The Topic of This Month Vol.20 No.1(No.227)

Multilocular echinococcosis in Hokkaido, Japan

Echinococcus multilocularis is a parasite; its intermediate hosts being Clethrionomys rufocanus bedfordiae of arvicolinae and such rodents as Microtus oeconomus and the final hosts Canidae animals. This parasite used to be distributed only in high-latitude regions in the Northern Hemisphere such as Alaska, Siberia, and some parts of Europe, but recently its distribution was caused to spread to central North America, China and the Middle East. Human infection can be acquired through oral ingestion of the infective eggs excreted in feces of the final host, and larval echinococcus (hydatid) develops mainly in the liver. Because of the extremely slow development of hydatid, it takes as long as up to 10 years for subjective symptoms to develop. Although serological or imaging diagnosis is utilized as a powerful method, confirmation depends on histopathological detection of the hydatid. Surgical removal of the lesion is the only radical therapy; no chemotherapeutic agent has proved satisfactory at present. Unless treated adequately, echinococcosis is a disease with poor prognosis. According to the Vital Statistics of Japan, Ministry of Health and Welfare, the fatal cases during 1985-1997 of echinococcosis numbered 26 (nine males and 17 females; average age 62.9 years).

The Committee for Echinococcosis Control in Hokkaido, with its office at the Health Department of Hokkaido Prefecture, collects information on echinococcus infection of humans and animals in Hokkaido Prefecture. This Committee provided the materials for the following article.

An outbreak of multilocular echinococcosis was reported in Rebun Island for the first time in Japan(figure); foxes imported during 1924-1926 may have served as the initial source of infection. The epidemics on Rebun Island have already come to an end (for further detail of the epidemics on the Island, please refer to p. 6 of this issue). Multilocular echinococcosis patients were found again in 1965 in Nemuro City, that is situated in the easternmost area a long way from Rebun Island. This episode indicated an independent entrance of E. multilocularis to the eastern part of Hokkaido. At first, patients were concentrated in the eastern area; from around 1980, the epidemic region expanded rapidly from the eastern to the western and northern regions (Table 1). Approximately 10 new patients per year have recently been reported occurring in Hokkaido (Fig. 1). From the first occurrence on Rebun Island, a total of 373 patients had been confirmed before 1997 (Table 1). The infection rate among foxes, a final host, and that among voles, an intermediate host, are shown in Fig. 2. Since the area for fox catching differed from year to year, the rate may not represent the overall picture of the whole Hokkaido, but it is undoubtedly increasing in recent years. A similar tendency can be seen among voles. Since foxes in not all regions of Hokkaido were subjected to the examination from the beginning, the exact picture is not known, but from around 1980, the life cycle of this parasite, that had been restricted to the eastern region, has rapidly spread to all regions of Hokkaido. The dog may also serve as a final host, but a very few cases of infection have been confirmed (see p. 3 of this issue and for the detailed information before 1991, p. 100 of IASR, Vol. 14, No. 5)

Since the occurrence of human patients in Nemuro City in 1965, an echinococcosis control program has been enforced on the basis of the manual prescribed by the Committee on Echinococcosis Control with the following five main slogans: 1. Hygiene education. 2. Medical examination. 3. Host animal control. 4. Safety water supply. 5. Research work. Medical examination aims especially at early diagnosis and early therapy of this disease. Regions for conducting and the method of examinations have changed; at present, two-step examinations are being practiced on all those who wish receiving examinations in all regions of Hokkaido. The city, town, and village offices bear the expenses for the primary examinations. The serum antibody test by enzyme immunoassay is being made for the primary screening (see p. 4 of this issue). If one is found to be positive or indeterminate in the primary screening, the serum antibody test by a more accurate Western immunoblot method and abdominal sonographic examinations will be made by specialists as the secondary screening, the cost of which will be defrayed by Hokkaido Prefecture. Finally, patients and watching-needed cases are confirmed by the Expert Subcommittee on Echinococcosis Diagnosis under the Committee for Echinococcosis Control (Table 2). This examination system, being unique to Hokkaido, has been working successfully and attention is being paid by the whole world. Cases that are found at general medical institutions are increasing in number; none of them has received the primary examination.

For host animal control, proper keeping and disposition of garbage, livestock waste, and fishery waste that foxes may feed on are essential so as to keep foxes away from the private houses. In line with this purpose, a comprehensive policy concerning the trash disposal is being discussed on an administrative level. Such a behavior of tourists to feed wild animals should be taken up as a significant subject of further discussion. In 1998, an expert subcommittee on host animals was organized under the Committee to discuss how to live together with foxes and to control the disease.

A few human cases of echinococcosis had been reported in Honshu (the main island of Japan), so investigation on actual infection status of host animals in Honshu was started by a study group of Ministry of Health and Welfare in 1997. No infected animals have so far been found south of Aomori Prefecture.

From April 1999, echinococcosis is becoming one of the category IV infectious diseases, i.e. notifiable diseases, in compliance with the enforcement of the new Law Concerning Prevention of Infectious Diseases and Medical Care for Patients of Infections.

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