MRSA Found on U.S. Dairies

Prevalence low; caution, not alarm, is warranted

Methicillin-resistant Staph. aureus (MRSA) has been found in U.S. dairy cattle, but with prevalence rates low, there is no need for panic.

Rates of MRSA colonization in some countries are as high as 49% in swine and 45% in the humans caring for them. On U.S. dairies, those rates are still less than 5%.

MRSA is cross-resistant to virtually all antibiotics, and people with impaired immune systems are the most vulnerable. In the Netherlands, dairy and veal farmers and their employees are screened prior to admittance to hospitals to ensure that MRSA carriers do not infect patients.

In the U.S., a University of Minnesota study of 50 dairy farms sampled bulk tanks over three seasons in 2009 and found Staph. aureus antibiotic susceptible strains on 42 farms. Only two farms were positive for MRSA, and, over the 150 bulk tank samples, those two positive samples equate to a prevalence of 1.3%.

"These are low numbers, and are consistent with other U.S. studies. We have very, very low prevalence, and shouldn’t be panicking," says Sandra Godden, a University of Minnesota veterinarian.

MRSA bacteria isolated from milk tend to be less virulent than other strains of Staph. aureus, notes John Middleton, a veterinarian with the University of Missouri. "Somatic cell counts from MRSA-infected cattle are similar to other Staph. aureus infections, and there is no major, increased pathogenicity," he says.

The problem comes in treatment. All stains of Staph. aureus tend to be difficult to treat, but MRSA strains are even more so because of their antimicrobial resistance. "The two MRSA isolates [in the Minnesota study] displayed resistance to beta lactams, cephalosporins and lincosamides and were multiresistant," Godden notes.

The critical point in preventing cow-to-cow and cow-to-human spread of any Staph. aureus specie is hygiene: Ensure clean milking routines and be vigilant about postmilking teat dipping. Culling older, chronic cows is often best for preventing those animals from infecting others.

"To reduce selection pressure for antimicrobial resistance, apply antimicrobial drugs only where there are clearly demonstrated production and animal welfare benefits," Middleton says.

In humans, the most common site for MRSA colonization is the nasal passage. Middleton urges milkers to wear latex gloves and avoid touching their face while milking.

Organic herds are not immune. Antibiotic-resistant bacteria can persist on these dairies even after many years of antibiotic-free management.

"Pasteurization should kill Staph. aureus and MRSA in milk, so food safety is ensured unless people are drinking raw milk," Middleton says.