What does the fox say? Monitoring antimicrobial resistance in the environment using wild red foxes as an indicator

Abstract

The objective of this study was to estimate and compare the occurrence of AMR in wild red foxes in relation to human population densities. Samples from wild red foxes (n = 528) included in the Norwegian monitoring programme on antimicrobial resistance in bacteria from food, feed and animals were included. All samples were divided into three different groups based on population density in the municipality where the foxes were hunted. Of the 528 samples included, 108 (20.5%), 328 (62.1%) and 92 (17.4%) originated from areas with low, medium and high population density, respectively. A single faecal swab was collected from each fox. All samples were plated out on a selective medium for Enterobacteriaceae for culturing followed by inclusion and susceptibility testing of one randomly selected Escherichia coli to assess the overall occurrence of AMR in the Gram-negative bacterial population. Furthermore, the samples were subjected to selective screening for detection of E. coli displaying resistance towards extended-spectrum cephalosporins and fluoroquinolones. In addition, a subset of samples (n = 387) were subjected to selective culturing to detect E. coli resistant to carbapenems and colistin, and enterococci resistant to vancomycin. Of these, 98 (25.3%), 200 (51.7%) and 89 (23.0%) originated from areas with low, medium and high population density, respectively. Overall, the occurrence of AMR in indicator E. coli from wild red foxes originating from areas with different human population densities in Norway was low to moderate (8.8%). The total occurrence of AMR was significantly higher; χ2 (1,N = 336) = 6.53, p = 0.01 in areas with high population density compared to areas with medium population density. Similarly, the occurrence of fluoroquinolone resistant E. coli isolated using selective detection methods was low in areas with low population density and more common in areas with medium or high population density. In conclusion, we found indications that occurrence of AMR in wild red foxes in Norway is associated with human population density. Foxes living in urban areas are more likely to be exposed to AMR bacteria and resistance drivers from food waste, garbage, sewage, waste water and consumption of contaminated prey compared to foxes living in remote areas. The homerange of red fox has been shown to be limited thereby the red fox constitutes a good sentinel for monitoring antimicrobial resistance in the environment. Continuous monitoring on the occurrence of AMR in different wild species, ecological niches and geographical areas can facilitate an increased understanding of the environmental burden of AMR in the environment. Such information is needed to further assess the impact for humans, and enables implementation of possible control measures for AMR in humans, animals and the environment in a true "One Health" approach.

Fig 1. Distribution of sampled wild red foxes per municipality.

Each municipality was categorised according to the human population density (in green) and the proportion of samples within each municipality is displayed as blue circles.

Fig 2. Occurrence of antimicrobial resistance among Escherichia coli (N = 434) isolated from wild red foxes in Norway in 2016.

The isolates are categorized according to human population density in the area where the foxes were hunted, i.e. in low population density (n = 98), medium population density (n = 268) and high population density (n = 68).

Fig 3. Resistance profiles of the fluoroquinolone resistant Escherichia coli isolates (n = 76) isolated by selective screening from wild red foxes in 2016 in Norway.

No. of AR = Number of additional resistances to other antimicrobial classes than quinolones (including nalidixic and/ or ciprofloxacin), CIP = ciprofloxacin, NAL = nalidixic acid, AMP = ampicillin, TET = tetracycline, TMP = trimethoprim, SXT = sulfamethoxazole, CHL = chloramphenicol, CTX = cefotaxime, CAZ = ceftazidime, GEN = gentamicin.