Schistosoma intercalatum is a parasitic worm found in parts of western and central Africa. There are two strains: the Lower Guinea strain and the Zaire strain. S. intercalatum is one of the major agents of the rectal form of schistosomiasis, also called bilharzia. It is a trematode, and being part of the genus Schistosoma, it is commonly referred to as a blood-fluke since the adult resides in blood vessels.
Humans are the definitive host and two species of freshwater snail make up the intermediate host, Bulinus forskalii for the Lower Guinea strain and Bulinus africanus for the Zaire strain.
Morphology
The clinically defining characteristic of most schistosome species are their eggs' size and shape. The eggs of Schistosoma intercalatum have a terminal spine and tend to be moderately larger than those of S. haematobium (approximately 130 × 75 μm). The origin of the name 'intercalatum' is from the observation that their eggs are of an intermediate range between the smaller S. haematobium and larger S. bovis. These eggs are unique because they will stain red when exposed to the Ziehl–Neelsen technique, aiding in identification. When viewed using scanning electron microscopy, it can be observed that the S. intercalatum's surface has a much lower amount of integumental elevations, or bosses, than S. mansoni. This feature is consistent with the tegument appearance of other terminally spined schistosomes.
Life cycle
Schistosoma intercalatum's life cycle is very similar to that of S. haematobium, except for some key differences. To start the life cycle, the human host releases eggs with its feces. In water, the eggs hatch to become miracidia, which penetrate the freshwater snail intermediate host. S. intercalatum has two major strains, each with its own preferred bulinid host. The Zaire strain will use Bulinus africanus, while the Lower Guinea strain will use the extremely common B. forskalii as its intermediate host. The miracidia penetrate the snail tissue, and inside they become sporocysts and multiply. The sporocysts then mature into cercariae inside the snail host and are ready to leave. The cercariae are free-swimming in the surrounding water until they find their definitive host: a human. If there is a small temperature change, the cercariae of S. intercalatum will form concentrated aggregates near the surface of the water. This mechanism for body heat detection of a potential host restricts the formation of viable cercariae to small streams and slow moving bodies of water because of their high sensitivity.
Epidemiology
S. intercalatum is at risk of endangerment in large part due to the introduction of invasive species into its native habitat. Since 1973, both S. mansoni and S. haematobium have been found in places that have been traditionally inhabited by S. intercalatum. This is thought to be because of the increase in transportation accessibility and the increase in forestry jobs in these habitats. In 1999, the noted number of S. intercalatum infections was 1.73 million.
Distribution
There are two major strains of S. intercalatum, both living in forested areas of Africa. One strain lives in the Congo area, particularly Zaire, and the other strain lives in the Lower Guinea area, mainly in Cameroon. Cameroon is a place of scientific interest because three species of human schistosomes live there. S. intercalatum is associated with lower morbidity than the other schistosomes that infect humans. In a study done on schoolchildren in the Republic of São Tomé and Príncipe in western Africa—where S. intercalatum and S. haematobium are endemic—the only schistosome present in the sample was S. intercalatum, an overall prevalence of 10.9 percent in stool specimens.
Unlike the more pathogenic species, infection with S. intercalatum is usually only associated with bloody stool, and sometimes splenomegaly. Chronic schistosomiasis results in granulomata forming around eggs in the mesenteric vessels.
Serologic testing looks for the presence of antibodies against the adult schistosome in the blood. This can only take place 6 to 8 weeks after initial infection in order for the parasite to reach the adult stage and the immune system to produce antibodies against it. However, serologic testing is not useful for patients with previous infections.