Veterinarians swapped ideas on hot topics in the realm of equine infectious disease in the Infectious Disease Forum at the 2004 American Association of Equine Practitioners Convention, held Dec. 3-8 in Denver, Colo. Steve Conboy, DVM, a private practitioner in Lexington, Ky., and Maureen Long, DVM, PhD, Dipl. ACVIM, assistant professor of large animal veterinary medicine at the University of Florida, monitored the session.

USDA Equine Study to Begin in 2005
Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, of Colorado State University, began the session by announcing that a new USDA National Animal Health Monitoring System (NAHMS) equine industry study will launch in 2005. The study will primarily focus on health management factors relating to the control of infectious disease and will have two components--an event component and an on-farm component.

In 1998, the USDA completed a NAHMS study that included valuable information on the health and health management practices of the U.S. equine population, testing practices for equine infectious anemia, equine identification, infectious upper respiratory disease, colic, equine protozoal myeloencephalitis, causes of equine death, vaccination practices, and biosecurity on equine operations. These reports can be found online at

"We want to know what's done about disease control strategies and how successful it would be to contact people at an event (at which data is collected) if something (a disease) were detected," said Traub-Dargatz. "The emphasis of the on-farm survey is going to be infection control strategies. There is a much more narrow emphasis to this study compared to the NAHMS 98 study, and no biological samples will be collected. The same 28 states that were used for NAHMS 98 will be used for this study."

Events organizers will provide booklets on animal identification and an educational packet including a questionnaire to find out the participant's familiarity with various disease prevention strategies. For example, Long is curious to find out how many people vaccinate their horses for West Nile virus (WNV) on a regional rather than a state-by-state basis. Objectives for the Equine 2005 study can be found at

National Identification Program
According to Amy Mann, director of regulatory affairs for the American Horse Council, the proposed national animal identification program "is intended to be a standardized animal identification system. We know that the way you identify cattle is not the same way you identify horses."

The goal of the program is to provide traceback on animals within 48 hours if it is suspected that an animal has been exposed to a foreign animal disease that could be a threat to the industry.

Information on the program and its progress can be found at

West Nile Virus
Since the 1999 AAEP Convention, WNV has remained a hot topic on which practitioners have shared their experiences. California's fight with WNV in 2004 was a rough one--538 cases were reported. "Our mortality rate seemed a little higher," said one California veterinarian. "As we suspected, toward the end of the year, we saw some underreporting of cases. A lot of horsemen did not watch the map (of case locations). It started in the north and the south, and vaccination practices left many horses unprotected. Eleven of the cases were what we considered completely vaccinated--two injections plus annual boosters.

"The procrastinators were using a lot of the RECOMBITEK (Merial's canarypox-vectored vaccine), and since they weren't ready, used only one dose of Recombitek to protect instead of priming doses. It's difficult to say what the impact of late vaccination was," added the California veterinarian. "Some out there might have ethical reasons not to vaccinate. We will have a significant population that will never be vaccinated against WNV."

The veterinarian guessed that 70% of California's horses were vaccinated, whereas another veterinarian in the room thought 50% might be more accurate.

On being able to forecast problems with an encephalitic disease, "We have no ability to predict whether an epizootic will come," said Long. "Eastern (equine encephalitis, EEE) is a really good example--we had 200 Eastern cases in Florida last year. I talked to vets in New Hampshire who experienced EEE this fall, and most of the arbovirologists are scratching their head over it. Not only was it a horse issue, the state's birds were affected too (birds generally don't get sick with EEE). Horses affected were two years old or less or greater than six months, and most were not vaccinated."

Long said the New Hampshire horse population just happened to be in that spot during the season where the bird/mosquito cycle of the disease spilled over into the mammals. She urged caution on behalf of the horse owners. "These diseases are here to stay and just because we don't see them for 10 years, doesn't mean that we won't have them here," she added. The good news is that cyclical patterns can be detected and preparations can be made; for example, roughly every 30 years there is an outbreak of St. Louis encephalitis in the Midwest, and Florida is on a 10-year cycle for EEE.

A veterinarian in the room pointed out that vaccination has shown a reduction in the occurrence of equine disease as evidenced by WNV human cases being up and the number of horse cases decreasing.

On the other hand, the veterinarians were surprised to see that Mexico had not reported the many cases of equine WNV that were expected. One veterinarian suggested that Mexican areas might have been infected late in 2004 and results had not yet been tabulated. Throughout 2004, some scientists had suggested that perhaps the strain of WNV spreading in the United States had changed and become less virulent, therefore explaining the small number of cases in Mexico. However, one veterinarian said, "The changes (in the strain) aren't what they would call significant--the actual base pair changes that they analyzed have not really shown a significant change from the NY99 avian isolate."

It was also suggested that since Mexico is already an endemic area of flavivirus (a subtype of arbovirus) that is carried by mosquitoes, there could be some cross-protection for WNV. For example, in Europe, there are more small, focal outbreaks, or "hot spots" of WNV than in the United States, where WNV has steadily swept across the United States. It was speculated that the tendency for European hot spots could be from cross protection from other flaviviviruses. Central and South America have several flaviviruses including dengue, yellow fever, and St. Louis encephalitis virus to which horses and humans may have been exposed and have some immunity.

Rob Holland, DVM, PhD, of Pfizer Animal Health, and a local equine practitioner in Lexington, had been working with a committee on establishing AAEP recommendations for WNV vaccinations. The new recommendations were published in early January 2005 and can be found at

Another important issue is the safety of the veterinarian who is working with a WNV horse. "Needlesticks are important," Long cautioned. "The level of the virus in the central nervous system (CNS) is very high. For those of you collecting cerebrospinal fluid (for diagnosis) and CNS (tissue), act like you're examining a rabies case. Certain viruses have been known to go across nasal epithelium. It's a big issue, and the day is coming for pathologists and veterinarians in the field when they could hurt themselves," she added.

The APHIS web site that instructs veterinarians on how to perform a post-mortem examination--this protocol should be followed (

Long reminded veterinarians of a study by Julie Wilson, DVM, Dipl. ACVIM, associate professor in clinical and population sciences at the University of Minnesota's College of Veterinary Medicine. The study looked at residual effects of West Nile virus and how animals can be permanently affected and sometimes even dangerous (see She believes this study gives us veterinarians yet another reason to encourage vaccination.

It is important not to overlook vaccinating equine blood donors at veterinary hospitals for WNV, since the disease has been shown to be transmittable via blood infusions in humans.

Vesicular Stomatitis
At the time of this meeting, Texas had wrapped up its vesicular stomatitis investigations and had no premises under quarantine. Colorado and New Mexico were still managing cases (both states have since been declared free of vesicular stomatitis).

The USDA's Animal and Plant Health Inspection NVSL on Jan. 1, 2005, started using a different piroplamosis test--the competitive enzyme-linked immunoabsorbent assay (CELISA). However, it came to light during the AAEP meeting that the CELISA test had been shelved due to technical problems that occurred once USDA/APHIS NVSL began producing the test reagents on a large scale and using the test on a routine basis.

The former test, a compliment fixation test, has problems with false negatives, meaning a horse could be a carrier of piroplasmosis in its blood, but not show up positive on the test and be allowed entry into the United States.

A roundtable discussion had been scheduled at AAEP to approach the issue. Currently, a research group in Poland is doing some transmission studies on pirpolasmosis. (For more on this see the AAEP Forum from February 2005,

A rash of influenza cases was noted in Florida and Kentucky in 2004. It was recommended that veterinarians swab or flush the nasal passages of horses that are in the high fever stage of influenza infection to verify the strain affecting the animals. According to Holland, Tom Chambers, PhD, of the University of Kentucky would be willing to help in surveillance for equine influenza, since Chambers heads one of the three world-wide OIE (Office International des Epizooties) reference laboratories for equine influenza. (OIE is the international advisory body that establishes guidelines and standards for animal health testing, monitoring, and trade. It also collects and disseminates information on occurrence and treatment of animal diseases.)

An equine influenza strain that had appeared in horses in Florida that was detected in dogs appeared to kill some of the dogs. "We now have a flu that has jumped species," said one veterinarian. "Does that make it more infectious to horses?" These types of questions should be addressed.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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