Continuous spread: Raccoon roundworm detected in nine European countries

FRANKFURT. While the spread of raccoons in Europe is often discussed, their companion tends to remain unnoticed: The raccoon roundworm Baylisascaris procyonis arrived in Europe at the beginning of the 20th century with the first raccoons from North America. Since their release or escape from fur farms, raccoons have spread uncontrollably across large parts of Central Europe – and their parasite with them. Germany is now considered the main distribution area for both species in Europe.

Dangerous companion of the raccoon

"This parasite can also infect humans and cause so-called larva migrans, in which migrating larvae damage tissues and organs," explains Prof. Dr. Sven Klimpel from Goethe University Frankfurt and the Senckenberg Biodiversity and Climate Research Centre. Humans become infected by accidentally ingesting infectious eggs found in soil, water, or on objects contaminated with raccoon feces.

A research team from the collaborative research project ZOWIAC (Zoonotic and Wildlife Ecological Impacts of Invasive Carnivores) has now investigated how far the parasite has already spread in Europe.

Insidious life cycle

The parasite's life cycle is complex: Adult roundworms live in the small intestine of raccoons. Females produce up to 180,000 eggs daily, which enter the environment via feces. At so-called raccoon latrines – preferred defecation sites – the resistant eggs accumulate. In the environment, they develop into infectious larvae within two weeks under adequate temperature and humidity conditions, and can survive for several years.

Children particularly at risk

Human infection with the raccoon roundworm is called baylisascariasis. Anne Steinhoff from Goethe University Frankfurt and first author of the study explains: "If the larvae enter the central nervous system, the disease can have severe consequences. Due to frequent hand-to-mouth contact, young children are primarily affected." Most known cases occur in North America, the natural distribution area of raccoons and the roundworm. There, in most documented cases, the disease led to permanent neurological damage or even death.

"Furthermore, it is assumed that many cases remain undetected or are misdiagnosed due to non-specific symptoms," Klimpel adds. "In Europe, diagnosis in humans is further complicated by the lack of specific diagnostic testing options." Definitive diagnosis is currently only possible at the Centers for Disease Control and Prevention (CDC) in the USA and Canada.

First comprehensive Europe-wide analysis

The aim of the study was to provide a current overview of the parasite's distribution in Europe and to identify research needs. To this end, the team led by Klimpel and Steinhoff examined raccoons from Germany by necropsy and supplemented these new data with a comprehensive analysis of available scientific studies and infection data from Europe.

Of the 146 raccoons examined, 66.4 percent were infected with Baylisascaris procyonis: 77.4 percent in Hesse, 51.1 percent in Thuringia, and 52.9 percent in North Rhine-Westphalia. The study provided prevalence data for Thuringia for the first time. "The results show both an expansion of the roundworm's distribution area and stable infection occurrence at high levels in German raccoon populations," Klimpel explains. The analysis revealed that the roundworm occurs in wild raccoons in nine European countries, primarily in Central Europe – in some cases with extremely high infection rates. In three additional countries, infections were detected in raccoons or other animal species in captivity.

Spread coupled to raccoon populations

"The studies show a steady expansion of the distribution area in Europe. The distribution of the roundworm is linked to the steady spread of its definitive host, the raccoon, which now occurs throughout Europe," Klimpel continues. "The actual distribution of the roundworm is likely significantly underestimated due to insufficient or absent data collection."

Particularly concerning: The urbanization of raccoon populations increases the likelihood of contact between humans and contaminated areas. Three documented cases of baylisascariasis in Europe are known – all resulted in permanent visual impairment.

"The results of this study make it clear that further research on the raccoon roundworm in Europe is urgently needed – particularly in light of growing raccoon populations and their increasing adaptation to urban habitats," Klimpel concludes.