West Nile en­cephali­tis:

Vac­ci­na­tion is the best de­fense against this mos­quito-borne vi­ral disease, which in just two decades has be­come a health threat for horses across the coun­try.

EQUUS - - Contents -

Vac­ci­na­tion is the best de­fense against this mos­quito-borne vi­ral disease, which in just two decades has be­come a health threat for horses across the coun­try.

hen warm spring weather brings out the mos­qui­toes, you’ll need to take mea­sures to pro­tect your horse against a num­ber of dis­eases these pests can carry. One of these is West Nile en­cephali­tis, al­though peo­ple typ­i­cally re­fer to the disease by

the name of the virus that causes it: West Nile virus (WNV).

First iden­ti­fied in North Amer­ica in 1999, WNV spread across the con­ti­nent within just a few years, and now it re­mains a per­sis­tent threat. Horses (and peo­ple) be­come in­fected with WNV when bit­ten by in­fected mos­qui­toes, which pick up the virus from wild birds. Horses are con­sid­ered dead-end hosts, mean­ing that they can­not pass the disease on to oth­ers.

Most horses who are in­fected with WNV show few, if any, signs of disease. Some might de­velop a low fever and weak­ness that passes quickly. In roughly 10 per­cent of in­fected horses, how­ever, the virus crosses into the horse’s cen­tral ner­vous sys­tem and causes a more se­ri­ous ill­ness. Signs of the disease may in­clude high fever, loss of ap­petite, de­pres­sion, mus­cle twitch­ing, stum­bling or in­co­or­di­na­tion, weak­ness or paral­y­sis, and re­cum­bency.

There is no spe­cific treat­ment for WNV beyond in­tra­venous flu­ids, an­ti­in­flam­ma­tory med­i­ca­tions and other sup­port­ive care. Re­cum­bent horses may ben­e­fit from be­ing sup­ported in a sling. Neu­ro­log­i­cal signs may be­gin to sub­side within five to seven days, al­though in some cases they may per­sist for

sev­eral weeks, and it may take up to a year for a horse to re­turn to his pre­vi­ous level of work. Roughly onethird of horses who fall ill with WNV ei­ther die or are put down.


By far the most ef­fec­tive way to pro­tect your horse against WNV is to keep his vac­ci­na­tions up to date. In fact, WNV is in­cluded on the list of “core vac­cines” the Amer­i­can As­so­ci­a­tion of Equine Prac­ti­tion­ers rec­om­mends for the ma­jor­ity of horses.

Sev­eral vac­cines are avail­able, us­ing both mod­i­fied-live and killed virus for­mu­la­tions. An­nual boost­ers, prior to the start of mos­quito sea­son, are rec­om­mended for pre­vi­ously vac­ci­nated adult horses. Those who have never been vac­ci­nated be­fore will re­quire a fol­low-up dose four to six weeks after the ini­tial in­jec­tion. Boost­ers may be needed at six-month in­ter­vals for very young and very old horses, as well as those who have com­pro­mised im­mune sys­tems

By far the most ef­fec­tive way to pro­tect your horse against WNV is to keep his vac­ci­na­tions up to date.

and/or who live in ar­eas with large pop­u­la­tions of mos­qui­toes year-round.

Your vet­eri­nar­ian can sug­gest spe­cific prod­ucts and sched­ules that are ap­pro­pri­ate for your horse.


Any steps you can take to re­duce the pop­u­la­tions of mos­qui­toes on your farm will re­duce the risk of spread­ing disease to your horses as well as to peo­ple and other an­i­mals. For the most part, this means re­duc­ing or elim­i­nat­ing stand­ing wa­ter where the mos­qui­toes lay their eggs:

• Dis­pose of old tires, un­used buck­ets and other de­bris that can catch rain­wa­ter. Keep garbage cans cov­ered, and prop wheel­bar­rows against a wall or turn them over when not in use. Make sure tarps placed over trail­ers, boats or other items are not col­lect­ing wa­ter in

their folds. Empty wa­ter troughs in un­used fields and pad­docks.

• Keep all gut­ters and drainage ditches un­clogged and flow­ing freely. Gut­ter screens and other types of cov­ers can help pre­vent fallen leaves from block­ing the flow of wa­ter. Also make sure pud­dles are not form­ing un­der down­spouts. If they are, con­sider in­stalling down­spout ex­ten­sions or dig­ging chan­nels to keep wa­ter drain­ing away from the foun­da­tion.

• Re­pair drip­ping faucets and spig­ots. Not only do drips waste wa­ter, but they can form per­sis­tent pud­dles.

• Change your an­i­mals’ drink­ing wa­ter reg­u­larly. Ev­ery few days, empty and change the wa­ter in buck­ets as well as drink­ing bowls for dogs and cats and bird­baths. Mos­quito life cy­cles vary by species, but most take one to two weeks to reach ma­tu­rity after hatch­ing, so chang­ing wa­ter in larger troughs once a week ought to re­duce their num­bers.

• Use lar­vi­ci­dal prod­ucts when nec­es­sary. When stand­ing wa­ter is dif­fi­cult to change or re­move en­tirely, such as in out­door drain traps, or when drain­ing and re­fill­ing troughs weekly isn’t a vi­able op­tion, con­sider adding lar­vi­ci­dal prod­ucts to the wa­ter. Many com­mon prod­ucts con­tain Bti--- Bacil­lus thuringien­sis sub­species is­rae­len­sis, a bac­terium that pro­duces tox­ins that kill mos­quito lar­vae. Ac­cord­ing to the En­vi­ron­men­tal Pro­tec­tion Agency, Bti is not toxic to peo­ple and is ap­proved for use in or­ganic farm­ing. Talk to your vet­eri­nar­ian for rec­om­men­da­tions on spe­cific prod­ucts that are safe to use in wa­ter that your horses will drink.

• Keep nat­u­ral water­ways healthy. Fish and other preda­tors will feed on lar­vae as well as adult mos­qui­toes in and around ponds, streams and other water­ways. If nat­u­ral ponds and streams on your prop­erty are pro­duc­ing too many mos­qui­toes, ask your lo­cal ex­ten­sion agent for ad­vice on how to re­duce stag­na­tion and at­tract and re­tain ben­e­fi­cial species.

• Ap­ply re­pel­lents. Read the label on your fly sprays to make sure they also work against mos­qui­toes, then ap­ply as needed be­fore turn­ing horses out or trail rid­ing.

• Place fans in­side barns. Mos­qui­toes are not strong fliers. A few well-placed fans that keep the air mov­ing through your stalls will pre­vent the pests from ap­proach­ing and feed­ing on your horses.

Re­duce or elim­i­nate stand­ing wa­ter where the mos­qui­toes lay their eggs.


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