Linda Benedict, Foil, Lane D., Owens, William E.
Dairy heifers are at risk for mastitis long before parturition. Recent studies document that these animals can become infected with a variety of mastitis pathogens. If undetected and untreated, these infections can often persist through calving and into the first lactation.
Staphylococcus aureus, a major mastitis pathogen, is often present as a cause of heifer mastitis, and, in some herds, this microorganism infects more than a third of the heifers. How and when these infections become established is unknown. Studies suggest flies may be involved in this disease, but no direct data implicating flies exist.
In this preliminary study, two heifers were exposed to horn flies fed blood that contained Staph. aureus to determine if mastitis could be induced by fly bites. Results indicate that the horn fly (Haematobia irritans) can be colonized with Staph. aureus during feeding and can remain colonized for up to four days, with substantial numbers of organisms present. When colonized horn flies were allowed to feed on teats of uninfected dairy heifers, mastitis with the same Staph. aureus strain resulted. This indicates that the horn fly can transmit Staph. aureus to heifers’ teats if a source of organisms is present. That source was shown to be present in the existing scabs on the teat ends of heifers. High concentrations of Staph. aureus were found in scab material present on heifers’ teats. When uncolonized flies were allowed to feed on this material, they became colonized with Staph. aureus just as readily as flies that had fed on experimentally infected blood. Thus, a vector capable of transmitting the infection is readily present, and when a source of Staph. aureus exists, such as scabs on heifer teats, the potential for dissemination of mastitis from heifer to heifer via horn flies exists.
The threshold number of flies required to be present to transmit mastitis is unknown. Because fly populations can rapidly increase to several thousand per animal under favorable conditions, however, the need for early fly control on dairy heifers in herds with a Staph. aureus problem is apparent. Once scabs are obvious and fly populations are high, spread of new infections is highly likely.
William E. Owens, Professor, Hill Farm Research Station, Homer, La.; Lane D. Foil, Professor, Entomology Department, LSU Agricultural Center, Baton Rouge, La.; C.H. Ray, Research Associate, and Stephen C. Nickerson, Professor, Hill Farm Research Station
(This article was published in the summer 1999 issue of Louisiana Agriculture.)
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