Post-surgical seroreversion in a case of equine cutaneous leishmaniosis by Leishmania infantum.

Equine leishmaniosis, caused by Leishmania infantum and transmitted by infected sand flies, has been increasingly reported in Europe, although clinical and immunological data remain scarce. We describe a 10-year-old mare from southern Spain presenting with periocular nodular lesions. Histopathology and immunohistochemistry confirmed granulomatous dermatitis with intrahistiocytic Leishmania amastigotes. Additionally, Leishmania infantum DNA was detected in paraffin embedded skin biopsy. Following surgical removal of the lesions, sequential serological monitoring using enzyme-linked immunosorbent assay, and immunofluorescent antibody tests were performed over 90 days. Enzyme-linked immunosorbent assay and Western Blot results varied depending on conjugate type (Protein A versus Protein A/G). Western Blot revealed immunoreactivity against multiple Leishmania infantum antigen fractions, including early infection markers 14-16 kDa by conjugated Protein A/G and the absence of band by conjugated Protein A. Immunofluorescent antibody test using an anti-horse IgG fluorescein-labeled conjugate, where titers declined from 1:160 at 45 days to seronegativity by day 90, indicating antibody seroreversion within three months post-surgery. Similar results were detected by enzyme-linked immunosorbent assay with the absence of detection by Protein A but seropositivity was detected by Protein A/G. Finally, no parasitemia was detected by molecular test during the follow-up. This case represents the first documented seroreversion kinetics in equine leishmaniosis and highlights the low and transient humoral response in horses compared to dogs. Our findings underscore the importance of combining histopathology and immunohistochemistry, for the accurate equine leishmaniosis diagnosis, and emphasize the need for further studies to clarify the epidemiological role of horses in Leishmania infantum transmission.