West Nile Virus Q&A

In a question and answer session following her West Nile Virus presentation during the Western Veterinary Conference (held Feb. 15-19 in Las Vegas, Nevada), Eileen Ostlund, DVM, PhD, head of the equine and ovine viruses section at the Diagnostic Virology Laboratory, National Veterinary Services Laboratory in Ames, Iowa, offered the following answers to the audience's questions.

Can you use Merial’s recombinant canarypox vaccine to booster horses after using the killed Fort Dodge vaccine?
We don’t know yet. We need to measure antibody levels before and after the booster vaccination to see if it is protective. I’d want to see the data before saying yes or no. The same proteins are targeted, but how the two different formats interact is not really known.

Does an infected, previously vaccinated horse need a booster?
If you couldn’t vaccinate all of your horses, I’d skip these. In general, vaccination won’t hurt, and it really only helps. It’s a cost-benefit issue, but I would probably just do it. We don’t really know enough to recommend not vaccinating these horses.

If you have a horse with a suspected reaction to one vaccine, would he have the same reaction to the other vaccine?
It’s hard to say--it depends on if the reaction was to the virus component, culture, adjuvant, vector, etc. Most likely, the reaction is to something other than the virus component, so it’s worth a try. Have a bottle of epinephrine on hand just in case.

How long does immunity last in a naturally exposed horse?
Currently we have no good model for infection. We haven’t seen any horses infected more than once yet, so we expect that the duration (of immunity) is quite long.

How fast do WNV signs progress?
It takes only hours to days from the appearance of clinical signs to their peak. There is a wide range of signs, from a little lip twitch to unusual aggression to narcolepsy.

Should you wait to get the IgM test result to treat the horse?
No, don’t delay the treatment. There is a time delay to get the test back. Your action might also vary with the situation--it might be different if you have a ward full of cases vs. the first case in the state.

Is interferon effective?
It has been looked at for humans and horses, but we don’t know yet how well that works.

Where can you get the antibody treatment?
Novartis and Lake Immunogenics make these products. Base your decision to use these products on case-by-case basis, depending on how sick and valuable the horse is, as the treatment isn’t cheap and might not save a horse that is already down.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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