Epidemiology of vancomycin-resistant enterococci in the community and the relevance of farm animals to human infection

Abstract

Several reports have documented the presence of vancomycin-resistant enterococci (VRE) in the stools of asymptomatic individuals from the community who have neither recently been in hospital nor received antibiotics. Such findings were contrary to the then existing perception of VRE as a strictly hospital-acquired infection of debilitated and immunocompromised patients on specialized units. Community-acquired infections with VRE are extremely rare but those that do occur may be conspicuous because of their serious nature, for example, endocarditis. If asymptomatic faecal carriage of VRE is present in the community, individuals admitted to hospital and subjected to the selective pressures of antibiotics on the normal gut flora, may act as the source of hospital outbreaks. VRE have also been found in sewage, from stools of healthy farm animals and animal products. Avoparcin, a glycopeptide showing cross-resistance to medically important glycopeptides, has been used in the European Community as a growth promoter in animal feeds. A possible link between the use of avoparcin, the selection of VRE, and humans becoming colonized via the food chain exists. To prove such a link is beset with many difficulties: it is necessary to explain the presence of VRE in the United States where avoparcin is not used, and the predominance of the VanA gene over the VanB gene. It is also proving difficult to show that animal and human strains are identical by means of molecular typing. To date, molecular typing of strains is only suggestive of a link, but epidemiological studies of farms that use avoparcin have shown a significant association with the presence of VRE in animal stools. As long ago as 1969, the Swann report declared that an antibiotic of medical importance should not be used as a growth promoter in animal feeds. The vasy array of antibiotics now being used in animal husbandry and fish-farming, and the cross-resistance of some antibiotics to their medically important counterparts is a real cause for concern. The emergence of multi-resistant enterococci causing human infections and the possibility of the transfer of the VanA gene from VRE to methicillin-resistant Staphylococcus aureus (MRSA) emphasizes the importance of this problem.