Johannesburg: Humans carry more antibiotic-resistant bacteria than the farm animals they handle, a new study on dairy farmers has found.
One of the most common and costly diseases faced by the dairy industry is bovine mastitis, a potentially fatal bacterial inflammation of the mammary gland (IMI).
Researchers studying staphylococcal populations responsible for causing mastitis in dairy cows in South Africa found that humans carried more antibiotic-resistant staphylococci than the farm animals with which they worked.
"The rise of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) and reported cases of bacterial transmission between dairy cows and humans has raised concerns from both the agriculture/veterinary sector and public health officials," said lead investigator Tracy Schmidt, from the University of Pretoria.
Staphylococcus aureus is a contagious udder pathogen that readily spreads between cows at milking.
The main source is milk from infected quarters, with milking machine teat liners playing a significant role in the transmission of the bacteria among cows and mammary quarters.
Other Staphylococcus species, collectively referred to as coagulase-negative staphylococci (CNS), often exhibit extensive resistance to antimicrobials and may serve as a reservoir of resistance genes that can transfer and supplement the genome of more pathogenic bacteria like Staphylococcus aureus.
This research analysed the diversity of Staphylococcus populations responsible in South Africa for IMI in dairy cows and assessed the susceptibility of different species to antimicrobials commonly used in the veterinary field as well as human medicine.
Individuals working in close contact with the animals were sampled and the diversity and susceptibility profiles of staphylococcal isolates determined and compared with isolates of animal origin.
With respect to staphylococcal diversity the results showed the clear predominance of Staphylococcus chromogenes among the CNS causing IMI, while Staphylococcus epidermidis was most commonly recovered from the human specimen.
The study found a relatively low occurrence of antimicrobial resistance among the bovine staphylococci.
"This is encouraging as it indicates the responsible usage of antimicrobials within local dairies and provides our veterinary practitioners and animal owners valuable information going forward with respect to the treatment of infected animals," said Schmidt.
Furthermore, all isolates tested negative for the presence of vancomycin-encoding genes - vancomycin being one of the front-line antimicrobials used for the treatment of methicillin-resistant staphylococcal infections in humans.
The results indicate the low potential health risk posed to close contact workers and milk consumers through exposure to antibiotic-resistant staphylococci originating from milk.
The research is published in the Journal of Dairy Science.