Study of the snail intermediate hosts of urogenital schistosomiasis in Niakhar, region of Fatick, West central Senegal

Abstract

BackgroundSchistosoma haematobium is the most widespread schistosome species in Senegal and occurs in several regions of the country especially in the Sudan-Sahelian zone. The aims of the study were i) to determine the freshwater species ii) to ascertain the role of the identified snail species in the transmission of S. haematobium and iii) to study the impact of drought on the snails.MethodsSnails were sampled each year in 17 sites from July to November-December 2012 and 2013. At each snail survey, snails were grouped by village, counted, identified according to shell morphology and the rates of schistosome cercarial shedding recorded. The shell height of the snails collected in July was measured and classified into four groups according to their size in order to determine those that are open to aestivation.ResultsB. senegalensis and B. umbilicatus were the only snails intermediate hosts collected in the Niakhar study area. B. senegalensis is found in all the 17 sampling sites while B. umbilicatus was only found in one site out of the many surveyed. The total number of B. senegalensis collected in 2012 and 2013 was 1032 and 8261 respectively. A total of 901 and 6432 B. senegalensis were tested for Schistosoma spp. infestation in 2012 and 2013 respectively. For B. umbilicatus, 58 snails were collected and tested in 2012. In 2013, 290 were collected and 281 tested. The overall rates of schistosome cercarial shedding were 0 % in 2012 and 0.12 % in 2013 for B. senegalensis and 13.79 and 4.98 % in 2012 and 2013 respectively for B. umbilicatus. For both species collected in July, size group 3 individuals (7–9.9 mm) were the most numerous, 63.6 and 57.8 % for B. senegalensis and B. umbilicatus respectively. B. umbilicatus was reported for the first time in the region of Fatick located in the old ecological zone of Sine-Saloum, is able to maintain Schistosoma spp. larvae during 7 months of drought and may transmit the disease in early July, increasing the period and the risk of transmission.ConclusionThis study recommends an adaptation of snail control strategies at pond cycles and ecology of the snails in these seasonal foci. Malacological control strategies must take into account these phenomena of drought resistance and the capacity of some snails to maintain parasite during aestivation. The treatment of ponds with Bayluscide at the end of the rainy season in November and upon onset of rains in July would be more advantageous to the control of snails thereby reducing transmission of urogenital schistosomiasis in the Niakhar area.

The Effect of Climate Change and the Snail-Schistosome Cycle in Transmission and Bio-Control of Schistosomiasis in Sub-Saharan Africa

“…It was further posited that individual Bulinus species, ranging between 7-9.9 mm in size, were present in the month of July; 63.6% and 57.8% for B. senegalensis and B. umbilicatus, respectively. For the first time, this study reported that B. umbilicatus can maintain Schistosoma larvae for as long as seven months of drought, thereby resulting in the transmission of schistosomiasis in early July leading to an increased risk of schistosomiasis transmission in the study area [15]. Therefore, it is safe to deduce that prolonged drought periods for more than nine months can stop the transmission of schistosomiasis at the foci, while drought periods lasting less than seven months can aid the transmission of the disease in other parts of the region due to maintenance of the larval stage.…”

“…Studies have shown that during aestivation, uninfected Bulinus snail species have the ability to withstand longer periods of drought than infected snails [56,57,82]. This implies that during drought, infected snails die off and this alters the transmission of schistosomiasis in drought-stricken areas [15,56,57,82]. Mutuku and co-workers [56] analyzed temporal changes on the spatial transmission pattern of S. haematobium worms on different age groups and their relationship to ponds infested with Bulinus snails in coastal Kenya.…”

“…In another related study by Senghor and co-workers [15], the impact of drought on the snail intermediate hosts of urogenital schistosomiasis in Niakhar, West-Central Senegal was studied from July to November-December in 2012 and 2013 respectively. It was observed that out of the two Bulinus species (B. senegalensis and B. umbilicatus) collected in the study area, only B. senegalensis was found in all 17 sampled sites, while B. umbilicatus was found in only one out of the 17 surveyed sites.…”

“…infection within the stipulated time periods. On the other hand, a total number of 58 and 290 B. umbilicatus snails were also collected during the years 2012 and 2013, respectively, while a total number of 58 and 281 snail species were tested for infection within the respective designated time periods [15]. Overall cercarial shedding of 0.0% and 0.12% was observed for B. senegalensis, as well as 13.79% and 4.98% for B. umbilicatus during these years.…”

“…Reoccurrence of infection over the years may additionally cause cancer of the bladder, pulmonary hypertension, and blockage in the urinary tract that may ultimately lead to other related complications and even death. In recent times, several studies [12][13][14][15][16][17] have deduced that climate and the changes associated with it such as temperature change, precipitation, humidity, flooding, salinity and drought have adverse effects on the Schistosoma life cycle, especially in sub-Saharan Africa. Hence, understanding the relationship between the variability in climate change and the life cycle of the schistosome worm will be a promising and predictive target in the control of schistosomiasis in the sub-Saharan Africa region.…”