Dermatophytosis (ringworm) is a common cutaneous fungal infection in both cats and dogs. As many as 90% of cats with dermatophytosis are infected by Microsporum canis. This strain is highly contagious and has zoonotic potential, but it is not life-threatening and it is treatable and curable. Due to the highly contagious nature of this disease, an accurate and timely diagnosis of dermatophytosis is very important.

The laboratory diagnosis of dermatophytes routinely includes direct microscopic evaluation of clinical specimens followed by culture on special media considered to be the gold standard. It is considered sensitive, is able to speciate, but requires 21 days of plate observation to yield a negative result.

Real-time polymerase chain reaction (PCR), also called quantitative PCR (qPCR) is an advanced molecular technology ideally suited for the detection of infectious pathogens. qPCR tests were developed targeting the ITS-1 gene sequences for genus-specific Trichophyton spp. and Microsporum spp. and a beta-actin-specific Microsporum canis assay. A clinical validation included 273 samples collected from 214 cats. Two hundred and one samples were collected from 195 exposed cats without lesions; 72 samples were collected from 19 cats with confirmed dermatophytosis lesions. Culture was positive in 17 lesion samples from 7 dermatophytosis-confirmed cats. PCR was positive in 15 of those samples, as well as in an additional 7 positive lesion samples from a total of 8 cats. Culture was 87% sensitive and 88% specific when compared to PCR on lesioned samples. Cats without lesions but exposed to diseased cats were also tested to determine if PCR is too sensitive by detecting fomite carriers. PCR detected 7 positive samples from 4 exposed cats, compared to 10 culture positives from 10 exposed cats. Of the 19 cats with lesions, 6 were monitored at regular intervals during systemic and topical therapy. The average time to resolution was 47 days. The shortest duration to resolution was 23 days. It is therefore recommended that PCR monitoring should start at around 3 weeks of treatment.

In summary, qPCR compared to conventional culture showed excellent sensitivity in clinical cases with lesions, did not pick up significant false-positive signals in cats exposed to but not infected with dermatophytes and can be used to monitor therapy success. Paired with its speed to result, the qPCR panel has excellent utility in the diagnosis of dermatophytosis.

Disclosures

Disclosures to report
Author and coauthors are employees of IDEXX Laboratories, Inc.