First West Nile Virus Vaccine For Horses Released

The equine industry called out for a way to protect its horses from the deadly neurological disease West Nile virus (WNV), and researchers and federal authorities responded.

The United States Department of Agriculture (USDA) and Fort Dodge Animal Health announced Wednesday, Aug. 1, the approval and release of the very first WNV vaccine for horses. The virus had already begun to appear in horses in the panhandle of Florida. This is the southernmost and earliest onset of equine cases in the three years since the virus first appeared in the Western Hemisphere. To be conditionally licensed, the vaccine had to meet federal standards of purity, safety, and reasonable expectation for efficacy. Each state will have to give approval for the vaccine's use, and most states have conditionally licensed products available only through a veterinarian.

"There's no question that Florida will need every dose it can get its hands on to protect its horse population," said Lee Coffman, DVM, Director of the Division of Animal Industry in Florida's Department of Agriculture and Consumer Services. He said the virus is moving steadily south, and although the virus is currently active in an area with a moderate density of horses, central and southern Florida have many more horses which would be susceptible to the virus.

West Nile virus hit the northeastern United States in 1999, baffling public health officials and epidemiologists when it killed birds, horses, and humans. Officials are still not sure how the disease was introduced in the United States, but they believe after studying the genetic makeup of many virus samples it was a single introductory occurrence. The virus is spread by the bite of an infected mosquito and harbored by birds. West Nile virus killed 36% of its diagnosed equine victims in 1999, and 38% in 2000.

Officials hope the vaccine will give veterinarians the upper hand in preventing the appearance of equine cases, which typically do not occur until late summer or early fall. (Learn more about the history of WNV in this country at

In February of 2000 when the USDA was forming its WNV Strategic Plan, authorities agreed to expedite the processing of applications for conditional licensure of a WNV vaccine. Several pharmaceutical companies were working to make the vaccine available to the horse industry, but Fort Dodge was the first to complete field trials to test the safety of the vaccine in horses and submit their product to the USDA Veterinary Services' Center for Veterinary Biologics in Ames, Iowa, for conditional approval.

Ernest W. Zirkle, DVM, New Jersey's state veterinarian, explained that roughly 100 doses of the experimental WNV vaccine were released in early summer to practitioners in his state and seven other states that had experienced the virus in horses in 1999 or 2000. The vaccine didn't have any adverse affects in these tests for safety. "If the practitioners were really having problems with it, we would have heard about it, so it seemed to go smoothly," said Zirkle.

"(When testing a vaccine for safety) we look at everything from lumps and bumps post-vaccination, to more serious reactions such as stiffness, lethargy, loss of appetite, or anything out of the ordinary," explained Tom Overbay, DVM, Director of Professional Services at Fort Dodge Animal Health. "We definitely have a product that does no harm. We certainly don't want unrealistic or unreasonable fear of the virus; we want horse owners to use a product because they think it is good medicine."

The killed vaccine consists of a two-dose initial series of injections given intramuscularly. The vaccine is a "first generation" product, meaning that other approaches are already in the works to provide a vaccine that will allow better differentiation between horses naturally exposed to WNV and horses which have been vaccinated for the virus. According to industry sources, there should be 25,000 doses of the vaccine immediately available, and 4,000 more available at the first of September. In mid-September, Fort Dodge should release another 200,000 doses of the vaccine, followed by a possible 600,000 by the first of November. The vaccine price should remain in the ballpark of the costs of routine equine vaccines.

"The USDA was extremely proactive in working with this particular organism," added Overbay. "They recognized the seriousness (of WNV) to the horse industry, and were extremely diligent in their efforts to review data and to expedite (the requests) of companies that are working on the vaccine.

"Because of the speed with which the USDA has approved this vaccine, the initial supplies will be somewhat limited," Overbay explained. "We recognize the importance of this vaccine in protecting horses, so we're going to produce vaccine as quickly as possible, recognizing that we want to provide two things -- a quality product, and production in an environment that is safe for our production department." Since the virus is a human pathogen, Fort Dodge employees adhere to strict guidelines in handling the agent in its live form.

The obvious question is will the vaccine it prevent WNV in horses? Overbay explained that the vaccine has been shown to stimulate an immune response in the animal against the virus. "Whether the immune response that is stimulated is the correct immune response to prevent the disease, that's something continual testing will answer for us," he said. Proof of an immune response has been used to grant full approval to other equine vaccines used to prevent Eastern/Western/Venezuelan encephalitis, and this represents the same criteria in approving human vaccines.

But as with all health products, the vaccine will need to be part of a sound health management disease program, according to Overbay. "It's just a part of the program, along with sound nutrition, the horse's housing, and as much protection from mosquitoes as possible. All these things roll together."

Horses affected by parasites or suffering from inadequate nutrition cannot mount as strong an immune response against WNV as horses which are healthy and on a good preventive health care program.

Only time will tell whether this vaccine will be a solution to the problem of disease in horses caused by West Nile virus.

(Editor's Note: Veterinarians with questions about the WNV vaccine should contact their Fort Dodge Animal Health representative.)