17th June 2024

Trematode infections

Trematodes, also known as flatworms or flukes, are parasitic worms that can cause human disease.

Blood flukes – Schistosomiasis

Schistosomiasis is a condition caused by trematodes of the genus Schistosoma. Various species are widely distributed across the tropics. Infection is caused by the larval stage cercariae penetrating the skin in freshwater. This can cause a pruritic rash known as “swimmer’s itch”, though this is less likely to occur with repeated infection. Likewise, the acute inflammatory reaction associated with initial egg production several weeks after larval entry is more likely in immune-naïve patients. Signs and symptoms of this inflammatory reaction, known as Katayama fever, include fever, rash, wheeze, and hepatosplenomegaly. Eosinophilia can be noted on a full blood count. The release of eggs from the body causes the main chronic symptoms. Schistosoma haematobium resides in the pelvic veins and causes urogenital schistosomiasis, with haematuria often being present. Chronic S. haematobium infection can lead to reproductive tract pathology, obstructive uropathy, and squamous cell bladder cancer. Other species of Schistosoma, including Schistosoma mansoni and Schistosoma japonicum, reside in the mesenteric veins. Egg passage through the bowel can cause small amounts of blood loss and anaemia over time. Portal hypertension can result from eggs and associated inflammatory responses within the portal veins. This, in turn, can lead to hepatosplenomegaly and oesophageal varices. Diagnosis may involve microscopic visualisation of eggs in urine or stool, though this can lack sensitivity. Stool and urine PCR can improve sensitivity, and rapid antigen detection point of care tests for urine samples are also available. Serology can be useful for returned travellers but is less useful for patients in endemic areas. Praziquantel is used to treat all species causing schistosomiasis.

Lung Flukes

Ingestion of freshwater crustaceans can lead to infection with the lung fluke genus Paragonimus. Paragonimus westermani is the main species to know about, being largely responsible for paragonimiasis in Asia. The disease is more rarely documented in Central Africa and South America. The immature flukes excyst through the duodenum and reach the lung via the peritoneum. This may cause a pneumothorax. The mature flukes then release eggs in the lung, which are coughed up and either released through the mouth or swallowed and released after passing through the digestive tract. Symptoms include initial abdominal pain followed by fever, wheeze, cough, and haemoptysis. Diagnosis is made either serologically or by identification of eggs in sputum or stool. Treatment is with praziquantel.

Liver Flukes

The main causative fluke for fascioliasis is Fasciola hepatica, which is found on every populated continent. Infection is via ingestion of contaminated freshwater plants. In a similar fashion to Paragonimus, immature Fasciola flukes excyst through the duodenum, though eventually invade the biliary ducts via the peritoneum. Similar symptoms are present during this stage, such as fever and abdominal pain. The infection may then be asymptomatic, aside from occasional pain from intermittent biliary obstruction. Pancreatitis and cholecystitis can also occur. Serology and stool culture can be useful in diagnosis. CT imaging can show hypodense liver lesions. Similarly, ultrasound can highlight hypoechoic liver lesions. Praziquantel – usually a safe bet for trematode treatment – is not effective for fascioliasis; triclabendazole is used instead.

Trematodes of the genus Clonorchis and Opisthorchis cause a similar clinical presentation to fascioliasis. Encysted larvae are introduced through the ingestion of freshwater fish. Infection occurs in Asia only, except in the case of rare international imports. Larvae enter the biliary tract through the ampulla of vater rather than through the peritoneum and inhabit smaller bile ducts than Fasciola. Chronic infection can lead to the development of cholangiocarcinoma. Praziquantel is the treatment of choice.