Intramammary coliform infection after heavy external contamination of teats.

Teats of lactating dairy cows were exposed to massive, repeated external contamination by application of a freshly prepared broth culture of Escherichia coli that was maintained in wet contact with the teat ends during 14 or 21 successive entire intermilking periods. When contamination was restricted to the intermilking periods by dipping teats in an iodophor germicide before each milking, 18 new intramammary infections occurred among the 93 mammary quarters at risk. The rate of infection achieved was 1/89 milkings. Transient residence of E coli in streak canals and frequent occurrence of sterile inflmmation of mammary quarters made diagnosis of new infection difficult. Teats also were exposed to E coli without sanitization before milking. The milking machine was modified to maximize the likelihood of contaminated milk droplets impacting on teat orifices through back-jetting. Diagonally opposed milking maching inflations were fitted with rifled bore short milk tubes designed to be protective against back-jetting. The rate of new E coli infections was 1/291 milkings among control and protected quaters. When contamination was extended into the milking operation, the new infection rate was not greater than that achieved when exposure was limited to intermilking periods; therefore, the protective value of rifled-bore short milk tubes was not adequately tested and a role of the machine as a vector was not demonstrated.