MRSA Research Center

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MRSA & Animals

Veterinarians, Livestock Handlers, and Pet Owners

Domestic pets, livestock, and wild birds and animals have recently been identified as carriers of MRSA in a number of countries and settings.  The role of animals as vectors for human MRSA infection and colonization is being studied carefully and its importance is not yet fully determined.  One genetic background, ST398, has been identified in Europe and more recently in the U.S. as very common among livestock, particularly pigs.  In addition to the risk posed to farmers and animal workers, the general population may be placed at risk for exposure to pathogens in these animal groups from poor hygienic practices observed at petting zoos.

Livestock and farming

Farm animals and farm workers have been found in many studies to have frequent colonization with MRSA, often of the same background genotypes, suggesting animal-to-person or person-to-animal transmission.  For example, of 110 European horses attending a clinic in Belgium in 2007, 10.9% had nasal colonization with ST398 strains of MRSA.  In a separate study of 25 horses with MRSA infections in 2006-7 at an Austrian veterinary hospital, the most common MRSA strains, ST398 (7 horses) and ST254 (15 horses) were also found among MRSA strains colonizing 18 of 131 tested personnel in the nares.  However, colonization in animals is far from universal; one study of 300 healthy horses in Europe did not identify a single episode of MRSA carriage.

In a study in Iowa, investigators found that 49% (147/299) of tested pigs and 45% (9/20) of tested pig farm workers carried ST398 MRSA bearing SCCmec V in the nares; no other background types of MRSA were identified.  Younger pigs were more likely to carry MRSA.  In a study of 20 pig farms in Ontario, Canada, 24.9% (71/285) of pigs carried MRSA in the stool or nares and 20% (5/20) of pig farm workers.  Pigs on 45% (9/20) of tested farms carried MRSA.  In the Netherlands in 2004, >20% of a sample of pig farmers were colonized with MRSA as were 39% of pigs in slaughterhouses.  The animals carried MRSA with ST398.  A study in the Netherlands in 2006 showed MRSA colonization of pigs from 7/31 (23%) tested farms.  ST398 was also found in cultures from poultry in Belgium in 2006.  In Denmark, a study demonstrated that exposure to pigs was associated with a CC398 MRSA infection, and found that among 50 pigs tested from farms of case-patients, 46% carried CC398 MRSA.

ST398 is not limited to pigs; human infections with this strain and others linked to contact with farm animals have also occurred.  In 2007, at 15/50 (30%) of Dutch pig farms tested, at least 1 farm worker tested positive for nasal carriage of MRSA; 24% of 139 people tested were colonized.  Testing of pigs and fomites in pig pens demonstrated a 56% prevalence of MRSA colonization or contamination on farms tested.  On each of the 15 farms with colonized workers, the spa type of MRSA from workers matched that of MRSA isolated from pigs on the same farm.  A pig handler in Belgium developed an MRSA SSTI after being bitten by a pig; he was found to have nasal colonization with a different strain of MRSA.  A case report found that a farm worker was colonized by the same MRSA strain that caused subclinical mastitis in milk cows in Hungary in 2002-2004.  In a study of 1721 healthcare workers in the Netherlands, colonization was 10-fold higher (1.7%) among those reporting contact with pigs and veal calves than among those without such contact (0.15%) although this difference was not statistically significant.

At a hospital in a region of the Netherlands with 7000 pig farms, screening of health care workers and high-risk individuals admitted to the hospital was routinely performed in 2002-2006.  In this period 73 newly colonized individuals were identified of whom 31.5% (23) carried “nontypable” MRSA (i.e., its genome is resistant to digestion with SmaI, the standard enzyme used for PFGE analyses), presumed to be ST398.

Food-borne transmission

Although MRSA has been isolated from food samples derived from animals, there have been few cases reported of food-borne MRSA infection. 

Veterinary clinics

Veterinary clinics may be a setting in which MRSA spreads, and veterinarians have been identified as a high-risk group for MRSA carriage.  For example, at a veterinary clinic for small and exotic animals in Berlin, 27 cases of MRSA infection in animals (18 in dogs) were recorded between May 2003 and December 2004.  All isolates were PVL negative and were either ST239 (2 isolates) or ST22 bearing the SCCmec IV element (25), both recognized human pathogenic strains.  Among 3372 horses admitted to a veterinary teaching hospital in Ontario, Canada, in 2002-5, 69 (2%) were colonized with MRSA; risk factors for colonization included antimicrobial use in the previous 30 days, previous MRSA colonization or infection, coming from a farm with known MRSA colonization or infection in the past, being admitted to the surgery service, or being on “foal watch” (a neonatal intensive care unit).

At a veterinary conference in 2005 in Baltimore, Maryland, 6.5% (27/417) of tested attendees carried MRSA in the nares, including 15.6% (15/96) of personnel caring for large animals.  At an international equine veterinary conference in 2006 in San Antonio, Texas, 10.1% (26/257) attendees tested carried 1 of 3 pulsotypes of MRSA (USA300, USA100, or USA500); risk factors for MRSA carriage identified in multivariate analysis included history of caring for a horse with a MRSA infection within the previous year, the index attendee having had a MRSA infection in the past year, and not washing hands between farms and between infectious cases.  Among 152 Dutch veterinary doctors and students with a history of contact with livestock, 4.6% had nasal colonization with MRSA.  This trend has not been observed in all countries of the world; for example, at a veterinary conference in the Czech Republic, only 2 of 280 (0.7%) of the attendees screened carried MRSA.

Pets

It is possible that pets may be a reservoir of antibiotic-resistant bacteria, including MRSA.  Of 70 S. aureus isolates from 46 cats treated at a clinic in Philadelphia, no clinical characteristics of infections or their outcomes differentiated MSSA from MRSA infections; all 15 MRSA strains tested carried SCCmec II, suggesting that the cats may be a reservoir of MRSA.  In a study from Hong Kong, only 17 of 736 tested dog owners were colonized with S. aureus and had a dog with S. aureus colonization; only 6 of these pairs shared the same genotypes, and 2 of these owners were health care workers. MRSA strains were isolated from 8.2% of dogs and from 2.3% of their owners, but only 1 pair was co-colonized with a MRSA strain.  An outbreak of MRSA in a family was linked to its pet cat, and clearance of a furunculosis patient's colonization with MRSA did not occur until her cat, colonized with the same ST80 strain, received systemic antimicrobial drug therapy, suggesting an animal reservoir for MRSA in homes.  Little data are collected on the use of antimicrobial drugs in pets in the U.S. but overuse of these drugs may contribute to the carriage of drug-resistant strains by domestic animals.

Visit the Pew Charitable Trusts Human Health and Industrial Farming's website for more information: www.SaveAntibiotics.org