Udder orf infection and its role in ovine clinical mastitis caused by Pasteurella haemolytica.

During an experimental study of ovine subclinical mastitis caused by coagulase-negative staphylococci, an outbreak of contagious ecthyma occurred among ewes unvaccinated against parapox virus. The same group of ewes developed a high rate (43.7%) of clinical mastitis caused by Pasteurella haemolytica. The rate of clinical mastitis among ewes vaccinated against parapox virus was very low (3.7%) suggesting that the presence of orf in the unvaccinated ewes was contributing to the high rate of clinical mastitis. An examination of the iron, sodium, potassium and albumin concentration of milk collected from 16 unvaccinated and nine randomly selected vaccinated ewes before experimental infection with coagulase-negative staphylococci or their uninfected control mammary glands indicated significant differences in the iron (p < 0.0001) and sodium (p = 0.01) concentration. Increased iron concentration in the milk may have assisted in the development of udder infection caused by P. haemolytica as iron is easily utilised by this bacterium.