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Tuesday, 17 August 2010 08:26

“Doc – I Have Heard There Are Two New Vaccines Available For The Prevention Of West Nile In The Horse.”

Written by  A.J. Neumann, D.V.M.
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You are absolutely correct. At this time two new vaccines have been made available to be used in the prevention of West Nile as it occurs in the equine family. During the past several years, a vaccine for this purpose, produced and marketed by Fort Dodge Laboratories, was the only one available for use until the recent introduction of two new West Nile vaccines by Merial and Intervet Laboratories, respectfully.

The West Nile Virus is not gone. This past season has seen rising numbers of reported cases, in humans and equines, occurring on the east and west coast regions while its overall occurrence in the Midwest has been down. One exception to this Midwestern statistic is the state of Missouri where an unusually high number of cases has occurred in both the human and equine species.

There are probably several reasons why this is occurring but I believe natural and acquired immunity against the virus by susceptible animals, birds and humans are a great part of the answer. The West Nile Virus first appeared on the East Coast near New York City and spread west, reaching the western region of our country in late 2004 and early 2005. Horses and mules were vaccinated in large numbers in those affected areas as the virus moved west. Here in Iowa, it has been estimated that as many as 90% of the horse and mule population was vaccinated for West Nile. I like to infer that the immunity gained by the individuals, who were vaccinated, be known as an acquired immunity. Of those animals which were not vaccinated, a percentage of them contracted the disease and died. Others recovered or showed very few, if any, symptoms of the disease. These animals developed a natural immunity to the virus. We know that for a period of time, most of the vaccinated and recovered animals will have immunity to the West Nile Virus. The number of those that remain immune, however, will decrease if the previously vaccinated animals are not re-immunized. The addition of new animals to the ranks of the non-vaccinated horses (including those lacking natural immunity) will also contribute to the herd's susceptibility to the virus.

With these facts in mind it is imperative that, as owners and breeders of horses and mules, we should continue to vaccinate for West Nile. If your animals are not vaccinated they should be closely watched for any symptoms of the disease since we know that the West Nile Virus is present in the environment.

Through the years as a veterinarian, I have logged hundreds of thousands of miles on country roads during all four seasons of the year. I have come to know and understand the habits and lifestyles of the local wildlife as well as observing the migration in spring and fall of the various species of ducks, geese and birds.

The fall migration of the raptors or hawks has always interested me. It is occurring right now, especially the migration of the red-tailed hawks. Before the advent of the West Nile Virus, during the autumn hawk migration, I could often count one or two red-tails per mile as they soared above the road or perched on some electric pole or tree along the right-of-way. The little “sparrow” hawk, or American Kestrel, was very abundant. Often I could count five or six per mile during their migration. Not so anymore! The red-tail numbers, as well as some of the other species of hawks and falcons, are way down. The little “sparrow” hawk; well, I can drive for miles and only count a few during its migration.

This has all occurred following the introduction of West Nile Virus into the Midwest several years ago.

The crow population has also suffered. At one time, about four years ago, our city was plagued by huge flocks of crows. They were into everything including the destruction of songbird nests. They virtually destroyed the jenny wren population as well as the robins in town. I have seen crows perched on a wren house, reaching into it and retrieving and killing the baby wrens. Shooting some of the crows didn’t alleviate the problem but the West Nile Virus took care of it.

The crow has really been hit hard by the West Nile Virus and their numbers have dropped dramatically from the big flocks of 25 to hundreds down to flocks of five or less, and they are gone from the city.

Other bird numbers have dropped as well. Early in the morning, about dawn, I liked to hear the booming hoot of the great horned owl as well as that of the barred owl. During the late summer months I could always hear the sound of a screech owl in our backyard. Not any more. It is very rare to hear them here. Again the West Nile Virus has taken its toll on the owl population. There may be other factors working in the reduction of these bird populations but I believe the West Nile Virus is the causative agent for the decline of these species. This decline occurred almost immediately after the introduction of the West Nile Virus into our area.

Now on to the West Nile vaccines: everyone who has used Fort Dodge’s West Nile vaccine is probably familiar with its action. It has been highly advertised, promoted and millions of doses have been used to prevent West Nile in our horse and mule populations, with which I might add, a great deal of success. A 1 cc dose is given twice at a three to four week interval to establish a primary immunity. Immunity to the West Nile Virus is said to be established five weeks after the last dose is given. The animal must be revaccinated annually to keep its immunity in force. The vaccine has been proven to be safely given to breeding stallions and mares in foal. In areas where the mosquito operates all year, many practitioners have recommended that horses and mules be vaccinated every six months after primary immunity has been established. This is a practitioner’s point of view and may or may not be supported by Fort Dodge.

Recombitek Equine West Nile Virus is a relatively new vaccine on the market, by Merial, for the immunization of equines against the ravages of West Nile Virus.

Merial uses recombinant canary-pox vectored technology to safely deliver quick protection against West Nile Virus in horses after a single injection of the vaccine. To quote Merial’s advertising,”A recent study showed the onset of immunity just 26 days after the initial dose, with horses protected against development of viremia when challenged with West Nile Virus-infected mosquitoes.” The company claims season-long immunity after two doses of the vaccine are given. An annual booster will provide another year of immunization. According to Merial, one dose annually of this vaccine will provide protection in horses previously vaccinated with other killed virus vaccines such as Fort Dodge's West Nile Virus vaccine.

Intervet Inc. has introduced the only modified-live one-dose West Nile vaccine for horses labeled "PreveNile West Nile Virus Vaccine." According to the company, “PreveNile is the first and only one-dose USDA approved equine West Nile Virus vaccine labeled for prevention of viremia and as an aid in the prevention of disease and encephalitis caused by West Nile Virus infection.”

This vaccine requires only a 1 cc dose for primary immunization, with a resulting one-year duration of immunity in yearlings or older horses.

According to Intervet, PreveNile is “99.9% reaction-free when administered to horses of various ages, breeds and sex. Of the 919 horses vaccinated, 229 were four months of age or younger and 302 were pregnant mares, including 17 mares in the first trimester of gestation, 11 mares in the second trimester of gestation and 274 mares in the third trimester of gestation. No post vaccinal adverse events were observed in any of the foals or mares. The results of this study show the vaccine to be safe when used to vaccinate horses four months of age or older by the intramuscular route.” In another study, “five or six yearling vaccinates receiving a full dose were not affected when challenged 10 days after vaccination.” The company further states “that no vaccine virus was shed by the vaccinated horses.”

Intervet states that “vaccination of healthy horses be given at five months of age. For primary vaccination, administer a single dose only. Revaccinate annually with a single dose. For horses previously vaccinated against West Nile, administer a single dose only.”

Another factor exists in evaluating these three vaccines especially by the person giving the “shots.” The West Nile vaccines by Fort Dodge and Intervet can be withdrawn from the container with a syringe for immediate use, while the vaccine by Merial must first be mixed and then withdrawn to be used.

So now you have a summary of the facts about three excellent vaccines which can be used to prevent the incidence of West Nile Virus in your mules or horses. Analyze the information and make your choice. With the proper use of a vaccine you should not have a single case of West Nile occurring in those animals under your management and care.

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