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The usual suspects
The trials of diagnosing neurologic disorders.


DVM360 MAGAZINE


Breakdown

Christmas passed with a return of unremarkable blood work results and a report that Rockett was not better, but no worse. The next day, however, an emergency call came in, and Rockett was now collapsed and wedged into a space in the corner of the pasture. On arrival, Dr. X found her standing in the yard. She was minimally responsive and blind. "She was quiet for a while," explained her owner. "Then she got anxious, bolted forward into the fence and went down. She laid that way for a bit, then got anxious and got up again."


Photo 1: The cerebellum and the brainstem of a horse affected with cryptococcal meningoencephalomyelitis. The granulomatous aspect of this disease process can be appreciated by the diffuse nodular appearance of the cord.
A physical examination now revealed severe cranial nerve deficits, severe ataxia and rapidly progressing disease. Rockett had two more similar "episodes" within a short time, and it was obvious more aggressive care was needed. For a while, Dr. X and the owners struggled to get Rockett to step into a trailer. She could no longer see and could not control her limbs. Rockett was referred to a nearby equine hospital at a veterinary college. She was unable to rise on arrival, with even more progressively worsening signs during the trip of less than two hours. Humane euthanasia was elected for Rockett, leaving Dr. X to ponder his differential list.

Epilogue


Photo 2: The spinal cord, seen with its myelin covering reflected, is also noted to be diffusely nodular. The extensive nature of the cryptococcal infection in this horse is also unique given that no respiratory or nasal sinus source of the organism was found during necropsy.
Necropsy results revealed that Rockett had severe, chronic pyogranulomatous meningoencephalomyelitis due to an infection with Cryptococcus neoformans (Photos 1-3). Far from a "usual suspect," this infection is a rare condition in horses with only a small number of reported cases in the literature outside of Western Australia, where it is endemic and carried by certain bird species. Most reported cases in the United States involve the respiratory tract and sinuses, and it is thought that this fungal infection is acquired by breathing in the cryptococcal yeast spores. The horse in this case, however, showed a complete lack of any respiratory or sinus involvement, and there was no indication about the mechanism of infection.


Photo 3: CSF taken from the horse in this case shows large blue cryptococcal yeasts present throughout the slide.
"The prognosis for horses with cryptococcal infections is not well defined because of their seemingly infrequent occurrence and the low number of reported cases," says Allen Morris, DVM, regarding a case report and literature review of cryptococcal meningitis in the horse.

"In order for treatment (fluconazole at 14 mg/kg PO loading dose, then 5 mg/kg PO q 24 h x 6 weeks) to be instituted in a timely fashion," Morris explains, "a definitive diagnosis of Cryptococcus infection at an early stage is probably critical to a positive outcome." Drs. Riley, Mills, Bolton and Thomas of the School of Veterinary and Biomedical Sciences at Murdoch University in Western Australia made this comment in their review of seven cases of cryptococcosis: "Cases of equine cryptococcosis carry a poor prognosis and treatment was not attempted in any of these cases."

Conclusion

Similar clinical signs and generally poor prognoses make neurologic diseases a diagnostic and treatment challenge for equine veterinarians. A broad differential list should be considered. Even though most cases will be caused by the usual suspects, clinicians should always be open to other more unique and obscure causes. As more cases of fungal encephalitis are described and possibly considered earlier in the disease process, greater numbers of cases may be treated with potentially better results.

Dr. Marcella is an equine practitioner in Canton, Ga.


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Source: DVM360 MAGAZINE,
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