Biomedical Engineering Reference
In-Depth Information
of the liver and to an increased risk of liver cancer. Human infections of
Opisthorchis felineus occur most often in Siberia.
Clonorchis viverrini, the Southeast Asian liver fluke, is likewise transmit-
ted when humans ingest metacercaria in undercooked fish, and adult flukes
live in the bile ducts.
Class Plagiorchiida contains two infectious genera: Dicrocoelium and
Paragonimus.
Two species of Genus Dicrocoelium (Dicrocoelium dendriticum and
Dicrocoelium hospes) are rare causes of liver fluke disease in humans. The
fluke is small, narrow and long; hence, the alternate name, lancet liver fluke.
The adult flukes live in the distal branches of the biliary tree, where they
tend to produce mild disease (compared with the biliary infections produced
by members of Class Opisthorchiidae, see above). The distinguishing feature
of Dicrocoelium infections is the second intermediate host: the ant, Formica
fusca. Humans become infected when they ingest ants infected by metacer-
cariae; hence, the rarity of human disease.
Paragonimus westermani, along with dozens of less common species
within Genus Paragonimus, causes the condition known as paragonimiasis.
Paragonimus species are lung flukes. Adult flukes live in the respiratory tree
of various infected animals (including rodents, pigs, and humans). Humans
become infected when they eat undercooked crabs or crayfish that are
infected with metacercariae. Adult flukes and their eggs produce pulmonary
inflammation. Paragonimiasis is a common cause of hemoptysis (coughed
blood) in endemic areas. Paragonimus eggs can be found in the sputum of
infected individuals. About 22 million people are infected worldwide, with
most cases occurring in Southeast Asia, Africa, and South America. Cases
also occur in the United States.
Trematode infections by pathogenic members of Class Schistosomatidae
are fundamentally different, in their method of transmission, from the previ-
ously discussed fluke infections. All the other subclasses of trematodes infect
humans when the metacercaria (cysts) are eaten. The schistosomes infect
humans when cercaria (the phase of larval development that precedes meta-
cercaria), swimming in water, actively penetrate the skin of humans.
Schistosomiasis is the disease caused by any of the five pathogenic species
of Genus Schistosoma: Schistosoma haematobium (urinary schistosomiasis),
Schistosoma intercalatum (intestinal schistosomiasis), Schistosoma japonicum
(schistosomiasis), Schistosoma mansoni (intestinal schistosomiasis), Schistosoma
mekongi (Asian intestinal schistosomiasis). About 200 million people are
infected by schistosomes. Infections are common in developing countries,
with about half of the infections occurring in Africa.
Trichobilharzia regenti is another member of Class Schistosomatidae.
Humans are an accidental dead-end host for this species. As in schistosomia-
sis, the cercaria penetrates the skin of humans, but the cercaria cannot
develop further. The disease is a localized infection known as swimmer's
