Metagonimus yokogawai (Katsurada, 1912) Katsurada, 1912
ETYMOLOGY:Meta = posterior and gonimus = genitalia along with yokogawai = for Dr. Yokogawa.
SYNONYMS:Heterophyes yokogawai Katsurada, 1912; Loxotrema ovatum Kobayashi, 1912; Metagonimus ovatus Yokogawa, 1913; Loossia romanica Ciurea, 1915; Loossia parva Ciurea, 1915; Loossia dobrogiensis Ciurea, 1915.
HISTORY: This fluke was originally described as Heterophyes yokogawai by Katsurada, but later, the same author renamed the worm Metagonimus yokogawai. The original description was based on material obtained by Dr. Yokogawa from human beings and from experimentally infected cats and dogs in Taiwan.
GEOGRAPHIC DISTRIBUTION: This is the most common heterophyid trematode in the Far East. It has also been reported from Siberia, the Balkans, and from human beings in Spain.
LOCATION IN HOST: Small intestine.
PARASITE IDENTIFICATION: Members of the genus Metagonimus can be identified by the position of the ventral sucker and genital opening which are fused and displaced to the right of the midline of the body. The testes are close together at the posterior of the body where one is slightly anteriad to the other. The ventral sucker is larger than the oral sucker. The adults of Metagonimus yokogawai are 1 to 1.5 mm long with eggs that are 26 to 28 µ by 15 to 17 µm.
LIFE CYCLE: The miracidium within the egg, like that of Heterophyes heterophyes, does not hatch upon contact with water, but rather, only after it is infected by the appropriate fresh-water snail, e.g., Semisulcospira libertina. The cercariae that are produced have long tails with thin dorso-ventral tail fins. The cercariae infect fish between the scales, and metacercariae develop predominantly within the muscles. Fresh-water fish that have been shown to be intermediate hosts include Plectoglossus altivelis, Odontobutis obscurus, Salmo perryi, and Tribolodon hakonensis. Animals become infected when they ingest the raw flesh of these fishes.
CLINICAL PRESENTATION AND PATHOGENESIS: Shallow ulcers may be present in the mucosa of the jejunum where parasites live within the villus epithelium. There is a shortening of the villus length and adhesions formed between villi. During the first 5 to 15 days after infection, there is a decrease in the number of goblet cells present in the areas around the trematodes; this number then returns to normal levels (Kim et al., 1983). Heavy infestations are likely to cause small bowel diarrhea.
EPIZOOTIOLOGY: Cats become infected by eating raw fish. Other hosts that ingest infected raw fish are also likely to become infected.
HAZARD TO OTHER ANIMALS: Dogs an be infected through the ingestion of infected raw fish; however, due to the requirements for two intermediate hosts, it is unlikely that an infected cat would pose a direct threat to other animals.
HAZARD TO HUMANS: Human beings have been infected with this parasite on numerous occasions and are the host from which the parasite was first recovered. People, like cats, obtain their infections by the ingestion of raw fish.
CONTROL/PREVENTION: The prevention of the ingestion of raw fish.
Kim BW, Lee JB, Cho SY. 1983. Attitude of goblet cells in small intestine of experimental cat metagonimiasis. Chung-Ang J Med 8:243-251.
Rho IH, Kim SI, Kang SY, Cho SY. 1984. Observation on the pathogenesis of villous changes in early phase of experimental metagonimiasis. Chung-Ang J Med 9:67-77.
Figure 2-27.Metagonimusyokogawai collected from a dog in Japan in 1919 by Dr. Yokogawa. Note the paired testes in the posterior of the body. The displace genito-acetabulum is difficult to observe in this figure.