37.1.2 Mode of transmission of schistosomiasis
Let us now focus on the mode of transmission of schistosomiasis. The major reservoirs of Schistosoma parasites are infected humans (the primary hosts) and freshwater snails (the intermediate hosts).
What do you understand about the term reservoir in the context of communicable diseases? (Think back to Study Session 1 of this Module.)
A reservoir is any location where infectious agents live before they infect new human hosts, and which is important for the survival of the disease-causing organism. Reservoirs can be living (e.g. infected humans or other animals, such as dogs, cows, insects or snails), or non-living things in the environment (e.g. water, food).
Figure 37.3 shows how Schistosoma parasites are transmitted from infected people to new human hosts, via the intermediate hosts – freshwater snails. Figure 37.4 shows the lifecycle of the parasites in more detail, highlighting the immature forms that can be found in the water.
The immature form of the parasite penetrates the skin of a new host when he or she is swimming, washing or standing in infected water. They pass to the liver, where they mature into adult worms. Male and female adult worms mate (look back at Figure 37.1) and deposit their eggs in the blood vessels of either the intestine (Schistosoma mansoni) or bladder (Schistosoma haematobium). The eggs pass out into the water in either the faeces or urine, to continue the infection cycle.