Study: WNV plagues 1 in 12 horses
Atlanta-More than 100 horses became infected with West Nile virus (WNV) after receiving the equine-targeted vaccine, according to a federal official, and in a separate study, one in 12 horses became critically ill when infected with the virus.
Dr. Randall Crom, U.S. Department of Agriculture staff veterinarian,presented findings on the USDA conditionally licensed vaccine at a conferencethe Centers for Disease Control and Prevention hosted earlier this spring.
More than 1 million doses of the vaccine, manufactured by Fort DodgeAnimal Health, have been distributed to date, according to Crom. The antibodyresponse: no IgM; high levels of neutralizing Ab after second dose.
At least 132 horses developed cases of WNV after receiving the vaccine,Crom reports. The state breakdown is: Fla. (107), Ga. (18), N.J. (2), Conn.,Del., Ill., R.I., Va.
"Of those occurring after the first dose of vaccine, onset appearedto be at random intervals after inoculation," says Crom. "Of thoseoccurring after the second dose of vaccine, 14 had onset at 15 or fewerdays after inoculation; five had onset at 20 or more days after inoculation."
Crom's prediction is that areas that have had WNV in horses, will haveit again (although in different horses).
"The efficacy of equine vaccine, and the number of horses vaccinated,will determine the impact of vaccination on all of the above," saysCrom.
Already this year, Florida has reported five cases of West Nile (birdsand horses), Louisiana has cited two, one in a cardinal, and Illinois, onedead crow.
In separate research, Wellington, Colo.-based Dr. Richard Bowen, of theCDC, helped conduct an experimental infection of horses with West Nile virus.
The study was reported in the Journal of Emerging Infectious Diseases(2002 Apr; 8(4):380-6.
The team infected horses with bites from infected mosquitoes and thenevaluated them twice daily to note when they had virus in their blood. Aboutone out of 12 became critically ill.
"That seems to be what happens in the real world too one of10 horses that become infected show clinical signs. It's a serious threat,"says Bowen.
Now the researchers plan to study dogs, cats and pigs.
"Dogs and cats seem to become infected readily. We've only (infected)four dogs. We didn't see anything in terms of clinical signs," he says. "In cats, we saw mild clinical signs in a couple of them.
"I think the tale will come from the field over the next coupleof years as West Nile spreads across the country and more animals becomeinfected. If we don't see any cases of West Nile in dogs or cats, then itwould indicate they're not very susceptible to the lethal form of West Nile,"says Bowen.