one class of phylum Platyhelminthes from the order Digenia and are generally referred to as flukes. Metagonimiasis is of the family Herterophyidae. Transmission Transmission requires two intermediate hosts, the first of which is snails, most commonly of species Semisucospira libertina, Semiculcospira coreana, and Thiara granifera. Infection is acquired through the secondary intermediate host, fish, that have not been thoroughly cooked. Metacercariae encyst under the scales or in the flesh of fish from fresh or brackish water. Sweetfish (Pecoglossus altevelis) is one of the most common fish species infected, but others include the golden carp (Carassius auratus), common carp (Cyprinus carpio), Zacco temminckii, Protimus steindachneri, Acheilognathus lancedata, and Pseudorashora parva. Definitive hosts include humans and various fish-eating mammals, primarily dogs, cats, and pigs. Fish-eating birds may also be infected with metagonimiasis. Reservoirs Reservoirs include fish-eating mammals such as dogs, cats and pigs as well as fish-eating birds. The presence of heterophyid infection in humans is generally caused by a lack of host specificity by the parasites, as seen in the many non-human reservoirs for metagonimiasis. The many reservoirs also have negative implications on the efficacy of prevention and eradication efforts of the disease. Incubation period The incubation period is around 14 days and infestation may persist for more than one year. Morphology Eggs The morphology of the eggs is very important for diagnosis, but is difficult as eggs are very small. Eggs have a smooth, hard shell that is transparent and yellow-brown with a more conventional, ovoid egg shape. They are about the same size as those of Heterophyes and Clonorchis, usually measuring 26-28 μm length and 15-17μm width. The egg also has a very slight opercular shoulder, marking the line of cleavage between the shell and operculum, an "escape hatch" for the mircidium. The Clonorchis has more distinctive tapering and a seated operculum that help distinguish it more readily from Metagonimus species. Adult flukes The body of the adult disease-causing agent of metagonimiasis is often described as leaf-shaped, similar to most trematodes. It is one of the smallest intestinal flukes, and is only slightly larger than Heteropheres. The most prominent feature is that its ventral sucker is deflected to the right of its midline and is closely associated with the opening of the genital pore. The testes are large and diagonal to each other while the smaller ovary is anterior to the testes and the uterus is filled with eggs.