Echinococcus spp. and echinococcosis

Abstract

Echinococcus spp. are cestode parasites commonly known as small tapeworms of carnivorous animals. Their medical importance lies in the infection of humans by the larval stage of the parasites, predominantly including Echinococcus granulosus, which is the causative agent of cystic echinococcosis (CE) and Echinococcus multilocularis, which causes alveolar echinococcosis (AE). A few other species or genotypes are only very rarely or not at all found in humans. Due to the emerging situation in many parts of Europe, the present article will predominantly focus on E. multilocularis . The natural life cycle of E. multilocularis involves predominantly red and arctic foxes as definitive hosts, but domestic dogs can also become infected and represent an important infection source for humans in highly endemic areas. In the definitive host, egg production starts as early as 28 days after infection. After egg ingestion by a rodent or a human, larval maturation will occur practically exclusively within the liver tissue. The geographic distribution of E. multilocularis is restricted to the northern hemisphere. In Europe, relatively frequent reports of AE in humans occur in central and eastern France, Switzerland, Austria and Germany. Within the past ten years, the endemic area of Europe now includes many more countries such as Belgium, The Netherlands, Italy, and most former Eastern countries as far as up to Estonia. The Asian areas where E. multilocularis occurs include the whole zone from the White Sea eastward to the Bering Strait, covering large parts of Siberia, western and central parts of China and northern Japan. Worldwide there are scant data on the overall prevalence of human AE. Some well-documented studies demonstrate a generally low prevalence among affected human populations. The annual mean incidence of new cases in different areas including Switzerland, France, Germany and Japan has therefore been reported to vary between 0.1 and 1.2/ 100,000 inhabitants. The incidence of human cases correlates with the prevalence in foxes and the fox population density. Recently, a study documented that a fourfold increase of the fox population in Switzerland resulted in a statistically significant increase of the annual incidence of AE cases [1] (Schweiger et al., 2007). This dramatic increase in red fox populations has also been reported throughout Europe, especially in urban areas. The so-called city-fox phenomenon and, thereafter, the increased proximity of foxes with humans and an urban domestic dog – rodent cycle may, therefore, have significant public health implications [1-3]. In infected humans the E. multilocularis metacestode (larva) develops primarily in the liver. Occasionally, secondary lesions form metastases in the lungs, brain and other organs. The typical lesion appears macroscopically as a dispersed mass of fibrous tissue with a conglomerate of scattered vesiculated cavities with diameters ranging from a few millimeters to centimeters in size. In advanced chronic cases, a central necrotic cavity containing a viscous fluid may form, and rarely there is a bacterial superinfection. The lesion often contains focal zones of calcification, typically within the metacestode tissue. Histologically, the hepatic lesion is characterized by a conglomerate of small vesicles and cysts demarcated by a thin PAS-positive laminated layer with or without an inner germinative layer [4]. Parasite proliferation is usually accompanied by a granulomatous host reaction, including vigorous synthesis of fibrous and germinative tissue in the periphery of the metacestode, but also necrotic changes centrally. In contrast to lesions in susceptible rodent hosts, lesions from infected human patients rarely show protoscolex formation within Correspondence: bruno.gottstein@ipa.unibe.ch Faculty of Medicine, Institute of Parasitology, University of Bern, Bern, Switzerland Gottstein Acta Veterinaria Scandinavica 2010, 52(Suppl 1):S5 http://www.actavetscand.com/content/52/S1/S5