Geographic distribution. Korea, Japan, China, Taiwan, and Russia
Infection rate. Unknown in worldwide.
Life cycle. The first intermediate hosts are fresh water snails: Semisulcospira coreana or S. libertina (M. yokogawai), S. coreana or Koreanomelania nodifila (M. takahashii), and S. globus (M. miyatai). The second intermediate hosts of M. yokogawai include the sweetfish (Plecoglossus altivelis), dace (Triolodon sp.) and perch (Lateolabrax japonicus). The fish hosts for M. takahashii are the carp (Cyprinus carpio), and dace (Tribolodon taczanowskii); those for M. miyatai are found under the scale of the sweetfish, dace, pale chub, dark chub, and common fat-minnow. Dogs, rats and cats are reported to be naturally infected final host.
Morphology. The flukes of the Genus Metagonimus are characterized by their small body, 1.0-2.0 mm long by 0.4-0.6 mm wide, laterally located ventral sucker, absence of genital sucker or ventrogenital apparatus, and egg size. Metagonimus takahashii differs from M. yokogawai in the position of two testes (anterior testis separated from the posterior testis), the distribution of vitelline follicles (more abundant and crossing over the posterior-most end), and by larger size of eggs (M. yokogawai, 28-30 mm; M. takahashii, 32-36 mm). Metagonimus miyatai morphologically differs from M. yokogawai and M. takahashii in the position of the posterior testis (separated greatly from the anterior one), the distribution of vitelline follicles (never crossing over the posterior testis), and the intermediate size of eggs (28-32 mm).
Pathology and clinical symptoms. Except for heavy infections, there is neither significant injury of the intestine nor marked symptoms. In heavy infections, irritation of the intestinal mucosa may result in a chronic intermittent mucous diarrhea with colicky pains, abdominal discomfort and tenderness. There is eosinophilia, but no anemia. When he worms penetrate the intestinal wall, the eggs may get into the lymphatics and venules and causes granulomatous lesions in such distant foci as the heart and brain.
Diagnosis. Identification of eggs by stool examination. Differentiation from other heterophyid flukes and Clonorchis eggs is needed.
Prevention. Prohibit eating raw, undercooked, or recently salted fish in endemic areas.
Treatment. The drug of choice is praziquantel (Distocide®) as a 10 mg/ kg single dose.