Bovine chronic besnoitiosis in a calf: Characterization of a novel B. besnoiti isolate from an unusual case report.

Bovine besnoitiosis, caused by the apicomplexan Besnoitia besnoiti, is a chronic and debilitating disease characterized by cutaneous and systemic manifestations that primarily affects adult beef cattle. Previous studies have reported that clinical besnoitiosisis is rare in calves. However, we isolated B. besnoiti from a chronically infected calf for the first time. The identity of the Besnoitia species was determined after parasite isolation and molecular genotyping. According to the results obtained in vitro the new isolate, named as Bb-Spain3, was characterized in a reproducible in vitro model and was categorized as a low invader and low prolific isolate with a slower lytic cycle compared to Bb-Spain 1 isolate. Specific traits that differentiate isolates obtained from adult animals from those infecting calves were not found. Next, we described the first case report of chronic besnoitiosis in a female calf less than 6 months-old with a low body condition. The disease was confirmed by the presence of specific anti-B. besnoiti antibodies and parasite detection in the skin. At post-mortem examination, tissue samples were collected for histological, immunohistochemical and molecular analyses. DNA-parasite was detected in 31 different calf's tissues, being the most highly parasitized tissues the skin and the respiratory and reproductive tracts. In addition, the parasite was also present in heart, eyes, lymph nodes and brain. The high parasite load, a wide intra-organic parasite distribution and the presence of both viable and degenerated cysts, were indicative of a rapid progression of the disease. This case report underlines the need to include the inspection of young animals in besnoitiosis control.