The Emergence of Cryptococcus gattii in British Columbia: Veterinary Aspects

by Duncan, Colleen

A series of presumed or confirmed Cryptococcus gattii cases diagnosed between 1999 and 2003 was compiled through review of records from veterinary laboratories and human diagnostic services. There was a continual increase in the annual number of animal, but not human, cases diagnosed; no seasonality was observed. Animal cases exceeded human cases by almost 75% even though it was hypothesized that animal cases are more likely to go undiagnosed or unreported when compared to humans. Animal cryptococcosis cases were identified on Vancouver Island prior to 1999 suggesting the organism may have emerged in the region prior to its identification as a causative agent for human disease; therefore animals may serve as a good sentinel for human cryptococcosis infection.

There were 50% more feline than canine cases and disease appeared more commonly in middle aged cats and younger dogs. There was no sex predilection for either species. The primary system involved was most commonly respiratory, followed by central nervous system (CNS) in both cats and dogs. There was a higher proportion of CNS disease in dogs relative to cats, and cats were much more likely to have subcutaneous or dermal masses relative to dogs. Multivariate survival analysis identified only the presence of neurological symptoms as a statistically significant predictor of mortality; those animals exhibiting CNS symptoms were over four times more likely to die than those never showing neural signs. A case-control study identified host and environmental risk factors for clinical C. gattii infection in dogs and cats suggesting that where an infectious agent is not uniformly distributed, individual risk increases when the organism is re-distributed through large scale environmental disturbance, or when the animal has increased opportunities for exposure through travel or activity level.

Serum samples and material for fungal culture were collected from dogs, cats, horses and terrestrial mammal species residing within the region where clinical cases had been diagnosed. Nasal colonization was identified in squirrels ( Sciurus carolinensis), horses, dogs and cats. Most of the animals sampled had no signs of systemic infection however asymptomatic infection, defined as the presence of cryptococcal antigen in the bloodstream in the absence of clinical symptoms, was identified in a small number of dogs and cats. Fourteen months of follow-up testing of asymptomatic animals revealed that animals can progress to clinical disease, remain sub-clinically infected, or clear the organism.