A comparative study on vector competence of Anopheles stephensi from geographically distinct malarious and a non-malarious urban area to Plasmodium vivax.

BACKGROUND OBJECTIVES Anopheles stephensi is responsible for the transmission of malaria in urban areas. Vector competence of An. stephensi from a non-malarious (Coimbatore) and highly malarious (Chennai) urban areas in Tamil Nadu state of India, was investigated to find the reason for non-transmission of malaria in Coimbatore. METHODS Vector competence (susceptibility/refractoriness) of An. stephensi mosquitoes from Chennai (malarious) and Coimbatore (non-malarious) to Plasmodium vivax (Chennai) was investigated. Bioassays were carried out concurrently in both these strains by artificial membrane feeding technique using the same malaria-infected blood. An. stephensi were dissected to observe infection in midgut and salivary gland. The parasite infection, oocyst and sporozoite positivity rate, the oocyst load, correlation between male-female gametocyte ratio and infection, and Survival Analysis of parasitic stages during sporogony were analyzed and compared. RESULTS The overall infection rate was 45.8 and 41.2 percent in Chennai and Coimbatore, respectively. Oocyst count ranged from 1-80 and 1-208 respectively and not statistically significant. Oocyst positivity was high from Day 8-21 in both strains. The Mean Survival Day (MSD) for oocyst was Day 14 in both strains. Sporozoite was observed in four experiments in each of the strains and the MSD for sporozoites was Day 20 in Chennai and Day 17 in Coimbatore. INTERPRETATION CONCLUSION An. stephensi of Chennai and Coimbatore are equally susceptible to P. vivax infection and non-transmission of malaria in Coimbatore can be attributed to external factors such as the presence of preferential breeding habitat, vector density, vector survival, and weather. The only difference observed was the comparatively shortened oocyst maturation time in the Coimbatore strain which requires further investigation.