by Arnold Plotnick MS, DVM, ACVIM, ABVP
Originally published in Catnip Magazine Volume 22, No. 2, 2014
About 10 months ago, I received a phone call from a cat owner seeking a second opinion. Their 4 year old male orange tabby, Teddy, has always been a bit of a troublemaker, knocking things off counters, chasing imaginary mice, and leaving no houseplant un-nibbled. Over the last few weeks, however, Teddy had been battling a stubborn upper respiratory infection (URI), and it was only getting worse, despite treatment. I asked about his treatments, and they read me a list of about eight antibiotics that had been tried, all to no avail. “We sometimes see a little improvement, but the sneezing and nasal discharge never goes away. Now his eye has become very runny.” I asked if the cat had been tested for the feline leukemia virus, or the feline immunodeficiency virus, as both of these viruses can suppress the immune system, making it difficult for some cats to defeat an infection. “He was tested twice”, I was told. “Negative both times.” I asked if the cat was ever treated with antiviral medication, instead of antibiotics. “Yes. We gave him famciclovir for two weeks.” I asked if there was any improvement. “None whatsoever”, his owner told me. Clearly, this was not a typical URI. I recommended that they bring Teddy in for me to see him in person. We scheduled an appointment for that Friday.
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Cryptococcosis is the most common systemic fungal disease of domestic cats worldwide. It is caused by a fungus belonging to the genus Cryptococcous.
The organism is present worldwide, although it is most commonly found in Australia, western Canada, and the western United States. People can also be infected with Cryptococcus, however, infection in cats is not considered to be a public health hazard, as there is no proven spread between animals and people.
Diagnosis is usually made by running a test called the latex agglutination test. This is a blood test in which the serum is evaluated for the presence of antigens that are part of the capsule that surrounds each Cryptococcus organism. The levels are often very elevated in infected cats, although even low levels are considered diagnostic. The test is very accurate and false positives are rare. Because the test is fast, reliable, and minimally invasive, it is the diagnostic test of choice. Not surprisingly, Teddy’s test came back extremely high. Another common diagnostic test is cytology. A sample of the nasal discharge, or material aspirated from a skin nodule, is applied to a slide and is stained and evaluated under a microscope by a cytologist. The organism has a characteristic appearance under the microscope, and identification of the organism confirms the diagnosis. Not finding the organism, however, doesn’t mean that the cat isn’t infected. If no organism is seen via cytology, a portion of the sample can be used for culture. The Cryptococcus organism can be isolated easily on standard culture media. In Teddy’s case, culture wasn’t necessary. I made several slides from a sample of Teddy’s nasal discharge and sent them to our laboratory for analysis. The cytology report confirmed the presence of numerous Cryptococcus organisms. Another diagnostic test would be histopathology. This differs from cytology in that instead of obtaining a sample of discharge (from the nose, or from a draining nodule), a biopsy specimen (for example, a small piece of tissue from a skin nodule) is submitted to the laboratory and a pathologist evaluates it under the microscope. In most cases, a large number of organisms will be identified on the microscope slide, confirming the diagnosis.