Tracing Mastitis Pathogens—Epidemiological Investigations of a Pseudomonas aeruginosa Mastitis Outbreak in an Austrian Dairy Herd

SIMPLE SUMMARY: Pseudomonas aeruginosa is an opportunistic pathogen in humans and animals. In ruminants, it is responsible for mastitis that may occur sporadically or as an outbreak within dairy herds. It can be refractory to treatment, and therefore, economic implications are high. To avoid spread within herds, it is important to identify the sources of infection or transmission ways and to implement effective prophylactic measures. Intra-mammary infections with P. aeruginosa can occur from both, a point source, or from a continuous exposure to a common contamination, e.g., from dirty, or soiled environment. We describe a P. aeruginosa outbreak in an Austrian dairy herd were genotyping of isolates helped to identify reusable udder towels as source of subclinical and mild clinical intramammary infections with P. aeruginosa. Bacteriological examination revealed that in nine out of 20 lactation cows at least one quarter was infected with P. aeruginosa. Genotyping of isolates with multiple locus variable-number-tandem repeat analysis (MLVA) revealed that all strains were closely related to each other. The same genotype was additionally confirmed in strains isolated in pure culture from a cleaning and disinfectant solution for udder towels indicating that there is a strong evidence that transmission from cow to cow occurred via those udder wipes. ABSTRACT: The present study describes an outbreak of Pseudomonas (P.) aeruginosa mastitis in a 20-cow dairy herd where throughout genotyping of isolates reusable udder towels were identified as the source of infection. Sampling of cows during three herd surveys and bacteriological culturing showed that P. aeruginosa was isolated from nine cows with a total of 13 infected quarters. Mastitis occurred as mild clinical or subclinical infection. P. aeruginosa was additionally isolated from a teat disinfectant solution, containing N-(3-aminopropyl)-N-dodécylpropane-1,3-diamine 1 as active component, and microfiber towels used for pre-milking teat preparation. Disc diffusion antimicrobial resistance testing revealed that all isolates were susceptible to piperacillin, piperacillin-tazobactam, ceftazidime, cefepime, aztreonam, imipenem, meropenem, tobramycin, amikacin, and ciprofloxacin. Thirty-two isolates of milk samples and 22 randomly selected isolates of one udder towel and of the teat disinfectant solution were confirmed as P. aeruginosa with matrix-assisted laser desorption, ionization time-of-flight mass spectrometry (MALDI Tof MS). Isolates were further characterized with rep-PCR and randomly amplified polymorphic DNA (RAPD) as well as with multiple locus variable-number tandem repeat analysis (MLVA). Results obtained in this study suggested that one single strain was responsible for the whole outbreak. The transmission occurred throughout a contaminated teat cleaning solution as a source of infection. The farmer was advised to change udder-preparing routine and to cull infected cows.