Schistosomiasis, a trematode parasite, is an agent of significant human and veterinary disease. It infects over 207 million people globally, with 700 million at risk of infection, mostly in Sub-Saharan Africa and is endemic in 74 countries. Classified as a Neglected Tropical Disease (NTD), it results in chronic health problems, and causes 200,000 deaths a year. NTD’s are usually found in developing countries and are most prevalent in the poorest communities; in wealthier regions they have been contained so are less visible than other diseases such as HIV AIDS, tuberculosis and malaria (‘the big three’). Schistosomiasis requires water contact for transmission and therefore thrives in areas where there is poor sanitation and unsafe (or contaminated) water. While schistosomiasis has lower mortality rates than the big three it causes high levels of morbidity, pain and disability.
Schistosomes belong to the Schistosoma genus (Phylum: Platyhelminthes, Class: Trematoda, Order: Strigeiformes, Family: Schistosomatidae). Many Schistosoma species have a specific relationship with their definitive vertebrate host and there are over 20 species infecting mammals, birds and crocodiles. Six main species infect humans, Schistosoma haematobium, S. mansoni, S. japonicum, S mekongi S. guiniensis, and S. intercalatum. belong to the Schistosoma genus (Phylum: Platyhelminthes, Class: Trematoda, Order: Strigeiformes, Family: Schistosomatidae). Many Schistosoma species have a specific relationship with their definitive vertebrate host and there are over 20 species infecting mammals, birds and crocodiles.
Unusual among flatworms, which are mainly hermaphroditic, schistosomes are dioecious, and the adult worms pair together. Males are approximately 6-15mm x 0.5-1mm (depending on species), and their lateral sides curve inwards to create a fold called a gynaecophoric canal, where the female resides, approx. 20-30mm x 0.25 – 0.5mm. Both sexes have oral and ventral suckers.
The Life cycle
Schistosomes have a complex lifecycle, with sexual reproduction occurring within the definitive vertebrate host and asexual reproduction occurring in the intermediate snail host.
Reproduction occurs post-pairing of adult worms in the venous/arterial system of the vertebrate host. Females may produce over 1000 eggs a day, which are passed from the vertebrate host in urine and/or stool. On contact with water they hatch into miracidia, the first (non-feeding free-living) larval stage. To survive, the miracidium must locate a susceptible snail and penetrate it within 24 hours, where it transforms into the first stage sporocyst. Second stage sporocysts, produced from the primary, develop in the hepatopancrea of the snail and divide into cercariae; the second free-living larval stage. Cercariae pass from the snail into the water column and use light and chemical cues to potentially find a suitable definitive host within 24 hours. The cercariae penetrate the definitive host through the skin; and developing into schistosomula within a capillary, they migrate to the liver via the lungs where they begin to feed on red blood cells and to pair together, this process takes around 10 days. Next they migrate to the capillaries around the intestines in the case of S. mansoni and S. japonicum, or to the capillaries around the bladder wall or kidney for S. haematobium. The worms reach maturity at 6- 8 weeks, and adult worms generally live for 4 years, but can persist for much longer.
Intermediate host snail species
The intermediate hosts of schistosoma species all belong to the class Gastropoda mainly to the Planorbidae, Lymnaedae and Pomatiopsidae families . The relationship between schistosomes and their snail host is very specific.
Planorbidae: Bulinus (hosts of S. haematobium group)
Biomphalaria (hosts of S. mansoni)
Lymnaeidae: Lymnaea and Radix (hosts of certain animal schistosomes)
Pomatiopsidae: Oncomelania (hosts of S. japonicum) and Neotricula (hosts of S.mekongi) species
Bulinus sp. Biomphalaria sp.
Transmission and symptoms
Transmission requires contact with freshwater together with the presence of a suitable intermediate snail host in the water body. Children generally have higher rates of infection, as they generally have more frequent exposure by swimming and playing in infected water, also the immune response to schistosomiasis tends to increase with age.
Disease pathology is caused by the body’s immune response to antigens coating the eggs circulating the body. The disease covers a range of clinical symptoms according to species. Two main forms of human schistosomiasis are recognised, both with differentially associated pathology: urogenital schistosomiasis, resulting from infection by S. haematobium, affects over 112 million people in sub-Saharan Africa and is associated with haematuria, bladder damage, hydronephrosis with a risk of progression to severe disease such as kidney failure and bladder cancer. Intestinal schistosomiasis in sub-Saharan Africa is mainly caused by infection with S. mansoni, estimated to be the cause of bloody diarrhoea in 4.4 million people, and hepato-splenomegaly, a severe clinical manifestation of the disease is estimated to affect 8.5 million people. In Aisa, S. japonicum is the main causal agent of intestinal schistosomiasis.
urine samples showing haematuria, distribution map, potential transmission site
The usual procedure to diagnose schistosome infection is microscopy. Stool and/or urine samples are collected from the patient and microscope slides are prepared using filtration or smearing techniques which are examined for the presence of schistosome eggs. If evident, the eggs are counted as an estimate of worm burden, termed infection intensity. For stool samples, this technique is known as Kato-katz, and a stain is often used for the eggs. Other diagnostic tests include the circulating cathodic antigen CCA test for S. mansoni and hemastix dipstick test for presence of hematuria for S. haematobium infection. Both allow the potential for quick identification of infection from a urine sample.
S. mansoni egg.
Slides prepared from urine samples to check for S. haematobium infection, kato-katz slides prepared from stool to ascertain S. mansoni infection, showing malachite green staining
The main treatment for schistosomiasis is Praziquantel (PZQ). The number of tablets taken is dependent on the weight of the patient, and a dose pole has been developed to help judge the amount of tablets needed without having to weigh everyone. Mass drug administration (MDA) is often carried out within schools or communities as part of treatment/control programmes. As praziquantel is only effective against adult worms, timing of treatment may needs to be considered. Currently, no vaccine is available for schistosomiasis.
Praziquantel tablets shown with Abednazole, another antihelmitic medication, the WHO standard praziquantel dose pole, and a pitchford funnel, used for processing stool samples