The Topic of This Month Vol.25 No.5(No.291)

Foodborne helminthiases as emerging diseases in Japan

(IASR 2004; 25 : 114-115)

The Parasitosis Prevention Law (enacted in April 1931) was abrogated in November 1994, 10 years ago. The causative parasites of the four diseases included in this law are helminths, namely roundworm (Ascaris ), hookworm (Necator and Ancylostoma ), Chinese liver fluke (Clonorchis ), and Oriental blood fluke (Schistosoma japonicum ). "Helminth" is used to denote parasitic worms, distinguishing from protozoa.

Ascaris and hookworms shed eggs in stools of infected people, dispersed in external environment, and contaminated food and water, which serve as the source of infection. In the years when human wastes were used as manure, it was difficult to avoid contamination of raw vegetables and pickles with parasite eggs or hatched infective larvae; particularly the infection rate of Ascaris during the unstable period after the World War II was as high as 50-80% with no difference between rural and urban areas. Since then, preventive measures had actively been taken by nationwide practice of stool exams and mass treatment through school and local health, and also by improving the human excreta disposal system. In addition, with extensive use of chemical fertilizer, Ascaris - and hookworm-infected people dramatically decreased in number in the latter half of 1960, being very rare today.

Clonorchis used to be transmitted through eating raw small cyprinids such as minnows and bitterlings. Therefore, there were many endemic areas all over Japan. As fresh-water snails, the first intermediate hosts, decreased sharply due to the environmental changes, the life cycle of Clonorchis itself has been gradually interrupted in Japan.

Diversification of foodborne helminthiases: Thus, domestic cases of helminthiases included in the Parasitosis Prevention Law have become very few; nevertheless attention had been paid to the occurrence of other foodborne helminthiases before abrogation of this law. Occurrence of foodborne helminthiases in Japan is closely associated with the Japanese food culture of liking to eat raw fish, shellfish and meat. By the rapid economic growth in the 1970s and the progress in food transportation system in Japan, foods used to be consumed locally have spread all over the country, resulting in the dissemination of the diseases. Also, by the international food circulation, development in food preservation technology and introduction of new food items, species of causative helminths have apparently increased to as many as more than 20 kinds. After 1960s, the following helminthiases have newly been reported in Japan in connection with particular food items:

Anisakiasis: This is caused by eating raw marine fish and squid. In Japan, the first report of this larval infection came out in 1964, though it is conceivable that this disease has been prevalent from long time ago. Many other cases have been reported since then throughout the country. During this period, Anisakidae and the closely related species were identified as the cause of acute gastric symptoms. Since then it has became possible to remove the larvae under a gastroscope, making diagnosis and therapy relatively easy. Recently, by the investigation conducted by the laboratory of a market in Saitama Prefecture, Anisakis larvae have been detected at a high rate in the liver of imported angler fish (see p.118 of this issue). It has also been reported that anisakiasis can be explained by allergic reactions against Anisakis larvae (see p. 119 of this issue).

Echinostomiasis: After 1970, the disease occurred by eating raw loach served at restaurants.

Angiostrongyliasis: Angiostrongylus cantonensis , expanding its distribution to the Japanese Islands through Taiwan by 1970, used to elicit cases by eating raw African snails or slugs, the intermediate hosts of this parasite, in Okinawa. From the latter half of 1970s, the epidemic areas spread over the mainland. In 2000, an outbreak of infection occurred without eating intermediate hosts in Okinawa, probably due to ingestion of vegetables naturally contaminated with infective larvae. Epidemiological investigation was conducted on this episode (see p. 120 of this issue).

Paragonimiasis: Since 1971, cases of Paragonimus miyazakii infection have occurred due to eating raw or undercooked Japanese freshwater crabs. After 1975, cases of Paragonimus westermanii infection have occurred due to eating raw meat of wild boar. Recently, cases of paragonimiasis have occurred among some foreigners staying in Japan (Koreans, Thais, and Chinese) by bringing their own eating habit of dishes using freshwater crabs into Japan (see p. 121 of this issue).

Trichinellosis: After 1974, cases have occurred from eating raw bear meat in Aomori, Hokkaido, and Mie Prefectures. Recently, infection with Trichinella in a Japanese who traveled to Kenya was reported (Kansenshogaku Zasshi, Vol. 78, Abstract of the 78th Annual Meeting of Japanese Association for Infectious Diseases, p. 100)

Intestinal capillariasis: After 1982, four cases of Capillaria philippinensis infection have been reported. Although the route of infection has been undetermined, eating raw freshwater fish is suspected. Fatal cases have been reported in the Philippines.

Gnathostomiasis: Gnathostoma spinigerum infection was known to be caused by eating raw snakehead in the past. G. hispidum , G. nipponicum and G. doloresi infections due to eating imported raw loach in 1970, domestic raw loach and raw masu salmon in 1988 were reported, respectively.

Spiruroid larva migrans: Since the latter half of 1980s, this disease has been prevalent all over the country from eating raw firefly squid (Watasenia scintillans ) that had been marketed only in the fishing districts before (see p.116 of this issue). The type X larva of the nematode suborder Spirurina causes ileus and creeping eruption after invasion from the intestines of humans, being not a natural host.

Countermeasures against foodborne helminthiases: In September 1997, the working group for food poisoning surveillance, the Section of Food Poisoning, the Food Sanitation Council, assessed countermeasures against foodborne parasitic diseases. After due consideration of the following three terms, the 10 species listed below were cited as the helminths requiring special countermeasures: (a) helminths causing nationwide outbreaks, or those tending to increase in recent years, (b) helminths prevalent in foreign countries and increasing in Japan is anticipated, (c) such helminths anticipating serious health damage, although the disease rarely occurs.

(1) Those transmitted through fresh fish and shellfish: They are Anisakis , Spiruroid nematode, Pseudophyllid cestode(s), Diplogonoporus grandis , Metagonimus yokogawai (intestinal fluke), and Gnathostoma . (2) Those transmitted through other foodstuffs (raw meat, etc): They are lung fluke (Paragonimus ), Sparganum mansoni , Cysticercus cellulosae and Trichinella .

For countermeasures against these foodborne helminthiases, the following four have been proposed: (1) edification of people with safe way of consumption of food, (2) development of methods of detection of parasites from food, (3) education of public health personnel for the knowledge of parasites and training in methods of parasite detection from food, and (4) surveillance of parasites in food produced in and out of the country and of incidence of parasite infection.

Notification as food poisoning is required: The Food Sanitation Law Enforcement Regulation was partly amended at the end of 1999 [Ministerial Ordinance No. 105, the Ministry of Health and Welfare (MHW), 1999]. At the same time, the manual for reporting the statistics of food poisoning was partly amended. To clarify the notification of health hazard caused by protozoa and helminths originating from food and drinks as food poisoning, such as Cryptosporidium , Cyclospora and Anisakis have been shown as the category of "others" in classification of etiological agents of food poisoning in the reporting form of food poisoning (a notice from the Food Sanitation Division, MHW, December 28, 1999).

In the recent statistics of food poisoning, cases of food poisonings caused by helminths are very few (Table 1). This reflects lack of understanding by medical personnel and public health officers for the necessity of notification of health hazard caused by helminths such as Anisakis as food poisoning.

From a questionnaire survey for anisakiasis in the whole country, it has been estimated that there are at least 2,000 cases per year (Ishikura, H., Progress of Medical Parasitology in Japan, Vol. 7, p. 439-464, 1999). On this occasion, it should be pointed out that when food poisoning due to a helminth such as Anisakis occurs, a physician must notify to a nearby health center within 24 hours after diagnosis.

Preventive measures against spiruroid larva migrans: The source of infection of spiruroid larva migrans, an emerging foodborne helminthiasis, is restricted generally to firefly squid. Infection due to consumption of raw firefly squid was first reported in 1987. In 1994, mass media made a wide campaign on warning the danger of eating raw firefly squid, and accordingly, the fisheries started to ship firefly squid after freeze treatment. Therefore, a decreasing tendency was seen, but then cases increased again (see IASR, Vol. 21, No. 6). Then, MHW issued recommendations on the requirement for processing to control hazards from parasites in firefly squid in 2000 (a notice from the Food Sanitation Division, MHW, June 21, 2000), showing preventive measures by freezing (at -30 for 4 days or longer, or conditions with equivalent or higher potency to inactivate parasites).

During the nine years from 1995 through 2003, 159 suspected cases of spiruroid larva migrans were subjected to serological tests at four laboratories, of these 31 cases were positive (see p. 116 of this issue). During 2000-2004, raw firefly squid on market were tested for larvae of the nematode suborder Spirurina type X. The average detection rate of larvae for the five years was 4.3%; the parts in which larva were detected were the viscera in 76%, the trunk in 14%, and the head and arms in 10% (see p. 117 of this issue).

This disease occurs preferentially from March through June every year. It is important to prevent the occurrence of this disease by promoting safe ways of providing and consuming firefly squid to both selling people and consumers as a standard practice.

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