Geographic distribution of Echinococcus multilocularis

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Alveolar hydatid disease is a highly lethal zoonotic infection caused by the larval stage of E. multilocularis. The life cycle of E. multilocularis involves, most often, transmission between foxes (host to adult worm) and rodents as intermediate host (larval form). Echinococcus multilocularis is found in the Northern Hemisphere with a high prevalence in northwestern Canada, Alaska, Eurasia (former USSR), Japan, and China. Hunters, trappers, and people who work with fox fur are at greater risk. In the northern tundra zone, E. multilocularis occurs in foxes, mainly the Arctic fox, Apollo lagopus and the rodents that they prey on. Echinococcus multilocularis is also present on some sub-arctic islands including St Lawrence Island. The cestode is found on some islands of the Canadian Arctic Archipelago but is not present on the northernmost islands of Canada or Greenland, possibly due to the unsuitability of the local rodents as intermediate hosts (Rausch 1986).

However, E. multilocularis has, prior to the 1960s, spread from the northern tundra zone and become established in central North America. Only two human cases have been reported to date within the central North American focus. No other human cases are known from this region. Hyperendemic foci has been described in some Eskimo villages in the North American tundra zone where local dogs feed upon infected commensal rodents and AHD have been diagnosed in Eskimos of a limited number of communities in Alaska (Anonymous 1991; Wilson et al. 1995). There are two other species within the genus Echinococcus, which are considered valid, E. vogeli and E. oligarthrus. Infection in humans with E. vogeli or E. oligarthrus results in a polycystic form of disease. Only a couple of cases with E. oligarthrus have been reported.

Role of Echinococcus strains

The understanding of the nature and diversity of variation within the genus Echinococcus has increased rapidly in the last years through successful characterisation of the nuclear and mitochondrial genomes of representative isolates of the strains (Thompson and Lymbery 1990). The new data, based on genome patterns, generally support previous characterisations based on morphological and biological criteria. The current data suggest that about seven genetically distinct populations exist concerning E. granulosus. Concerning the different populations of E. multilocularis from Europe, Alaska, and North America few data are up to now available. However, large morphological studies and experimental infection trials concluded that the cestode in central North America appeared to be indistinguishable from that of the northern tundra zone. The only exception was indications of differences in the ability of the larvae to develop in rodents of various species. It is now important to recognize that biological differences may exist between populations identified as E. granulosus and that these differences may account for local differences in patterns of transmission and public health significance of the disease.

Diagnosis and treatment in human echinococcosis

Clinical diagnosis of cystic or alveolar echinococcosis in human is based on clinical findings, epidemiological data, morphological changes revealed by ultrasonography, computerized

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tomography (CT), magnetic resonance imaging (MRI), and immunological and other laboratory tests. Surgical intervention is the basic form of treatment of cystic echinococcosis but chemotherapy with benzimidazole carbamates can be regarded as an alternative treatment in patients not suitable for operation or most often as a supplementary treatment pre- and postoperative to prevent relapses of the disease (Czeckowski et al. 1992). Concerning treatment of alveolar echinococcosis, surgical resection of the involved liver segment and of metacestode lesions from other affected organs is indicated. Postoperative chemotherapy with benzimidazole carbamates is often given for several years. However, long-term prognosis is generally bad. Liver transplantation has been performed in a limited number of patients with alveolar echinococcosis.

Prophylactic measures

Vaccination of intermediate hosts may in the future reduce the prevalence of human echinococcosis. In immunisation trials of sheep with onchosphere secretions or with an onchosphere homogenate 97-98% resistance to a challenge infection of the sheep with E. granulosus eggs have been achieved. The vaccine has the potential to be used as a tool for control of transmission of E. granulosus through its natural intermediate hosts (particularly domestic ungulates).

Traditional methods of hydatid control (antihelminthic treatment of dogs) include regular treatment within the prepatent period for prolonged periods (20-30 years) to achieve satisfactory control (Bowles and McManus 1993; Heath and Holeman 1997).

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