Georgia Neurologic Case Highlights Difficulty of Rule-Outs

A 24-year-old pleasure mare in Brunswick, Ga., was recently diagnosed with concurrent infections of equine protozoal myeloencephalitis (EPM) and equine herpesvirus type-1 (EHV-1), both of which can cause clinical signs of neurologic disease. The mare's treating veterinarians have said the neurologic deficits the mare has shown are likely caused by the EPM more than the EHV-1. No other horse in the mare's barn has shown any signs of illness.

This case highlights the complexity of pinpointing neurologic problems, since differential diagnoses in such cases can run the gamut from Eastern equine encephalitis to West Nile virus (WNV). It's important that the veterinarian be able to distinguish what the disease is, because some neurologic diseases pose a danger to other horses (such as EHV-1) and some even to humans (such as rabies and Venezuelan equine encephalitis), while others such as EPM are not contagious to other animals. Equine herpesvirus, also called rhinopneumonitis, can cause respiratory signs (relatively common in transient herds), abortion in pregnant mares, or neurologic signs.

Mary Jo Davis, owner of the Thoroughbred mare "Louie" (who was retired from racing 20 years ago), said Louie wasn't herself on Jan. 13, when it appeared she had been restless or cast in her 15'x18' stall at a small boarding facility with a relatively stable population of horses that do not leave the farm often. On Jan. 14, she was moving stiffly, but Davis attributed this to a chronic stifle problem. Louie became uninterested in her feed, was eating only little hay, and became lethargic and depressed by Jan. 16, and by Jan. 18, she was ataxic (uncoordinated) and showed muscle fasciculations (twitching). Louie's treating veterinarian referred the mare to Equine Associates in Hawkinsville, Ga., on Jan. 19.

Jennifer Baker, DVM, co-owner of Equine Associates, said, "Louie was very bright and alert when she came in, but they act a little more bright and alert when they come in from a trailer ride. She was circling to the right--always to the right, and she never went to the left even when you tried to force her to the left. She had mouth motions where she would hold her tongue out to the side of her mouth with her lips pursed like a kissing motion. She was depressed, wouldn't eat (her grain), and only ate a little hay. She had Grade 3 (out of 5) proprioceptive deficits in all four feet--she didn't know where her feet were. She never got down and stayed--this was not one of those down herpes cases that you hear about."

A confounding factor in Louie's diagnosis was that she had recovered from WNV in 2003 and some WNV survivors have later shown neurologic deficits.

Tests on blood serum came back with high antibody titers of EHV-1 (indicative of active infection). Baker said, "We were thinking more of the Eastern (equine encephalitis), Western (equine encephalitis), West Nile virus, or EPM on this case. We're very surprised that it's such a high serum-positive titer (for EHV-1) and we're not sure where she contracted the virus. This mare could have been harboring it for a long time," meaning that she might have been a latent carrier of EHV-1.

"There's such a variance in how it affects the horse," Baker added, while Louie was still at the clinic. "You hear about how serious it can get, how horses become paralyzed, and I'm wondering why she didn't get it worse than she did. I guess we're very fortunate."

Louie responded well to three days of intense anti-inflammatory treatment, improving to about a Grade 1 proprioception in all four feet. "If you put her outside, that really stimulates the circling again, so we haven't taken her outside because we don't want to stress her," added Baker. Louie stayed at the clinic in Hawkinsville for two weeks. In the meantime, both serum and cerebrospinal fluid tests showed Louie was also positive for EPM, and the veterinarians prescribed Marquis treatment for Louie.

Louie has been making slow progress in her recovery since she returned from the clinic. Constance Hatchard, DVM, associate veterinarian at Cheek to Cheek Hospital for Animals in Brunswick, Ga., and the mare's regular local veterinarian, said, "I think it's pretty much the EPM that is causing the current problem, and she probably was just a carrier of herpesvirus." Louie still has muscle fasciculations from her withers forward.

If a horse is infected with two diseases that can cause neurologic signs concurrently, how do you know which disease is causing the neurologic signs? Or might it be both? Hatchard believes EPM is causing the signs, because "The neurologic signs seen with herpes are not being seen in Louie--she hasn't had urinary problems or fecal incontinence," and are more characteristic of a horse with EPM. Regardless, barn personnel have been limiting the traffic in and out of Louie's stall and have reduced the contact between horses in the barn. Several owners had their animals vaccinated against EHV-1, including the owner of a pregnant mare at the barn. No other horses at the facility have become sick.

Baker mentioned that a farmer in the area with 30 broodmares had logged a few abortions in prior weeks, and he hadn't vaccinated his mares against EHV-1. She's been pushing owners to remember to vaccinate, even when they haven't heard of a case in awhile. None of the available EHV-1 vaccines are labeled as protective against neurologic signs of EHV, although they might be able to reduce severity of signs.

Baker said, "You don't hear a lot about (herpesvirus) until you get the outbreaks at the tracks. The backyard and the small stables, you don't think about rhinopneumonitis--it's usually a minor, transient little cold thing, and then all the sudden we see a case like Louie and it scares you."

It looks like Louie has weathered the worst of the EHV and EPM. It's hard to know whether the EPM weakened her immune system so that she was vulnerable to the EHV-1, or vice versa. Baker said, "They compounded each other and both played a part, but I don't know which came first and I guess we'll never know."

Hatchard emphasized to owners, "As soon as you start to see signs (of illness), get someone out to look at them. Early detection and treatment means a better prognosis. Vaccines are never 100% but it's important to have a good vaccine protocol and to vaccinate at least twice yearly for the respiratory and abortive forms of EHV."

Davis added, "I want Louie to get well, but I'm also very realistic knowing she's 24. But she bounced back from the West Nile by 99%, and I'm hoping she'll bounce back from this. Support from her vets is critical to her care at this point, and I have received a lot of helpful insight from fellow horse owners who have dealt with EPM in the online Yahoo EPM community group.  When owners like me have a horse and read a story (about EHV) and think about it being on the tracks, we never think to vaccinate our horses against rhino, because they never go anywhere. That's something horse owners need to rethink."

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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