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Comparing PCR with fungal culture in diagnosing ringworm in cats

Jacobson LS, McIntyre L, 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 Feb;20(2):103-107.  (Winn-funded study)

Ringworm, or dermatophytosis, is a fungal infection of the skin found in most species, including cats, dogs, large animals, and humans. In cats, the disease is most common in kittens and animals with weak immune systems. While the disease is not fatal, it causes significant cosmetic lesions and may be spread to humans, where it causes an itchy rash. Due to the highly contagious nature of the disease, rapid diagnosis is important. The gold standard of diagnosis is a fungal culture; while this is highly accurate it may take several weeks to attain results and requires facilities for handling potentially biohazardous materials.

PCR is a technology used to detect the DNA of organisms. It allows for a faster turnaround time and highly sensitive and specific identification of an organism. The risks of culturing a fungus are also not present.

The purpose of this study was to determine the sensitivity and specificity of dermatophyte PCR for diagnosis of ringworm infection and for confirming mycological cure when compared to  fungal culture. The study was designed as a prospective cross-sectional observational study on cats presenting to a Humane Society in Toronto, Canada in a 17-month period.

Hair samples for both fungal culture and PCR were collected by plucking from the margin of a lesion and brushing the coat with a sterile toothbrush. Each sample was divided in half and submitted for both PCR and fungal culture. Fungal cultures were performed on a routine fungal culture medium using standard techniques and incubation, with samples checked for growth at 3, 7, 10, 14, and 21 days. Samples with growth were transferred to a slide and confirmed as ringworm with microscopy. Real time PCR was performed at a commercial reference laboratory for Microsporum and Trichophyton species.

Positive cats were treated with a combination of lime sulfur dips and oral itraconazole. Mycological cure was defined as two consecutive negative cultures.

Cats were enrolled in the study if they had skin lesions consistent with ringworm (alopecia with or without crusting) or if they had exposure to known positive cats. 150 cats were enrolled in the study, of which 132 were included in the final analysis.

Of these cats, 28 (21.2%) were culture positive, and 104 (78.8%) negative. All of the culture positive cats were also PCR positive. Of the culture negative cats, 92 of them were also PCR negative. 9 of 12 of the culture-negative, PCR-positive cats were re-cultured, of which two were positive. Of the 7 cats that were negative on repeat culture, 5 had recent exposure to positive cats.

Based on these results, the sensitivity of the test was determined to be 100% (95% CI 97.7%-100%) and the specificity 88.5% (95% CI 80.7–93.9).

When assessing for mycological cure, at the time of the first negative culture, 82.4% were PCR positive, and at the time of second culture 64.7% were PCR positive.

The results of this study suggest that there is good sensitivity and specificity for initial diagnosis of ringworm in cats, however this may not be as accurate in confirming  cure status. These results are not surprising, as PCR is widely known to have a very high sensitivity, to the extent of detecting organisms even in animals without clinical disease. Cats with too low of a burden to cause clinical disease, or with dead organisms present in their coat will likely test positive by PCR but not be culturable. Real-Time PCR attempts to circumvent this limitation by setting a limit of detection, but was apparently unsuccessful in this study.

Data from one PCR is not necessarily transferrable to another test at another lab, and so these results may not apply to all PCR tests on the market. Proper sample collection is also important, and so improper collection may change the results of PCR testing.

The conclusion of the authors is that PCR may be used for rapid detection of disease in cats at risk of dermatophytosis, and may be especially useful in ruling out infection. When confirming mycological cure, fungal culture should remain the standard of care. (MRK)

See also:

Frymus T, Gruffydd-Jones T, Pennisi MG, et al. Dermatophytosis in Cats: ABCD guidelines on prevention and management. J Feline Med Surg 2013; 15: 598–604.