The Bactericidal Action of Bovine Blood for Brucella and Its Possible Significance.

In a herd of cattle in which several cows are infected with Brucella abortus, it has been observed for many years that seldom, if ever, do all adult cows become infected. Very often a considerable percentage of animals in such a herd even fail to show by diagnostic tests indications of having been exposed to infection. It has been assumed that they were resistant to infection, this resistance being attributed to a hypothesized natural immunity or an immunity acquired through exposure to the causative microorganism. In this connection there are available considerable data from experimental infection studies that have a direct bearing on the occurrence of resistance to infection under experimental conditions. Rettger et al. (1926) exposed 26 bred and unbred heifers to Brucella abortus organisms by the oral, subcutaneous, and vaginal routes. Twelve (46 per cent) withstood the exposure, and a large percentage failed even to develop specific serum agglutinins. It is hardly conceivable that the failure of these animals to become infected was due to the size of the exposure doses of organisms. It is logical to assume that a high degree of resistance was already in operation before exposure, or was acquired immediately thereafter. In an experiment conducted to determine the influence of an adequate and a low mineral intake in cattle on their susceptibility to brucellosis by artificial exposure, Hadley and Welsh (1931) obtained data indicating that many of the animals used in the experiment were resistant to infection. Of a total of 33 used in the experiment, 13 failed to become infected. Many of the 13 developed specific serum agglutinins in low titer for only a short time. The animals were kept under observation for two years, and during this time 29 (88 per cent) ceased to show any evidence of being infected, as indicated by the absence of specific serum agglutinins and by negative bacteriological findings. Five years later Beach and Humphrey (1935) used 14 of these animals in an experiment to determine their resistance to experimental infection. Only one of the group became infected. The results of their experiment are adequate proof that immunity against brucellosis can be acquired through exposure to infection. The mechanism of the immunity in the adult cow still lacks adequate explanation. It is also known that calves up to 6 months of age are highly resistant to Brucella abortus infection (Huddleson, 1916; Quinlan, 1923). Calves not only resist infection, but often fail to develop specific serum agglutinins after repeated ex-