Stray cats or feral cats are nowadays frequently found in almost every place of the Republic of Korea, including coastal areas. They are suggested to be a natural definitive host for G. seoi, since they find foods including de-shelled raw oysters in a waste box or, in some cases, steal seafood in the kitchen of villagers in coastal areas. A raccoon dog with a similar life pattern to cats was also suspected as a source of G.seoi or other parasite eggs.

Heterophyid flukes, in particular Heterophyes nocens and Pygidiopsis summa, are another group of intestinal trematodes that infect humans in coastal areas [8,9]. The source of human infections is brackish water fish, including the mullet, goby, and perch produced from estuaries [9]. Cats purchased from a market in Seoul [10] and in Busan [11] were found to be infected with H. nocens and P. summa. However, no studies have been progressed on the infection status of cats caught in other localities of the Republic of Korea. The present study aimed to investigate the infection status of feral cats and a raccoon dog on Aphaedo Island, Shinan-gun with intestinal helminths. Aphaedo is a well-known endemic area of G. seoi and H. nocens infections [12,13].

The carcasses of 4 feral cats and a raccoon dog caught on Aphaedo Island by a villager were purchased. It was informed by the villager that the raccoon dog and a cat were obtained in January 2003, and 3 other cats were caught in February 2004. Their intestines were longitudinally opened with a pair of scissors in 0.85% saline and washed with the same solution until the supernatant became clear. The sediment of the intestinal contents was carefully examined for parasites using a stereomicroscope. The collected worms were fixed in 10% neutral formalin solution and identified using a light microscope after acetocarmine stain.

Adult G. seoi were recovered in 2 of 4 cats examined. One cat had 307 G. seoi adults and the other had 3 adults (Fig. 1; Table 1). These cats were co-infected with H. nocens (a total of 1,527 worms in 4 cats) and P. summa (131). In a raccoon dog, no adults of G. seoi were collected; however, 55 specimens of echinostomes (species undetermined) were obtained (Table 1). Other helminths, including toxocarids, hookworms, Stictodora fuscata, Acanthotrema felis, and Spirometra erinacei were also collected from the animals examined.

There had been no studies on intestinal parasites of raccoon dogs in the Republic of Korea. Therefore, this is the first report on intestinal parasites of raccoon dogs. As for cats, infections with several helminth species, i.e., Clonorchis sinensis, Paragonimus westermani, H. nocens, Metagonimus yokogawai, P. summa, Stellantchasmus falcatus, Heterophyopsis continua, Centrocestus sp., Echinochasmus perfoliatus, Echinoparyphium sp., A. felis, Pharyngostomum cordatum, S. erinacei, Taenia taeniaeformis, Anisakis simplex (larva), and Toxocara cati, have been reported [14-16]. In addition, an experimental study showed that cats were a fairly suitable host for G. seoi infection compared with ducks, chicks, and 6 kinds of mammals, including gerbils, hamsters, rats, dogs, guinea pigs, and mice [5]. The present study first demonstrates that feral cats are a natural definitive host for G. seoi infection. However, the epidemiological significance of feral cats in maintaining the endemicity in the surveyed area needs to be further clarified.

It is of note that infection of a clam with metacercariae of a gymnophallid, Meiogymnophallus fossarum, could change their host's orientation, behavior, and distribution [17,18]. The infected clams could be more easily visible in the sand substrate by the definitive host than non-parasitized clams, thus could facilitate predation. Oysters infected with metacercariae of G. seoi could be exposed more easily to oystercatchers or other definitive hosts, although this speculation has not been confirmed. It is well accepted that oystercatchers have the natural capacity of destroying oyster shells and eating the animal part. However, it is questioned how cats can open the shell of an oyster and consume the animal part. A possible explanation is that they may have consumed the wasted animal part of oysters, since stray or feral cats could easily find wasted oysters discarded in the trash box or in the yard in endemic areas.