Despite the definite cases of clinical paragonimiasis detected among Indochinese refugees in the United States, Australia, and other countries after 1975, little attention has been paid to the relationship between host and Paragonimus sp. in Laos [7]. For the purpose of preparing the basic information on paragonimiasis, field investigations were conducted in Laos between February 2003 and November 2005 under the auspices of the Korea-Lao Parasite Control Project.

During Korea-Laos project of the Korea Association of Health Promotion (KAHP) and the Korea International Cooperation Agency (KOICA) until 2003, Namback District in Luangprabang province was recommended for a survey area of paragonimiasis by Prof. V. Sengdara (Faculty of Medical Sciences, National University of Laos, Vientiane, Laos). An epidemiological study was conducted in the Phonsavanh and Phonkeo villages, as well as nearby schoolchildren to provide basic information about paragonimiasis. A total of 308 villagers (166 females and 142 males), and 633 schoolchildren (302 females and 331 males) were subjected to intradermal skin tests. Veronal buffered saline-extracted whole worm antigen of Paragonimus westermani (Shinpoong Pharm. Co. Ltd., Seoul, Korea) was inoculated into the forearm, and the wheal size was measured 15 min later. A wheal measuring in excess of 60 mm² was considered as a positive reaction according to the manufacturer's instructions.

Among the 308 villagers, 156 (62 females and 94 males) evidenced a positive reaction, and the male villagers exhibited a higher positive rate (66.2%) than was seen in the female villagers (37.3%). The higher positive age groups were the 45-49 (68.4%), 55-59 (62.5%), and 25-29 (62.2%) yr age groups, respectively. The most prevalent positive age group was males aged 45-49 (88.9%), in which 8 of 9 tested village men had positive reactions on the skin test (Table 1).

The skin test positive reaction rate in schoolchildren was 14.5% and with the exception of class 4 in the primary school, the positive rate of the higher classes was approximately twice as high as that detected in class 3 in the primary school. Unlike what was observed with the villagers, no difference in positive reactivity was seen between boys and girls in the tested schoolchildren (Table 2). Additionally, the sputum of 128 skin test-positive reactors was examined for the presence of Paragonimus sp. eggs. Unlike the relative high rate of Paragonimus skin test results, eggs were observed in the sputum of only 4 of 128 total positive cases, and 3 of these egg-positive samples were from school boys (2 were 13-yr-old and 1 was 14-yr-old) (data not shown). These dissonances of positive rate between skin and sputum test of paragonimiasis may be speculated due to incorrect collection of sputum by young children.

The majority of schoolchildren reported numerous experiences of eating roast crabs in the field. On the other hand, the adult villagers reported frequent consumption of seasoned crabs (Tan Cheoy Koung) and papaya salad (Tammack Koung) with crushed raw crab. In addition to this characteristic feature of the villagers' food culture, the denizens of this area drink fresh crab juice as a traditional cure for measles, and this was also thought to constitute a route for infection. Some of the positive reactors, both the schoolchildren and the villagers, complained of chronic cough, bloody sputum, mild chest pain, fish smell, and lassitude. These symptoms disappeared after treatment with praziquantel (25 mg/kg, 3 times a day for 2 days).

Crab was the only second intermediate host for Paragonimus sp. in Namback District. In order to identify the second intermediate host of Paragonimus sp. in Namback District, several types of crabs, i.e., the rock, rice-field crab, and iron crab in common names, were collected from streams, rice fields, and the Namback market, all of which are located near the residences of the villagers and schoolchildren.

In our examination for Paragonimus sp. metacercariae, the collected crabs were crushed individually or as a batch, then digested with artificial gastric juice (0.6% pepsin in 0.85% NaCl) for 2 hr, and washed 3 times with normal saline. After filtration with a fine sieve, the digested crab materials were examined with a stereomicroscope. Metacercariae of Paragonimus sp. were isolated from rock crabs only, and not from any other kinds of crabs, and the rock crabs were identified as Indochinamon ou by its morphology [8,9]. Both the infection rate and density of metacercariae in the crabs were generally low.

Paragonimiasis still occurs all over the world, including Laos. In the present study, human paragonimiasis was confirmed in Namback District of Luangprabang province, Lao PDR via skin tests, sputum studies, and identification of Paragonimus sp. metacercariae. After the Vietnam War, paragonimiasis was frequently reported in Laotian refugees throughout the world. In Chicago, 3 Laotian refugee children with chronic pulmonary complaints were diagnosed as paragonimiasis via the identification of P. westermani eggs in stool, sputum, and bronchoscopic examinations [10]. In California State, 4 pulmonary paragonimiasis cases were reported in refugees from Southeast Asia, and 3 of these were Laotians [3]. A 25-yr-old Laotian farmer living in Australia demonstrated numerous Paragonimus eggs upon a cytological examination of bronchial washing, which was undertaken in the process of tuberculosis treatment [11]. At least 2 Paragonimus sp. (P. heterotremus and P. westermani) have been identified as causative agents of human paragonimiasis in Laos [12,13]. Moreover, due to the fact that Paragonimus infection is frequently accompanied by tuberculosis or pulmonary complaints, and the annual risk of tuberculosis infection in Luangprabang has been estimated to be 0.6%, further study into Paragonimus sp. in Luangprabang, Laos is still necessary [14,15].

Until the present, several studies of Paragonimus infection have been conducted in Vientiane province. Among several species of crabs, freshwater snails, and prawns in the Nam Set River near the Vientiane province, 4 types of Paragonimus metacercariae (P. westermani, P. heterotremus, P. harinasutai, and P. bangkokensis) have been identified [16]. Moreover, Yahiro et al. [13] collected Paragonimus eggs from the sputum of 6 paragonimiasis patients and identified them as P. heterotremus [13].

In our study, we confirmed the presence of human paragonimiasis in Luangprabang province, Lao PDR, and the life cycle of Paragonimus sp. was shown to be perpetuated through the rock crab (Indochinamon ou). Although this study provides some basic information regarding paragonimiasis in Luangprabang province, the ecology and roles played by snail and crab intermediate hosts remain to be clearly elucidated. Furthermore, the clinical characteristics of human infection with this parasite remain unclear. For this reason, an integrated study of Paragonimus infection in this region should be undertaken in order to better understand the host-parasite relationships.