Numerous trematodes cause disease in humans. Flukes that cause schistosomiasis, paragonimiasis, fascioliasis, clonorchiasis, and opisthorchiasis are included in the World Health Organization (WHO) list of neglected tropical diseases (NTD) to which interventions for poor and marginalized populations are prioritized given the significant health burden.
Although this article focuses on intestinal trematodes, a limited discussion on liver flukes (Clonorchis sinensis, Opisthorchis viverrini, Fasciola hepatica, Fasciola gigantica) is provided given the similarity in the mode of acquisition (foodborne) and the challenge in terms of diagnostic differentiation with the intestinal flukes.
Intestinal trematodes can be classified into at least 14 families (with their corresponding sources of infection found below), as follows:
Brachylaimidae (from terrestrial snails)
Cathaemasiidae (from a yet unknown source)
Diplostomidae (from snakes, frogs, and tadpoles)
Echinostomatidae (from freshwater fish, frogs, mussels, snails, and tadpoles)
Fasciolidae (from aquatic vegetables and contaminated water)
Gastrodiscidae (from aquatic vegetables, crustaceans, molluscs, and amphibians)
Gymnophallidae (from oysters)
Heterophyidae (freshwater fishes)
Lecithodendriidae (from dragonflies)
Microphallidae (from shrimps and crabs)
Nanophyetidae (from salmonid fishes)
Paramphistomidae (from aquatic plants)
Plagiorchiidae (from insect larvae)
Strigeidae (from a yet unknown source)
Intestinal trematodes are flat hermaphroditic worms that vary in length from a few millimeters to many centimeters (see the image below). Of the approximately 70 species known to colonize the human intestine, only a few species are known to cause actual infection. Globally, it is likely that more than the estimated 40-50 million people are infected with intestinal trematodes, primarily infected via the foodborne route. Populations in Southeast Asia appear to be most vulnerable. An exhaustive 2009 review of these infections in this region provides detailed information on the large number of species infecting humans, their pathogenicity, diagnostic issues, and treatments.
In 2012, the various manifestations, methods of diagnosis, and management of foodborne trematodiasis, which include the intestinal flukes, were detailed in the British Medical Journal
and the European Journal of Microbiology and Infectious Diseases
. In the former, the species of intestinal flukes considered to be of public importance include the following:
Adult fluke of Fasciolopsis buski. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.
The most common human intestinal trematode was said to be F buski, which causes fasciolopsiasis,
and should be differentiated from F hepatica and F gigantica, which are liver flukes that cause fascioliasis. Conversely, the genus Echinostoma is the largest, with about 500 species of echinostomatid flukes. About 20 species belonging to 10 genera have been reported to cause human disease.
The genus Echinostoma is considered the largest, which includes Echinostomahortense, Echinostomaangustitestis, Echinostomacinetorchis, Echinostomaechinatum, Echinostoma ilocanum, Echinostomamacrorchis, and Echinostomarevolutum. E ilocanum was said to be the most common cause of infection in humans.
Heterophyes heterophyes, Metagonimus yokogawai, and Gymnophalloides species are less-common causes of human intestinal fluke infection.
Other intestinal flukes that rarely cause human intestinal infection include Gastrodiscoides hominis, Phaneropsolus bonnei, and Prosthodendrium molenkampi. Intestinal flukes have likely infected humans for hundreds of years, if not longer. Evidence of G seoi infection has been traced back to the 17th century based on discovery of G seoi eggs in a Korean mummy.
See Common Intestinal Parasites, a Critical Images slideshow, to help make an accurate diagnosis.