The role of Hydrotaea irritans in the transmission of summer mastitis.

A series of experiments to investigate the possible role of the sheep head fly, Hydrotaea irritans, in the transmission of summer mastitis is reported. These showed that the pathogens Actinomyces pyogenes and Peptococcus indolicus may be harboured in the gut of the fly for up to 96 h following a contaminated meal; A. pyogenes survived on the surface of the fly for a similar period. H. irritans fed on the pathogens can contaminate a subsequent meal by regurgitation. However, no infections resulted from repeated exposure of the teats of heifers or cows to H. irritans fed on blood containing A. pyogenes and P. indolicus or on pus from a case of summer mastitis. In these experiments the frequency of feeding of the flies on the teat was high but subsequent bacterial contamination of the teat skin was low. Artificial contamination of the teat skin with greater than 10(6) cfu A. pyogenes led to a low incidence of mastitis even in the absence of cutaneous damage. Repeated applications of lower doses (less than 10(4) cfu) did not produce infection.