A comparison of antibiotic susceptibility patterns for Staphylococcus aureus in organic and conventional dairy herds

Mastitis is considered to be the most costly disease affecting the dairy industry with annual losses in the United States exceeding $2 billion (Philpot, 1984). Losses stem from milk discard, drug costs, veterinary care, increased labor, and premature- culling. Staphylococcus aureus is one of the most important pathogens causing intramammary infections in dairy cattle (Gonzalez et al.,1988) and continues to be one of the major causes of mastitis in dairy herds worldwide (Barkema et al. 1998, Gonzalez et al. 1988, Nickerson et al. 1999, Osteras et al. 1999, Sol et al. 1997, Zecconi and Picini 1998). This microorganism causes both clinical and subcllnlcal mastitis, and appears to be present in both well managed herds and herds with a general management and hygiene problem (Barkema et al. 1998 Zecconi, A., and R. Piccini 1998). To reduce the impact of Staph aureus on farms, it is important to both reduce the incidence of infections and shorten the duration of existing infections. Shortening the duration of infection occurs through either successful treatment or culling of the infected animal. Treatment with antibiotics is either done during lactation or at dry off. Use of antibiotics for dry cow treatment is a routine part of dairy herd health-management schemes (Gonzalez 1988, Schukken 1993). On farms with Staph aureus mastitis problems, (cl)oxacillin is the drug of choice for dry cow treatment (Sol and Sampimon 1995, Browning et al. 1990, Cummins and McCaskey 1987) and currently, cloxacillin is widely used throughout the United States and Europe. Selective pressure created by use of antibiotics offers resistant strains a survival advantage (Chambers 1997). Thus, dairy farms constitute an environment where Staph aureus is highly prevalent, where antimicrobials (such as cloxacillin) are routinely used, and where selective pressure favors the survival of resistant strains.

Seventeen million pounds of antibiotics are used in animals each year in the United States (Animal Health Institute 2001). Approximately 90% of the antibiotics used in agriculture are given as growth-promotants and prophylactic agents (e.g. dry cow therapy) (Khachatourians 1998). Many of the antibiotics used on farms are given by farm workers that may not be familiar with principles of antibiotic therapy and may not adhere to recommended therapeutic regimens.

There is concern that antibiotics that are used in animals have created a reservoir of resistant bacteria that may be transferred to humans via various food product (Bates et al. 1994). In Europe, use of avoparcin has been used as a feed additive in many species as a growth promotant (Witte 1997). In separate studies by Aarestrup (1995) and Witte (2000), vancomycin resistant enterococci (VRE) were isolated from manure from pig and poultry farms using avoparcin in the feed, -whereas on farms where avoparcin was not used, VRE were not detected. Many of the antibacterial resistance genes are on plasmids that may transfer themselves to other genera and species of bacteria (Tenover and McGowan 1996). In vitro transfer of VR from enterococci to staphylococci (in particular, Staphylococcus aureus) has already occurred in the laboratory (Noble et al. 1992).

Routine, non-therapeutic use of antibiotics and growth-promotants in animals raised on certified organic farms is discouraged. Antibiotics are used in rare cases for the welfare of the animal to treat a specific disease. Withdrawal times following this use are extensive (California Certified Organic Farmers 2000). This is in contrast to conventional farms where antibiotics are used as growth promotants (milk replacers), routine therapeutics (mastitis and other diseases) and prophylaxis (dry cow treatment) -

In a previous small scale project done at our laboratory at Quality Milk Promotion Services, differences in the antibiotic susceptibility patterns of StaphyIococcus aureus isolates obtained from an organic dairy herd were compared with the patterns obtained from isolates from a conventional dairy herd. Statistically significant differences were noted with the isolates from the organic farm exhibiting a higher degree of susceptibility than those from the conventional farm.

Busato, et al.(2000) looked at udder health and risk factors for subclinical mastitis on dairy farms in Switzerland. They observed no difference in antibiotic resistance between conventional and organic farms, however, only a few samples were analyzed for antibiotic susceptibility. They also acknowledged that antibiotics were allowed on these organic farms for therapy of clinical mastitis. There are very few studies that touch on antibiotic resistance on organic farms in a valid comparison with conventional herds.

Consumer interest in the health and safety of their food is ever growing and is creating a high value market for food free of possible exposure to pesticides, hormones and antibiotics. To meet these increased interests, organic farming has become one of the fastest growing segments of US agriculture (Economic Research Service 2000). Total organic dairy sales were the most rapidly growing segment of the US dairy industry last year at $200 million dollars. The additional benefit of producing organic milk (with less antibiotic resistance) would create a stronger competitive edge for organic products with the health conscious public.


Our objective was to compare the antibiotic susceptibility patterns of all Staphylococcus aureus isolates obtained from composite milk samples from all cows from 22 organic dairy herds with to a similar number of isolates from 16 conventional dairy herds.