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