The advantages of PCR in dermatophyte testing

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Dermatophytosis is a common superficial fungal skin disease in dogs and cats. It is a relevant condition as it is contagious, infectious and can be transmitted to people. In the canine and feline species, the three most common causative fungi of dermatophytosis are Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes.

Dermatophytosis is diagnosed by utilizing a number of complementary diagnostic tests, including Woods lamp and direct examination to detect active hair infection, dermatophyte culture by toothbrush technique to diagnose fungal species involved and monitor response to therapy, and histopathology examination with special fungal stains for nodular or atypical infections.

Woods lamp and direct examination have a high rate of false negative and false positive results. Only 50% of M. canis isolates are detectable with the Wood’s lamp and other dermatophytes don’t fluoresce at all. Direct microscopic examination lacks sensitivity because spores are often hard to see and can also lack specificity due to the presence of saprophytic fungi along the hair shaft. False positive and false negative results are most commonly due to inadequate equipment, lack of magnification, patient compliance, poor technique or lack of training. Dermatophyte culture is a very sensitive test and allows species identification. However, fungal culture requires specialized knowledge for correct evaluation. In addition, fungi can take up to 40 days to be cultured.

Molecular diagnostic assays provide speedy, sensitive and specific detection of dermatophytes. The panel detects Microsporum spp. and Trichophyton spp. using PCR tests and has greater than 95% sensitivity and 99% specificity. A positive PCR test can be the result of active infection, fomite carriage or nonviable fungal organisms from a successfully treated infection. There is no evidence that PCR is affected by systemic treatment. False negative test results are very rare and can occur because sampling techniques have not been optimized or if a global dermatophyte marker is not used, as many infections in dogs are due to pathogens other than M. canis.

Dermotophyte PCR test is now available at Battlab providing results within 2-4 working days versus 1-3 weeks using conventional fungal culture. Samples consist of hair roots, skin scrapings and skin squames. All positive results will automatically be followed by a specification PCR. This makes possible the identification of the most common dermatophytes, including: Microsporum canis, M. gypseum, M. persicolor, Arthroderma benhamiae, Trichophyton mentagrophytes, T. equinum, and T. verrucosum.