Studies on the transmission of simian malarias. II. Transmission of the H strain of Plasmodium knowlesi by Anopheles balabacensis balabacensis.

Five species of anopheline mosquitoes were examined in regards to their susceptibility to infection with the H strain of Plasmodium knowlesi. Anopheles balabacensis balabacensis, Anopheles freeborni, Anopheles maculatus, and Anopheles stephensi were readily infected to the oocyst stage but only in the first-named species were salivary glands heavily infected. Only A. quadrimaculatus failed to become infected. Transmission of this parasite to Alacaca mulatta monkeys was obtained by the bites of A. b. balabacensis mosquitoes on two occasions. The extrinsic incubation periods in these lots of mosquitoes were 12 and 13 days and the prepatent periods in the infected monkeys were 8 and 7 days, respectively. The usefulness of the malaria parasites of nonhuman primates as laboratory models for the study of pathology, physiology, immunity, and drug response has been well established. Some of these species have been easily adapted to laboratory transmission with a variety of anophelines while considerable difficulty has been encountered in finding a suitable laboratory vector for others. Plasmodium knowlesi is one of the monkey parasites which has proven most difficult to adapt to routine laboratory transmission. The development of this parasite in mosquitoes to the point of sporozoite-positive salivary glands has been demonstrated in Anopheles annularis (Singh et al., 1949), in A. atroparvus (Weyer, 1937; Hawking et al., 1957), in A. aztecus (Gamrnham et al., 1957), and in A. stephensi (Singh et al., 1949; Hawking and Mellanby, 1953; Hawking et al., 1957; Garnham et al., 1957). The parasite has been passaged through mosquitoes to susceptible monkeys by the intravenous inoculation of dissected infected salivary glands and/or guts by several workers. The mosquitoes used for this purpose included A. stephensi (Singh et al., 1949; Garnham et al., 1957; Hawking and Mellanby, 1953; Hawking et al., 1957), A. annularis (Singh et al., 1949), A. aztecus (Gamham et al., 1957), and A. atroparvus (Hawking et al., 1957). None of these investigators reported attempts to transmit the infection through the bite of infected mosquitoes, and to our knowledge such natural transmission has not been reported elsewhere. The present report is concerned with the Received for publication 20 March 1967. susceptibility of five species of Anopheles to Plasmodium knowlesi and the transmission of this parasite by the bite of Anopheles balabacensis balabacensis. MATERIALS AND METHODS The P. knowlesi used was isolated from a naturally acquired human infection (Chin et al., 1965) and had been passaged by blood to humans and Macaca mulatta monkeys. It has been designated by us as the "H strain. " Malaria-free M. mulatta monkeys from North India were used throughout the course of these experiments. In Al. mulatta monkeys parasitemias of P. knowlesi develop to extremely high levels and the infection is usually fatal if not treated. The Anopheles quadrimaculatus mosquitoes were the Q-1 strain which was obtained from Technical Development Laboratories, CDC, Savannah, Georgia. The A. freeborni (F-1 strain) were from Marysville, California and have been maintained in the laboratory since 1944. The A. stephensi were from Delhi, India and were obtained from the London School of Hygiene and Tropical Medicine, London, England in 1963. The A. maculatus were obtained from the Institute for Medical Reearch, Kuala Lumpur, Malaysia in 1964 and have been maintained in our laboratory since then. The A. balabacensis balabacensis colony was originally from Thailand and was recently obtained from the Walter Reed Army Institute of Research, Washington, D. C. through the courtesy of Col. W. D. Tigertt and Major J. E. Scanlon. The mosquitoes most often used in the experiments were A. freeborni and the susceptibility of the others was, in each case, compared with this species. After the infective feeding, six to 43 specimens of each species were examined for oocysts. Only when A. freeborni was found to be infected were the results used in determination of the comparative susceptibility of the anopheline species. The details of the procedures used in this study are presented in the first report in this series (Collins et al., 1966).