Jpn. J. Infect. Dis., 57, 184-186, 2004

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Laboratory and Epidemiology Communications

Changing Epidemiology of Angiostrongyliasis Cantonensis in Okinawa Prefecture, Japan

Ryuji Asato*, Katsuya Taira, Masaji Nakamura, Jun Kudaka,
Kiyomasa Itokazu and Masanori Kawanaka1

Okinawa Prefectural Institute of Health and Environment, Okinawa 901-1201 and 1Department of Parasitology, National Institute of Infectious Diseases, Tokyo 162-8640

Communicated by Tetsutaro Sata

(Accepted July 16, 2004)

*Corresponding author: Mailing address: Okinawa Prefectural Institute of Health and Environment, Ozato 2085, Ozato-son, Okinawa 901-1202, Japan. Fax: +81-98-945-9366, E-mail:

Okinawa Prefecture experienced an outbreak of angiostrongyliasis in January of 2000 (1). The origin of the infection's outbreak could not be identified. We examined the past records of Angiostrongylus cantonensis (Ac) infection outbreaks and investigated the current distribution of Ac's intermediate and paratenic hosts with infective third-stage larvae in Okinawa. In order to find the infective larvae of Ac in the giant African snail, Achatina fulica, the pallial organ (lung) of the snail was compressed between two glass plates and examined under a microscope (2) (Figs. 1A, 1B). In other host animals, the whole body was digested in artificial gastric juice (1%pepsin/1%HCl), and the digested material was allowed to sediment; the sediment thus formed was then examined microscopically. In particular, albino rats were given larvae from Platydemus manokwari (Fig. 2) and Parmarion martensi (Fig. 3) orally with the specimen, and identification was made based on the morphology of the adult Ac recovered at 59 days post-inoculation.

As shown in Table 1, the Ac epidemic showed different features before and after 1990. Before 1990, 17 in 21 (80%) of the infections were traced to their infection sources, while after 1990 only 2 in 14 could be traced to its source. The infection was more frequent (15/21) in April - November before 1990, although it was more frequent (11/14) in December - March after 1990.

Table 2 shows the Ac-positives among the intermediate and paratenic hosts collected in the present and in past field surveys. In the 2000s, among six intermediate host species that were positive in the 1970s, Satumamercatoria mercatoria, Acusta despecta, and Bradybaena circulus became negative, and the positive rate of A. fulica, the prevailing host, that was 39% in the 1970s decreased to 10% in the 2000s. Meanwhile, P. martensi whose presence was not recognized in the 1970s became prevalent in the 2000s, particularly in the northern part of the mainland of Okinawa, and its infection rate was as high as 20.3%. P. manokwari, whose presence in Okinawa was noticed recently in 1993 (3), also showed a high infection rate of 14.1%.

Ac infection is mediated not only by ingestion of the Ac-carrier intermediate or paratenic hosts but also through the ingestion of vegetables, drinking water, and by contact with fingers that are contaminated by the infective larvae of Ac (4).

In the outbreak in January 2000 in Okinawa that involved seven patients, the clinical symptoms and immunological reactions were too weak for typical angiostrongyliasis and were somewhat similar to those of infection by low doses of Ac larvae. In the epidemic, larvae-contaminated fresh vegetables such as lettuce and cabbage were suspected as the source of infection. The first Ac infection case via a fresh vegetable salad had been reported for a patient who developed symptoms 7 days after a short trip to Okinawa in December 1999 (5). Among the 52 Ac cases reported so far in Japan, 35 were from Okinawa Prefecture (6).

As already indicated, the peak season of infection was displaced from April-November to December-March, and tracing the infection to its source has become more difficult in recent years. The displacement of the outbreak season coincided with the decline in the infection rate in A. fulica that is more active in April-November and the appearance of new hosts P. martensi that is more active in the winter season, while P. manokwari is active throughout the year. In relation to the displacement of the outbreak season, it should be noted that the harvest season for lettuce is November-May and that for cabbage is November-July.

Ac-infected P. martensi and Veronicella alte were examined histologically. Ac larvae were present in the muscular layer just beneath the body surface of these snails. However, the former species appeared to be more easily infected because of the less dense muscular layer and tended to contain a greater number of larvae. Platydemus manokwari is frequently observed adhering to the lower side of cabbage leaves. It is quite possible that the larvae are released from the hosts when sliced together with the cabbage leaves in the preparation of fresh salad.

The authors thank Prof. H. Yoshikura, Emeritus Member of the National Institute of Infectious Diseases, for advice in preparing this manuscript.

The present case communicated in Japanese appeared in Infectious Agents Surveillance Report (IASR), vol. 24, p.120-121, 2004.


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