In a field study in South Carolina, Anopheles quadrimaculatus mosquitoes were applied to native cases of malaria in Negroes to determine the infectivity of such malaria cases.
Of 142 feedings upon Plasmodium falciparum patients, 21 showed infections. Of 12,606 mosquitoes dissected, 2.5 per cent were infected. These infections developed normally in the mosquito and in one out of two trials transmission was successful.
The majority of the mosquito infections occurred during the months of October through January. Mosquitoes kept at outside temperatures during October and November became infected.
One patient remained infective to mosquitoes over an 8-month period.
Of the 25 patients showing gametocytes, 64 per cent infected mosquitoes. The gametocyte densities resulting in infections were relatively low, none being over 90 per cmm. Thirteen of the infections resulted when the gametocytes were fewer than 10 per cmm.
Patients with asymptomatic parasitemias infected mosquitoes at about the same rate as did those who were symptomatic. As the asymptomatic group is considerably larger than the other, it is concluded that the former is mainly responsible for the transmission of malaria.
There were 14 feedings upon P. malariae. Four cases showed gametocytes of a low density at feeding and three lots of mosquitoes were infected. Of 586 mosquitoes dissected, 1.7 per cent were infected.
Of nine feedings upon P. vivax, none became infected nor were gametocytes present.
It is concluded that the patient with an asymptomatic parasitemia, usually with a relatively low gametocyte density, is the important factor in the transmission and maintenance of P. falciparum and P. malariae malaria in the area studied.
Malaria Investigations, Division of Tropical Diseases, National Institute of Health, Columbia, S. C.
Communicable Disease Center, Manning, S. C.
Now with Bureau of Entomology and Plant Quarantine, Department of Agriculture, Washington, D. C.
Now with Chemurgic Corporation, Turlock, California.