Trematodes

Trematode infections occur worldwide. Trematodes have complicated life cycles, with alternating asexual and sexual developments in different hosts. Depending on the habitat in the infected host, flukes can be classified as (see links for separate articles):

• Blood flukes: Schistosoma haematobium, Schistosoma mansoni, Schistosoma japonicum, Schistosoma mekongi, Schistosoma intercalatum.

• Liver flukes: Fasciola hepatica, Clonorchis sinensis (also called Opisthorchis sinensis), Opisthorchis viverrini.

• Lung flukes: Paragonimus westermani.

• Intestinal flukes: Fasciolopsis buski, Metagonimus yokogawai, Heterophyes heterophyes.

Pancreatic flukes (Eurytrema pancreaticum, Eurytrema coelomaticum, and Eurytrema ovis) are parasites in the pancreatic ducts and occasionally the bile ducts of sheep, pigs, and cattle in Brazil and Asia. The intermediate hosts are terrestrial snails and grasshoppers. There are no obvious clinical signs of infection.¹

Epidemiology

Trematode infections are becoming more prevalent worldwide but are only rarely seen in the UK, where they usually occur in people returning from endemic areas.

Investigations

• Diagnosis is made after microscopic demonstration of eggs.

• Other methods of diagnosis include antigen detection using enzyme-linked immunosorbent assay (ELISA) and serological antibody tests using indirect haemagglutination, indirect immunofluorescence, and ELISA.

• FBC:
  

  • May show eosinophilia in patients with fasciolopsiasis, schistosomiasis, heterophyiasis, metagonimiasis, early stages of paragonimiasis, and acute clonorchiasis (disappears in chronic clonorchiasis).

  • Anaemia may be seen in patients with schistosomiasis, fascioliasis, and paragonimiasis.

  • Neutropenia may be seen in patients with fasciolopsiasis.

• Urine: gross or microscopic haematuria may be found in individuals with schistosomiasis.

• Cerebrospinal fluid (CSF): elevation of CSF pressure and pleocytosis and eosinophilia in the CSF may occur in individuals with cerebral paragonimiasis.

• Imaging tests include CXR, ultrasound, CT, MRI and cholangiography.

Management

• Praziquantel remains the drug of choice for all trematode infections except fascioliasis, for which bithionol is the drug of choice.

• Surgical management may be needed for complications of trematode infection, which include bladder carcinoma in patients with urinary schistosomiasis, fibrosis and thickening of the intestinal wall in those with intestinal schistosomiasis, ascending cholangitis in those with fascioliasis, and cholangiocarcinoma in those with clonorchiasis.

Prevention

• Elimination of molluscan hosts to control infection with schistosomes and intestinal, liver, and lung flukes.

• Proper sanitary disposal of human and pig excreta to avoid water contamination.

• Drug treatment for infected persons.

• Avoidance of consumption of contaminated water, water plants, fruits, fish, crab, and raw liver.

• Thorough cleaning and washing of raw vegetables and aquatic fruits to prevent infection with intestinal flukes.

• Thorough cooking of water-grown vegetables before eating.

• Cooking of crab and crayfish before eating in order to prevent infection with lung flukes.