West Nile fever

The West Nile fever virus is trans­mit­ted from mosquitoes to peo­ple and horses. The dis­ease can be fa­tal and so horse own­ers should fo­cus on preven­tion, says Dr Mac.

Farmer's Weekly (South Africa) - - Horse Talk - FW

The West Nile fever (WNF) virus is pri­mar­ily a bird-borne ill­ness, and is trans­mit­ted by mosquitoes to horses and peo­ple, who are con­sid­ered dead-end hosts (these an­i­mals can­not trans­mit the virus to oth­ers). In Europe and the US, out­breaks of WNF have been linked to the ar­rival of in­fected mi­gra­tory birds that in­fect lo­cal mosquitoes, which in turn bite hu­mans and horses when their nat­u­ral hosts (the birds) die.

WNF af­fects 20 species of indige­nous wild birds in South Africa, and the cy­cle be­tween bird and mos­quito does not nec­es­sar­ily cause the birds to die. More­over, WNF has also been found in soft-bod­ied and hard­bod­ied ticks that feed on birds in South Africa. In ef­fect, this sug­gests that WNF cir­cu­lates be­tween lo­cal mosquitoes and birds, and that the dis­ease is thus en­demic in South­ern Africa.


In South Africa, the mos­quito Culex uni­vit­ta­tus, which feeds mainly on birds, is the main vec­tor of WNF, al­though the virus has been found in sev­eral other species of Culex and Aedes mosquitoes. It can also be trans­mit­ted into a new gen­er­a­tion of mosquitoes through in­fected eggs (transo­var­i­ally) that were laid the pre­vi­ous sea­son. These eggs over­win­ter in the dried mud of pools where the mosquitoes bred and re­main a source of WNF virus.


In horses, symp­toms oc­cur three to 14 days af­ter be­ing bit­ten by a mos­quito. More than 80% show no signs of dis­ease.

Sick horses may have a fever, but the main signs are neu­ro­log­i­cal, with tremors of the face and neck mus­cles. Hy­per-ex­citabil­ity may also be demon­strated. Horses stag­ger or wan­der aim­lessly in cir­cles and ap­pear blind, or show paral­y­sis of the hind legs, thus as­sum­ing a sit­ting po­si­tion. Some have weak­ness or paral­y­sis of their fa­cial mus­cles and tongues and are un­able to swal­low. Just be­fore death, the in­fected horse may convulse and go into a coma.

Al­though there is no spe­cific cure, you need to con­sult a vet­eri­nar­ian who can help you treat and nurse less se­vere cases to im­prove the chance of sur­vival.

Al­though WNF can be trans­mit­ted to vets by the blood of an in­fected horse, peo­ple are mainly in­fected only if bit­ten by a mos­quito that fed on an in­fected bird.

In an out­break, up to 90% of peo­ple de­velop an­ti­bod­ies, while only a few de­velop neu­ro­log­i­cal signs, which can be fa­tal.


Mos­quito con­trol is very im­por­tant in pre­vent­ing WNF. Make sure there are no old tins, pots or tyres where mosquitoes can breed. Drain pools of stag­nant wa­ter af­ter rain. There are dif­fer­ent lamps and mos­quito traps you can use in and around sta­bles. As it was re­cently found that mosquitoes that bite hu­mans who have been treated with Iver­mectin die within a few days, it may be a good idea to strate­gi­cally de­worm horses with this drug when there ap­pears to be a lot of mosquitoes around.

The most ef­fec­tive preven­tion against West Nile fever is vac­ci­na­tion. The vac­cine is in­ac­tive, and vac­ci­na­tion con­sists of a pri­mary course of two in­jec­tions four to six weeks apart, fol­lowed by an an­nual booster in De­cem­ber ev­ery year. There may be some pain and swelling at the vac­ci­na­tion site and horses gen­er­ally need to be off work for about two days af­ter the vac­ci­na­tion.

• Dr Mac is an aca­demic, a prac­tis­ing equine vet­eri­nar­ian and a stud owner. Email her at farm­er­[email protected]­ton. co.za. Sub­ject line: Horses.



BE­LOW: The trans­mis­sion cy­cle of the West Nile virus. In­fected mosquitoes feed on un­in­fected birds. The virus pro­lif­er­ates in the body of the bird, the virus’s nat­u­ral host. An un­in­fected mos­quito then feeds on the in­fected bird, thus be­com­ing a vec­tor. The mos­quito trans­mits the virus when feed­ing on horses or other an­i­mals. As the virus does not pro­lif­er­ate as read­ily in horses, they are dead-end hosts and can­not trans­mit the virus to other an­i­mals.

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