Summer mastitis in heifers: studies on the seasonal occurrence of Actinomyces pyogenes, Peptostreptococcus indolicus and Bacteroidaceae in clinically healthy cattle in Denmark.

With the aim of investigating the seasonal occurrence of Actinomyces pyogenes, Peptostreptococcus indolicus, Bacteroides melaninogenicus ss. levii and Fusobacterium necrophorum, and thus the potential for development of summer mastitis, clinically healthy Danish Holstein-Friesian heifers due to calve in the autumn were sampled from the teat tip, the conjunctiva and the oral cavity at 2-6 week intervals from 1979 to 1981. The overall isolation rates of F. necrophorum, P. indolicus and B. melaninogenicus ss. levii, in order of significance, were significantly higher during the pasture period whereas no differences in isolation rates of A. pyogenes between housed and pastured animals were detected. F. necrophorum was recovered almost exclusively from the oral cavity, P. indolicus and A. pyogenes occurred most frequently in samples from the teat skin, whereas isolates of B. melaninogenicus ss. levii were evenly distributed between conjunctiva and teat tip samples. A distinct seasonal pattern of the isolation rates of summer mastitis pathogens was recorded, which corresponded closely to the seasonal activity of symbovine insects, in particular the headfly Hydrotaea irritans (Fallén). However, the high proportion of clinically healthy bacterial carriers as compared with the incidence of clinical disease strongly suggests that as yet unknown contributing or triggering factors, apart from the mere presence of the relevant bacterial species, are required for the establishment and development of clinical summer mastitis.