The raccoon roundworm (Baylisascaris procyonis) is a gastrointestinal parasitic nematode of the raccoon (Procyon lotor). It is common in its native range in North America, where its prevalence in raccoons can reach 82% (1). Through their feces, infested raccoons can shed millions of B. procyonis eggs, which may remain infective in the environment for years (2). Paratenic hosts can acquire the parasite when ingesting nematode eggs from raccoon latrines (3).

B. procyonis infestations are usually benign in the raccoon but can be fatal in paratenic hosts, including humans (1). Since 1980, several fatal cases of neural larva migrans have occurred in humans in the United States (3); infants have been frequently affected because of fecal–oral transmission (4). Increasing raccoon densities in close proximity to humans has increased public health concern about B. procyonis roundworms (2).

As a result of joint translocation with raccoons, B. procyonis roundworms have increased their geographic range (5). Raccoons are common in Germany and Luxembourg (Figure, panel A). All raccoons in Germany are assumed to have stemmed from a small number of founders and 2 separate introduction events in western Germany (Hesse) during the 1930s and eastern Germany (Brandenburg) during the 1940s (8). However, genetic analysis has inferred a minimum of 5 founder events (6). In addition to 2 genetic populations clustered around the known introduction sites (referred to as the Hesse and Brandenburg populations), distinct raccoon populations were identified in Saxony (eastern Germany), around the Harz Mountains in central Germany, and in Luxembourg and neighboring regions (Figure, panel B).

B. procyonis roundworms occur in the Hesse and Harz populations (5) but are absent from Brandenburg (9). No information is available about the remaining 2 populations in Luxembourg and Saxony. Although only a few human cases of baylisascariasis have been reported from Germany (9), a detailed overview of the parasite’s geographic distribution is needed to identify potential risk areas.

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Figure. Characteristics of the geographic distribution of the raccoon roundworm (Baylisascaris procyonis). A) Geographic origin of 8,184 dissected raccoons and the German administrative districts (Landkreise) in which raccoons were harvested during 2017–2018....

During 2008–2018, we collected 8,184 legally harvested or road-killed raccoons from Germany and Luxembourg (Figure, panel A), focusing on different regions every year or every few years, and investigated their intestines for the presence of B. procyonis roundworms. We plotted the presence of the parasite onto the 10 × 10–km ETRS89-LAEA5210 EEA reference grid, a base map provided by the European Environment Agency (https://www.eea.europa.eu/data-and-maps/data/eea-reference-grids-2). We calculated the proportion of infested raccoons for 69 of Germany’s 294 administrative districts where B. procyonis roundworms were present and >25 raccoons had been sampled. We generated maps by using ArcMap v.10.3 (ESRI Inc. https://www.esri.com).

B. procyonis roundworms were widespread in central Germany, their distribution probably corresponding to the geographic extent of the Hesse and Harz genetic populations (Figure, panels B, C). However, we did not detect the parasite in Luxembourg and western areas of Germany or in a northern/eastern region that included the federal states of Brandenburg, Mecklenburg-Western Pomerania, Schleswig-Holstein, northern parts of Lower Saxony, Saxony-Anhalt, and eastern Saxony. In other words, the parasite was not detected in the areas covered by the Luxembourg, Brandenburg, and Saxony genetic populations (Figure, panels B, C). A median of 43.6% (interquartile range 34.4%–49.7%) of raccoons were infested in the 69 administrative districts where the parasite was present and >25 raccoons had been sampled.

Identification of risk areas for human B. procyonis roundworm infestation is necessary because of the frequent proximity of raccoons to human populations. Our results show that the nematode is widespread and prevalent in central Germany. Given that B. procyonis eggs remain infective for years, the nematode is likely to pose a public health risk in its distribution area (10). To reduce the risk for B. procyonis infestation, protective measures (procedure masks, gloves, handwashing) should always be applied when raccoons or their feces in the risk area are handled. In this context, educational material should be made available to schools and daycare centers and to persons who have occupational contact with raccoons.

The match between the distribution of the roundworm and the extent of the different genetic populations of the raccoon suggests that the absence of the parasite results from the founder animals’ parasite-free status. However, we cannot exclude the possibility that ecologic or geographic differences between the introduction sites also contributed to the lack of parasites in some populations. Further research and monitoring are needed, especially in view of a possible spread of the parasite into nematode-free raccoon populations. Also, because of the rapid spread of raccoons, assessment of the status of the parasite in northwestern and southwestern Germany and at the periphery of its current distribution more generally should be considered.

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