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By SallyAnne L. Ness, DVM, DACVIM

Besnoitiosis is an emerging infectious disease of donkeys in the United States. Besnoitiaspecies are cystforming protozoan parasites that infect multiple host species worldwide. Equine besnoitiosis was first described in horses and donkeys in Sudan in the 1920s. Until recently, reported equid cases in North America have been limited to few isolated donkeys and an outbreak in a Michigan herd of Miniature Donkeys in 2005. 

In the fall of 2010, two Pennsylvania donkeys were donated to the Cornell University Hospital for Animals after being diagnosed as infected with Besnoitia bennetti— a rare member of the protozoan family closely related to Sarcocystis, Toxoplasma, and Neospora. In the wake of those two cases, researchers at Cornell University diagnosed multiple infected donkeys across several Northeastern states, plus Tennessee, Texas, Minnesota, Michigan, Oregon, and Washington. Currently, the natural host for besnoitiosis and the mode of transmission within donkey populations is unknown. As the number of identified cases continues to grow, it appears that besnoitiosis might be emerging as an important infectious disease of donkeys in the United States. 

Clinical Signs 

Donkeys with besnoitiosis develop multifocal (occurring in more than one location) pinpoint parasitic cysts, approximately 1.0 mm in diameter, in the skin over the nostrils, ears, face, and body. One of the most unique features of besnoitiosis is the development of “sclera pearls,” which are tiny cysts on the surface of the eye. Some infected animals remain otherwise healthy, while others become thin and debilitated as a result of the disease.

The reason for this difference in host response to infection is unknown. Infected donkeys might also have generalized crusting dermatitis (skin inflammation), poor hair coat, and/or focal areas of hair loss. Often affected donkeys have a history of chronic skin disease nonresponsive to treatment with antibiotics, ointments, medicated baths, etc. 


The current gold standard for diagnosisis identification of Besnoitia cysts within a skin biopsy sample. Upper airway endoscopy of affected donkeys might reveal extensive involvement of the nasal mucosa, pharynx, and larynx. Researchers are currently investigating the prevalence of upper airway lesions in a population of infected donkeys. 

We can detect antibodies to B. bennetti via Western blot and indirect fluorescent antibody testing (IFAT). Although these tests are not commercially available at this time, studies at Cornell University are currently under way to assess using serology to diagnose donkeys with besnoitiosis, and there might soon be a simple blood test available for diagnosing this disease in individual donkeys. 


There are currently no known effective besnoitiosis treatments. Reported attempts to treat infected donkeys with various antimicrobial and antiprotozoal agents have had variable results. Attempted treatment of infected donkeys at Cornell with ponazuril (Marquis), trimethoprim sulfa, and Oroquin 10 were not effective—at least at the dose and duration of treatment used. 

Further investigations are needed to gain a better understanding of disease transmission, treatment, and ultimately prevention. Although besnoitiosis has not yet been reported in North American horses, cases have been described in Africa, and the potential for similar infections in the United States cannot be excluded.  

About the Author: SallyAnne L. Ness, DVM, is an internal medicine specialist at the Cornell University College of Veterinary Medicine’s Equine and Farm Animal Hospital.  

Article provided courtesy of AAEP Media Partner, The Horse.

Reviewed by original author in 2016.