Leishmaniasis: No Longer Just a Foreign Animal Disease

There's a scary new disease out there that could potentially harm not only horses, but also humans. Less than two years ago a team of Florida-based veterinarians published a report of the first equine cutaneous (affecting the skin) leishmaniasis case diagnosed in the United States in a horse without history of international travel. And recently, Sarah M. Reuss, VMD, Dipl. ACVIM, shared her experience with the disease to raise awareness about this potentially dangerous problem among her fellow practitioners.

Reuss, a clinical assistant professor at the University of Florida's College of Veterinary Medicine, reviewed the condition and its clinical signs, diagnostic methods, and treatment options with veterinarians at the 2013 American Association of Equine Practitioners' Convention, held Dec. 7-11 in Nashville, Tenn.

Leishmaniasis is a zoonotic (meaning it is transmissible between animals and people) disease well-documented (and potentially fatal) in humans and dogs worldwide. But it wasn't until 2012 that Reuss and colleagues identified the United State's first published autochthonous case (meaning one that's not related to foreign travel) in a 10-year-old Morgan mare. Recently, she said, Florida veterinarians confirmed a second leishmaniasis case in a middle-aged rescue mare, also with no history of international travel.

"It's only two cases, but our interest was piqued," she said.

Reuss said the disease is endemic in many tropical and subtropical climates and can be caused by more than 30 species of Leishmania protozoal parasites. The parasite identified in the Florida horses' cases, L. siamensis, has previously been identified in southern Thailand and central Europe, but hadn't been previously reported in North America.

Researchers know that sand flies can transmit leishmaniasis to horses, Reuss said, and there are sand flies that are native to the United States. In horses, Reuss said, leishmaniasis presents as nodules on the head, pinnae (the upper portion of the horse's ears), scrotum, legs, and neck—the areas where sand flies commonly feed, she pointed out. These nodules can occur in groups or be solitary, and they often ulcerate, she said.

Veterinarians must take several steps to diagnose leishmaniasis, Reuss said, including:

  • Direct observation of nodules;
  • Cytology (the examination of cells under a microscope allowing high magnification of single cells);
  • Histopathology (the microscopic examination of diseased tissue allowing lower magnification of groups of cells);
  • Electron microscopy (allowing the highest magnification to look at discrete structures within the cells);
  • Immunohistochemistry (the process of detecting antigens—or substances foreign to the body that evoke an immune response—in cells by using antibodies against those antigens);
  • Polymerase chain reaction testing (looking for the DNA of the protozoa); and
  • Culture (testing of samples for pathogens by trying to grow those pathogens).

Reuss said practitioners can also serology (blood tests) in the diagnostic process, but this is more commonly employed when working with humans and dogs than with horses.

When it comes to treatment, veterinarians in the United States don't currently have access to pentavalent antimony compounds—the drugs most commonly used to treat leishmaniasis in other parts of the world. In the United States, pentavalent antimony can only be obtained for the military or investigational use from the Centers for Disease Control and Prevention, thus limiting clinical access, she explained.

Fortunately, many equine leishmaniasis lesions regress spontaneously, Reuss said: "If you do nothing, they'll likely go away."

She said veterinarians have applied several treatment options—including pentavalent antimony therapy, surgical resection, and amphotericin and fluconazole (antifungal agents) administration—to successfully treat affected horses, "but it is unclear how many would have resolved with no treatment," she noted.

Reuss said both of the affected Florida horses were treated with fluconazole and local amphotericin injections, and their clinical signs resolved.

Take-Home Message

"Equine cutaneous leishmaniasis can no longer be considered just a foreign animal disease," Reuss concluded. "With climate change and the spread of vector habitats, emerging diseases will continue to infiltrate the United States equine population.

"We have no idea what the current prevalence in the United States is," she said, "but I'd bet it’s more than just the two horses."

She also cautioned veterinarians that, "although this is not a fatal disease in horses, we can and should consider our equine patients as sentinels for this potentially fatal zoonotic disease."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.