Mastitis in female mammals reduces milk yield, is irritable for the animal, and is the most common and costly infectious disease of dairy cattle. Lack of diagnosis is not the limiting factor in controlling the disease. Rather it is such matters as ensuring pre-milking udder hygiene and remaining uncertainty over seasonal and environmental influences and disease organisms. Disease factors include pathogens, toxins and mechanical impairment. The most frequently isolated mastitis pathogens are Staphylococcus aureus, Escherichia coli and Streptococcus agalacatiae, S. dysgalactiae, S. uberis and other Streptococcus species. Unusual mastitis pathogens are Anaerobic bacteria, Mycobacteria, Leptospirae, Mycoplasmata, Rickettsiae, Chalmidiae, Fungi, Yeasts and Virus, all of unknown virulence and importance. Vaccination with a foot-and-mouth vaccine by itself is not considered to increase the incidence of mastitis, but stress occasioned by the herd vaccination, or by transport and change of feeding or management systems, may increase the risk.
Norway had an outbreak of clinical mastitis in 1988, when 10% of herds were found to be infected. A 3-year survey of ewes in England and Wales, found that the majority of flocks had an incidence of less than 6%, although the variation between years and between flocks was between 0 and 23.5%. Almost 90% of samples were bacteriologically-positive, with Staphylococcus spp in 80% of positive samples. Comparatively few new cases of mastitis developed after weaning; most cases of so-called "post-weaning" mastitis had been undetected during lactation.