RT‐qPCR for the diagnosis of dermatophilosis in horses

Background Dermatophilus congolensis causes a crusting dermatitis that affects horses. Diagnosis requires the identification of the organism with cytological evaluation of crust samples. This method can lack sensitivity in chronic cases. Hypothesis/Objectives To develop a probe-based real time quantitative polymerase chain reaction (RT-qPCR) test to assist with the diagnosis of dermatophilosis in horses. Animals Twenty six privately owned horses and seven horses from a research colony were used. Methods Crust samples, collected from 14 horses with suspected dermatophilosis and 12 horses with crusting skin disease not characteristic of dermatophilosis, were evaluated by cytological evaluation and RT-qPCR; the latter was also performed on hair samples collected from seven healthy horses. Results Cytological evaluation revealed organisms consistent with Dermatophilus congolensis from nine horses with suspected dermatophilosis, with only a few organisms seen from five samples. Cytological evaluation of all other crusts was negative for Dermatophilus. Other bacterial organisms were detected on cytological evaluation from 15 samples. RT-qPCR for Dermatophilus was positive from 11 crusts, whereas all other samples were negative. Two samples were cytologically negative but RT-qPCR positive for Dermatophilus. No samples were cytologically positive but RT-qPCR negative for Dermatophilus. Conclusion Results of this study show that RT-qPCR may be a more sensitive and easier method than cytological evaluation for the diagnosis of dermatophilosis in horses.