Farm Horizons, May 2003

Return of the 'state bird,' ­ and West Nile Virus

By Julie Yurek

Our "state bird" will soon be amongst us, buzzing about dawn and dusk with bloodthirsty vigor.

The return of mosquitoes will also mean the return of West Nile Virus, which infected 1,000 horses in Minnesota last year.

Thirty percent of infected animals died from the disease, according to the Minnesota Board of Animal Health.

Horse owners should already be vaccinating their horses against the disease, since the vaccination requires two shots, three to six weeks apart to take effect.

This was a problem last year, since many horses died between the first and second shots, since both sets are required to make the vaccination effective.

Horses who received the two-shot vaccination will only need a booster shot this year.

The booster needs to be done annually to keep the horse protected, said Heather Case, a veterinarian and horse specialist at the Lester Prairie Vet Clinic and St. Boni Pet Hospital.

Even after the second shot, the horse will not be protected from infection until at least two to three weeks after, Case said.

Horses who have not received any vaccinations should be given it as soon as possible, she said.

The vaccine uses a dead virus, and the horse cannot contract the virus from the shot.

Minnesota horses need to be vaccinated only once annually.

"In places such as Florida where there is no break from mosquitoes, horses there are vaccinated more than once per year," Case said.

The earlier the vaccination is given to horse, the higher its antibodies will be when mosquito appear, she said.

"July was when it (the virus and mosquitoes) hit hard," she said.

The clinic has been doing a lot of vaccinating already, she said.

Symptoms of the virus include lethargy, hindquarter weakness, involuntary muscle contractions, loss of coordination, head tilt, circling, convulsions, paralysis,

and coma.

Another deadly disease that horse owners should vaccinate horses for is eastern and western encephalitis, Case said.

"Eastern encephalitis has a 90 percent fatality rate, while West Nile is 40 percent," she said.

Latest counts

The latest count of infected birds, horses, and humans affected by the West Nile virus as of January 2003 includes:

Carver County ­ five birds, 15 horses, and two humans

McLeod County ­ three birds, 10 horses, and three humans

Meeker County ­ three birds, 20 horses

Sibley County ­ one bird, eight horses

Wright County ­ four birds, 67 horses, and one human

"Many horses infected with West Nile Virus do not develop any illness, but of horses that become ill, about one-third die or need to be euthanized," according to the Animal and Plant Health Inspection Service, which is part of the U.S. Department of Agriculture.

Common sense prevention

The most important step any property owner can take to control mosquito populations is to remove all man-made sources of stagnant water in which mosquitoes might breed.

Turn over wading pools, get rid of tires that may be laying around, drill holes in the bottom of container that are left outside, and clean livestock watering troughs monthly.

It is also important to prevent horses from being exposed to adult mosquitoes.

Other suggestions include:

· housing animals in structures with well-maintained insect screening can be useful to reduce exposure to mosquitoes.

However, precautions must be first taken to eliminate mosquitoes from inside the structure, otherwise, use of such mosquito resistant structures may actually lead to mosquito exposure.

Mosquito adulticides can be used to clear the structure of the insects, as well as the use of fans to reduce the mosquitoes' ability to feed on horses.

· the use of insect repellents is another option to reduce exposure to mosquitoes.

Due to practical limitations in the coverage area and the limited duration of some repellents, repellents should not be solely relied upon to prevent mosquito exposure.

Topical application of a product containing a synthetic pyrethroid compound, i.e. permethrin, as the active ingredient may offer the best combination of safety and efficacy.

· reduce outdoor exposure.

Since it is not know which exact mosquitoes are responsible for the transmission of the virus, it is hard to determine when animals should avoid outdoor exposure.

A recently completed epidemiologic study of West Nile Virus suggests that keeping horsed in stalls at night may be helpful in reducing their risk of infection.

Effect on humans

A total of 41 infected humans were reported in the state, according to the Minnesota Department of Health web site,

Twenty-seven were male and 14 were female, with an average age of 48 years old (ranging from four to 85). Twenty-seven humans were hospitalized for a duration ranging from one to 40 days.

Twenty-four patients had a fever, nine had encephalitis, seven had aseptic meningitis, and three had acute flaccid paralysis.

Fortunately, there were no fatalities. All of the hospitalized patients were discharged, however, most of the encephalitis and acute flaccid paralysis required extensive post-hospitalization rehabilitation.

Two patients who were hospitalized in mid- to late-August are still in long-term rehabilitation facilities.

None of the cases acquired their infections from blood transfusions or organ transplants.

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