Metacercarial Density Research Articles

Survey of Gymnophalloides seoi Metacercariae in Natural and Cultured Oysters from Several Western Coastal Areas, Korea.

Gymnophalloides seoi (Digenea: Gymnophallidae) is a human intestinal trematode contracted by eating raw oysters (Crassostrea gigas) in the Republic of Korea (=Korea). It has been known to be highly endemic in Aphae Island, Shinan-gun, Jeollanam-do (Province). However, recent epidemiological status of G. seoi has not been reported since the 1990s. In this study, we investigated the prevalence of G. seoi metacercariae in natural and cultured oysters collected from 3 islands and 2 coastal areas in western parts of Korea. The oysters were examined using the artificial digestion method followed by stereomicroscopy. The overall positive rate of G. seoi metacercariae in natural oysters was 66.0% (99/150), and the oysters collected from Yubu Island showed the highest infection rate (74.0%). However, the metacercarial density per oyster was relatively low (1.5–2.4 per oyster). By contrast, no metacercaria was found in cultured oysters purchased from 2 coastal areas in Chungcheongnam-do. Thus, we could confirm that natural oysters produced from 3 western coastal islands are infected with G. seoi metacercariae, whereas cultured oysters purchased from 2 coastal areas were free from infection.

Hypolobocera guayaquilensis (Decapoda: Pseudothelphusidae): A New Crab Intermediate Host of Paragonimus mexicanus in Manabí Province, Ecuador.

To determine that Paragonimus sp. is actively transmitted in a tropical area of the Pacific region of Ecuador where human cases of pulmonary paragonimiasis have recently been documented, a total of 75 freshwater crabs were collected from 2 different streams in the Pedernales area of Manabí Province, Ecuador. All collected crabs were identified as Hypolobocera guayaquilensis based on morphological characteristics of the male gonopods. The hepatopancreas of each crab was examined by compressing it between 2 glass plates followed by observation under a stereomicroscope. Excysted Paragonimus metacercariae were detected in 39 (52.0%) crabs and their densities varied from 1 to 32 per infected crab. There was a positive relationship between crab size and metacercarial density. Sequences of the second internal transcribed spacer region of the ribosomal RNA gene of the Paragonimus metacercariae obtained in this study were identical to those of Paragonimus mexicanus deposited in the DDBJ/EMBL/GenBank database. Thus, the present study is the first to confirm that the crab species H. guayaquilensis is the second intermediate host of P. mexicanus in Manabí Province, Ecuador. Because this crab might be the possible source of human infections in this area, residents should pay attention to improper crab-eating habits related with a neglected parasitic disease, i.e., paragonimiasis.

Pygidiopsis summa (Digenea: Heterophyidae): Status of Metacercarial Infection in Mullets from Coastal Areas in the Republic of Korea.

To know the infection status of zoonotic trematode metacercariae in brackish water fish, we surveyed mullets collected from 18 coastal areas in the Republic of Korea. The metacercariae of Pygidiopsis summa were detected in 236 (68.2%) out of 346 mullets examined. They were found in mullets from 15 areas except for those from Boseong-gun (Jeollanam-do), Pohang-si, and Uljin-gun (Gyeongsangbuk-do). Especially in mullets from Taean-gun (Chungcheongnam-do) and Geoje-si (Gyeongsangnam-do), their prevalences were 100% and 95.5%, and the average metacercarial density was more than 1,000 per fish. They were also detected in mullets from 3 coastal lakes, Gyeongpoho, Songjiho, and Hwajinpoho, in Gangwon-do, and their average densities were 419, 147, and 672 per infected fish, respectively. The metacercariae of 5 other heterophyid species, including Heterophyes nocens, Heterophyopsis continua, Metagonimus sp., Stictodora fuscata, and Stictodora lari, were found in the mullets examined. The metacercariae of H. nocens were detected in 66.7, 100, 28.6, 81.6, 3.9, 61.5, and 27.3% of mullets from Muan-gun, Shinan-gun, Haenam-gun, Gangjin-gun, and Boseong-gun (Jeollanam-do), Hadong-gun, and Geoje-si (Gyeongsangnam-do), and their metacercarial intensities were 64, 84, 119, 99, 1, 24, and 24 per fish infected, respectively. From the above results, it has been confirmed that P. summa metacercariae are heavily infected in mullets from coastal areas of Korea. It is suggested that residents who frequently consume raw mullet dish can be easily infected with heterophyid flukes.

Prevalence of Haplorchis taichui among humans and fish in Luang Prabang Province, Lao PDR

This study confirmed the prevalence of the intestinal fluke Haplorchis taichui (Trematoda: Heterophyidae) among people and fish in Luang Prabang Province, Lao PDR. Fecal specimens were collected from 559 riparian people (229 males and 330 females), residing in 4 Districts (Luang Prabang, Xieng Ngeun, Pak Ou, and Nam Bak) and were examined by the Kato-Katz fecal smear technique. The overall helminth egg positive rate was 64.9%. The positive rate for small trematode eggs (STE), which may include H. taichui and other heterophyids, Opisthorchis viverrini, and lecithodendriids, was 15.2%. For recovery of adult helminths, 10 STE-positive people were treated with 40mg/kg praziquantel and 15mg/kg pyrantel pamoate, and then purged. Mixed infections with 3 Haplorchis species (H. taichui, H. pumilio, and H. yokogawai), a species of cestode (Taenia saginata), and several species of nematodes including Enterobius vermicularis and hookworms were found. The worm load for trematodes was exclusively high for H. taichui with an average of 7691 specimens per infected person, followed by H. yokogawai (8.3 specimens) and H. pumilio (4.1 specimens). Out of 207 freshwater fish (17 species) purchased in a market in Luang Prabang District, 138 (67%) harboured H. taichui metacercariae (metacercarial burden per fish; 520). Lower prevalence of fish and lower metacercarial density were observed for H. yokogawai (52% and 50 per fish, respectively) and H. pumilio (18% and 3 per fish, respectively). STE found in the surveyed population of Luang Prabang Province were verified to be those of intestinal fukes, particularly H. taichui.

Heterophyid metacercariae in free living and farmed fish of El-Max Bay, West of Alexandria, Egypt

BackgroundThe role of fish living freely in their natural habitats in the transmission of fish-borne trematodes is well recognized. Moreover, the role played by aquaculture fish has also gained great attention in the last few years.ObjectivesTo investigate the rate, density, distribution of infection, and infectivity of heterophyid metacercariae in free living and farmed fish collected from El-Max Bay, a Mediterranean coastal bay in Egypt. The influence of freezing duration on the infectivity of the detected metacercariae was also evaluated.Materials and methodsTilapia nilotica and Mugil cephalus from both habitats were examined for encysted heterophyid metacercariae using a compression method. The density of infection was estimated by the number of metacercariae per gram of trunk tissue following artificial digestion. The distribution of infection was studied in snips taken from the head, gills, trunk, viscera, and tail. Infectivity of the collected metacercariae was tested in rats. The susceptibility of metacercariae to freezing was evaluated by assessment of their infectivity to rats after they were kept frozen at −15°C for 4, 7, and 14 days.ResultsRates of infection with heterophyid metacercariae ranged from 11 to 23% in the different groups of fish. Free living fish had a significantly higher rate of infection and/or density as well as higher infectivity of metacercariae than farmed fish. Higher metacercarial density was observed in the trunk and viscera of the studied fish compared with the head, tail, and gills. Infectivity of the detected metacercariae decreased gradually with increasing duration of freezing.ConclusionBoth free living and farmed fish can transmit Heterophyes parasites, the former being somewhat more important. The potential risk of human infection is considered to be high. Freezing for 2 weeks is an effective means of inactivating the parasite. Our results underscore the need to raise awareness among public health agencies, consumers, and aquaculture managers of the measures needed to reduce transmission of this intestinal fluke.

Foodborne Trematode Metacercariae in Fish from Northern Vietnam and Their Adults Recovered from Experimental Hamsters

The prevalence of foodborne trematode (FBT) metacercariae was investigated in fish from 2 localities of northern Vietnam in 2004-2005. Freshwater fish (9 species) were collected from local markets in Hanoi City (n=76) and Nam Dinh Province (n=79), and were examined for FBT metacercariae using the artificial digestion technique. Adult flukes were obtained from hamsters experimentally infected with the metacercariae at day 8 post-infection. Three (Haplorchis pumilio, Centrocestus formosanus, and Procerovum varium) and 6 (Haplorchis taichui, H. pumilio, C. formosanus, P. varium, Stellantchasmus falcatus, and Heterophyopsis continua) species of FBT metacercariae were detected in the 2 regions, respectively. Overall, among the positive fish species, H. pumilio metacercariae were detected in 104 (80.0%) of 130 fish examined (metacercarial density per infected fish; 64.2). C. formosanus metacercariae were found in 37 (40.2%) of 92 fish (metacercarial density; 14.7). P. varium metacercariae were detected in 19 (63.3%) of 30 fish (Anabas testudineus and Mugil cephalus) (metacercarial density; 247.7). S. falcatus metacercariae were found in all 10 M. cephalus examined (metacercarial density; 84.4). H. continua metacercariae (2 in number) were detected in 1 fish of Coilia lindmani. Morphologic characteristics of the FBT metacercariae and their experimentally obtained adults were described. The results have demonstrated that various FBT species are prevalent in northen parts of Vietnam.

Prevalence of Opisthorchis viverrini infection in humans and fish in Kratie Province, Cambodia

Opisthorchis viverrini is a medically important foodborne parasite in the Indochina Peninsula. In Cambodia, the prevalence of this trematode has been reported in Takeo Province, but not in other areas. In this study, we investigated the prevalence of O. viverrini infection among people in seven riparian villages along the Mekong River, Kratie Province. We also examined the status of metacercarial infection in fish hosts. Fecal specimens were collected from 2101 residents and schoolchildren, and were examined by the Kato-Katz technique. The average O. viverrini egg positive rate was 4.6%, with the highest prevalence found in Roka Kandal A village (10.4%) followed by Talous village (5.9%). In these villages, adult residents showed higher prevalences (19.4% and 9.0%, respectively) than schoolchildren (6.4% and 1.4%, respectively). O. viverrini adult worms were recovered from 2 egg-positive cases (18 and 4 specimens) after praziquantel treatment and purgation. In addition, three of seven freshwater fish species caught near the villages were positive for O. viverrini metacercariae. A total of 367 metacercariae were harvested from 19 infected fish (metacercarial density; 19 per fish). The species of the metacercariae was confirmed through adult worm recovery by experimental infection to hamsters. The results provide evidence that the surveyed areas of Kratie Province, Cambodia, are endemic for O. viverrini infection.

Prevalence ofMetagonimusMetacercariae in Sweetfish,Plecoglossus altivelis, from Eastern and Southern Coastal Areas in Korea

The present study was performed to determine the current infecion status of Metagonimus yokogawai metacercariae in sweetfish, Plecoglossus altivelis, collected from several streams in eastern and southern coastal areas of Korea. The sweetfish collected were artificially digested with pepsin-HCl solution and examined under a stereomicroscope in August and September, 2007. Out of 145 sweetfish collected from 10 streams in eastern coasts (Gangwon-do and Gye-ongsangbuk-do), 88 (60.7%) were infected with Metagonimus metacercariae. The average metacercarial density was 61 per infected fish. Among 141 sweetfish collected from 10 streams in southern coasts (Gyeongsangnam-do, Ulsan Metropolitan city, and Jeollanam-do), 140 (99.3%) were infected with Metagonimus metacercariae, and their average density was 949 per infected fish. The present study confirmed that M. yokogawai metacercariae are still prevalent in the sweetfish from several eastern and southern coastal localities. The prevalence and metacercarial density were much higher in the sweetfish from streams of southern coasts than in those of eastern coasts. Therefore, attention should be paid to this small fluke infection, and consumption of raw sweetfish naturally produced in these areas should be prohibited.

A survey of Heterophyes nocens and Pygidiopsis summa metacercariae in mullets and gobies along the coastal areas of the Republic of Korea

The infection status of mullets Mugil cephalus (n = 139) and gobies (n = 35) Acanthogobius flavimanus with metacercariae of Heterophyes nocens and Pygidiopsis summa was examined in 11 western, southern, and eastern coastal areas of the Republic of Korea, using a digestion technique. Heterophyid metacercariae were highly prevalent in mullets from western and southern coastal areas; Shinan-gun (100% for H. nocens and 100% for P. summa), Muan-gun (93% and 100%), Buan-gun (42% and 75%), Seocheon-gun (73% and 53%), Ganghwa-gun (47% and 100%), Sacheon-shi (47% and 77%), and Gangjin-gun (50% and 70%, respectively). Only 1 (10%) of 10 mullets from an eastern coastal area, i.e., Donghae-shi, was positive for P. summa metacercariae. Metacercarial densities were the highest in the trunk of mullets for H. nocens and the gill for P. summa. Gobies from Muan-gun were positive for H. nocens (40%) and P. summa metacercariae (40%), and gobies from Seocheon-gun revealed H. nocens metacercariae (20%). The metacercarial density was remarkably higher in mullets than in gobies. The results revealed that H. nocens and P. summa metacercariae are prevalent in mullets and gobies from coastal areas of the Republic of Korea, and the prevalence and intensity of infection vary according to geographical locality.

A new endemic focus of Gymnophalloides seoi infection on Aphae Island, Shinan-gun, Jeollanam-do

A new endemic focus of Gymnophalloides seoi infection has been discovered on Aphae Island (Shinangun, Jeollanam-do), Republic of Korea. This area, which is referred to as Bokyong-ri, is a small seashore village located in the northern portion of the island. Fecal samples were collected from a total of 57 residents and examined by the Kato-Katz and formalin-ether sedimentation techniques. Helminth eggs were detected in 37 samples (64.9%); 33 samples were positive for G. seoi eggs (57.9%), 4 for Pygidiopsis summa (7.0%), 13 for other heterophyids (22.8%), 1 for Clonorchis sinensis (1.7%), and 1 for Trichuris trichiura (1.7%). Women (70.4%) revealed higher rates of G. seoi infection than did men (46.7%), and individuals older than 50 years of age also evidenced higher rates of G. seoi infection than in other age groups (P < 0.05). In worm collection from 13 G. seoi egg positive cases, G. seoi (total 86,558 specimens), Heterophyes nocens (278), Stictodora sp. (10), Heterophyopsis continua (3), P. summa (3), and unidentified flukes (42) were collected. Oysters, the source of G. seoi infection, were collected from an area proximal to the village and 50 were examined for metacercariae; 47 (94%) were found infected and the observed metacercarial density was 9.5 +/- 8.9 per oyster. The results of this study indicate that Bokyong-ri is a new endemic area of G. seoi infection, with high rates of infection in humans and oysters.

Infection status of Paragonimus westermani metacercariae in crayfish (Cambaroides similis) collected from Bogildo (Islet), Wando-gun, Chollanam-do, Korea.

During the period from October 1996 to November 1998, the infection status of Paragonimus westermani metacercariae in freshwater crayfish (Cambaroides similis) collected from Bogildo (islet). Wando-gun, Chollanam-do, which is known for an endemic area of P. westermani in Korea, were examined. The average infection rate of Paragonimus metacercariae in crayfish was 88.6%, and mean number of metacercariae per infected crayfish was 30.2. This metacercarial density was the highest in the group of weight in 7.1-9.0 g. These results suggest that the natural life cycle of P. westermani is still well-preserved in Bogildo.

Discovery of Gymnophalloides seoi metacercariae in oysters from islands of the West Sea known as the habitats of paleartic oystercatchers.

An epidemiological survey was performed to know the infection status of oysters with Gymnophalloides seoi metacercariae in 7 islands of the West Sea known as the habitat of paleartic oystercatchers, Haematopus ostralegus osculans, in Korea. The surveyed areas were Aphaedo (Shinan-gun, Chollanam-do), Jangjado, Sonyudo and Munyodo (Okdo-myon, Kunsan-shi, Chollabuk-do), Yubudo (Changhang-up, Sochon-gun Chungchongnam-do), and Polumdo and Chumoondo (Sodo-myon, Kangwha-gun. Inchonshi). The oysters collected from Aphaedo, the known endemic focus, were examined monthly from August 1995 to October 1996 for observation of any seasonal variation of the metacercarial density. The average metacercarial burden was 761-2,077 by month, but the seasonal variation of the metacercarial density was not obvious. A total of 54 metacercariae was detected in 63 oysters collected from Yubudo. Out of 30 oysters from Sonyudo, 25 (83.3%) were infected with 1-66 metacercariae (12.6 in average). All of 50 oysters (100%) from Munyodo were infected with 3-162 metacercariae (53.5 in average). Only 4 metacercariae were detected in 100 oysters from Chumoondo. However, no metacercariae were found in 55 oysters from Jangjado and 50 oysters from Polumdo. From the above results, it was confirmed that G. seoi is still highly prevalent in oysters from Aphaedo, and several islands of the West Sea known as the habitats of paleartic oystercatchers are new endemic areas of this fluke.

Oysters, Crasostrea gigas, as the second intermediate host of Gymnophalloides seoi (Gymnophallidae).

Gymnophalloides seoi has drawn medical attentions since the discovery of the first human case and a highly endemic area on a southwestern coastal island of Shinangun, Korea. Marine bivalves especially oysters were strongly suspected as the source of infection. In this study the oysters, Crassostrea gigas, naturally produced from the endemic area were examined whether they contain gymnophallid metacercariae. All of 50 oysters examined were infected with the metacercariae of a gymnophallid, with the metacercarial density per oyster of 610 on average (2-4, 792 in range). Later they were identified as G. seoi by obtaining adult worms from experimental mice. The metacercariae were unencysted, and firmly attached on the mantle surface of the oysters with their oral sucker. In sectioned specimens they were equipped with the ventral pit, a peculiar organ of the genus Gymnophalloides, and non-muscular genital pore which was connected dorsally to the seminal vesicle. The seminal vesicle was in a great majority mono-sac. By this study, it has been confirmed that the oyster is a 2nd intermediate host of G. seoi as well as the major source of human infection with this fluke.

Study on Centrocestus armatus in Korea. I. Infection status of Zacco platypus and Z. temminckii with the metacercariae of C. armatus

This study was performed to observe the infection rate and infection intensity of fresh water fish such as pale chub (Z. platypus) and dark chub (Z. temminckii) with the metacercariae of C. armatus. The fish were caught in several rivers and streams from June to September, 1988 and examined by artificial digestion technique. Total 370 fish were caught at 19 rivers. The metacercarial (C. armatus) infection rate and average burden in Z. platypus were 86.8% and 224, and those in Z. temminckii were 78.5% and 131. The infection rate and burden per fish were 73.4% and 32 in the fish caught from the Han river, and 88.0% and 44 from the Youngsan river. All fish caught from the Keum and the Tamjin river were infected and their mean metacercarial density was 45 and 59 respectively. The infection rate and density in fish caught from the Seomjin and Nagdong rivers were 100% and 119, and 81.0% and 348 respectively. From this study, it is confirmed that C. armatus is widely distributed along 5 major rivers in Korea.

Infection status of Sesarma dehaani collected from Sumjin river delta with the metacercariae of Paragonimus iloktsuenensis

This study was performed to observe the recent infection status of Sesarma dehaani with the metacercariae of P. iloktsuenensis in the well-known enzootic focus, Sumjin river delta. A total of 74 Sesarma dehaani were collected from a focus near the mouth of the Sumjin river in November, 1986 and February, 1987. The crabs were examined for P. iloktsuenensis metacercariae by the method of Seo and Kwak(1972). The metacercariae of P. iloktsuenensis were found in the liver of the crabs. Among the 74 crabs examined, 47(63.5%) were found infected with 1-102 metacercariae (18.2 per crab). The infection rate and metacercarial density increased as the size of the crab was increased. From the results, it is suggested that the life cycle of P. iloktsuenensis is actively maintained in the Sumjin river basin.

Studies on intestinal tematodes in Korea: XVI. Infection status of loaches with the metacercariae of Echinostoma hortense.

The infection status of the loaches, Misgurnus anguillicaudatus, with the metacercariae of Echinostoma hortense, was studied in Korea. A total of 154 loaches purchased at 4 local makets (Seoul, Kimhae, Naju-gun and Kangjin-gun) were examined their infection rate as well as the density and location of the metacercariae in the fish body. The results are as follews: The loaches carrying the metecercariae of E. hortense were 64 (41.6%) in total number and the metacercarial density ranged 1-29 per infected loach with an average value of 8.1. The highest infection rate and metacercarial density were obtained from the loaches purchased at Kangjin-gun, Jeonranam-do. The metacercaria of E. hortense were chiefly distributed in the distal intestinal wall and the adjacent mesentery, the perianal tissues, and the head and gill of the loaches examined. From the results, it is concluded that the loach is one of the important second intermediate hosts of E. hortense in Korea, and their infection rate and metacercarial density are considerably high.

Seasonal variations of metacercarial density of Clonorchis sinensis in fish intermediate host, Pseudorasbora parva.

The seasonal variations of the rate and intensity of metacercarial infection of C. sinensis in P. parva were observed. The fish were collected at Sun-Am river which located in Kim-Hae City, Kyong-Sang-Nam Do(=Province), Korea, from March 1983 to February 1984 every month. A total of 788 fish was examined. The number of metacercariae in each fish was individually counted after the individual digestion by artificial gastric juice. The result was as follows: During one year, 513(65.1%) out of 788 fish were infected with metacercariae. In May, June, July and September, the infection rates ranged from 82. 0 % to 98. 6% whereas the rates was relatively low in March, April, November and February raning from 11. 4% to 64.7%. The intensity of infection was similar with those of infection rates. The mean intensity per infected fish was 103.0 and standard deviation was 118.9 throughout one year. The highest mean intenstiy was in June(294. 8) and the lowest in Novebmver(11.1). The observed frequency of fish with certain intensities of metcercariae were fitted to theoretical equations derived from negative binomial distribution in March, April, November and February(p > 0.05). Meanwhile, the equation of lognormal distribution were fitted with the observed frequencies in May, June, July and September(p > 0.05, p > 0.75). The variance/mean ratio varied by month. The value was the highest in July(814.3) and the lowest in November(158.8). Unlike our hypothesis, the metacercarial density of Clonorchis sinensis in its the most favourable fish host, Pseudorasbora parva showed considerable seasonal variations in the hyperendemic area. The possible factors were discussed.

Studies on intestinal trematodes in Korea XIII. Two cases of natural human infection by Heterophyopsis continua and the status of metacercarial infection in brackish water fishes.

Two cases of natural human infection by Heterophyopsis continua (Heterophyidae) were identified by collection of adult worms in 1983 in Korea. And in order to know the source of infection a study on fish intermediate hosts was performed along the southwestern coastal areas. The cases were 24 and 50-year old males residing in southern coastal areas. They had the clinical complaints of non-specific gastrointestinal symptoms such as epigastric pain and one of them also complained the discharge of tapeworm segments. Praziquantel in single dose of 10~15 mg/kg was given followed by purgation with mangnesium salt and 2 and 46 specimens of H. continua were collected from the diarrheal stools. They said to have eaten raw flesh of several kinds of brackish water fishes. The results of fish examination for metacercarial infection were as follows: Seventeen (63.0%) out of 27 perches (Lateolabrax japonicus) and 10 (29.4%) out of 34 gobies (Acanthogobius flavimanus) harboured the metacercariae but none of 42 mullets (Mugil cephalus) were found to harbour them. The average metacercarial density in perches was 55.4 and 18.7 per fish respectively. And the majority of the metacercariae were collected from the muscle of body portions in these fishes. After the metacercariae were experimentally fed to two rats and one young dog, two adult worms were recovered from the dog while none from the rats. These worms were also indentified as H. continua. From the results it was concluded that the brackish water fishes which the human cases said to have eaten were the source of infection.

An ecological study of Paragonimus in Malaysia.

An ecologic study on Paragonimus in Malaysia was attempted from May to September 1967. Seven streams located in various directions and distances from Kuala Lumpur were surveyed for the study of intermediate hosts, snail and crab. One Malayan village and one aborigine village where infected crabs were found, and two tuberculosis hospitals in K.L. were surveyed for the study of human population. Intradermal tests along with sputum or stool examination to detect human infection by Paragonimus were employed. Wild animals, only a few, were shot in the vicinity of the aborigine village and several domestic cats from the Malayan village were bought. These animals were autopsied and examined for adult Paragonimus. Among five species of crab collected from the study areas, only two species, Potamon jahorenes and Parathelphusa maculata were found to be infected with Paragonimus. P. maculata seemed to be better crab host for the Paragonimus because this species had higher infection rate and metacercarial density than the other in the very same area. Three out of seven streams had infected crabs and the infection rate as well as the infection intensity varied from one stream to another. Only avilable snail in the streams was identified as Brotia costula. The infection rate of the snail was very low, six snails out of 11,898, which is about the same rate reported from other countries. Infected snail, however, had thousands of rediae uncountable containing about twelve microcercocercariae in each redia, sufficient enough to maintain the life cycle of the parasite even with only a few infected snail, the amplifier. This is the first confirmed report on the snail host of Paragonimus from Malaysia where the existence of Paragonimus had been reported in 1923. The first trial to study human population by means of intradermal test, sputum and/or stool examinations in Malaysia showed no evidence of human infection of Paragonimus. The number of animals, wild and domestic, examined for natural infection was too small to draw any statement. These examined animals were all negative for adult Paragonimus. Even though more extensive studies on wild animals and human population may be necessary for the definite conclusion, the facts that infected crabs from jungle stream where human contacts are extreamely rare, and also highly infected crabs from the area where none of humans or domestic animals were infected, strongly suggest the life cycle of Paragonimus in this area may be maintained by wild animal hosts rather than by human host. The morphology of all stages of the parasite, the pattern of penetrating glands, flame cells and excretroy bladder of cercaria, lancet shaped single cuticular spines and 6 branched ovary of adult worm obtained from experimentally infected cat, and the shape of egg including all measurements agree well with the characteristics of Paragonimus westermani.