Abstract: AIM: To study the infectivity of different densities of P.vivax to An.sinensis. METHODS:Venous blood samples from 38 volunteers artificially infected with P.vivax of South Yunnan collected at the time of initial parasitaemia, primary attack and relapse, were used to infect Anopheles sinensis by feeding on parasitaemia blood in vitro. The finding of oocysts in the mid gut of the mosquito by microscope was taken as criteria of positive infection. RESULTS: P. vivax in the initial parasitaemia did not infect the mosquito . In the cases of primary attack, when the density of parasite was more than 100/μl blood, the infectivity and intensity rose with the prolongation of the course of the disease and the increase in the density of the parasite . The infect ivity of the P. v ivax in the relap se cases w as h igher than in the
primary attack cases, the mosquitoes could be infected at a parasite density of 1/μl blood.
The mosquitoes acquired a higher infection rate, a higher positive rate of mosquito mid-gut and a higher oocyst index in the primary attack cases whose parasite density were higher than 1 000/μl blood and in the relapse cases whose parasite density were higher than 100/μl blood. CONCLUSION: The P. vivax in the period of clinical attack was one of the most dangerous period of the spread of malaria . The cases of relapse were the more dangerous infective sources in malaria transmission.

Key words: Plasmodium vivax, Anopheles sinensis oocyst index, positive rate of mosquito mid gut