Parasites are a ubiquitous phenomenon in the marine environment, and it is probable that all marine fish are infected with parasites [1]. Among these, fishborne zoonotic parasites, especially the trematodes (flukes) are a public health and food safety risk throughout the world [2]. While zoonotic parasites transmitted by freshwater fish are well-documented and have received considerable attention, especially the liver flukes, zoonotic intestinal flukes from marine and brackish water are far less characterized, although in many regions they are well-documented as major public health problems [3]. The zoonotic flukes identified in countries, such as Korea, Japan, China, and Thailand, indicate that digeneans belonging to the Heterophyidae and Echinostomatidae are the most common intestinal flukes seen in humans [2]. A recent review demonstrates that the number of intestinal flukes transmitted to humans specifically from marine and brackish water fish is substantial [4].

Recently zoonotic species of fishborne intestinal flukes were reported for the first time in both humans and freshwater fish in Vietnam [5-9]. However, no reports on these parasites in marine or brackish water fish in Vietnam have appeared. Because of the growing economic importance of marine and brackish water aquaculture in Vietnam, it was of interest to investigate the trematode food safety risk associated with grouper (Epinephelus coioides and Epinephelus bleekeri), a very high value cultured fish. For comparison, wild-caught grouper seed which are captured for grow-out in culture, and wild-caught mullet were also investigated.

The results of the present survey for zoonotic metacercariae in grouper and mullet demonstrate that in the Nha Trang marine and brackish waters 3 potentially zoonotic trematodes, H. continua, P. summa, and P. varium are present. The first 2 species are new metacercariae records for Vietnam. Adult H. continua have been reported previously from a sea gull (Larus genei) in Vietnam [12]. Procerovum varium was only recently reported from freshwater fish in Vietnam [9]. Although P. varium is reported as a human parasite in Japan [2], Chai [4] reports that it has only been demonstrated to be infective for humans experimentally. No previous reports of P. summa have appeared in Vietnam.

The presence of these metacercariae in grouper represents a significant food safety concern, since raw grouper is a popular fish dish in Vietnam and elsewhere. Our data on prevalence rates in grouper raised in aquaculture indicate that options for prevention of infection of grouper may be limited. The prevalence of H. continua and P. varium did not increase significantly overall from the levels seen in the wild-caught seed during the grow-out period in either ponds or cages, indicating that the major risk to infection occurs when the smaller fish live in the brackish water environment. Control efforts in these circumstances are severely handicapped by lack of basic information on the natural life cycles of the parasites. Information on the snail vectors (taxonomy, ecology, and host-parasite dynamics) for these fluke species is nearly completely lacking. The generally broad host ranges for heterophyid trematodes make interventions to prevent egg contamination of the aquatic ecosystem problematic. For example, gulls, ducks, cats, dogs, and humans are listed as hosts for P. summa, the reservoir host ranges for H. continua and P. varium, however, are poorly characterized [4]. One example of the potential benefit of research on the ecology of these parasites could be in changes in selection of sources for wild-caught seed. Currently, brackish water wild-caught seed is the main source of grouper for grow-out in the Nha Trang area. Sourcing of wild-caught seed from marine water or their production by culture (nursery) could offer viable prevention options as the snail vectors may be absent in the saline marine waters. Without more research effort on these aspects of these zoonotic flukes, prevention of human infection is limited to targeting risk factors such as attempting changes in cultural eating behavior, and/or the development of post-harvest fish inspection and parasite decontamination.

Overall, there were no differences between the 2 grouper species in metacercarial prevalence, irrespective of size. However, in wild-caught seed grouper, the prevalence of P. varium was significantly greater than H. continua in E. coioides, but not in E. bleekeri. Although there have not been any spawning habitat studies reported on these 2 grouper species in the Nha Trang aquatic ecosystem, based on their biological characters there is no indication of differences in spawning and subsequent development habitats; for example, the juvenile/fingerlings are caught from the same aquatic locations. But because of the difference we observed in P. varium infection rates, ecological studies are needed, particularly on snail vectors occupying the same habitat as that of the grouper, and exploring a possible microhabitat preference by different grouper species that causes differences in exposure to P. varium cercarial infection. The explanation for the significant increase in P. varium only in E. bleekeri in the pond grow-out stage is also not obvious, but as with the wild-caught seed, ecological investigations on snail vectors and grouper behavior in grow-out pond habitats are needed.

In the Republic of Korea, the intestinal fluke Pygidiopsis summa is found in mullets (Mugil cephalus) and gobies (Acanthogobius flavimanus) along the coastal areas [13] and H. continua is recorded from perch, goby, and shad; these fluke species have all been documented to cause human infections [14,15]. Chai [15] provides estimates that there are 10,000 cases of H. continua and 50,000 cases of P. summa in Korea. The variation in P. summa prevalence between geographical locations seen in this study is not unusual, and was also reported by Guk et al. [13]. The prevalence rates in the Vietnamese mullets are very high, compared to those reported in Korea [13,16]. The important definitive and intermediate hosts for these intestinal flukes in Vietnam need to be identified to better understand the factors responsible for the infection rates found, and the strong seasonal tendency for transmission. As cited above, sea gulls and cats are definitive hosts for H. continua, and they are also known reservoir hosts for P. summa [17].

Unpublished observations from Korea suggest that brackish water snails belonging to the genera Cerithidea and Tympanotonus are vectors for P. summa in Korea [15]. This information could be helpful in guiding investigations on the hosts for the 3 intestinal flukes identified in grouper and mullet in Vietnam. Without a reliable understanding of the life cycles and host ranges, the development of effective approaches to prevention of fishborne fluke transmission to fish and humans will be severely handicapped. The actual risk of infection to humans from consuming these fish species also needs to be determined; the recent publication of Dung et al. [6] on recovery of intestinal flukes from humans infected from eating freshwater fish in Vietnam can serve a guide to designing such studies.