Comparison of microscopy and real-time pcr in diagnosis of snail schistosoma infection in river Asao, Kisumu county, western Kenya

Abstract

Schistosomiasis is the third most devastating parasitic infection of the genus Schistosoma. It causes about 200 million deaths globally per annum which is passed by infected intermediate snail hosts of the genus Biomphalaria and Bulinus. Western Kenya is among the endemic regions in Kenya with a high prevalence of the disease. The disease control is mainly based on mass drug administration of praziquantel to the final host, with little attention paid to the role of snail intermediate host on the prevalence and transmission of schistosomiasis. Specifically, there is limited data on the species diversity of snails, distribution and parasite infection rates of snails, in inland waters flowing to Lake Victoria. Additionally, less sensitive approaches such as microscopy have mostly been used in studies related to the intermediate hosts, Bulinus and Biomphalaria snails. Therefore, this study aimed to determine: the distribution of the intermediate host snails along River Asao; prevalence of infection in intermediate host snails among the sampling sites; the correlation between the effectiveness of microscopy and advanced molecular tools such as the real-time PCR (RT PCR) in detection and quantification of the parasitic load among infected snails and finally, to quantify infection in snails as the parasite multiplies within the snails and off-host miracidia quantification. The methods involved sampling of snails from sampling sites along river Asao. A GPS visualizer tool was used to map the location of the intermediate snail hosts and focal points of infection. The locations of the snails collected was recorded and used to map the distribution of both infected and uninfected snails following screening in the lab. The snails were then identified morphologically using identification keys as well and standard Polymerase Chain Reaction technique. The parasite load of infected snails was determined by use of microscopy through counting of cercariae shed and by real-time PCR through quantification of the parasite DNA present in the snail host. Only Biomphalaria pfeifferi and Bulinus globosus were present in river Asao. Biomphalaria snails were evenly distributed, while Bulinus snails were focally distributed. Microscopy detected 16% infected Bulinus and 2% infected Biomphalaria snails while real-time PCR detected 28% and 17% infected snails of the Bulinus and Biomphalaria species, respectively. Microscopy-positive snails also tested positive for real-time PCR. There was no correlation between the cercariae number by microscopy and RT-PCR (r= -0.261, P= 0.438) and also between miracidia number by microscopy and RT-PCR value (r = -0.312, P = 0.192). However, an increase in number of Schistosoma sporocysts inside infected snails was significantly shown by real-time PCR ((χ² = 14.18, df = 4, P = 0.0067). Microscopy is suitable for quantification of Schistosoma cercariae and miracidia by counting but is not as sensitive as real-time PCR in the detection of infected snails. Real-time PCR is a useful tool for quick results when identifying and quantifying infected snails in a certain region and thus easy to carry out proper control measures to prevent disease transmission and snail breeding. The infection distribution map can be used to raise awareness of active transmission sites to lower chances of the disease transmission.