Vaccination Update: 06/02

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West Nile Vaccine Results
If you’re considering using the West Nile Virus vaccine, talk with your vet. We would start the series now, rather than waiting for mosquito season to peak in late summer. One dose doesn’t appear to be protective (see Florida sidebar below) and protection may be short-lived.

Fort Dodge’s Rob Keene, DVM, reported that the USDA won’t grant full approval for the vaccine until challenge studies are complete, which are currently underway. Challenge studies basically vaccinate previously unexposed horses, then deliberately expose them to the virus to assess protection. The current status of the vaccine is still “provisional.”

The total number of 2001 WNV cases in horses was 738 cases, which occurred in 130 counties in 20 states. The outcome is known for only 470 horses, but of those, 156 were euthanized or died, for a mortality rate of 33.2%.

The WNV vaccine’s effectiveness is getting a tough trial in Florida. Over one-fourth of the Florida cases that occurred after vaccine use began in late August were in horses that received only one dose of the vaccine. Twenty cases occurred in horses that had both inoculations. This means 11% of the new cases were in fully vaccinated horses. Half of those became symptomatic within 10 days of receiving the second dose.

The WNV vaccine probably didn’t have time to become effective in some of these horses, especially those cases that occurred after only one dose.

Vaccinations can also sometimes precipitate obvious signs in an animal that was recently exposed to a disease but is not yet showing symptoms. In addition, four of the 14 confirmed or suspected cases for 2002 have occurred in horses that received both doses of the WNV vaccine.

As of now, the University of Florida and the Florida Department of Agriculture and Consumer Services, division of animal industry, recommend Florida horses receive boosters three times a year because of the prolonged mosquito season.

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Will WNV Hit California In 2002'
Many California residents are being encouraged to vaccinate for West Nile virus, because they’re being told there’s a good chance WNV will hit California this year. While you’d need a crystal ball to know for sure, we think it’s unlikely this year.

WNV is definitely spreading. However, the pattern appears concentric from the location of the first cases in New York in 1999. Migratory birds, which could bring the virus to California, may have helped, but the pattern isn’t what you would expect if the birds were the major factor.

Migrating birds in North??America typically follow one of four well-described “flyways”: Pacific, Central, Mississippi and Atlantic. Birds tend to stick to established flyways. ??Migratory birds on the Pacific Coast have little chance of contacting birds that have been east of the Rocky Mountains.

Consider eastern equine encephalitis (EEE). ??Like West Nile, this virus is harbored by birds and transmitted to other animals via mosquitoes. Since it was first identified in the 1930s, there has never been a case of naturally occurring EEE in a horse in California (see related story page 20). The Rockies are an effective physical barrier, and migratory birds from other flyways simply don’t have contact with birds traveling the West Coast.

That’s not to say a freak occurrence couldn’t bring infected mosquitoes into California or some Jonathan Livingston Seagull wannabe couldn’t go off established routes. After all, WNV somehow made its way from Israel to New York. However, if EEE is an indicator, the odds of this happening are pretty slim. Obviously, you can vaccinate anyway, but we wouldn’t consider West Coast horses at high risk yet.

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Florida’s WNV Vaccine Record
It’s not news that Florida was the state hardest hit by WNV. Florida had 492 cases confirmed for 2001 vs. 39 for the endemic EEE virus. The next highest state was Georgia with 68. Massachusetts was third with 45 cases, 38 of them in Plymouth County.

Once the vaccine was released in late August, it quickly became apparent that the first dose of the two-dose series wasn’t protective. Of the confirmed cases in Florida from September to December 2001, 27.6% occurred in horses that received one dose of the vaccine. From October through December, 11% of the confirmed WNV cases were in horses that received both doses.

The onset of symptoms after the second dose ranged from the same day the horse received the vaccination up to 40 days after the second shot of the series. Most of the vaccinated horses in 2001 that still became ill were symptomatic within 22 days of the last vaccination dose.

Of the 14 Florida horses confirmed/suspected of having WNV so far in 2002, seven died, including two of the four cases that received both doses of the vaccine. Of the vaccinated horses, one completed the series six weeks prior to showing symptoms, one completed it two months prior and the other two horses completed it four months prior.

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Encephalitis Cases In 2002
Viral encephalitis is off to an early start.?? In Florida alone, 2002 has seen three EEE cases, five West Nile cases (with nine more suspected) and 37 encephalitis cases with status undetermined or pending. Louisiana reports EEE in one bird and one horse and West Nile in two horses.