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Does My Donkey Have Besnoitia

Judy Marteniuk, DVM and Charles Mackenzie, BVSc FRCVS

Michigan State University

What is Besnoitia?

Besnoitia is a protozoan parasite that is found worldwide and can affect many species of animals, such as cattle, sheep, goats, reindeer, equids, rabbits, opossums, and rodents.  Besnoitia bennetti is the species that has been found in horses, donkeys and mules.  Other protozoan parasites in this family that you may recognize are Toxoplasma gondii (causes abortions in multiple species, including humans) and Sarcocystis neurona (causes Equine Protozoal Myelitis - EPM). 

B. bennetti was first reported by Bennett in 1927, and 1933 in four horses in the Sudan.  Initially, the organism was thought to be in the family Sarcocystis, but was reclassified as Besnoitia in 1932.  Over the years, sporadic reports of B. bennetti have been published.  In 1955, Schultz and Thorburn found tissue cysts in a horse in S. Africa. By 1960, Pol determined that cutaneous besnoitiosis in horses was not able to affect cattle.  Then, Bigalke in 1970 suggested that Besnoitia in horses, mules and donkeys in S. Africa was a separate species. Van Heerden described the unique ultrastructure for B. Bennetti in 1993. The tissue cysts are between 100-200 μm in diameter and the organisms (slowly replicating bradyzoites) in the cysts are 8.7 X 1.9 μm. 

Interestingly, B. bennetti has been reported primarily in Africa and only more recently in the United States.  In the U.S., sporadic recognition of the parasite has been made in Florida, Maryland, Massachusetts, Michigan, Montana, Texas; it has also been reported in West Virginia in the past year. Thus, it is probable that the infection has a much wider distribution in the US than was first thought.

What are the signs of Besnoitia?

Donkeys clinically affected with Besnoitia will have hair loss with thickening and crusting of the skin.  This abnormal skin is most prominent over the face, ears, neck. perineal area, and lower legs. Some donkeys are reported to have pruritus (itchiness) while others show no signs of being irritated by the skin lesions.  Most of the Michigan donkeys that we reported on in 2005 did not appear to be itchy; however, one jack had a significant degree of pruritus and hair loss..  Another prominent feature of this disease is the presence of “pinpoint” nodules in the sclera, and mucous membranes of the lips, nostrils and vulva. In the Michigan outbreak, the sclera nodules were close to the limbus and common toward the medial canthus (inner corner) of the eye.

Blood work on affected donkeys varied.  Some individuals had completely normal blood work, while other donkeys were anemic, and had an increased white blood cell count.  Total protein and globulins (proteins used to fight infection) also varied from normal to increased.  The severity of the clinical signs was not consistently correlated with the changes in the blood.

Interestingly, most of the donkeys in the Michigan outbreak and in the literature seem to be female.  This may be related to the fact that the farms affected with Besnoitia were breeding farms.  Also, affected donkeys tend to be the younger members of the herd.  The age breakdown for the Michigan outbreak is presented below.

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How is Besnoitia diagnosed?

Clinical presentation with the nodules in the sclera, vulva, and lip are very characteristic for the disease. The loss of hair and the thickening and crusting of the skin although present with Besnoitia, may be present in several other skin problems.  Therefore, a skin biopsy of the affected areas is needed.  Histologically, characteristic cysts are seen in the dermis and epidermis.  Each cyst is packet with hundreds of bradyzoite stage parasites.

With the outbreak in Michigan, all donkeys survived and recovered over a prolonged period of time.  However, in the literature a necropsy of a severely affected donkey is reported.  That donkey had hundreds of pinpoint white granular cysts (0.5 to 1.5 mm) in the epidermis, and dermis.  These cysts extended to the subcutaneous tissues and connective tissue under the affected skin.  Lymph nodes near affected skin were also enlarged, as was the lymphoid tissue surrounding the larynx.   When the tissues were examined under the microscope, lesions were seen in the thyroid, around the thyroid, around the laryngeal connective tissues, in the lung, colon, kidneys and lymph nodes. Interestingly, numerous free bradyzoites were seen in the skin crusts. It is suspected that these free, infective organisms may be readily picked up and transferred to other equids by biting insects. However, the route of transmission of this protozoan has never been fully defines.

Can I treat my donkey for Besnoitia?

There is no approved treatment for Besnoitia. The most common treatment used for Besnoitia is prolonged treatment with trimethoprim sulfa.  Treatment is often given continuously for several months, similar to treating a horse for EPM.  Others have described an alternating treatment protocol where the donkey is treated for several weeks, then not treated for several weeks.  This protocol may be repeated a number of times.  Another treatment option may be using Marquis (ponazuril) which is another drug licensed for EPM treatment.  Currently, there is no information on the efficacy of this drug for Besnoitia treatment.

Interestingly, Besnoitia cysts can still be found in skin biopsies even when treatment is extended over months and the donkey’s skin is almost normal in appearance.  It is currently unknown if these cysts are cleared with time. Also, in the Michigan outbreak, donkeys that were mild to moderately affected and did not receive prolonged treatment, improved with time to the point that it was very difficult to find any lesions. 

How is Besnoitia spread?

Currently, the lifecycle of Besnoitia in all hosts is poorly understood.   However, we do know that the lifecycle is what is termed an “indirect lifecycle” meaning that the parasite needs both a direct/definitive host and an indirect/intermediate host. Many of the members of the Besnoitia family are believed to have a member of the cat family as the definitive host. To date, researchers have not been able to find or reproduce bradyzoites of B bennetti in mice, rabbits, gerbils or cats.

With this type of parasite, the most common method of spread is when the direct host eats tissue from an infected, intermediate host. However, there is also the possibility that the organism can be spread from intermediate host to intermediate host ( directly between donkeys - termed horizontal spread) by insects since the bradyzoites can be found in sloughing, crusty skin debris. To further support the potential for horizontal spread is the fact that Besnoitia has been shown to remain infective for 50 hours in Culex spp (mosquitos).

Finally, even though understanding of the lifecycle is incomplete, you can still implement preventive measures. Just good sanitation and fly control will be beneficial in reducing the potential for horizontal spread.  Also, keeping your grain bin covered will discourage wildlife from setting up residence in your barn, thus reducing the risk of the definitive host depositing B. bennetti oocysts in the environment.

Future considerations
If you think that your donkey may be affected with Besnoitia, please let us know.  Without the help of donkey owners, learning more about this organism will take more time than is necessary. Even with everyones help, it will take a lot of work, patience and luck to document the lifecycle of Besnoitia bennetti.


  Reprinted with permission from NMDA and The Asset Magizine.

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