WHO Model Prescribing Information: Drugs Used in Parasitic Diseases - Second Edition
(1995; 152 pages) [French] [Spanish] View the PDF document
Table of Contents
View the documentPreface
Open this folder and view contentsProtozoa
Close this folderHelminths
Open this folder and view contentsCestode (tapeworm) infection
Open this folder and view contentsIntestinal nematode infection
View the documentTissue nematode infections
Open this folder and view contentsLoiasis
Open this folder and view contentsLymphatic filariasis
Open this folder and view contentsOnchocerciasis
Open this folder and view contentsSchistosomiasis
Close this folderIntestinal, liver and lung flukes
View the documentPraziquantel
View the documentSelected WHO publications of related interest
View the documentBack cover
 

Intestinal, liver and lung flukes

At least 50 million people, largely in southern and eastern Asia but also in central and eastern Europe, are infected by food-borne flukes (trematodes). Miracidia released from eggs excreted by infected individuals penetrate into the first intermediate host, a freshwater snail, where they develop in a matter of weeks into free-swimming cercariae that penetrate fish or crustaceans or attach to water plants. Each cercaria forms a cyst containing a metacercaria.

The cycle is completed when the cysts are ingested by humans and other mammalian hosts. The clinical sequelae of infection depend upon the pattern of migration and distribution of the mature worms.

Intestinal flukes

The intestinal flukes include Fasciolopsis buski, Metagonimus yokogawai, Heterophyes heterophyes, Echinostoma spp and Gastrodiscoides hominis. The encysted metacercariae are released in the duodenum, attach themselves to the mucosa and mature into adult worms over a period of 3 months. They rarely cause symptoms other than diarrhoea and abdominal discomfort, but patients heavily infected with Fasciolopsis buski sometimes develop allergic oedematous reactions.

Liver flukes

The liver flukes include Clonorchis sinensis, Opisthorchis viverrini, O. felineus and Fasciola hepatica. After their release in the duodenum the metacercariae ascend the biliary tree to reach the intrahepatic ducts where they mature into adult worms over a period of 2 months. Eggs produced by female worms are returned to the duodenum in the bile. Symptoms are rare, but heavy infections may give rise to fever, epigastric pain, hepatic enlargement, jaundice and relapsing cholangitis. In some areas Clonorchis sinensis and Opisthorchis spp infections are strongly associated with cholangiocarcinoma (cancer of the bile ducts).

Lung flukes

The lung flukes are of the genus Paragonimus. After their release in the ileum the metacercariae of these flukes penetrate the intestinal wall to reach the peritoneal cavity and then migrate to the lungs by crossing the diaphragm and the pleural space. Within the lung parenchyma each induces an intense but localized necrotic inflammatory reaction which ultimately gives rise to a cyst or abscess cavity. The capsule surrounding the parasite swells as the fluke matures, and after some 6-9 weeks the parasites start producing eggs that are liberated into the bronchioles and ascend the airways to reach the gastrointestinal tract. It is unusual to find more than 20 cysts, each harbouring one or two worms. These usually live for 6-7 years but some have been known to survive for up to 20 years.

Light infections are asymptomatic, but if large numbers of flukes remain untreated they can cause chronic bronchitis, bronchiectasis, pleural effusion and fibrosis. They can also develop to maturity in ectopic sites, particularly in the brain, where they may cause epilepsy, focal paresis and other signs of space-occupying lesions. Pulmonary paragonimiasis may produce clinical symptoms similar to or be associated with active tuberculosis.

Prevention1

1 For further information, see Control of foodborne trematode infections. Report of a WHO Study Group. Geneva, World Health Organization, 1995 (WHO Technical Report Series, No. 849).

Prevention of infection is dependent upon informing communities at risk of the danger of eating potentially infected or contaminated foods.

Treatment1

1 For further information, see Control of foodborne trematode infections. Report of a WHO Study Group. Geneva, World Health Organization, 1995 (WHO Technical Report Series, No. 849).

Praziquantel has transformed the therapy of most fluke infections. Parasitological cure has been obtained in virtually all cases (with the exception of Fasciola infections) without significant adverse effect but it needs to be taken for several days in the treatment of Paragonimus infections.

Limited evidence suggests that a single dose of the veterinary formulation of triclabendazole is effective and well tolerated in a high proportion of cases of both Fasciola and Paragonimus infections. However, further clinical trials need to be evaluated before triclabendazole can be registered for human use. It may be available for compassionate use on a named basis.

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