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.

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.

VECTORS

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.

SYMPTOMS AND TREATMENT

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.

PREVENTION

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 farmersweekly@caxton. co.za. Subject line: Horses.

JUST BEFORE DEATH, THE HORSE MAY CONVULSE

SOURCE: CENTRES FOR DISEASE CONTROL AND PREVENTION 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.