Case report: characterization of a persistent, treatment-resistant, novel Staphylococcus aureus infection causing chronic mastitis in a Holstein dairy cow

Abstract

Background Mastitis is the most common health concern plaguing the modern dairy cow and costs dairy producers estimates of two billion dollars annually. Staphylococcus aureus infections are prevalent, displaying varied disease presentation and markedly low cure rates. Neutrophils are considered the first line of defense against mastitis causing bacteria and are frequently targeted in the development of treatment and prevention technologies. We describe a case of naturally occurring, chronic mastitis in a Holstein cow (1428), caused by a novel strain of S. aureus that was not able to be cleared by antibiotic treatment. Case presentation The infection was identified in a single quarter, 2 months into the cow’s first lactation. The infection persisted for the following 20 months, including through dry off, and a second calving and lactation. This case of mastitis was associated with a consistently high somatic cell count, however presented with no other clinical signs. This cow was unsuccessfully treated with antibiotics commonly used to treat mastitis, consisting of two rounds of treatment during lactation and an additional round at the beginning of dry off. The chronic infection was also unchanged through an experimental mid-lactation treatment with pegylated granulocyte-colony stimulating factor (PEG-gCSF) and an additional periparturient treatment with PEG-gCSF. We isolated milk neutrophils from 1428 and compared them to two cows challenged with experimental S. aureus, strain Newbould 305. Neutrophils from 1428’s milk had higher surface expression of myeloperoxidase compared to experimental Newbould challenged animals, as well as increased presence of Neutrophil Extracellular Traps. This suggests a heightened activation state of neutrophils sourced from 1428’s naturally occurring infection. Upon postmortem examination, the affected quarter revealed multifocal abscesses separated by fibrous connective tissues. Abscesses were most common in the gland cistern and collecting duct region. Microscopically, the inflammatory reaction was pyogranulomatous to granulomatous and consistent with botryomycosis. Colonies of Gram-positive cocci were found within the eosinophilic matrix of the Splendore-Hoeppli reaction within granulomas and intracellularly within the acinar epithelium. Conclusions Collectively, we describe a unique case of chronic mastitis, the characterization of which provides valuable insight into the mechanics of S. aureus treatment resistance and immune escape.

Occurrence and aetiology of subclinical mastitis in water buffalo in Bangladesh

“…As the clinical signs remain unnoticed, affected animals may act as reservoirs that shed microorganisms continuously to the environment and affect their herd mates (Ali et al ., 2014). Persistent infection also limits the efficacy of antimicrobial treatment by creating a fibrous barrier between the organism and the antibiotic (Putz et al ., 2020).…”

Neutrophil and CD4+ milk cell count related to natural incidence of mastitis in Jersey cattle

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Antimicrobial susceptibility of bovine clinical mastitis pathogens in Japan and development of a simplified agar disk diffusion method for clinical practice

“…Given that microbiological breakpoints have been used previously, the criteria for the simplified ADD method were based on the microbiological breakpoints in this study. However, Staphylococcus aureus differs from other staphylococci in that it forms microabscesses in the mammary tissue and escapes antimicrobial therapy and immunity by forming slime, which is reportedly a factor leading to refractory Staphylococcus aureus mastitis [6,17,21].…”