Volume 6, Number 1—February 2000
Involving Ornithologists in the Surveillance of Vancomycin-Resistant Enterococci
To the Editor: Because migratory birds cross national or intercontinental borders, they are possible long-range vectors for human pathogens such as viruses, Borrelia burgdorferi sensu lato, and enteropathogenic bacteria with antibiotic resistance or virulence factors (1,2). Enterococci are ubiquitous in humans and animals and have a propensity for uptake and transfer of glycopeptide antibiotic resistance (3); therefore, the emergence of glycopeptide-resistant enterococci (GRE) in humans is a public health concern. Low-level vancomycin resistance (genotype vanC-1-3) is intrinsic in enterococcal species (e.g., Enterococcus gallinarum, E. flavescens, and E. casseliflavus) that may normally occur in the intestinal flora of some birds. However, the finding of high levels of GRE in wild birds suggests acquisition from an environmental source.
In March 1998, we obtained fecal samples while banding 318 northbound migrating gulls in Malmö, southern Sweden. Using a selective culture procedure with enrichment broth (bile esculin azide broth, Acumedia, LabFab, Ljusne, Sweden) containing vancomycin (8 µg/ml) and aztreonam (60 µg/ml), we isolated vancomycin-resistant E. faecalis from a black-headed gull (Larus ridibundus). High-level glycopeptide resistance (>256 µg/ml) was demonstrated by E-test (AB Biodisc, Solna, Sweden), and a vanA genotype was found by polymerase chain reaction amplification (4). This survey protocol can also be used to detect medium to low levels of glycopeptide resistance. Using the same procedure in a study of 230 sub-Antarctic birds on Bird Island, South Georgia, in 1996, we found four GRE isolates with vanC1 genotype (MIC 3-8 µg/ml).
Many species of gulls have moved into urban areas, where they commonly feed on human trash and deposit feces. The black-headed gull with GRE described above was banded as a fledgling in Malmö in 1995. Birds of this population spend the winter mainly in Western Europe (5), where they forage at garbage dumps, sewage outlets, and agricultural areas. This bird may have acquired GRE in such an area. VanA genotype E. faecium and E. faecalis have been found in poultry and pigs in the Netherlands and Denmark, where the vancomycin analog avoparcin has been used as a growth promoter (6). Manure from such farms may be a GRE source accessible to wild birds.
We have previously reported the introduction into Sweden of multidrug-resistant Salmonella Typhimurium by migratory birds (7). The present report further emphasizes the possibility of migratory birds as long-range vectors of bacteria potentially associated with human disease. The risk to humans for GRE from migratory birds may seem insignificant compared with such risk from hospitalization or from eating meat products from GRE-colonized animals. However, if the frequency of birds carrying high-level GRE increases and if amplification in a secondary reservoir or spread through polluted water takes place, spread by migratory birds may become a problem. Bacteriologic surveys of birds may provide vital information for assessing the environmental dispersion of GRE from farms and hospitals. In combination with data about migration patterns and reports of banding recoveries from ornithologists, the potential sources of GRE might be deduced.
This work was supported by the Center for Environmental Research, the Medical Faculty of Umeå University, the Swedish Council for Agricultural Research, and the Swedish Society of Medicine.
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