Possible human origin of methicillin-resistant Staphylococcus aureus isolated from cows in a dairy herd in New...

In a 520-cow, spring-calving, pasture-based Waikato dairy farm, the bulk milk somatic cell count (SCC) increased by approximately 20% from 203,000 cells/mL in the 2020/21 season to 245,000 cells/mL in the 2021/22 season. Analysis of herd test SCC data found that the incidence of intramammary infection increased more than 10-fold from 1.8 to 19.2 cows/30 days at the March 2021 and March 2022 herd tests, respectively.

Milk sampling of all quarters of 43 cows with a SCC > 200,000 cells/mL following herd testing in March 2022, identified 12 quarters from 10 cows infected with Staphylococcus aureus. Antimicrobial sensitivity testing by disc diffusion testing found nine of these isolates (from seven cows) were methicillin resistant.

Whole genome sequencing demonstrated the presence of mecA, a gene encoding an alternative penicillin-binding protein (PBP2a) associated with methicillin-resistant Staph. aureus (MRSA). These isolates were sequence type 5, spa type t002, and all were carrying one or more of the immune human immune evasion cluster genes, scn, chp, sak and sea.

Due to concerns for public safety, the Ministry for Primary Industries, the milk processor and the farm's staff were notified. A comprehensive management plan was implemented which focused on reducing the risk of human exposure by wearing gloves, discouraging consumption of raw milk, and smoking or eating in the milking parlour, alongside measures designed to minimise cow-to-cow transmission such as improving teat spray application, milking machine maintenance, additional sampling, and segregation of likely infected (that is, high SCC and confirmed presence of Staph. aureus) cows. Implementation of the plan resulted in a significantly lower bulk milk SCC and reduced incidence rate of new infections in the subsequent two lactations. However, MRSA was isolated from cows for at least a further 13 months, with some cows remaining infected this entire period despite having been treated with cloxacillin-based antimicrobials at the end of lactation. One staff member had a history of being hospitalised during 2021/22 for significant soft tissue infections.

Given the sequence type, spa type, and presence of the immune evasion cluster genes, it is probable that the MRSA was introduced into the herd from a human. Veterinarians need to be aware of the possibility of MRSA within dairy herds and the possible human origin for these infections.

Abbreviations: BMSCC: Bulk milk somatic cell count; MRSA: Methicillin-resistant Staphylococcus aureus; NAS: Non-aureus staphylococci; PBP: Penicillin-binding protein; SCC: Somatic cell count; spa: Staphylococcal protein A; ST: Sequence type.