Sometimes subtle and often dramatic, infectious neurologic disease cases are things veterinarians never want to see, although most would like to understand them better and more easily recognize them when they appear in clients' horses. From rabies to West Nile virus (WNV), these diseases garnered their own table topic discussion at the 2005 American Association of Equine Practitioners Convention, held Dec. 3-7 in Seattle, Wash.

Moderators Debra Sellon, DVM, PhD, Dipl. ACVIM, professor of equine medicine at Washington State University, and Maureen Long, DVM, PhD, Dipl. ACVIM, assistant professor in the large animal section at the University of Florida, first reviewed handling rabies cases, an age-old topic that never loses its importance. A veterinarian in attendance described two recent extraordinary rabies cases in his practice. One horse showed a mild lameness and deteriorated, ending up down in the stall without any frantic movements that can be characteristic of rabies cases. Another case, a pony mare, was referred for colic after aborting her fetus. She exhibited "full blown self-mutilation and foaming at the mouth" in addition to her colic signs. Hyperesthesia (over-sensitivity to noise and touch) and odd "neurologic tics" (muscle twitches) are what led veterinarians to believe this case was much more than just a colic case.

The human factor of rabies cases is very important. "There were 20-some odd humans exposed to the animal by the time it walked through that door," described the veterinarian involved in those cases. "There is huge human exposure, as happens often with cats (which might not show the aggressive or frantic signs usually linked to rabies)." If there is any doubt whatsoever over whether the horse might have had rabies, the brain should be sent out for testing.

Some of the veterinarians in attendance have rabies protocols in practice, some of which include the entire staff having a current rabies vaccination. In some areas it can be difficult to find an emergency room or doctor's office that has rabies vaccine available, so if you aren't going to vaccinate ahead of time, it's good to research where to go if inoculations are needed post-exposure.

"About one (equine) case in 10 years is about the average veterinarians see," Sellon said. "It's not going to happen frequently, but when it does, everyone is sent scattering. Having a program in place makes things easier, because backtracking is a big headache."

At this point there have been no published, confirmed reports of a person getting rabies from an infected herbivore, but veterinarians and pathologists are at an increased risk of exposure and contracture of disease when performing necropsies on infected brains. "I don't think I can stress (safety when testing these animals) enough," said Long. "When you're working with bone and CNS (central nervous system) tissue, you can cut yourself so easily. One of the most common forms of occupational exposure (to infectious neurologic diseases) is during post-mortems, when you're dealing with rib cages and skull bone." Veterinarians should wear special masks when performing these necropsies because certain flaviviruses (diseases such as WNV that are spread by mosquitoes) might be able to spread via aerosolized droplets that the pathologist inhales.

Mosquito-Borne Diseases
Continuing to arboviruses, attending veterinarians commented on their 2005 experiences with Eastern equine encephalitis (EEE). One Virginia practitioner said he and other veterinarians had seen many EEE cases in unvaccinated animals around Chesapeake, and most had to be euthanatized.

Long said, "EEE has been very active over the past three to four years. It's moving farther north--New Hampshire reported it this year--and seems like it's spreading. Just because you're not in an area where you normally see it," don't let your guard down. She emphasized the importance of vaccinating young, unexposed horses early, and she has noticed more cases in young Quarter Horses than other breeds in Florida. One pathologist noted the virus was even detected in alpacas in 2005.

Cases of Western equine encephalitis (WEE) haven't been detected recently, but Sellon says the virus is still showing up in mosquito pools and birds, especially in California. She's not sure if the virus is remaining quiet in a long cycle, but regardless, it's important for horse owners and veterinarians in the West to remain on guard for the disease. The same goes for Venezuelan equine encephalitis (VEE), especially since mosquitoes can infect humans after feeding on infected horses (unlike with EEE, WEE, and WNV).

West Nile virus is still a headline in many states, especially California, where there is a 44% case fatality rate (scientists believe there is underreporting of mild cases, which could be driving up the fatality rate). California researchers have begun collecting information on owner-administered vaccinations to determine their effect on vaccine reliability.

Veterinarians are taking their cues on vaccinating for West Nile virus by watching surveillance information--in Florida, for example, equine cases begin showing up two to three months after WNV is detected in mosquito pools and birds. It's important to make sure the horses are fully vaccinated before the mosquito season picks up and if the mosquitoes are a continued threat, it's important to booster every three to four months where there is a lot of mosquito and virus activity.

A short discussion ensued on maternal antibody interference and how early to begin vaccinating young horses for some of the more debilitating encephalitic diseases (EEE, WNV) that pose a current risk. Foals less than 90 days old don't respond well to the vaccines, so Long encouraged veterinarians to commence vaccinating against these diseases as soon as possible at three months of age.

EPM
Fort Dodge's EPM vaccine is nearing the end of its run as a conditionally licensed product (the conditional license can only be renewed annually for five years). Many veterinarians in the room give the EPM vaccine to their clients' horses if asked, and offer to keep the clients informed about study progress.

"Young and old horses are the populations at risk, and those are the ones I push," said one veterinarian. "Any horse coming from places with a different exposure (a horse being moved from a place without opossums, the definitive host for the agent that causes EPM, Sarcocystis neurona, to a place with opossums), I feel that justifies them having the vaccine."

Some veterinarians have expressed concern that the vaccine renders the cerebrospinal fluid positive for EPM when the horse isn't actually infected. Other veterinarians made the point that many horses are positive anyway, due to S. neurona exposure.

It's been suggested that perhaps some horses are more prone to EPM than others. "I don't think we know why certain horses get this disease," Long said. "I do think there's immune predisposition because only certain horses show signs and then some of these seem to recur with disease," Long thinks that some of these horses appear to get re-infected after recovery rather than it never being resolved the first time around. 

Sellon added that she's seen some genetic predisposition to EPM in mice studies. "There are black mice and white mice strains that are immune deficient in the exact same way," she explained. "Black mice get neurologic disease when infected with S. neurona and white mice don't. No one has yet been able to show a breed disposition to EPM, but that doesn't rule it out and I think it may be possible."

Long currently uses Marquis to treat her EPM cases and has become more aggressive in treating cases that are rapidly declining at admission with both Marquis (ponazuril), a potentiated sulfonamide compound, and pyramethamine.

Equine Herpesvirus
There were several outbreaks of neurologic equine herpesvirus type-1 across the country and in Canada in 2005, and several research announcements. Scientists revealed that there are genetic differences between neurologic and respiratory EHV-1 strains, and they currently are under closer examination. The strains seem to have a replicative aggression that wasn't noted in former strains, meaning they multiply and spread more rapidly. In a small preliminary study in 2005, researchers showed possible protection against development of neurologic signs of virus with the modified-live vaccine.

One very important take-home message from the past few years of herpes outbreaks is that if you suspect you have a herpes case, it needs to be isolated immediately. "What we learned (in veterinary school) is that if they're neurologic and don't show the respiratory form, they were no longer infective, but that's not the case here," Long said. Also, "if they're negative by culture or PCR (polymerase chain reaction tests), that does not rule out whether or not you actually have herpes."

The current gold standards for herpesvirus testing are nasal swabs, buffy coat samples, and CSF samples for culture of virus. A horse's clinical signs can range from hindlimb ataxia and proprioceptive deficits in the forelimbs (lack of physical awareness of limbs and their placement) to urine dribbling.

Sellon concluded by reminding veterinarians to consider every possibility of neurologic disease when diagnosing a clinical case, because it might not always be what it seems.

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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