Journal Scan: Screening shelter cats for ringworm doesnt have to take forever

The authors investigated commercially available PCR as a potentially more time-efficient method of diagnosing dermatophytosis. (Adobe Stock)

Dermatophytosis, usually an infection by Microsporum canis, is a relatively common skin condition in cats. It's most frequently diagnosed in kittens, making its prevalence in a shelter situation high, especially among the most adoptable population. It's also zoonotic, which presents the challenge of identifying and treating infected cats before releasing them to their new adoptive homes.

The traditional method of diagnosing dermatophytosis is fungal culture. However, a fungal culture can take 10 to 21 days to perform. Commercially available polymerase chain reaction (PCR) testing can return results in one to three business days. The authors investigated commercially available PCR as a potentially more time-efficient method of diagnosing dermatophytosis by looking at sensitivity, specificity and correlation with fungal culture results.

What they did

Researchers selected 132 at-risk cats from a shelter situation by either the identification of skin lesions, their history of exposure or both. Hair samples were taken from identified cats and both fungal culture and PCR were performed. All positive culture samples were subsequently submitted for PCR as well.

What they found

The authors found 28 cats that cultured positive out of the 132 sampled. All of the cats that cultured positive were also PCR-positive, making the sensitivity 100%. An additional 12 cats were PCR-positive but were negative on the initial fungal culture. Nine of those cats were available to be recultured, and two were positive on a second fungal culture. This made the specificity 88.5%.

Take-home points

Commercially available fungal PCR proved a reliable, fast method of diagnosing dermatophytosis. However, the authors found a significant number of cats that were PCR-positive and culture-negative. The authors postulated that this may be due to the PCR detecting low levels of fungi, including contaminants, nonviable spores and cats with very low fungal burdens. The authors believe that PCR has a place in the shelter environment. Although it has a higher initial cost, it decreases the time the cat needs to spend in the shelter, which potentially reduces overall costs.

Jacobson LS, McIntyre L, and Mykusz J. Comparison of real-time PCR with fungal culture for the diagnosis of Microsporum canis dermatophytosis in shelter cats: a field study. J Feline Med Surg 2018;20(2):103-107.

Link to abstract: https://www.ncbi.nlm.nih.gov/pubmed/29172910

Dr. Michael Nappier is assistant professor of community practice in the Department of Small Animal Clinical Sciences at the Virginia-Maryland College of Veterinary Medicine in Blacksburg, Virginia.