Stable flies, Stomoxys calcitrans (Linnaeus) (Diptera: Muscidae), are common blood-feeding ectoparasites of cows and thus potential vectors of the skin-dwelling bacterium Staphylococcus aureus, a causal agent of bovine mastitis which inflicts udder inflammation in cows. Our objectives were to determine whether stable flies (i) are attracted to disease-causing strains of S. aureus, and (ii) transmit S. aureus from infected blood to sterile blood. In 3-chamber olfactometers, five of eight S. aureus strains grown on agar and tested versus sterile agar attracted female stable flies. When flies ingested droplets of blood inoculated with S. aureus at doses of 0 (control), 10(5) (low), 10(7) (medium), and > 10(9) (high) colony-forming units per milliliter and subsequently ingested sterile blood, they transmitted S. aureus to the sterile blood. The dose of S. aureus in blood droplets fed upon by flies during their first feeding bout dose-dependently affected the amount of bacteria that flies transmitted to sterile blood during their second feeding bout, but the time elapsed between feeding bouts (0 h, 1 h, 8 h, and 24 h) had no effect on the amount of microbes transmitted to sterile blood. Our data infer the existence of a positive feedback loop. First, stable flies carrying S. aureus and feeding on cows transmit S. aureus, thereby causing mastitis. As S. aureus bacteria of afflicted cows proliferate, they attract even more flies which, in turn, worsen the infection. This type of feedback loop underscores the need for effective stable fly control tactics that curtail the incidence of bovine mastitis in cows.
