Loss of filarial larvae in a natural mosquito population.

Analysis of log normal parasite densities of Wuchereria bancrofti in the mosquito Culex pipiens fatigans collected in the suburbs of Colombo, Sri Lanka, has shown a decreasing parasite load with age of infection. The median density of microfilarial intake in the natural population was 10.3, but this had decreased to 2.6 by the infective stage. Of the total microfilariae ingested, 51.9% were found in the thorax 12--17 hours after infection of the mosquitoes in the laboratory. Further decrease in parasite density during development could not be attributed to filarial mortality in the mosquito but could be accounted for by increasing mosquito mortality dependent on the density of parasite infection. Dissection of recently blood-fed house-resting mosquitoes gave a prevalence rate of 7% microfilarial carriers in the human population compared to a rate of 4% by routine blood-filming.