Laboratory and Field Evaluation of a Direct Filtration Technique for Recovery of Schistosome Cercariae *

Abstract

The use of cercariometry in epidemiological studies has been limited by turbidity, difficulty of recovering cercariae at low concentrations in natural waters, and by complex apparatus that requires a power source. The technique of differential filtration developed by Theron has been modified and tested in the laboratory and in the field in Upper Egypt for detection of Schistosoma haematobium cercariae. A recovery filter with a pore size of 30 micrometer was found to give the best results. The recovery rate in the laboratory was 51% on filtration of 5-liter samples of formalin-treated water, and 19-30% with 10- and 20-liter samples. This moderate efficiency is offset by simplicity of the technique, rapid filtration, and ease of reading the recovery filter, permitting sampling of large volumes of water in the field by persons with little training. Average densities of 0.024 cercariae per liter of water were found at the Nile and 0.029 cercariae per liter in the irrigation canals. Most cercariae were recovered between 0.700 and 0900 hours.

The effectiveness of water treatment processes against schistosome cercariae: A systematic review

“…Cercariae may be stained with a dye prior to being filtered to aid with the detection. Some studies fixed cercariae before filtration to facilitate the counting [ 30 , 31 ]. This may affect results as cercariae are highly mobile and may be able to move through pores that dead cercariae would not pass through [ 31 ].…”

“…Membrane filters have been used for cercariometry (to determine the density of cercariae in water. Micro-fabrics with pore sizes of 23–30 μm have successfully filtered out cercariae [ 30 , 31 , 66 – 71 ]. When filtering natural water, several pre-filters were used to prevent the clogging of the filter.…”

Chlorination of Schistosoma mansoni cercariae

“…This may be due to the clumping reducing the surface area of cercariae exposed to chlorine, thereby shielding cercariae at the centre of the agglomeration. This occurrence was very rare, only affecting two out of more than 500 experiments, and is potentially less likely in natural water where the cercarial concentration is much lower [27,28].…”

“…For example, water with a concentration of less than 1 cercaria/ 100L (as found in the field on St Lucia [41]) would enable the safe treatment of up to 10,000L, as this volume would contain less than 100 cercariae. Cercarial concentration can vary significantly in and between water sources based on the time of day, distance to shedding snails and many other factors [27,28,42]. It is therefore recommended that water intended for treatment is abstracted in the morning [42] away from vegetation and snail habitats to minimize the initial cercarial concentration (e.g.…”

Malacological and parasitological surveys along the Xe Bangfai and its tributaries in Khammouane Province, Lao PDR

“…Indeed, cercariae causing dermatitis in a village might have originated from a snail population far upstream and not detected by our snail collections. A simple filtration technique has been developed for cercariometry (Kloos et al, 1982;Prentice, 1984;Théron, 1986) and widely adopted in transmission studies (Prentice & Ouma, 1984;Kimura et al, 1994;Aoki et al, 2003). In the present study the technique adopted was that of Muhoho et al (1997), briefly described as follows.…”