Adaptation of Escherichia coli to the bovine mammary gland.

Clinical mastitis in six Somerset dairy herds was monitored over a 12-month period. Escherichia coli was implicated in 34.7% of all clinical cases. Forty-one percent of all clinical E. coli mastitis cases occurred in just 2.2% of the population. A total of 23.9% of clinical E. coli cases occurred in quarters suffering recurrent cases of E. coli mastitis. The genotypes of strains involved in recurrent cases of clinical E. coli mastitis were compared by DNA fingerprinting with enterobacterial repetitive intergenic consensus primers. In 85.7% of cases of recurrent quarter E. coli mastitis, the same genotype was implicated as the cause of disease, suggesting persistence of the organism within the mammary environment. The same genotype as that in the original case was also implicated in 8.5% of recurrent cases occurring in different quarters of the same cow, suggesting spread between quarters. These findings challenge our current understanding of the epidemiology of E. coli mastitis and suggest that pathogen adaptation and host susceptibility may be playing a part in the changing pattern of clinical mastitis experienced in the modern dairy herd.