Besnoitiosis is a protozoan disease. The cystic stages cause clinical changes in the skin, subcutis, blood vessels, mucous membranes, and various deeper tissues.
Etiology and Transmission
The parasite is relatively host specific. Besnoitia besnoiti is the causal agent of a primarily cutaneous disease in cattle, and B bennetti of a similar disease in donkeys and horses. B besnoiti has been reported in southern Europe, Africa, Asia, and South America; B bennetti has been reported from Africa, southern France, Mexico and the USA. B jellisoni and B wallacei are found in rodents; B tarandi infects reindeer or caribou; B darlingi is found in lizards, opossums, and snakes; and B sauriana in lizards. B caprae has been found in goats in several countries, including Iran, New Zealand, and Kenya. Rodents and other wildlife in Australia, as well as a range of antelope and wildebeest species in Africa, have also been shown to be carriers of this parasite.
The Toxoplasma-like organisms multiply in macrophages as well as in endothelial and other cells, producing characteristic large, thick-walled cysts filled with bradyzoites that develop into the invading tachyzoites.
Experimental cyclic transmission with intestinal sexual stages in a definitive host—the cat—has been reported for B besnoiti, B wallacei, and B darlingi. For others in this group, such as B bennetti, the definitive host remains unclear. Biting flies or ticks may transmit B besnoiti mechanically from chronically infected cattle, but this mode of transmission remains unproved for this or other Besnoitia spp. Some Besnoitia spp can be transmitted artificially to suitable hosts by needle inoculation of tissues containing cysts. However, the major route(s) of transmission for most of the species still remain undefined. Individual isolates appear to be relatively specific for their respective intermediate hosts.
Infected cattle often show no clinical signs other than a few characteristic cysts in the scleral conjunctiva. Illness begins with fever followed by warm, painful, ventral swellings that give way to sclerodermatitis. Swollen lymph nodes, diarrhea, inappetence, photophobia, rhinitis, and orchitis also are seen. The skin becomes hard, thick, and wrinkled and frequently develops cracks that allow secondary bacterial infection; often myiasis develops. Loss of hair and epidermis is common in severe cases. In addition to these skin lesions, there may be involvement of the musculoskeletal system and, in some cases, draining lymph nodes and organs such as the testes. The cystic lesions seen in the infected tissues are often associated with altered blood vasculature. Severely affected animals become emaciated. In general, a similar presentation can be seen in donkeys and horses infected with B bennetti.
A useful diagnostic finding in both cattle and horses is the appearance of cysts in the scleral conjunctiva and nasal mucosa; these “pearls” are most easily seen when an animal lifts its head. Diagnosis can be made by finding crescent-shaped bradyzoites in skin scrapings, biopsy, or conjunctival scrapings.
Although mortality is low, convalescence is slow in severe cases, and severely affected bulls can become permanently sterile. Affected ruminants remain carriers for life.
The disease in goats is similar to that in cattle (as is that of horses and donkeys) but tends to be less severe or invasive in these latter animals. Clinical manifestations are more common in younger animals.
Prevention and Treatment
B besnoiti infections are economically important to cattle owners in endemic areas because of mortality (although usually <10%), sterility (which may be temporary or permanent), hide damage, loss of condition, and lower market value. Treatment is difficult, but some success with oxytetracycline has been seen with cattle if given early in the course of the disease. However, no highly successful treatment has yet been defined for affected Equidae. Affected animals should be isolated and treated symptomatically.
In some countries, cattle are immunized with a live, tissue culture-adapted vaccine. Reduction of biting insects and ticks also may reduce transmission.
Last full review/revision March 2012 by Charles Mackenzie, BVSc, BSc, PhD, FRCVS, FRCPath, DEd