The risk of humans contracting schistosomiasis depends on their contact with cercariae; this can be quantified as, for example, the annual exposition time. Corresponding data are presented in the St. Lucia project (see this website). However, the density of the cercariae in permanently used bathing and washing places determine the accumulated worm load in the human population. At what intervals does a campaign with molluscicides have to be repeated in order to prevent the infection of snails newly immigrating thereafter? To what extent does a campaign against schistosomiasis by therapeutic mass treatment control snails?
The effectivity of infected snails causing a subsequent infestation of a human population in contact with them may be estimated quantitatively. In principle, the following two parameters can be obtained. First, the population density (abundance) of infected snails, as a rate of collected snails, can be estimated by food traps attracting snails. Second, the duration (days) of expulsion of cercariae (duration of patency of the infected snails) can be estimated in the laboratory with wild-trapped snails observed in glass tanks.
The product of the rate of infected snails and the days of expulsion corresponds to the amount of cercariae (cercarial abundance). The latter is not calculated from the snail population present. A trap may contain 50 snails, of which 20% extrude cercariae for 3 days = 30 expulsion days. The number, i.e. amount of cercariae expulsed remains unknown and is not estimated e.g. by experiments. However, cercarial abundance can be taken as a potential intensity of infestation (PII). The proportion of the abundances before a campaign, immediately thereafter and following an interval corresponds to the effectivity of the campaign. In practice, more data have to be determined, such as the exposure time of the traps and the age of the snails, i.e. the size of infected snails, because young snails have mostly not yet been contacted by miracidia.
A campaign using N-tritylmorpholine (Frescon®), carried out at two crater lakes in Cameroon with comparable general conditions (fig. 7.4, page 275) resulted in significant differences. At the first location (Mbo) with Bulinus rohlfsi as the single IH, the PII was 7.7% of the original value by 0-6 months after termination of control, if the molluscicide had been applied every 6 weeks. In the other location (Kotto), B. camerunensis and B. rohlfsi (but with lower abundance) were present. The PII remained between 16-48% and, subsequently, the applications had to be repeated every 2 weeks. When the campaign was definitively terminated, the PII for B. rohlfsi remained significantly lowered for at least one year, whereas that for B. camerunensis returned to the original situation after one month. In the second year after the campaign, more than a threefold value of the original situation was noted there (Duke and Moore, 1976).
The biology of the snails explains these differences convincingly: B. camerunensis grows quicker and has a higher reproduction rate. The egg batches contain twice as many eggs as those of B. rohlfsi and the snails are fertile again after 2 weeks, whereas B. rohfsi is fertile again after 3 weeks. Moreover, B. camerunensis delivers cercarie 2 days earlier than B.rohlfsi. However, in the laboratory, B. camerunensis is less susceptible to the miracidia of S. mansoni, whereas B. rohlfsi is highly susceptible in the laboratory and after exposure at the banks of lakes.
Some differences have been observed with respect to the pathogen. At Kotto (not at Mbo), people excrete eggs resembling those of S. intercalatum, which causes intestinal schistosomiasis, in their stools together with high excretion of S. haematobium in their urine. According to further investigations and laboratory experiments, a hybridisation of the two species is most probable (tab. 7.1, page 276).
The PII allows the quantification not only of the density of infected snails, but also of their delivery of cercariae. However, the absolute density of cercariae that a person may contact via water remains unknown. This is a difference from the annual transmission potential (ATP) of blackflies transmitting Onchcerciasis (see chapter 220.127.116.11,page 45), a value that determines the dosage, namely the absolute number of invasive filarial larvae a person may obtain during a certain exposure time; the PII gives a relative value for the risk of becoming infested by a Schistosoma species.