Bovine besnoitiosis, caused by a cyst-forming protozoan parasite called Besnoitia besnoiti (B. besnoiti), is widespread in Africa, Asia, southern Europe and South America. Where present, B. besnoiti leads to major economic losses in beef and dairy cattle as a result of decreased milk production, infertility of bulls, unthriftiness and mortality. Carcass trim and condemnation at slaughter would be a major issue under Canadian and U.S. meat inspection systems.
Although a form of the disease is common in caribou, reindeer and muskoxen (B. tarandii) in Canada, and has been diagnosed in donkeys from several U.S. states (B. bennetti), bovine besnoitiosis has not been reported in North America.
Could it become a new disease threat in North America? Like many other diseases that started elsewhere only to emerge on North American soil, besnoitiosis needs to be watched. The ingredients are here.
Over the last 10 years, B. besnoiti has spread from pockets where it traditionally occurred into new areas in Spain and France. It was diagnosed in Germany and most recently appeared in Switzerland and Italy. Animal health officials are concerned the parasite is becoming established in areas where it has never existed before. Recent epidemiological data confirms an increased number of cases and geographic expansion of besnoitiosis in cattle herds in EU states. With development of better diagnostic tools, scientists are keeping a close watch on the creep of subclinical disease in beef and dairy herds across Europe. U.K. officials are concerned about the potential introduction there. The European Food Safety Authority (EFSA) has recently issued a paper suggesting that bovine besnoitiosis should be considered an emerging disease in the EU. Because many aspects of the epidemiology of besnoitiosis remain uncertain, including the prevalence and incidence of infection, routes of transmission and risk factors associated with the disease, the paper highlights the urgent need for research programs to improve efficient and sustainable control methods.
Besnoitiosis is a disease that primarily affects the skin and subcutaneous tissues below the skin. Cysts are commonly found in the conjunctiva surrounding the eye and in the lining of the throat and external reproductive tract. Through early stages of the disease, fluid (edema) often collects along lower portions of the body associated with disruption of small blood vessels. Eye and nasal irritation is common.
During more chronic stages of the disease, a multitude of tiny skin cysts create very noticeable lesions. Skin nodules form and the skin becomes dry, thickened and hairless, sometimes described as elephant skin. As the disease progresses, animals become emaciated and have difficulty moving. At this point, their welfare is compromised. Cysts are hard and have a slight roughness that gives connective tissue below the skin the appearance of being sprinkled with “corn meal.”
The severity of the disease may vary between mild and severe. Seriously affected animals can die. Many infected animals remain asymptomatic, the only sign of disease being the presence of cysts in conjunctiva around the eye, or in the lining of the vulva in cows and nasal passages of all cattle. The testes of bulls can degenerate and atrophy, rendering them sterile.
Although mortality is low (less than 10 per cent), convalescence is slow in severe cases. Affected animals remain carriers for life. Up to 50 per cent of animals in a herd can be infected with most infected animals showing no, or only mild clinical signs. Herd productivity suffers.
The complete life cycle of the B. besnoiti remains unclear. Several routes of infection are probably involved.
The bovine is considered an intermediate host, capable of spreading infection through direct contact with herd mates. Wild carnivores, dogs and perhaps cats act as definitive hosts capable of contaminating pastures and feedstuffs with infective stages of B. besnoiti. Wild ruminants and probably rodents should not be disregarded as reservoirs of the parasite and a potential source of parasites to carnivores. Transmission between intermediate hosts independent of definitive hosts may occur through biting insects.
The seasonal pattern of besnoitiosis in beef cattle on summer pasture suggests transmission by direct contact between cattle (including natural mating) may be one possible route of transmission. The mechanical transmission of parasites by biting insects like horseflies and deer flies is considered another way parasites are transmitted between infected and non-infected cattle. At present, bovine besnoitiosis has not been reported to infect humans.
A common diagnostic finding is the appearance of cysts in the scleral conjunctiva at the junction of “white and colour” of each eye and in the nasal mucosa. The crescent-shaped organisms can be found in skin scrapings, skin biopsies, and conjunctival scrapings. Blood and PCR tests are also used. In some countries, cattle are immunized with a live, tissue-culture-adapted vaccine. There are no effective drugs or vaccines available in North America at present. Oxytetracycline may have some therapeutic value if given early in the course of the disease.
— Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).