Detection of Besnoitia besnoiti DNA by real-time PCR in individuals or in pools of Stomoxys calcitrans (Diptera: Muscidae), reveals parasite circulation on cattle farms

Bovine besnoitiosis is an emerging disease in Europe, transmitted mechanically by the stable fly Stomoxys calcitrans. Infected cattle harbouring large quantities of parasitic cysts in their dermis are considered super-spreaders. The objective of this study was to demonstrate that Besnoitia besnoiti DNA can be detected in stable flies captured in the immediate vicinity of these hosts. In this case, we wanted to assess how far away from the super-spreaders we could capture infected Stomoxys flies. On two cattle farms affected by besnoitiosis in the south of France, super-spreaders were identified with skin biopsies. In spring, these cattle were kept indoors, and Vavoua traps and BioFlyTrap were deployed in their immediate vicinity and then at increasing distances up to 130 m. DNA extracted from the abdomens of captured stable flies was subjected to real-time PCR to detect B. besnoiti DNA. The apparent densities of S. calcitrans were very high in the immediate vicinity of cattle, but fell sharply as the trap was placed further away, thus demonstrating the sedentary nature of this species. Besnoitia besnoiti DNA was detected in 3 out of 42 batches of Stomoxys flies on one farm (only in the vicinity of the cattle) and in 10 out of 134 batches in the other farm (five in the vicinity of the cattle, the others up to 70 m). No infected S. calcitrans were found 130 m away from the parasite carriers. The detection of pathogen DNA in S. calcitrans, captured on farms, could serve as a tool for epidemiological surveillance of their circulation.