Echinococcus multilocularis (Leuckart, 1863)

Mackenstedt, Ute, Jenkins, David & Romig, Thomas, 2015, The role of wildlife in the transmission of parasitic zoonoses in peri-urban and urban areas, International Journal for Parasitology: Parasites and Wildlife 4 (1), pp. 71-79 : 72-73

2. Echinococcus multilocularis

2.1. Background on transmission factors

Alveolar echinococcosis (AE), caused by the larval stage of the cestode Echinococcus multilocularis , is a zoonotic disease of increasing importance in the northern hemisphere ( Davidson et al., 2012). Incidence and prevalence of human AE vary widely across the expansive range of E. multilocularis for reasons, which are only partly understood. The wide geographical spread results from the ability of the parasite to use a large variety of local predator–prey systems for its transmission ( Eckert et al., 2001). Thus, the parasite is endemic in natural ecosystems like arctic tundra or Tibetan high-altitude grassland, as well as in highly anthropogenic central European farming landscapes or Japanese city parks. In the far north, E. multilocularis cycles between arctic foxes ( Vulpes lagopus ) and northern voles ( Microtus oeconomus ). In the temperate parts of Eurasia, red foxes ( V. vulpes ) are the principal definitive hosts, although other canids may regionally also contribute to the life cycle, e.g. Tibetan foxes ( V. ferrilata ), raccoon dogs ( Nyctereutes procyonoides ), golden jackals ( Canis aureus), coyotes ( C. latrans ), wolves ( C. lupus ) or domestic dogs ( Eckert et al., 2001). Concerning intermediate hosts, the situation is even more complex. In most of Europe, Microtus arvalis seems to be the most important species, while e.g. in central Asia, other grassland-adapted rodents are principal intermediate hosts. In contrast, Myodes spp. , living in dense forest undergrowth, maintain the life cycle in northern Japan ( Eckert et al., 2001).

Therefore, factors that drive the transmission of this parasite – and risk factors for human disease – are necessarily different across geographical regions, and possibly even between different ecosystems of the same area:

1 Definitive hosts: wild and domestic canids as competent hosts are widespread and occur in all endemic regions. However, canid species differ in their capacities to support worm populations ( Kapel et al., 2006), and their infection risk differs due to habitat and prey preference. Moreover, their population densities vary due to species-specific parameters and available food resources. As the age structure of the canid population is known to be important (juvenile red foxes have far higher worm burdens than adults – Hofer et al., 2000), hunting pressure or disease mortality has an impact on transmission. Repeated infections with E. multilocularis elicit intestinal immune responses, which act as a downregulating factor on parasite egg production in highly endemic areas ( Torgerson, 2006).

2 Intermediate hosts: different rodent species differ drastically in their susceptibility to, and tolerance of, the parasite, as well as in their habitat preference on a small spatial scale. Some species maintain rather stable populations (at different densities), while others tend towards cyclic population outbreaks and crashes ( Giraudoux et al., 2002). The amplitude of the population cycles, again, is determined by the landscape pattern ( Raoul et al., 2001). Varying population densities of the same species not only change the predation rates of the canids, but may also change their predation behaviour with respect to other food sources. Availability (microhabitats, diurnal activity patterns) and attractiveness as prey is different among rodent species.

3 Environmental conditions: climate and anthropogenic influences determine the vegetation type and, thereby, the species composition and density of host species ( Romig et al., 2006). Climatic factors (e.g. precipitation) and soil parameters act on the survival time of E. multilocularis eggs in the environment ( Veit et al., 1995). Weather conditions (e.g. snow cover in winter) influence the survival of rodents and their availability as prey.

4 Human behaviour: Attitude to wildlife, hunting pressure and rodent pest control act on host populations. Intentional or accidental introduction of new host species can change life cycle patterns, and the parasite can be introduced into non-endemic areas via travelling dogs or translocated wild animals ( Davidson et al., 2012). Dogs kept for various purposes may complement the life cycle ( Deplazes et al., 2011), or may act as a specific risk factor for human AE ( Kern et al., 2004). The presence or absence of good personal hygiene behaviour are likely to be key factors for the frequency of human disease.