The Opisthorchids are a group of trematode parasites that are found in the bile ducts and gall bladder of several classes of vertebrates. There are a seven different genera of these trematodes that are found in cats. Some of the species found in cats, Clonorchissinensis and species of Opisthorchis are also important parasites of humans in certain parts of the world and because a great deal is written about these parasites in human parasitology texts. The first opisthorchid was, however, found in a cat by Rivolta (1884). The life cycle of the Opisthorchids involves a fish as the second intermediate host, and therefore, cats around the world are commonly infected with this group of parasites. The Opisthorchids, like the Dicrocoelids, usually have the vitellaria confined to bands along the lateral edges of the body. Unlike the Dicrocoelids, the testes of the Opisthorchids are found in the posterior of the body. Usually, one being anterior the other rather than being alongside the other, int the dicrocoelids, the testes are typically next to each other.
The characters differentiating the seven genera are as follows. The genus Amphimerus differs from the genus Opisthorchis in that the vitellaria are divided into an anterior and posterior group with the posterior group extending into the posterior portion of the body to the level of the posterior testis. Also, in the genus Amphimerus, unlike in the genera Clonorchis and Opisthorchis, the ventral sucker is larger than the oral sucker. The genus Clonorchis is characterized by having testes that are highly branched. The vitellaria of this trematode are restricted to the sides of the body anterior to the testes. The genus Opisthorchis is very similar to that of Clonorchis, but differs in that the testes are not branched. The genus Paropisthorchis is characterized by having the ventral sucker and genital pore located on a pedunculated structure that extends out from the ventral surface of the body. Specimens of Metorchis differ from those of Opisthorchis, Clonorchis, Amphimerus, and Paropisthorchis in that the uterus is more bunched together, "rosettiform," and with branches that encircle the ventral sucker. Metorchis spp. also tend to be broader than the Opisthorchin relatives. The testes of Metorchis, Parametorchis, and Pseudamphistomum tend to be more spheroid than those of Clonorchis and Opisthorchis, although branching does occur in some species. The vittelaria of Metorchis are confined to the lateral margins of the body while in the genus Parametorchis, the vittelaria from the lateral sides become confluent anteriorly. In specimens of Pseudamphistomum the posterior end of the body is squared-off giving the ventral surface of the body the appearance of being a pseudo hold-fast structure.