Updating West Nile Virus Vaccine

Rob Keene, DVM, field veterinarian for Fort Dodge Animal Health, talked about the West Nile virus vaccine at the West Nile Virus workshop held March 7 at the University of Kentucky's Gluck Equine Research Center. Keene explained plans for packaging and marketing the vaccine--which received a full license from the USDA on Feb. 6--and described the efforts already underway to combine the WNV antigen with others in future inoculations to be marketed.

Veterinarians will continue to provide and administer the WNV vaccine (it will not be available over the counter). The vaccine with the conditionally licensed labels will probably last until summer. Fort Dodge will package the vaccine with the new full licensed label and sell it in 50-dose quantities (they will not be labeled for sale separately to clients). Fort Dodge will only sell the WNV (conditional and full license labeled) vaccine to veterinarians who maintain a doctor-patient-client relationship. The reason for the USDA1s strict packaging law is possible illegal sale and distribution of the vaccine, which could mean improper use. In the past, some people have tried to copy a vaccine label and sell it in smaller quantities. Keene encouraged the veterinarians in attendance to contact the company if they see this activity.

Another reason to regulate the packaging and distribution is so that Fort Dodge can track the manufacturing lots of the vaccine and know where they were used.

After two years of experience with the vaccine, and knowing results of challenge trials required to get a full license, veterinarians can educate their clients on the vaccine better than ever. "This is a great opportunity to share how (all) vaccines (alone) are not 100% effective," Keene said, and a good time to visit with clients about management, nutrition, complete health programs, vaccines, immune systems, and to clear any rumors that might be flying around about the disease, vaccine, etc.

Rumors have flown and will continue to fly. Keene says if a horse gets sick or acts strangely, the most recent "new" thing that was done to it will be blamed. Probably the most important thing for horse owners to understand is that the vaccine cannot cause the disease, since it is a killed-virus vaccine.

The vaccine is labeled for the prevention of viremia, which it was proven to do in 94.6% of horses in the study. Control horses in the challenge study (the ones not receiving the vaccine) which developed viremia didn1t show clinical signs characteristic of WNV after being exposed to the virus.. Nine out of 11 controls developed viremia, and only one out of 19 vaccinates developed viremia.

To give better perspective on the real-world results of the vaccine, Keene highlighted information from observations of WNV field performance in North Dakota, Minnesota, and Oklahoma. Of the horses that died of WNV in those states, the mortality averaged 25%, and greater than 95% of the horses which died were not vaccinated properly

To be adequately protected, horses must receive an initial shot, followed by a booster, three weeks later. There is a "magical time, three weeks post-vaccination," when the vaccine becomes effective, according to Keene. Many of the vaccinated horses that got sick were exposed early, when the horses weren't protected.

What's the future for WNV vaccination options? Fort Dodge currently is conducting field (safety) studies with a West Nile virus DNA vaccine. This vaccine will help officials have much more accurate surveillance of the disease. "You1ll know which horses are vaccinated, and which horses are sick," said Keene.

Finally, work is underway to include the WNV antigen in the various combination vaccines that are available. "Anything1s possible," Keene said, "work is proceeding nicely, and we don1t see any problems with continuing the development of multiple combinations with WNV included."