Cryptococcosis is a systemic fungal disease caused by Cryptococcus neoformans. Cryptococcus is a yeast-like fungus found most often in association with pigeon droppings. Cryptococcus does not cause disease in pigeons due to the high body temperature of these birds (107.6 degrees Fahrenheit or 42 degrees Celsius), which inhibits growth of the organism. Optimal growth occurs at 98.6 degrees F (37 C), which is the average temperature of mammals.

Cryptococcus neoformans has a worldwide distribution and is the most common systemic fungal infection of cats, although it also affects dogs. Doberman pinschers and Great Danes appear to be at increased risk in Australia, whereas cocker spaniels seem to be at higher risk in North America. However, dogs of any breed, as well as mixed breed dogs, can develop cryptococcosis. Young dogs less than four years of age seem to be predisposed. No gender predilection has been recognized.

Cryptococcus has a thick capsule surrounding it, which contributes to its virulence and resistance to treatment. Infection occurs after inhalation of the organism, when cryptococcus produces a thick capsule that interferes with the ability of the immune system to eliminate it. Immunosuppressed animals, such as animals suffering from malnutrition or receiving treatment with cortisone-like drugs are most susceptible.

Clinical Symptoms

Non-specific symptoms of systemic disease such as fever, lethargy, loss of appetite, and weight loss

Skin nodules in 40 percent of affected animals, in which the organism may be detected by microscopic examination of the exudate

Neurologic symptoms in 15 percent of affected animals, which include incoordination, seizures, lethargy, circling, head-pressing, head tilt, back-and-forth eye movements called nystagmus, paralysis of the facial nerve and blindness.

Eye abnormalities in 15 percent of affected animals, which include dilated, unresponsive pupils. Inflammation of both the front (anterior uveitis) and back (granulomatous chorioretinitis) of the eyes may be detected. Small hemorrhages may be detected in the back of the eye (retinal hemorrhages).

Other diseases of the nasal cavity and nervous system may produce similar signs and must be eliminated as diagnostic possibilities:

Nasal cavity tumors

Foreign bodies in the nasal cavity

Other fungal infections (aspergillosis)

Tooth root abscess

Chronic bacterial sinusitis

Nervous system diseases

Other infectious diseases such as distemper and toxoplasmosis

Cancer of the nervous system like lymphosarcoma

Granulomatous meningoencephalitis

Epilepsy

Certain metabolic diseases like hepatic encephalopathy

Drug or chemical toxicity