Spatial distribution and incidence trend of human alveolar echinococcosis in southwest Germany: increased incidence and urbanization of the disease?

Echinococcus multilocularis and AE occur in many countries of the northern hemisphere. In Europe, the epidemiology of E. multilocularis and AE is changing since the 1990s. AE has been known to be endemic in the Central European AE belt including eastern France, southern Germany, northern Switzerland, and western Austria since the second half of the 19th century. In the last decades, the parasite and the disease have spread to northern Germany and most of mainland Europe with the exception of southern European and most Scandinavian countries. In addition to the geographically spread, the biomass of the parasite has grown drastically in the epidemic regions due to sharply increasing fox populations and parasites prevalence rates [1, 2]. Moreover, foxes, for most of the 20th century not known to occur in larger towns and cities in continental Europe, established large self-sustaining populations within villages and towns since the middle of the 1990s with higher fox population densities than in rural settings and high prevalence of E. multilocularis [3]. Surveys in the late 90s in major European cities such as Stuttgart, Geneva, and Zurich in the Central European AE belt revealed parasite prevalence rates of 17%, 43%, and 44%, respectively [3, 4]. Elevated parasite biomass in densely populated urban areas results in a rising infection pressure and may lead to an increased incidence of human AE. Indeed, data from the 2000s show increasing incidence rates in Switzerland, France, Austria and Germany compared to the 1990s [5‐7] and in Switzerland human AE is increasingly reported from resident of urban areas but cases of human AE in urban dwellers in Germany remain rare [3, 8]. Due to this evolution, E. multilocularis is considered a serious public health threat in Europe [9].

In Germany, AE is a rare disease with 165 reported cases in the years 2012 to 2016, of those 80% were occurring in the epidemiological hot spots of the federal states of Baden-Württemberg and Bavaria [7]. Since 2001, AE is a notifiable disease but reporting is estimated to be poor, failing to detect 67% of AE cases in Germany [7, 10]. The early identification of emerging hotspots of human AE incidence is crucial for the implementation of preventive measures. Here we report the epidemiological data of human AE cases in treatment at a regional referral center at the Medical Center of the University of Freiburg situated in the epicenter of European AE epidemic area in the southwest of Germany.

A total of 58 patients with E. multilocularis infection were enrolled in the study. Table 1 summarizes the baseline characteristics. Overall, 48.3% of participants were female. At time of diagnosis of E. multilocularis infection, patients were in a median 63.0 years old (ranges: 12.9–86.0) and 43.1% of them had an immunosuppressive condition including 5 patients (8.5%) with solid organ transplant without variation in the observed time periods.

AE incidence in the years 2012 to 2019 increased by 63.6% compared to the years 2004 to 2011. Between 2004 and 2011 a mean of 3.69 patients per year were diagnosed with AE. Since 2012 numbers of patients with initial diagnosis of AE exceed the yearly mean considerably in 5 years. On a population level average annual incidence per 100,000 population increased from 0.12 between 2004 and 2011 to 0.20 between 2012 and 2019. Seven patients living in the city of Freiburg were diagnosed with AE, none of them in the years before 2012 (Fig. 1). All seven patients lived all their live or at least 25 years in the city of Freiburg. Five patients lived in the urban area of the town, two patients lived in suburban areas, and one of them was a farmer. None of the other 6 patients had classic risk factors for AE like owning a dog or vegetable garden.

Our study shows increasing numbers of human AE in the endemic area in southwest Germany since 2011 and in Freiburg city dwellers which was an unknown phenomenon until 2012. Increasing numbers of AE seem not be due to an increasing number of susceptible persons, as proportions of patients with immunosuppressive state remain stable over the time of observation.

In line with our results, there is clear evidence of increasing incidences of human AE in the neighboring countries Switzerland and France, as well as Austria since the beginning of the 20th century [6, 9, 12]. In Switzerland for example, a statistically significant twofold increase of the average annual incidence was reported from 0.10 during 1993–2000 to 0.26 during 2001–2005 per 100,000 population [6]. In Germany, numbers of registered AE cases are rising since the 90s, most probably to higher incidence of human AE but data is less robust as AE is a notifiable disease only since 2001 [7]. Increasing AE incidence in the Central European AE belt is believed to be founded in the steeply rising parasite biomass due to a sharply increasing fox population and parasite prevalence since introduction of oral vaccination campaigns against rabies in foxes in the early 80ies [13]. In southern Germany fast-growing fox population showed increasing parasite prevalence to up to 75% and 80% in 1995 and 2002/2003 in the most affected areas resulting in tenfold higher biomass estimates in the period 1995–2000 compared to estimates before 1990 [1, 2, 14, 15] (Fig. 2).

Simultaneously, foxes colonized urban areas. Foxes living in urban areas were a known phenomenon in Great Britain due to high fox population densities in the absence of livestock reducing rabies. In Germany foxes were regularly sighted in urban areas in the middle of the 90s and established resident self-sustained fox populations in German and other European cities in the 2000s with high population densities and E. multilocularis prevalence rates of up to 44% [4]. In contrast to the rising density of fox populations from rural through peri-urban to urban areas, E. multilocularis frequencies show the opposite trend probably due a decreasing availability of the intermediate hosts in increasingly urbanized areas [16]. Against this background, peri-urban areas, parks, urban gardens and wasteland with high population densities of foxes, intermediate hosts and humans might be the hotspot of E. multilocularis transmission. In a study in the outskirts of Zurich for example, up to 50% of 57 fox droppings were tested positive for E. multilocularis during winter with an important seasonality effect [16]. With the establishment of urban wildlife cycles of E. multilocularis dogs may be become an important source of human infection. Prevalence of E. multilocularis in dogs generally is much lower than in foxes (approximately 0.3–0.4%), but may be elevated in dogs with regular access to infected rodents. In an epidemic area in rural western Switzerland 7% of 86 dogs were identified as carriers of E. multilocularis [17]. Lower parasite prevalence may be outweighed by high dog population density in urban areas; it has been calculated that dogs may contribute to 7–19% of environmental contamination with E. multilocularis eggs under urban conditions [18]. Frequent and close contacts to humans further increases risk of transmission. In China where dogs became involved in the parasite’s life cycle, prevalence of human AE can reach up to 4% [3].

In light of the above, emerging cases of AE in residents of the densely populated outskirts and urban center of Freiburg give rise to concern. There is no established surveillance system or data on fox population and E. multilocularis biomass density in the region of Freiburg. But in parallel to supraregional and international trends, density of urban fox population in Freiburg drastically increased in the last decades suggesting an associated augmentation of E. multilocularis biomass especially in the gardens, local recreation areas, and outskirts of the city. Taking into account the incubation period of 5–15 years, the now observed cases of urban human AE may be a result of the increased infection pressure in the last decades and the first signal of elevated AE incidence.

Our survey shows more AE cases in the Freiburg region and the Upper Rhine Valley than previous epidemiological studies covering the years 1992 to 2016, which is probably due the recent emergence of AE in this region and delayed reporting [7].

Due to the study design, the small number of cases and the impossibility to determine the actual geographical location of E. multilocularis transmission, our data are not able to document a statistically significant increasing incidence and urbanization of human AE. But against the background of the changes in the ecology and epidemiology of E. multilocularis, the long incubation time and the devastating nature of human AE, we belief that the here presented observations should prompt measures to assess the extent of E. multilocularis-related public health threat in the densely populated region of Freiburg as guidance for future prevention strategies.