Articles: Press Release
West Nile Virus (WNV) and Encephalitis a Threat to Horses;
Vaccination Can Provide Protection
Texas Animal Health Commission
Box l2966 * Austin, Texas 78711 * (800) 550-8242 * FAX (512) 719-0719
Bob Hillman, DVM, Executive Director
For information, contact Carla Everett, information officer,
at 1-800-550-8242, ext. 710, or firstname.lastname@example.org
For Immediate Release - June 25, 2003
A few moments with your local veterinary practitioner can help protect
horses, mules and donkeys against mosquito-borne diseases, advises Dr.
Terry Conger, veterinarian and state epidemiologist for the Texas Animal
Health Commission (TAHC), the state's livestock health regulatory agency.
Vaccines are available to protect horses against three viral diseases that
pose a risk to Texas horses: West Nile Virus (WNV), and Eastern and
Western Equine Encephalitis (EEE) and (WEE). The three diseases are forms
of "sleeping sickness," that can cause infected horses to develop
debilitating fever, swelling of the brain, muscle tremors, weakness and
extreme fatigue, and in about a third or more of cases, death of the
"As of June 24, 26 cases of West Nile Virus (WNV) have been reported this
year by the Texas Department of Health's (TDH) Zoonosis Control Division,
which maintains the state's statistical database for reporting
laboratory-confirmed WNV cases in humans, mosquitoes, birds, and horses,"
said Dr. Conger. "According to the TDH, the 26 cases have been confirmed
in one horse each in Angelina, Calhoun and Van Zandt Counties. WNV also
has been detected in six birds and four mosquito pools in Harris County;
one bird in Orange County; one bird and two mosquito pools in Dallas
County; four mosquito pools in Tarrant County, three mosquito pools in
Jefferson and two in Nueces Counties."
"In June 2002, Texas had its first case of WNV, and by year's end, 1,699
equine cases had been reported. Denton County was hit hardest, with 69
equine cases," explained Dr. Terry Conger, TAHC's state epidemiologist.
"More unreported cases undoubtedly occurred across the state, as owners may
have been reluctant to ask their veterinarian or the TAHC for assistance.
Only laboratory-confirmed cases are included in the state's database. Most
private veterinary practitioners send blood or tissue samples for WNV
testing to the Texas Veterinary Medical Diagnostic Laboratory (TVMDL) in
College Station or Amarillo."
WNV infection was first detected in the U.S. in New York in l999, and since
then, the virus has swept across the country, spread by infected mosquitoes
that have fed on birds carrying the disease. Only four states on the U.S.
mainland have not reported cases: Arizona, Nevada, Oregon and Utah. Dr.
Conger said it is likely that these states also will eventually have cases
"Sporadic outbreaks of WEE and EEE also occur throughout the U.S., and this
year we may have cases in Texas, as South Carolina, Florida and Georgia
have already confirmed EEE in horses. Because Texas has a large mosquito
population and history of infection in past years, it is prudent to have
equine animals vaccinated against WEE and EEE every year," said Dr. Conger.
"It is important to note that the vaccines for WNV, WEE and EEE do not
provide cross-protection. For example, WNV vaccine protects only against
WNV." As of late June, 14 horses in South Carolina have been diagnosed
with EEE. In Florida, more than 110 EEE cases in equine animals have been
confirmed, and in Georgia, a dog, three birds and 16 horses have tested
positive for the disease.
"WNV, EEE and WEE are going to be with us always, as these diseases have
become endemic in our country. Even if you try to reduce the mosquito
population in your area, vaccinating against these mosquito-borne diseases
still needs to be part of routine horse health maintenance," said Dr.
Dr. Conger explained that the WNV vaccine for horses vaccination requires a
series of two shots, administered three to six weeks apart. After the
second injection, several more weeks must pass before full immunity is
developed. Equine animals vaccinated last year should receive a single
WNV booster this year. In areas where mosquitoes are a continual problem
year-round, animals should receive a WNV booster every six months. "Owners
should take action now to get their equine animals vaccinated, before cases
become more widespread," urged Dr. Conger.
"EEE and WEE vaccines are available in a combined product. To develop
disease protection, the animal will need two injections, 30 days apart. A
booster shot is then needed annually or biannually," said Dr. Conger. "In
most cases, horses can be vaccinated for WNV, EEE/WEE at the same time."
He said some veterinarians also employ the EEE/WEE vaccine in an off-label
use to protect ratites (emus, rheas and ostriches) against the disease.
"While no vaccine can guarantee 100 percent protection, it's much better
than no protection," commented Dr. Conger. "Last year, Colorado State
University followed more than 500 equine animals infected with the WNV in
Colorado and Nebraska. The death rate was greater than 36 percent in
animals that had not received vaccine. The death loss dropped to around 20
percent for animals that had received at least one dose of WNV vaccine. Of
the 13 animals properly vaccinated with two doses of vaccine, a dozen
Dr. Conger said equine animals infected with WNV, EEE or WEE, may exhibit
similar signs, including fever, irregular gait, teeth grinding,
drowsiness, an inability to swallow. Affected animals may be paralyzed and
be unable to rise. He said TAHC and public health officials advise owners
to have sick animals tested to diagnose the disease that caused illness or
death, because rabies always must be considered when an animal is
disoriented or staggers. If an owner has been exposed to rabies while
caring for a sick horse, appropriate preventive health measures must be
taken immediately, he said.
"WNV, EEE and WEE impact a community's animals, and owners should take
advantage of opportunities to protect their stock by reducing mosquito
populations and vaccinating," said Dr. Conger. "Although these "sleeping
sicknesses" aren't regulatory diseases, veterinarians are required to
report cases, so that disease trends or outbreaks can be followed and all
preventive and educational measures can be taken."