Pseudomonas aeruginosa is an infrequent cause of mastitis that typically causes sporadic cases in dairy herds. However, herd outbreaks in which a high proportion of cows are affected also occur.

In individual cows, the disease may vary from severe, and rarely even fatal clinical cases to those which are entirely subclinical. A common form is repeated episodes of moderately severe clinical mastitis. In the intervals between clinical episodes, the milk somatic cell count is usually elevated. When many cows are affected, bulk milk somatic cell counts may be increased.

An important feature of Pseudomonas mastitis is that these infections are difficult to eliminate by therapy. This is true even when antibiotics are chosen on the basis of antibiotic susceptibility testing. Cows with Pseudomonas mastitis are frequently culled after repeated treatment failures.

Pseudomonas is reported to be widespread in the cow's environment, and sporadic infections may result from chance exposure from the environment. Herd outbreaks, however, have been linked to a point source of the organism. One such source of contamination is the water supply used for washing cows. In milking parlors, drop hoses may be heavily colonized with pseudomonads and may be the source of high numbers of bacteria in the wash water. Addition of low concentrations of iodophor to the wash water does not insure that pseudomonads will be eliminated. Outbreaks have also been traced to pools of stagnant water to which cows have access. Other outbreaks appeared to result from use of contaminated teat cannulas or antibiotic preparations for intramammary treatment. When more than one case of Pseudomonas mastitis has been diagnosed in a herd, efforts should be made to identify and eliminate a point source of the organism.