Three species of neodiplostomula are known to inhabit the European grass snake, Rhabdophis tigrina , in the Republic of Korea: Pharyngostomum cordatum (large-sized neodiplostomula), an intestinal trematode of cats; Neodiplostomum seoulense (small-sized neodiplostomula), an intestinal trematode of humans and rodents; and Neodiplostomum leei (small-sized neodiplostomula), which migrates to the livers of rodents and is an intestinal trematode of birds. The present study describes a fourth species, Neodiplostomum ( Conodiplostomum ) boryongense n. sp. (Digenea: Neodiplostomidae), based on adult flukes recovered from the small intestines of chicks experimentally infected with small-sized neodiplostomula from the grass snake. The new species differs from 13 previously known species. It also differs from N. seoulense in its larger body size, severely bilobed testes, and smaller genital atrium, and from N. leei in its larger body size, smaller ventral sucker, presence of a genital cone, and vitelline follicles distributed chiefly in the forebody. The new species does not migrate to the livers of rodents nor does it develop to adulthood in the rodent intestines. However, the neodiplostomula of the new species are indistinguishable from those of the other 2 species. Results show that at least 4 species of neodiplostomula inhabit the grass snake in the Republic of Korea.

Neodiplostomum seoulense is highly pathogenic and lethal to experimental mice; most worms are expelled within 2 mo of acquisition. In this study, T-helper (Th) cell immune responses were studied in N. seoulense -infected BALB/c mice. Spleen and mesenteric lymph node (MLN) cells of infected mice proliferated in response to parasite antigens; CD4 + T cells proliferated more than CD8 + T cells. Antigen-induced interferon (IFN)-γ (a Th1 cytokine) secretion began to increase at day 7 postinfection (PI) in spleen and MLN cells, and this was maintained at day 28 PI, whereas interleukin (IL)-4 (a Th2 cytokine) secretion was somewhat lower. Similar results were observed for mRNA signals of IFN-γ and IL-4. Antigen-specific serum total immunoglobulin (Ig)G, IgG1, IgM, and IgA levels (Th2-induced) were elevated from days 7 to 14 to day 28 PI, and IgG2a (Th1-induced) was elevated at days 21 to 28 PI. Interestingly, the numbers of macrophages (Th1- or Th2-induced), which were found to kill N. seoulense worms in vitro, increased remarkably during days 14–28 PI in spleens and small intestines of infected mice. This study shows that mixed Th1 and Th2 responses occur during the course of N. seoulense infection in BALB/c mice. Heavy infiltrations of macrophages in the small intestine may participate in host damage and worm expulsion.

Meiogymnophallus sinonovaculae n. sp. (Digenea: Gymnophallidae) is described from metacercariae found in the razor clam, Sinonovacula constricta , in the Republic of Korea, and adults recovered from the small intestine of experimentally infected mice. The worms are characterized by paired clusters of vitelline follicles, a well-developed pars prostatica with abundant prostate cells, a voluminous undivided seminal vesicle (in adults and metacercariae), and large, grouped, domelike sensory papillae on the ventral surface anterior to the ventral sucker (in metacercariae). This new species resembles the type species Meiogymnophallus affinis , but differs in having compact, elliptical, and 4–5-lobed vitellaria, and an excretory vesicle with bicornuated anterior arms reaching to the oral sucker. This is the second documentation of the presence of a species of Meiogymnophallus in the Republic of Korea.

Of morphologically indistinguishable small-sized neodiplostomula in the grass snake Rhabdophis tigrina in the Republic of Korea, some, such as Neodiplostomum seoulense , mature into adults in the intestines of rodents, whereas others, such as Neodiplostomum leei , migrate to rodent livers and mature in the intestines of chicks. In the present study, we aimed to observe in more detail the extraintestinal migration and development of N. leei by using various animal models, i.e., mice, rats, hamsters, rabbits, cats, and chicks. In mammals, small-sized neodiplostomula ( N. leei ) inoculated orally penetrated the intestinal wall, entered the peritoneal cavity, and oriented to the liver without passing through any other organ. In rodent livers, the neodiplostomula were surrounded by host inflammatory cells and fibrotic tissues from day 10 postinfection (PI); the worms were found dead by day 56 PI. When neodiplostomula from rodent livers were transferred orally to mammals, they reoriented to the liver, although they were able to develop into adults in the chick intestine by day 6–7 PI. The possibility of human infections, i.e., liver migration, by N. leei neodiplostomula, among snake consumers, warrants investigation.

Susceptibility of inbred mouse strains to Metagonimus yokogawai , Metagonimus miyatai , and Metagonimus takahashii infections was studied using BALB/c, ddY, C57BL/6J, C3H/HeN, and A/J mice, with H-2 haplotypes d, s, b, k, and a, respectively. Two hundred metacercariae were orally fed to each mouse, and the worm recovery rates (WRR), worm dimensions, and intrauterine egg numbers were measured at days 3, 7, 14, 21, and 28 postinfection (PI). On day 14 PI, the WRR of M. yokogawai was highest in ddY mice (average, 62.2%); those of M. miyatai and M. takahashii were highest in ddY (19.5%) and BALB/c mice (10.4%), respectively; worm maturation was best in C3H/HeN ( M. yokogawai ), C57BL/6J ( M. miyatai ), and ddY mice ( M. takahashii ). All mouse strains showed higher susceptibility to infection with M. yokogawai than with M. miyatai or M. takahashii . The results show that susceptibility of mice to Metagonimus infection varies according to mouse strain and parasite species but is suggested to be independent of the mouse H-2 haplotype.