Spread of the Zoonotic Nematode Baylisascaris procyonis into a Naive Raccoon Population

Abstract

The raccoon roundworm (Baylisascaris procyonis), a gastrointestinal nematode of the raccoon (Procyon lotor), may cause a severe form of larva migrans in humans, which can lead to death or permanent neurological damage. Although roundworms were inadvertently introduced to Europe alongside their raccoon hosts, the parasite is not present in every raccoon population. It is important to understand the geographic distribution of B. procyonis, as early and rapid treatment can prevent severe pathologies in humans. We present evidence for the roundworm spreading into a naive raccoon population through natural dispersal of infected raccoons. We sampled 181 raccoons from Saxony-Anhalt, a German federal state containing contact zones of different raccoon populations, two of which were previously free of the parasite. We screened the raccoons for roundworms and used microsatellite-based assignment tests to determine the genetic origin of the raccoons and their parasites. We detected roundworms in 16 of 45 raccoons sampled in a previously roundworm-free area in the northern part of the state. The largest proportion of the genetic ancestry (≥ 0.5) of the 16 raccoon hosts was assigned to the previously naive raccoon population. Conversely, the genetic ancestry of almost all the roundworms was assigned to the nearest roundworm population in the southern part of the state. Infected raccoons have, therefore, spread to the north of the state, where they interbred with and infected local raccoons. It seems likely that the roundworms will continue to spread. Health authorities should consider continuous surveillance programmes of naive populations and raise public awareness.

Keywords: Genetic structure; Invasive species; Parasitic nematode; Public health; Range expansion; Zoonosis.

Figure 1

Population genetic structure of raccoons (Procyon lotor) in Germany and Saxony-Anhalt. a The spatial extent of the five main STRUCTURE clusters inferred by Frantz et al. (2021) and comparison of the geographic distribution of the raccoon roundworm (Baylisascaris procyonis). The roundworm data are based on the analysis of 8184 raccoons (Heddergott et al. 2020), and the presence/absence of the parasite is plotted using the 10 × 10-km ETRS89-LAEA5210 EEA reference grid. Different colours represent different genetic populations. b Population genetic structure of raccoons in Saxony-Anhalt. The 85 new samples were analysed together with 859 reference individuals in programme STRUCTURE, with the number of genetic clusters (K) set to K = 5. In both figures, the sizes of the pie charts are proportional to the number of individuals investigated from a specific locality. For further methodological details, please refer to the Material and Methods section.

Figure 2

Geographic distribution and infection status of the 181 raccoons (Procyon lotor) from Saxony-Anhalt sampled for this study. We refer to the samples above the dotted line as being from ‘the north of Saxony-Anhalt’. Inset: location of Saxony-Anhalt within Germany.

Figure 3

Population genetic structure of the raccoon roundworm (Baylisascaris procyonis) in Saxony-Anhalt and neighbouring regions. The 88 new samples were analysed together with 226 reference individuals in programme STRUCTURE, with the number of genetic clusters (K) set to K = 2, following Osten-Sacken et al. (2018). The background grid, based on the 10 × 10-km ETRS89-LAEA5210 EEA reference grid, indicates the presence/absence of the parasite, based on the analysis of 8184 raccoons (Heddergott et al. ; see also Fig. 1). Different colours represent different genetic populations, and the sizes of the pie charts are proportional to the number of individuals investigated from a specific locality. For further methodological details, please refer to the Material and Methods section. Inset: location of Saxony-Anhalt within Germany.