Equine West Nile
Vaccine Adverse Effects

From Patricia Doyle, PhD

Date: 22 Jul 2003
From: ProMED-mail <>
Source: Manitowoc Herald Times Reporter 20 Jul 2003
Stillborn, aborted, and deformed foals may be linked to West Nile Virus vaccine.
Reports are coming from across the nation of troubled births, including a foal born with no front legs. The foal's mother received her first injection for [prevention of ] West Nile Virus (WNV) last year, 30 days into her pregnancy and again 2 weeks later.
According to the U.S. Department of Agriculture (USDA), there have been several dozen cases of deformities and stillborn foals throughout the nation. The USDA is investigating whether a new West Nile vaccine could be responsible for these defects. According to the USDA, they have received enough information to warrant an investigation.
In 2002, a West Nile vaccine manufactured by Fort Dodge was approved and distributed. Nearly 7 million doses were given to horses the first year. In most cases the vaccine prevented or minimized symptoms and has saved the lives of many horses. It appears that the problem is showing up in the foals of mares that were vaccinated while in foal.
The West Nile Virus affects a horse's neurological system. The virus was found in 1999. It is becoming a nationwide epidemic. Transmission of the virus is by mosquitoes. Birds, horses, cats, squirrels, chipmunks, rabbits, and people serve as hosts after being bitten by an infected mosquito. All of these are considered dead-end hosts because they cannot pass this virus to others.
Controlling the mosquito population is the best line of defense. Additional preventive measures for protecting horses should include the following: Use insect repellents; Stable horses from dusk to dawn; Eliminate mosquito breeding area by filling in low areas; Empty buckets and birdbaths frequently; and Clean roof gutters
Clinical signs of a horse sick from West Nile Virus are as follows: Lack of coordination, stumbling and limb weakness; Sleepiness, dullness and listlessness; Facial paralysis, droopy eyelids and lower lip; Inability to get up; Mild fever, blindness and muscle trembling; and excitability and seizures.
The vaccine is an important step in the prevention of West Nile Virus (WNV). Without the vaccine horses affected with WNV have a 65 percent chance of recovery if treated. That means that 35 percent of all horses affected are [likely to die of the illness]. All non-pregnant horses should be vaccinated and research supports the need for 2 vaccines initially.
If your mare is bred, you may want to avoid the vaccine and take extra precautions. Please seek the advice of a veterinarian before giving this vaccine to pregnant mares.
[Byline: Linda Willman]
Date: 24 Jul 2003
From: ProMED-mail <>
Source: Fort Dodge Animal Health Website [edited]
Currently, the West Nile - Innovator vaccine does not carry a claim for vaccination of pregnant mares. However, in pre-release safety trials, 32 of 649 horses were pregnant mares which were closely monitored following vaccination. There were no ill effects demonstrated in the mares, their pregnancies, or their subsequent foals. Since its release, thousands of doses have been administered to pregnant mares. A low number of undiagnosed abortions have been reported from the field following use; these reports appear to be associated with individual animal responses due to stress, which can occur with any vaccine administered to pregnant animals. The safety of West Nile - Innovator vaccine is similar to that of other routinely used killed virus vaccines such as the sleeping sickness combination products. For additional information contact your veterinarian.
[Although not provided on the Fort Dodge website, but provided directly by Fort Dodge, was the following information, which is paraphrased from the 2 reports. - Mod.TG]
In separate press releases from the University of Wyoming and from Colorado State University, and in conjunction with the USDA, the veterinarians there say there is currently no scientific proof to link the West Nile virus vaccine to aborted, stillborn, or deformed foals and that the horse owners should continue to vaccinate their animals to protect them from the deadly disease.
Concerns rose from unfounded and erroneous claims made by a Denver newspaper that some pregnant mares may have been adversely affected by the popular vaccine, according to spokespersons from the University of Wyoming, the Colorado State University veterinary diagnostic laboratories, and the Wyoming and Colorado state veterinary offices, because none of the horses in question has been scientifically tested.
The Colorado paper made reference to some anecdotal reports of vaccinated mares delivering stillborn or deformed foals. There is no proof of this, and the horses may not have been vaccinated.
According to a spokesperson from the USDA APHIS, none of the kinds of material that would normally be tested for veterinary diagnosis has been submitted. Owners are encouraged to submit aborted or stillborn fetuses or deformed or dead foals to veterinary diagnostic laboratories for necropsy and determination of the problems, whether or not they are related to the vaccine. Presently, there is no such evidence to determine [whether there is a] relationship between the problem and any vaccine or disease.
Date: 24 Jul 03
From: ProMED-mail <>
Source: Aphis website [edited]
Some recent stories have suggested that the Fort Dodge Animal Health West Nile Virus Vaccine approved by the USDA may cause pregnant mares to abort or give birth to deformed foals. The misleading information in those articles has sparked many anxious phone calls from horse owners, veterinarians, and others involved with horses.
Horse owners should be assured that the vaccine is safe, and it should be used as protection against West Nile Virus. Millions of doses of the vaccine have been used since the USDA's Center for biologics approved its use in 2001.
The Center for Veterinary Biologics, within USDA's Animal and Plant Health Inspection (APHIS), maintains a toll-free telephone hotline (800-752-6255) and a mailbox on its web site and actively encourages veterinarians and other vaccine customers to report problems with vaccines. <>
To date, there have been a very small number of reports regarding a possible association between the use of West Nile Virus vaccine and abortions, birth defects, or other reproductive anomalies or failures. It does not appear that there is a relationship between West Nile Virus vaccine use and these reproductive problems or any other major problems. The Center and the vaccine manufacturer will continue to collect, monitor, and track the performance of this vaccine.
[The numbers cited in the vaccine trial and available on the website are not sufficient to detect adverse events that might be occurring at low rates. The press releases from the universities cited indicate that no necropsies have yet been performed. However, if evidence or data are available, ProMED-mail would be grateful if a knowledgeable and authoritative source would share that information. - Mod.TG]
Patricia A. Doyle, PhD
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