Relationship between previous history ofStreptococcus uberisinfection and response to a challenge model

Abstract

Streptococcus uberisis the most common cause of clinical mastitis at calving in pasture-based dairy cows. Results of experimental inoculations were compared with cows' previous history of infection to help define a model for susceptibility toStr. uberismastitis. Cows used had either no apparent history of intramammary infection (IMI) byStr. uberisor other major mastitis pathogens throughout their productive lifetime (‘apparently uninfected’; AUI), or had a confirmed history ofStr. uberisIMI (‘historically infected’; HI). Cows were exposed toStr. uberisin sequential steps: dipping of the teat end (DIP;n=53 cows); a teat canal inoculation (TCI;n=33 cows); and, finally, intramammary inoculation challenge (IC;n=7 cows). Only cows that remained free of infection at each step progressed to the next phase. Infection rates were similar between AUI or HI cows following the DIP (9 and 17% respectively), or the TCI (75 and 68% respectively). Physical and biochemical traits of cows were examined. Analysis of traits prior to inoculations implied that HI cows produced more milk fat, while AUI cows tended to have longer teat canals. Analysis of traits for cows that became infected following DIP, implied that there was a positive association with milk fat production and negative association with somatic cell count (SCC), while there was a positive association with the duration of p.m. milking, and negative association with SCC in those cows that became infected following TCI. Only AUI cows became infected following the IC inoculation. Similarity in response to experimental inoculation between the two groups suggests that the current dip or teat canal inoculation (using a 3-mm depth of inoculation) models are not good predictors of natural resistance toStr. uberis. However, a population of cows was identified that remained uninfected after DIP, TCI and IC, and may comprise a resistant phenotype.