Recently published work as described by the authors highlighted the extent of Complement activity in bovine milk. Localised mastitis infection occurring in the mammary glands of dairy cows is readily detectable by the levels of somatic cells in milk. Thus, it is opportune to monitor Complement activity in milks in association with the animal's innate immune response to mammary infection. Preliminary screening of milk samples taken randomly showed that milk with a high somatic cell count (SCC) reduced growth of the Complement-sensitive strain E. coli O111 to a greater extent (P < 0·05) than when the marker microorganism was grown in milk heated for the purpose of inactivating Complement. A follow-up study set out to determine the effect on Complement activity when a sub-clinical mastitis infection was induced in the mammary gland of four lactating dairy cows. The effect of Str. dysgalactiae spp. dysgalactiae inoculation into selected individual udder quarters of the mammary glands of each animal was followed by monitoring of SCC levels in the milks from the segregated udder samples during subsequent milking. At 72 and 96 h post inoculation (PI), the SCCs for the challenged quarter were increased compared to normal values. At the same time, the bactericidal sequestration assay identified increased E. coli O111 inhibition that can be directly linked to greater Complement activity in those quarter milks affected by induced inflammation. Thus, it can be identified that the high SCC milks were more effective in limiting E. coli O111 growth. Milks from the unchallenged quarters in all four cows were significantly less effective at reducing growth of the assay strain (P < 0·05).

An ELISA assay targeting specific activation components of the Complement pathways confirmed that greater bacterial inhibition observed during the bactericidal sequestration assay was attributable to higher Complement activity in the milk samples from the affected quarters, i.e., with higher SCC. The induced infection was confirmed as self-limiting in three of the affected animals and their SCC returned to normal levels within 14 d PI, while the fourth cow required brief antibiotic intervention.