Epidemiology

Geographic distribution of Echinococcus granulosus

The distribution of E. granulosus is cosmopolitan including North and Central Europe, the Mediterranean, Central Asia, north and north-western China, Australia, East Africa, and parts of South America.

In a recent publication, the infection by E. granulosus is regarded as the most frequent parasitosis in Europe (Seiferth et al. 1993). Information on cases of echinococcosis in Western Europe is relatively complete in comparison with other geographic regions where reporting systems are generally lacking and the quality of information varies.

The northern biotype of Echinococcus granulosus

Studies of populations of E. granulosus from different regions have demonstrated intraspecific variations in final and intermediate host assemblages together with other characteristics. According to Rausch (1986), two main biological forms of E. granulosus have been recognized, namely the 'European Form' and the 'Northern Form', which is considered to be ancestral to the 'European Form'. The 'Northern Form' occurs in Holarctic zones and under favourable conditions also at lower latitudes in North America (north of 45° N latitude) and in Eurasia.

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In the larval stage, the northern biotype of E. granulosus occurs in ungulate animals of the family Cervidae, 'cervid strain'. There is no evidence that its development takes place in domestic ungulates, excluding the domesticated reindeer. The dog, Canis lupus, may replace the wolf as a final host. The extent of involvement of other canids in the cycle in Eurasia is not clear. The reindeer and elk are the principal intermediate hosts in northern regions. In Eurasia, the range of the elk extends southward to latitude 50°N.

The Nordic countries

There is evidence that this form (the cervid strain) occurs in Norway, Sweden, and Finland in a dog-domestic reindeer cycle. In northern Norway, the prevalence of E. granulosus cysts in the lungs of reindeer is reported to have decreased after introduction of annual praziquantel treatment of dogs and their exclusion from slaughter places (Kummeneje 1982).

Echinococcus granulosus cysts have also been found in the lungs of slaughtered reindeer in northern Sweden (Roneus 1966) and also lately, in 1996 (A. Uggla, National Veterinary Institute, Uppsala, Sweden, personal communication). In the period in between there were no veterinary observations of E. granulosus cysts in reindeer. The latest human case of autochthonous E. granulosus in Sweden was detected in 1984 in a 20-year-old woman of Lappis origin, with a large liver cyst. Imported cases are annually diagnosed in Sweden especially from the Mediterranean region, Central Europe and the Near East (Czechowski et al. 1992).

There are no recent epidemiological reports from Finland, but in 1969 1.2% of reindeer were reported to be infected with cysts in areas bordering northern Sweden. Iceland is free of E. granulosus, and the last human case of cystic echinococcosis was diagnosed in 1960. Echinococcus sp. is not present in Greenland possibly due to the unsuitability of the local rodents as intermediate hosts.

The Baltic states

In Estonia, Latvia, and Lithuania, the notification of echinococcal disease is not mandatory and the prevalence is generally not well known.

In Estonia, echinococcosis has not been a reported disease for a long time. There are no official data on prevalence, however, single cases of E. granulosus in animals in 1993-1994 were seen. In Latvia, 10 human cases with E. granulosus in the liver were notified and one case of cerebral echinococcus since 1984. The latest cases were from 1995. There are also veterinary data from 1994 to 1995, which show a very low incidence in cattle (0.00l-0.002%) (Ludmila Jurevica epidemiologist, personal communication).

In Lithuania, the epidemiological situation is also uncertain. During the same period, 1992-1996, nine cases of human echinococcosis were recorded, two of which were fatal. From the same period, positive serology for echinococcosis has been reported in 108/533 (20%) patients with suspected disease. Furthermore, in 1997, 152 persons were examined during 10 months, 40 of whom turned out with a positive serology of echinococcus (personal communication, A. Laiskonis, Laboratory of Parasitology of

State Public Health Centre, Kaunas, 1998).

Russian Federation and adjacent countries of Central Asia

In north-eastern Siberia, rates of infection have been high (25-70%) in domesticated reindeer. The sheep strain, and perhaps other pastoral strains of E. granulosus, occur

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throughout most of Russia and the newly independent nations of the former Soviet Union. (Martynenko et al. 1988). High rates of infection in several domestic intermediate hosts, including sheep, cattle, camels, and pigs have been recorded.

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