Retention and attempted mechanical transmission of Ehrlichia risticii by Stomoxys calcitrans

The ability of adult Stomoxys calcitrans (L.) (Diptera: Muscidae) to retain viable Ehrlichia risticii (Rickettsiaceae), the aetiologic agent of Potomac horse fever (PHF), and mechanically transmit the pathogen from citrated bovine blood artificially infected with E. risticii to susceptible mice was studied. Viable E. risticii were found in the digestive tract of S. calcitrans 3 h after the flies had engorged to repletion on infected blood; however, no E. risticii were detected in flies > or = 2 days after feeding. Subsequently, groups of S. calcitrans were fed for 20 s on infected blood, then fed to repletion on mice 30, 60, 130, 180 or 220 min after having feeding interrupted. Mice displayed no clinical signs of PHF and did not produce anti-E. risticii antibodies when assayed 30 days after S. calcitrans had fed. Although S. calcitrans were able to harbour viable E. risticii for at least 3 h, transmission of the disease agent to susceptible mice during interrupted feeding was not demonstrated under these experimental conditions.