Submitted to: BioMed Central (BMC) Veterinary Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2020
Publication Date: 9/15/2020
Citation: Putz, E.J., Palmer, M.V., Ma, H., Casas, E., Reinhardt, T.A., Lippolis, J.D. 2020. Characterization of a persistent, treatment-resistant, Staphylococcus aureus infection causing chronic mastitis in a Holstein dairy cow. BioMed Central (BMC) Veterinary Research. 16:336. https://doi.org/10.1186/s12917-020-02528-8.
Interpretive Summary: The number one disease of dairy cattle is mastitis which includes any inflammation of the mammary gland. Mastitis is a serious concern both from an animal health perspective as well as a substantial cost to dairy producers and the industry. Most often, mastitis is caused by a bacterial infection in the udder. Common dairy practice is to treat the affected mammary gland with antibiotics, but research is underway investigating antibiotic alternative treatments. This study describes a case report of naturally occurring case of mastitis in a Holstein cow early in her first lactation. The causative bacteria was found to be Staphylococcus aureus (S. aureus). Traditional antibiotic therapies were not successful in resolving the infection, despite multiple rounds of treatment. The cow was dried off at the end of her lactation, only to return immediately with the same infection at the beginning of her second lactation. Upon euthanasia of the cow number, pathology of the infected mammary quarter revealed multiple abscesses. This case report uniquely characterized a S. aureus infection associated with a treatment-resistant phenotype. The behavior of S. aureus herein described sheds important light on bacteria persistence in the mammary gland and offers insight on the physical immune escape strategies of the pathogen which will continue to be important drivers of developing mastitis prevention and treatment technologies.
Technical Abstract: We describe a case of naturally occurring, chronic mastitis in a Holstein cow (1428), caused by a strain of Staphylococcus aureus (S. aureus). The infection was identified in a left hind quarter two months into the cow’s first lactation and persisted for the following 20 months, including through dry off, and a second calving and lactation. This case of mastitis presented with no signs of pain or inflammation, no changes in milk productivity, but with a moderate and consistent increase in somatic cell count (SCC). This cow was unsuccessfully treated with antibiotics used to treat mastitis. The chronic infection was also unchanged by mid-lactation or periparturient treatment with pegylated granulocyte-colony stimulating factor (PEG-gCSF), as well as persisted through common antibiotic dry-off treatments. Neutrophils are considered the primary responders to mastitis infections. We isolated milk neutrophils from 1428 and compared them to two cows challenged with experimental S. aureus, strain Newbould 305. We found that neutrophils from 1428’s milk had higher surface expression of myeloperoxidase (MPO) compared to experimental Newbould challenged animals, as well as increased presence of Neutrophil Extracellular Traps (NETs). 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 Splendore-Hoeppli reaction, within granulomas and intracellularly within acinar epithelium. Collectively, we describe a unique case of chronic mastitis, which the characterization thereof provides valuable insight into the mechanics of S. aureus treatment resistance, immune escape, and suggests options for hypothesis driven research which is critical in developing next generation, antibiotic alternative therapeutics.