Stomoxys calcitrans as a potential mechanical vector of Anaplasma phagocytophilum: assessment through original ex vivo feeding models.

Granulocytic anaplasmosis is a zoonotic disease that affects various domestic mammals (dogs, horses, and, more rarely, cats). In ruminants, it is better known as tick-borne fever (TBF) and is responsible for significant economic losses on European livestock farms, mainly due to a drop in milk production, abortions, and immunosuppression, which can lead to secondary infections. The disease is caused by the strictly intracellular bacterium Anaplasma phagocytophilum, whose biological vectors are ticks of the genus Ixodes. Other blood-feeding arthropods may be involved in transmitting this bacterium, notably Stomoxys calcitrans, a major ectoparasite of livestock that is implicated in transmitting other pathogens, including bacteria of the genus Anaplasma. This study aimed to evaluate the potential of S. calcitrans to act as a mechanical vector of A. phagocytophilum under laboratory conditions. Two experimental models were employed: one mimicking immediate transmission, and the other delayed transmission. In both models, A. phagocytophilum DNA and RNA were detected in S. calcitrans for the first time, but no traces of the bacterium's DNA or RNA were found in the glass feeder's blood. Further research is needed to confirm these findings through field studies investigating the presence of the bacterium in flies under natural conditions. This study also describes two original infection models of stable flies designed to reproduce their ex vivo blood-feeding, promoting alternative experimental approaches in accordance with animal welfare regulations and 4R principles.