Zoonotic Pathogens in Eurasian Beavers (Castor fiber) in the Netherlands

Successful reintroduction programs have resulted in an impressive repopulation of the Eurasian beaver (Castor fiber) throughout Europe (Wróbel 2020). In the Netherlands, reintroduction of beavers started in 1988. From 1988 to 2005, 144 beavers originating from the German Elbe area were released at five different locations, adding to beavers escaped from a nature park and beavers that naturally dispersed from the Eifel region into the Netherlands (Niewold 2005). The population has grown rapidly, with beavers extending their range to large parts of the Netherlands. Early in 2019, the beaver population in the Netherlands was estimated at 3,500 animals (Dijkstra 2019).

A variety of zoonotic pathogens has been described in beavers (Girling et al. 2019), and they could be a potential source of (waterborne) zoonotic diseases, posing a public health concern (Rosell et al. 2001; Fayer et al. 2006). With a growing beaver population, more insight into the presence of zoonotic pathogens in beavers in the Netherlands is necessary for risk communication and public education. Our study aimed to assess the potential zoonotic risk of beavers in the Netherlands.

Dead beavers were opportunistically collected from March 2018 to March 2020 and tested for zoonotic pathogens. These beavers were roadkill; found moribund and euthanized; or had been culled for population control. At necropsy, samples of kidney, spleen, heart, lung, tongue, thoracic fluid, and fecal swabs were collected and were stored at –20 C or –80 C until further testing. The liver was visually inspected for the presence of cysts that could be caused by Echinococcus multilocularis. Specific tests used for zoonotic pathogen detection are shown in Table 1. We extracted DNA in two ways: for the spleens of six beavers, the DNeasy Blood and Tissue Kit (Qiagen, Hilden, Germany) was used according to the manufacturer's instructions. For the other beavers and tissues, small sections (10–25 mg) of kidney, lung, and spleen were placed separately in Lysis matrix D tubes (Mpbio, Solon, Ohio, USA) with added Magna pure 96 lysis buffer (Roche, Basel, Switzerland), homogenized in a TissueLyser™ (MPbio), and stored at –80 C. We extracted and purified total nucleic acid using the MagNA Pure 96 System (Roche) according to the manufacturer's instructions.

In total, 24 beavers were collected; eight were roadkill, nine were culled, and seven died due to unknown causes. Due to autolysis and internal damage (especially roadkill), it was not always possible to collect all tissues. Most (n=20) originated from the province of Limburg in the southeast Netherlands, where population control by culling is practiced to protect local infrastructures (Fig. 1). The number of culled beavers increased from about 15 in 2018 to about 40 in 2020 (I. Janssen, Water Authority Limburg pers. comm.). A selection of the culled beavers was tested, with a requested maximum of five beavers per location per year due to logistics. Fourteen beavers were male and nine were female; for one beaver the sex was not recorded. Three beavers weighted <10 kg, 10 weighed 10–20 kg, and 11 weighed ≥20 kg.

One adult male beaver, 28 kg, collected in the south of the Netherlands in February 2019, tested positive for Toxoplasma gondii (n=18; 6%, 95% confidence interval [CI] 1–26%) in both the enzyme-linked immunosorbent assay and the magnetic capture-quantitative PCR (Table 1). One female beaver, 15 kg, collected from the central-east area of the Netherlands in April 2018, tested positive for Neoehrlichia mikurensis (n=16; 6%, 95% CI 1–28%; Table 1). One male beaver, 8.5 kg, collected from the south of the Netherlands in March 2018, tested positive for extended-spectrum-betalactamase or AmpC (ESBL/ AmpC-)-producing Escherichia coli (n=16; 6%, 95% CI 1–28%; Table 1). The ESBL/ AmpC-gene found was bla_CMY-2 in an E. coli ST6599.

Previous studies in Eurasian and North American (Castor canadensis) beavers have detected T. gondii infection (e.g., Jordan et al. 2005; Herrmann et al. 2013). The prevalence we found is within the 95% CI of previously reported prevalences. Beavers may become infected by ingesting oocysts excreted by wild or domestic cats or from contaminated foliage or water. Unless beavers are consumed by humans, the public health risk of this finding is low.

Beavers were not specifically examined for ticks during necropsy, but detection of the tick-borne pathogen N. mikurensis in one beaver corroborates previous findings that beavers are occasionally bitten by ticks and can become infected with tick-borne pathogens (Izdebska et al. 2016); N. mikurensis has not previously been reported in beavers. Beavers probably do not play a major role in N. mikurensis maintenance and transmission, as wood mice (Apodemus sylvaticus) and bank voles (Myodes glareolus), which are considered the reservoirs for this pathogen, are far more ubiquitous.

Enterobacteriaceae that are ESBL/AmpC-producing are resistant to beta-lactam antibiotics. They can be found in humans, many animal species, and surface water (Blaak et al. 2015). The occurrence of Enterobacteriaceae in beavers has been reported once previously: a beaver in Switzerland with sepsis from an infection with an ESBL/AmpC-producing Klebsiella pneumoniae strain (Pilo et al. 2015). Our study confirms that ESBL/AmpC-producing Enterobacteriaceae in the environment can lead to infections in beavers. Wildlife can play a role in the dissemination of the strains, plasmids, and/or resistance genes; therefore, infection of wildlife with ESBL/AmpC-producing Enterobacteriaceae has relevance for public health. However, only 1/16 beavers tested positive for ESBL/AmpC-producing E. coli, and contact between beavers and humans is limited; therefore, the contribution of beavers to the public health risk appears to be limited compared to other wildlife species such as wild birds (Dolejska and Papagiannitsis 2018) or to companion or farm animals (Mughini-Gras et al. 2019).

We did not detect any beavers with Francisella tularensis infection, though this pathogen has been reported in European beavers (Mörner et al. 1988; Schulze et al. 2016). Immediately following closure of this study, a dead beaver in poor condition was sampled at the Dutch Wildlife Health Centre (DWHC) and was found positive for F. tularensis (DWHC 2020). However, the 14 beavers in our survey that came from the same region in Limburg all tested negative, suggesting that F. tularensis is not widespread in this area and that infected beavers are likely to show symptoms of disease.

The presence of a zoonotic pathogen in beavers does not necessarily mean beavers are a direct risk for public health. The finding of T. gondii is merely indicative for the occurrence of that pathogen in the environment. For N. mikurensis and ESBL/AmpC-producing Enterobacteriaceae, beavers might play a (minor) role in the circulation and spread of the pathogen in the environment. Our survey did not find evidence that beavers are currently a public health risk in the Netherlands, although it is a small sample (0.6%) of the estimated Dutch beaver population and samples were collected opportunistically. Regular surveys should be performed to monitor potential changes in public health risk in the growing beaver population in the Netherlands.

We would like to acknowledge Margriet Montizaan and her colleagues of the DWHC for the collaboration and for collection of samples of beavers, the persons (especially from the Water Authority Limburg) who sent in beavers for the study, Manoj Fonville, National Institute for Public Health and the Environment (RIVM) and Tryntsje Cuperus (RIVM) for helping with collection of samples, Elisa Benincà (RIVM) for creating Figure 1, and the Dutch Mammal Society/ National Database Flora and Fauna for providing the beaver distribution data.