The isolation of MRSA from animals was first reported in 1972, following its detection in milk from mastitic cows . Since then, MRSA has been isolated from many different animal species, including dogs, cats, horses, sheep, pigs, dairy cows, veal calves and fowl. This was recently reviewed by Leonard and Markey, who suggested that MRSA may be an emerging pathogen in companion animals and horses . However, a distinction should be made between food production animals and individually housed animals, which are, predominantly, kept for companionship and leisure activities. When it is found in companion animals, the presence of MRSA is mainly due to transmission from a human reservoir [27–29]. In food production animals, a new strain of MRSA (ST398; see below) emerged recently [11,30,31], and people working with living pigs or veal calves were found to be colonized from these animal reservoirs. A French group that studied the occurrence of S. aureus in pigs and pig farmers at the turn of the century first mentioned this animal reservoir . They found that pig farmers were more frequently colonized with S. aureus, including MRSA, than non-farmers. In addition, the strains from non-farmers and farmers were different, whereas strains from pig farmers were identical to the strains in pigs. One of the types that was found only in pig farmers was multilocus sequence type 398 (ST398). A few years later, Voss et al.  reported ST398 in pig farmers and pigs in The Netherlands. This prompted additional investigations into the epidemiology of MRSA in pigs, showing that ST398 had spread extensively. Dutch studies have reported prevalences at the farm level varying from 23% to 81% [34,35] (Broens et al., Proceedings of the 1st American Society of Microbiology Conference on Antimicrobial Resistance in Zoonotic Bacteria and Foodborne Pathogens, 2008, Abstract A19). Later studies from Belgium (Dewaele et al., Proceedings of the 1st American Society of Microbiology Conference on Antimicrobial Resistance in Zoonotic Bacteria and Foodborne Pathogens 2008, Abstract A36), Denmark , Germany , France , the USA (Smith et al., International Conference on Emerging Infectious Diseases, 2008, Abstract A17) and Canada  also reported the occurrence of this strain in pigs and pig farmers. A case–control study confirmed that humans colonized with ST398 had a strong relationship with pig farms, defining MRSA as a zoonotic pathogen . In addition, this study showed a strong relationship with exposure to veal calves. Subsequent studies of veal farms revealed high prevalences of ST398 as well; 88% of the farms and 28% of the calves tested positive (Graveland et al., Proceedings of the 1st American Society of Microbiology Conference on Antimicrobial Resistance in Zoonotic Bacteria and Foodborne Pathogens, 2008, Abstract B84). Finally, ST398 has also been linked to poultry, but its relationship to human carriage must be confirmed in larger surveys [30,31,40].
Several observations have confirmed the potential of ST398 to spread and cause disease among humans. However, both the transmissibility and the virulence of ST398 are likely to be less than those of other MRSA types, according to two observational studies [41,42]. Therefore, the impact of ST398 on public health may be limited, but close monitoring of its evolution over time will be required.