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."
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.
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.
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 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.
"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.
Photo 3: CSF taken from the horse in this case shows large blue cryptococcal yeasts present throughout the slide.
"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
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.