at­tack red blood cells, caus­ing de­pres­sion, fever, ane­mia and other signs. Piro­plas­mo­sis can be fa­tal, but it’s of­ten mild with few prob­lems be­yond weak­ness and loss of ap­petite. Some horses be­come car­ri­ers, show­ing few signs.

Out­breaks of piro­plas­mo­sis are re­ported to the USDA. Horses who test pos­i­tive are quar­an­tined and may be eu­th­a­nized or shipped out of the coun­try to pre­vent spread. In 2009 a large out­break oc­curred on the vast King Ranch in south­ern Texas, af­fect­ing hun­dreds of horses. Sci­en­tists were able to iden­tify cayenne ticks as the main vec­tors on the ranch. And the size of the out­break gave them a chance to test what turned out to be the first ef­fec­tive treat­ment for piro­plas­mo­sis—a se­ries of high doses of the drug imi­do­carb.

“There has not been an out­break so large since,” Dr. Mit­tel says. Sev­eral small U.S. out­breaks have in­volved the use of shared hy­po­der­mic nee­dles or con­tam­i­nated blood prod­ucts, which can also spread piro­plas­mo­sis. The horses were rac­ing Quar­ter Horses with links to Mex­ico, where the in­fec­tion likely orig­i­nated.

When groom­ing your horse, pay spe­cial at­ten­tion to top tick-bite zones, such as the base of the mane. Tweez­ers are use­ful tools to re­move ticks safely and ef­fi­ciently.

Your vet­eri­nar­ian can per­form a quick stall­side test (the C6 SNAP) to di­ag­nose Lyme disease. A blood sam­ple will turn blue when pos­i­tive, in­di­cat­ing the pres­ence of an­ti­bod­ies to Bor­re­lia burgdor­feri bac­te­ria, but will not re­veal the level of...

A spike in your horse’s temperature (103–104° F) could in­di­cate anaplas­mo­sis, though a blood test is needed to con­firm the in­fec­tion.

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