Cercarial Density in the River of an Endemic Area of Schistosomiasis Haematobia in Kenya

Abstract

Abstract. The cercarial density in natural water and number of infected Bulinus globosus were monitored over a one-year period to identify the transmission foci in an endemic area of schistosomiasis haematobia in Kenya. Overall prevalence and intensity of infection of the study community were 59.2% and 10.9 eggs/10 ml of urine. Cercariometry was carried out on 456 occasions at 20 study sites while snail sampling was done on 465 occasions at the same sites over a one-year period. Cercariometry was exclusively done at flowing water habitats. The results showed the focality and seasonality of transmission. Cercariae were detected on 44 occasions at 11 sites. The detections were made on seven occasions at two study sites, six occasions at one site, four occasions at four sites, three occasions at one site, two occasions at two sites, and one occasion at one site. Densities of 1-4 cercariae/100 liters of water were found on 31 occasions. Five to nine cercariae/100 liters of water were found on seven occasions, 10-19 cercariae/100 liters of water were found on two occasions, and high cercarial densities greater than 20 cercariae/100 liters of water were found on four occasions. The highest count was 52 cercariae/100 liters of water. The presence of cercariae in natural water was shown to depend on the water temperature, but the intensity and duration of sunlight did not affect the presence of cercariae in water. The monthly variability of cercarial density was proportional to the number of infected snails. Cercarial density was highest in March and April, in the middle of the rainy season, whereas no cercariae were detected in cool dry season. The snail population peaked late in March, the beginning of the long rainy season, remained high for two months, and decreased rapidly late in May when heavy rain occurred. The overall infection rate of snails was 7.3% and the majority of infected snails were collected from March to May. There was no definite correlation between the presence or absence of cercariae and infected snails. Cercariae were frequently found where infected snails were absent and cercariae were sometimes absent where infected snails were present. Cercariometry and snail sampling remain quite complementary in identifying the transmission foci of schistosomiasis.Several technologies have been proposed for identifying water bodies that are transmission sites for schistosomiasis. The large-scale use of sentinel rodents to detect transmission foci has already been shown to be logistically difficult, ex pensive, and too slow for an effective response.1 Snail col lection shows the general pattern of transmission for indi vidual sites and for the area as a whole, but it provides in formation about the intensity of the parasite flux from the vertebrate to the snail vectors and thereafter the rate of de position of schistosome eggs in the sites.2 Cercariometry al lows direct evaluation of the cercarial density depending on the volume of filtered water, although the presence of cer cariae other than those of human ...

Cercariae (Platyhelminthes: Trematoda) as neglected components of zooplankton communities in freshwater habitats

“…Nevertheless, the number of cercariae present in a habitat is directly proportional to the number of infected molluscs (Upatham, 1976;Muhoho et al, 1997). However, it is difficult to establish a precise relationship between the cercarial densities and the number of infected snails because of the complexities of hydrodynamical processes, mollusc distribution and cercarial emission patterns found in any individual aquatic habitat (Aoki et al, 2003).…”

Advances in the Diagnosis of Human Schistosomiasis

“…Conventional microscopic quantitation is used to detect cercariae in natural water sources and has been applied to describe the diurnal variation, seasonal patterns, and spatial distribution of cercariae (333,334). However, this approach has the limitations common to the other conventional techniques discussed above.…”

Applying evolutionary genetics to schistosome epidemiology

“…These data suggested that high rates of new parasite recruitment were an alternative and likely explanation to treatment failure (Black et al, 2009). Another method of collecting transmission related data is to sample cercariae directly from water (Muhoho et al, 1997; Aoki et al, 2003; Hertel et al, 2004). Although traditional cercariometry studies have fallen out of vogue, they have the potential to detect the spatial partitioning of cercariae and cercarial diversity within transmission zones.…”