Cutaneous Equine Sarcoidosis: A Case Series and Investigation Into a Potential Association With Equine Gammaherpesviruses‐2 and ‐5 in 30 Equids

Background Gammaherpesviruses have been associated with granulomatous inflammation in the skin of various species. Equine sarcoidosis (idiopathic granulomatous disease) can cause granulomatous dermatitis, and its cause remains unknown. Objective This retrospective study aimed to investigate the possible association between equine gammaherpesviruses‐2 and ‐5 (EHV‐2, ‐5) and cutaneous equine sarcoidosis (cES). Animals Thirty client‐owned equids. Materials and Methods All animals had histologically and clinically confirmed cES. Signalment, geographical location and clinical features were recorded. Gram, Ziehl–Nielsen and Grocott–Gömöri methenamine silver histochemical stains, quantitative (q)PCR and in situ hybridisation were performed on formalin‐fixed paraffin‐embedded tissue samples. Results Quarter horses and thoroughbreds were overrepresented, and ages ranged from 10 months to 31 years old, with minimal difference in sex distribution. Localised, partially generalised, and generalised forms were noted in 13, 11 and six animals, respectively. The most common lesions were alopecia, crust and scale, and the limbs were the most affected site. Neither bacteria nor fungi were detected with histochemical stains. EHV‐5 and ‐2 DNA were detected by qPCR in eight and four of 30 animals, respectively. Of those eight horses, EHV‐5 hybridisation signals were found within skin lesions of five. ISH did not detect EHV‐2 in any of the 29 animals tested. Conclusion and Clinical Relevance Although the exact cause of sarcoidosis remains uncertain, these findings suggest that EHV‐5 may play some role in the pathogenesis of cES. This study also reports a single case of cES in a donkey.