Long-term study of MRSA ST1, t127 mastitis in a dairy cow

Context Methicillin-resistant Staphylococcus aureus (MRSA) has been identified in a wide range of animals and diseases, thus it is considered an emerging threat with a high zoonotic potential. Sequence type (ST) 1 and SPA type (t) 127 has been mostly isolated from community-associated infections, but t127 has seldom been identified in cattle and swine. The present report regards a dairy cow with an intramammary subclinical infection by MRSA ST1, t127 that was investigated over an entire lactation, to explore both bacterial molecular features and host immune response. Main conclusion Shedding of MRSA from the infected quarter was irregular and inflammatory response was characterized by the presence of neutrophils indicative of an acute infection, even though MRSA mastitis was chronic. Such result was in contrast to what observed in chronic S. aureus mastitis, in which macrophages are the principal inflammatory cell. Approach The cow was sampled three times on five consecutive days and cyto-bacteriological analysis of milk was performed. In the last sampling, milk cell population was evaluated by cytometer. At slaughterhouse, parenchymal tissue was collected from each mammary quarter and submitted to histological and immunohistochemical evaluation. Results and interpretation The microorganism could be detected in all milk samples only from the first and third follow-up periods, and Somatic Cell Counts fluctuated between 300 and 6,000 cells/μl. In both right quarters, PMNs were mainly high (60-75%), while the left quarters demonstrated high lymphocyte and low phagocytes levels. T cells were always the predominant lymphocyte subset. All isolates carried the genes for antibiotic multiresistance, those encoding different toxins and leukocidin D/E, but not Panton-Valentine. At histological examination, multifocal areas with groups of neutrophils were seen in alveolar lumens of the infected quarter. Immmunohistochemistry confirmed that MRSA-positive quarter had the highest scores for myeloperoxidase, a marker of neutrophils. Significance of findings The presence of this MRSA type suggests a human-cow transmission, since no animals had been introduced into the herd, and the cow had never been moved. The multifocal distribution of inflammatory infiltrates indicate a patchy progression of the infection and may suggest that clearance of a single inflammatory focus occurred, while others remained active or developed over time. As a result, SCC and MRSA counts fluctuated irregularly during the follow-up periods, sometimes showing low SCC values. The actual consequence is that misdiagnosis can occur. Thus, we suggest that all cases of recurrent mastitis by S. aureus should be screened for methicillin resistance.