Leishmaniasis is a zoonotic disease caused by the obligate intracellular protozoal parasite of the genus Leishmania. There are more than 30 known species of Leishmania that vary with region. The protozoa are endemic in many tropical and subtropical regions in both the Eastern and Western hemispheres, where it is well described in people and dogs. Leishmania infections in humans can result in cutaneous, mucocutaneous, or visceral (organ) lesions. Visceral leishmaniasis is the second leading parasitic cause of death worldwide. Signs include fever, anemia, diarrhea, darkening of the skin, spleen and liver enlargement, and lymphadenopathy.
Dogs are the most commonly affected domestic species and may act as a reservoir for disease. Canines can have visceral, ocular and cutaneous lesions. In endemic areas all breeds of dogs are affected, while in the United States this disease has been most significant in the foxhound population.
Cutaneous leishmaniasis has been documented in horses around the world. Lesions are most commonly observed as nodules on the head, external ear, scrotum, legs and neck. These nodules can ulcerate and are often mistaken for aural plaques or sarcoids. Visceral lesions have not been widely reported in the horse.
L. infantum has been reported as the causative agent of cutaneous leishmaniasis in horses in Germany, Spain and Portugal. Recently a report from central Europe identified L. siamensis (a species previously reported as a cause of visceral human leishmaniasis) in four horses. In South America, L. braziliensis has been identified as the causative organism in horses. Leishmaniasis has been recognized sporadically in the United States, primarily in horses with a history of international transportation. Recently, however, two horses in Florida were diagnosed with cutaneous leishmaniasis due to L. siamensis.
In all mammalian species affected with leishmaniasis, the mode of transmission is believed to be by various species of sand flies. In the Eastern Hemisphere, these are Phlebotomus spp, while Lutzomyia spp are predominant in the Western Hemisphere. The protozoa are transmitted by the female sand fly bite during salivation that occurs during blood feeding.
Diagnosis of cutaneous leishmaniasis in horses is often made by impression smear or biopsy of the lesion with protozoa identified within macrophages. Polymerase chain reaction (PCR) and sequence analysis can be used to confirm the diagnosis and identify the species. PCR targeting the internal transcriber spacer 1 (ITS1) is most sensitive.
Lesions in horses are often self-limiting, but several treatments have been used, including surgical excision and medications such as amphotericin, fluconazole, and pentavalent antimony compounds. The pentavalent antimony compounds (sodium stibogluconate or meglumine antimoniate) are the standard treatment for people and have been used in horses, but these drugs have potentially serious side effects and are not available in the United States. As lesions often spontaneously regress, it is difficult to determine the efficacy of any described treatment.
With increasing international transportation of horses, leishmaniasis should be considered in any horse with cutaneous nodules. While the disease is not fatal to horses, they can be infected with species of Leishmania that are capable of transmission to humans.
This article was written by Dr. Sarah Reuss of the University of Florida College of Veterinary Medicine. The Equine Disease Quarterly is funded by underwriters at Lloyd’s, London, their brokers and Kentucky agents.