Schistosomiasis: Further study on Cercaria Release and Infection to Man
Cercria is the 2rd stage larva of schistosome which when released from the snail intermediate host is capable of infecting man.
Factors that aid in release of cercaria and injecting into man are:
AGE OF SNIALS: Young snails are more susceptible to infection than old snails and hence young snails produce greater number of cercaria than old ones.
- INFECTION: The number of cercaria produced by a snail depends upon factors like level of compatibility of host and parasite, age of snail, size and nutritional status of snail and number of miracidia to which they has been exposed. Rate of infection depends on the population density of cercaria, rate of production, emergence, dispersion and availability of cercaria (Ukoli, 1984). Also longevity of cercaria in locating there definite host determines the infection rates as failure to locate the definite host within limited period results in high mortality of cercaria and disruption of the life cycle (Onwubuike and Nwoke 1982).
- SEASONS: Rainfall and temperature are two important physical factors affecting transmission patterns, depending on the intermediate host and habitat, active transmission can take place either over a long period or can be more intense over a short period. During rainy season there is increase in flow of water and the snail population is reduced, but at the end of any season the streams drop in level and the velocity is reduced and the remaining snails starts breeding with increasing temperature especially were patches of slack of water occurs again during this period giving risk to dry season transmission of schistosomiasis.(Onwubuike and Nwoke 1982).
- TIME OF THE DAY: Light is the main stimulus causing the release of cercaria of S. haematobium. Under natural condition the pattern of cercaria, emission of S. haematobium from Buclinus trauncatus corresponds to the curves for temperature and intensity of illumination commencing at about 09.00 hours and peaking around 15.00 hours and subsequently declining.
- VELOCITY OF WATER: velocity of water is an important factor affecting infectivity of cercaria. (Webbe, 1960), has shown that sentinel animals placed in water containing equal numbers of cercaria but at different velocities, the peak infectivity occurred when the velocity was about 30cml, below this fewer cercaria make contact with the hosts and above this cercaria which make contact tend to be swept away, (Webbe,1960).
Schistosona haematobium repairs a specific snail of the genus Bulinus for transmission. The following factor acts together to determine availability, growth and distribution of he snail intermediate host.
- COMPOSITION OF THE WATER: Calcium is important as it is the major constituent of the snail and it has been shown to be absorbed directly by some snail from the surround has been shown to be absorbed directly by some snail from the surrounding water (Nduka and Harrison, 1976). The presence of calcium has been shown to be important for both growth and fecundity as well as shell formation, (Beadle, 1984). Alkaline and Calcium rich waters of Surinam and Venezuclla are highly suitable for Bulinus globus and water with PH 5.3-9.0 enhances infection, (lasson,1978).
- TEMPERATURE: Bulinus truncates specie thrives best at 220c -260cand incidence rate increase at optional temperature within such range. Temperature range between 180c and 800c exists in the tropical forest and arid regions and that explains schistosomiasis prevalence in various geographical regions and more especially when other favorable factors are present.
- LIGHT: Moderate light is essential for photosynthesis and the growth of plants, so it is probable that lack of light removes an important food resource of snails and substratum for deposition of eggs (Ukoli 1984, Wright 1976).
- HABITAT: Middy substratum rich in organic content and emergent or submerged vegetation favours snails, (Appleton, 1975). Med encourages vegetation. Ecological studies have shown that there is a correction between some plants and certain species of snail. Still or slow flowing water is preferred by snail as in ponds, dams, lakes irrigation canals and slow streams and rivers (Appleton 1957) estimated the upper limits of tolerance for bulinus species to be about 0.3m/sec.
- SEASON: Study by (Okafor, 1989), in Amambra State shows a seasonal infection rate possibly because most snail population fall during the rainy season to increase in the river warmer month after the rains. In which case incidence rate would be high between end of rainy season and early dry season. Similar studies (Okafor and Anya, 1982) in Ishielu L.G.A of Ebonyi State showed transmission potential to be 89.2% at early dry season and 10.81% at peak of rainy season.
These studies explains that rainfall changes the condition of snail habitat in a number of ways such as water level, rate of flow and the chemistry of a given water body. The beginning of any rain may result to flooding of an otherwise suitable snail habitat, thereby dislodging and flushing them away. In such situation sudden fall in snail population result with low rate of infection.
Some water bodies completely dry up during dry season and such habitat are occupied by snail species capable of aestivation, e.g. Bulinus senegeinsis is able to aestivate for 6-7 months (Smithers, 1956). Usually only a small percentage of the total population survive desiccation, but on emergence from aestivation the snails posses at least twice the breeding activity of non aestivated snails. Interestingly, aestivated snails have been shown to maintain a viable parasitic infection and which is of epidemiological significance. (Smithers,1956).