Non-random distribution of udder infections among cows. Evaluation of some contributing factors.

Bovine udder infections were recorded in three herds throughout a lactation period with bacteriological examinations at 3 week intervals. For analysis, each cow was taken into account at the time when the greatest number of its quarters were infected. Distributions of cows having 0, 1, 2, 3, or 4 infected quarters, either by major or minor pathogens, differed significantly (P less than 0.01) from the random binomial distribution: the number of cows with 3 or 4 infected quarters was much higher than expected. As the two types of pathogens behave similarly, they were not distinguished and data were pooled. Distributions of infections among cows within each of the three herds were not random. Cross-infections (from a quarter to another of the same cow) could have given rise to less than one-third of the new infections. A greater proportion of cows became multi-infected with increasing age. This process was delayed in the herd with the lowest new infection rate. In the three most highly represented lactation ages (1, 2, and 4), differences between actual and theoretical distributions were all significant (P less than 0.01). Results suggested that together with the major contribution of concentration of infections in the older cows and the less important contribution of cross-infections, individual susceptibility played a part in the non-random distribution. Nevertheless the resistance of the less susceptible cows seemed to have been easily overcome when measures for control of new infections were not used.