Besnoitiosis in Donkeys: Skin Cysts, Eye Lesions, and Emerging Parasite Infection

Besnoitiosis is an infectious parasitic disease caused by Besnoitia organisms, most commonly Besnoitia bennetti in donkeys. It is considered an emerging equine parasite infection in some regions, including reported cases in the United States and Europe. The parasite forms thick-walled tissue cysts, which is why many affected donkeys develop visible white nodules in the skin, mucous membranes, and around the eyes.

In donkeys, the disease most often shows up as a chronic skin problem rather than a sudden emergency. Tiny cysts may be seen in the nostrils, muzzle, ears, genital tissues, or inner thighs. A very characteristic finding is the presence of "scleral pearls," which are small cysts at the border of the cornea and sclera. Some donkeys remain bright and stable, while others lose weight, fail to thrive, or develop more widespread skin changes.

This condition can be frustrating for pet parents because it may look like a stubborn skin disease that does not respond to antibiotics, ointments, or medicated baths. That pattern matters. When a donkey has persistent white nodules, chronic dermatitis, or unusual eye lesions, besnoitiosis should be on your vet's list of possible causes.

Besnoitiosis in donkeys is caused by infection with the protozoal parasite Besnoitia bennetti. This parasite is related to other cyst-forming coccidia, including organisms in the same broader group as Toxoplasma and Neospora. In infected donkeys, the parasite multiplies in one stage and later forms thick-walled tissue cysts packed with bradyzoites, which are the structures your vet may identify on biopsy.

One of the biggest challenges is that the full life cycle is still not completely understood. The definitive host for B. bennetti remains unknown, so experts do not yet have a complete map of how the parasite moves through the environment. That means prevention recommendations focus on practical herd biosecurity rather than a single proven route of control.

Researchers and clinicians suspect that transmission may involve close contact, mechanical spread by biting insects, or movement of infected animals, but the evidence in donkeys is still limited. Young donkeys may be at higher risk of developing visible disease. Because there are still gaps in what veterinary medicine knows about transmission, a careful herd history and examination of herd mates can be just as important as evaluating the affected donkey.

Diagnosis usually starts with a good physical exam and a careful look at the skin, nostrils, and eyes. Visible white nodules in the nares or scleral pearls can strongly raise suspicion. In chronic cases, your vet may also consider other causes of nodular or nonresponsive skin disease, including sarcoids, habronemiasis, fungal disease, allergic skin disease, bacterial dermatitis, and other parasitic conditions.

The most practical confirmatory test in donkeys is typically a skin biopsy with histopathology. Under the microscope, pathologists can identify the characteristic thick-walled tissue cysts. This is the current diagnostic approach most often referenced for donkeys in the United States. Depending on the case, your vet may also recommend basic bloodwork to assess overall health, an ophthalmic exam if the eyes are involved, or endoscopy if airway lesions are suspected.

Serologic testing has been studied and can help identify infected donkeys, but availability is limited and may depend on specialty or research access. Because no single test answers every question, diagnosis often combines lesion location, biopsy findings, and the donkey's clinical history.

Prevention centers on biosecurity and early recognition, because there is no licensed, reliably effective treatment for donkeys and no practical routine vaccine option for this species. Check donkeys regularly for white nodules in the nostrils, muzzle, genital tissues, and eyes. If you notice suspicious lesions, schedule a veterinary exam before moving that animal, breeding, or mixing it closely with new herd mates.

When adding a new donkey to the herd, quarantine and perform a careful visual exam. If lesions are present, your vet may recommend biopsy before full introduction. It is also sensible to avoid purchasing animals from herds with known besnoitiosis concerns when possible. Because the exact transmission route is still not fully defined, reducing unnecessary contact and avoiding shared exposure with visibly affected animals is a practical step.

Good fly and biting insect control may also help reduce risk, especially during warmer months, even though transmission details in donkeys are not fully settled. Work with your vet on a realistic herd plan that matches your facility, climate, and budget. In emerging diseases like this one, catching lesions early and limiting movement of suspect animals can be more useful than waiting for the disease pattern to become obvious.