Couple Fluke: Infection, Transmission & Diseases

Couple flukes, known as the causative agent of schistosomiasis (bilharzia) among other diseases, are parasitic separate-sex sucking worms that undergo a generation change via a specific freshwater snail. After copulation, the significantly thinner female remains for life in an abdominal fold of the male that has been formed for this purpose. Disease is not caused by the adult worms, which feed on components of the blood in the venous system, but by their eggs, which leave the bloodstream, infect organs, and trigger immune responses.

What are couple flukes?

Couple flukes (Schistosoma) belong to the genus of sucking worms, with more than 80 known species. They are the only species of separately sexed sucking worms. The much thinner female remains in a skin pocket of the male for life after copulation. The worms reside primarily in the venous vascular system of the intestine or urinary bladder of their end hosts. They feed parasitically on blood components and reach a length of up to 20 millimeters. Depending on the species, females produce 100 to 3,000 eggs daily, which leave the bloodstream and migrate to specific organs or are excreted in urine and stool. The excreted eggs develop into ciliated larvae that depend on a specific intermediate host for further development. In most cases, this is certain species of ramshorn snails. The larvae develop in the intermediate host into the mother sporocyst, which subsequently forms a large number of daughter sporocysts. The sporocysts grow into fork-tailed cercaria in the intestine of the snail. Once the excreted cercariae, which float freely in the water, come into contact with their final host, they penetrate through the skin and develop into adult worms. Depending on the species, the final host may be humans and other mammals, as well as waterfowl or crocodiles.

Occurrence, distribution, and characteristics

Schistosoma mansoni and Schistosoma haematobium, as the causative agents of schistosomiasis (bilharzia), are the most important and best-known representatives of the couple flukes, of which there are a total of five human-pathogenic species. Schistosomiasis is widespread mainly in tropical Africa and almost throughout the entire Nile Valley. Schistosoma mansoni depends on a particular posthorn snail for its alternation of generations, which is found primarily in stagnant and slowly flowing waters. Schistosoma haematobium, the second species of couple fluke with high human pathogenicity, also poses a high risk of infection to the population in some tropical regions of Africa. A certain species of Bulinus snail serves as an intermediate host. Another pathogenic couple fluke, Schistosoma japonicum, occurs in some regions of East Asia as the causative agent of intestinal bilharziasis. In Europe and North America, only species that parasitize exclusively in duck birds are common. However, any cercariae present in contaminated bathing lakes also penetrate the skin of humans. Although they subsequently die, they can cause an unpleasantly itchy bathing dermatitis. There is no direct risk of infection from person to person, since the cercariae larvae that emerge from the eggs are absolutely dependent on their specific intermediate host for their further development and transformation. For this reason, the worldwide spread of individual species of schistosomes is also not readily possible.

Diseases and ailments

Schistosomiasis is mainly caused by the eggs of the worms, some of which are excreted in urine or stool. Another part initially remains in the body and may invade the liver, intestine, or other organs. In rare cases, the central nervous system is also affected. For example, Schistosoma haematobium cercariae initially migrate to the lungs where, two to 10 weeks after penetration of the cercariae through the skin, they cause typical symptoms such as Katayama fever. It is manifested by edema, fever, dry cough and other symptoms. Depending on the type of schistosomiasis pathogen, the liver, bladder or intestine are mainly affected. As they pass through tissues, the eggs cause inflammatory reactions of the immune system and initiate repair mechanisms. They lead to the formation of fibrous granulomas. This means that functional organ tissue is partially replaced by connective tissue that can no longer perform organ-specific tasks.If organs such as the liver or spleen are affected, fibrous structures form and there is an increase in blood pressure in the portal vein of up to 100 percent and severe enlargement of the spleen. The greatest risk of contracting schistosomiasis is when bathing in waters contaminated with active cercariae. It is also the only route of infection for developing schistosomiasis or bilharzia. In many cases, the entry sites where the larvae have penetrated through the skin are visible. An itchy rash often develops there, which may indicate that cercariae have entered. The safest preventive measure is to avoid using waters known to be contaminated with cercariae for bathing or swimming. Once cercariae have penetrated the skin, further development into adult pair flukes can hardly be stopped. Only when the cercariae have developed into pair flukes that have established themselves in the venous vascular system is drug therapy possible. If left untreated, schistosomiasis can lead to serious health problems. Most notably, the liver, spleen, lungs, bladder, and intestines can suffer lasting damage from fibrotic tissue changes, some of which can be life-threatening.