McWilson Warren

THE VECTORS OF SIMIAN MALARIA: IDENTITY, BIOLOGY, AND GEOGRAPHICAL DISTRIBUTION.

Prior to 1960, no natural vectors were known for any species of simian malaria. In the course of investigations in Southeast Asia during the last 3 years, natural vectors for five of these malarias have been found; four have been transmitted in the laboratory. Natural vectors for the African and New World species are still unknown. In a series of tables, in the appendix, the authors summarize the literature on the potential experimental vectors. The ecology of simian malaria in Malaya is reviewed with particular reference to members of the Anopheles leucosphyrus group of mosquitoes in which three natural vectors of simian malaria have been identified. Because they are also vectors of human malaria, it is with this group of mosquitoes that the greatest possibility of a liaison between human and simian malaria exists in Southeast Asia. Interest in the vectors of simian malaria has been stimulated by the transmission of the Malayan monkey parasite Plasmodium cynomolgi bastianellii from man to monkeys by the bite of infected mosquitoes. The original infections in man were acquired accidentally in three different laboratories in the United States from experimentally infected Anopheles freeborni mosquitoes (Eyles et al., 1960; Schmidt et al., 1961). Subsequently other strains of Plasmodium cynomolgi, which is closely related to the human parasite P. vivax, were transmitted to man (Coatney et al., 1961) and it has been shown that man-to-man transmission of the parasite can occur under laboratory conditions. It is also known that the Central American monkey parasite, P. brasilianum, is transmissible to man (Contacos et al., 1963). These findings have led to efforts to determine the vectors of simian malaria in nature and to see whether there are ecological barriers Received for publication 8 October 1963. * Presented at the XVI International Congress of Zoology, Washington, D. C., on 21 August 1963 and sponsored by the American Society of Parasitologists and the British Society of Parasitology. t Present address: Division of Entomology, Commonwealth Scientific and Industrial Organization, Brisbane, Australia. which would prevent the transmission of monkey malaria to man under natural conditions in the field. These investigations have been centered mainly in Southeast Asia, particularly in Malaya, but it is pertinent at this stage to review briefly what is known about the vectors of simian malaria the world over. VECTORS OF SIMIAN MALARIA IN AFRICA Four members of the genus Plasmodium have been isolated from subhuman primates in Africa (Table I). Three species come from the great apes, Plasmodium reichenowi; a falciparum parasite of gorillas and chimpanzees; Plasmodium schwetzi, a vivax parasite of gorillas and chimpanzees; and Plasmodium malariae, a quartan parasite, similar to the human quartan parasite, of chimpanzees. The natural vectors of simian malaria are unknown in Africa, and information on the development of parasites under experimental conditions is scanty. Bray (1957) reported gut infections with Plasmodium reichenowi in experimentally fed Anopheles gambiae but mosquitoes dissected for gland infections were all negative. A similar observation was made by Rodhain and Lassman (1940) when they noted that A. atroparvus fed on chimpanzees infected with P. schwetzi developed gut infections, but no

The Chesson strain of Plasmodium vivax in Aotus monkeys and anopheline mosquitoes.

The Chesson strain of Plasmodium vivax was studied in Aotus trivirgatus monkeys. Parasitemia in intact and splenectomized animals was similar to that reported for this strain in man. Comparative infectivity studies with mosquitoes fed on infected monkeys indicated that the most susceptible was Anopheles freeborni, followed by An. balabacensis, An. culicifacies, An. maculatus, An. atroparvus, An. stephensi, An. quadrimaculatus, and An. albimanus. Transmissions via sporozoites from An. maculatus was demonstrated on two occasions; prepatent periods were 30 and 32 days.

Studies on the Santa Lucia (El Salvador) strain of Plasmodium falciparum in Aotus trivirgatus monkeys.

The Santa Lucia strain of Plasmodium falciparum was isolated from El Salvador, Central America, and established in Aotus trivirgatus monkeys. Transmission from monkey to monkey via the bites of infected Anopheles freeborni, A. maculatus, and A, albimanus mosquitoes was obtained in 20 of 27 attempts. Prepatent periods in the monkeys ranged from 17 to 46 days with a mean of 24.3 days. Parasitemias and mortality were higher following sporozoite inoculation into animals which had been previously infected with P. vivax than in those with no previous malaria experience. Monkeys previously infected with P. vivax and P. cynomolgi had lower maximum parasitemias than those previously infected with P. vivax only.

Infectivity of the Santa Lucia (El Salvador) strain of Plasmodium falciparum to different anophelines.

Anopheles freeborni mosquitoes were much more heavily infected with the Santa Lucia strain of Plasmodium falciparum from coastal El Salvador than were any of the other species tested. Of 5 strains of A. albimanus examined, the most heavily infected was the CA-109A and the least was the Melara, both of which come from coastal El Salvador. Of the exotic anophelines, the A. maculatus was infected at a slightly higher level than was the A. balabacensis. The incidence of highly infected individual mosquitoes was greatest in the Panama-Escobal strain of A. albimanus from the Republic of Panama; the incidence was lowest in the Melara strain from El Salvador. All strains of A. albimanus developed infected salivary glands, but the A. freeborni and A. maculatus mosquitoes appeared to develop infected glands more effeciently. Infection rates in A. freeborni mosquitoes were highest if mosquitoes were fed on Aotus trivirgatus monkeys between the 19th and 25th days of patent gametocytemia.

SUSCEPTIBILITY OF MACACA FASCICULARIS MONKEYS FROM MAURITIUS TO DIFFERENT SPECIES OF PLASMODIUM

Macaca fascicularis monkeys from Mauritius were shown to be susceptible via sporozoite inoculation to 7 species of Plasmodium (P. fragile, P. coatneyi, P. gonderi, P. inui, P. cynomolgi, P. knowlesi, and P. fieldi), indigenous to macaques in southeastern Asia. Four monkeys were sequentially infected with different species of Plasmodium to determine maximum and course of parasitemia. In 2 nonsplenectomized monkeys, P. fragile developed maximum parasite counts of only 134 and 155/microliters. For Plasmodium knowlesi, a parasite that is life-threatening to rhesus monkeys, maximum parasite counts were 4,278 and 7,440/microliters. Plasmodium coatneyi developed to what must be considered as moderate levels. After animals underwent splenectomy, parasite counts of P. coatneyi were 58,280, 89,094, 4,464, and 43,524/microliters. The maximum parasite counts for P. gonderi (13,508 and 21,576/microliters) and P. fieldi (1,767 and 17,836/microliters) were lower than would be expected in M. mulatta. In 2 monkeys that developed patent parasitemia with P. inui, the maximum parasite counts (95,046 and 728,748/microliters) indicated that this parasite may be the best adapted species for development in these animals once infection is established. Finally, the reinfection of 2 monkeys with P. cynomolgi suggested that some animals may be basically more resistant than others, whether splenectomized or not, to the production of high-density parasitemia.

Effect of sequential infection with Plasmodium vivax and P. falciparum in the Aotus trivirgatus monkey.

Aotus trivirgatus monkeys with prior experience with Plasmodium vivax were inoculated with P. falciparum via the bites of infected mosquitoes. The animals with prior malaria had higher parasitemias and significantly higher levels of mosquito infectivity than monkeys with no prior P. vivax experience. Monkeys with a history of P. falciparum that were inoculated with P. vivax had essentially the same parasitemias as those with no prior malaria. However, levels of mosquito infectivity were markedly increased in those monkeys with a history of P. falciparum. The results imply that the introduction of another malaria species into a malarious area may result in higher levels of mosquito infection and more rapid establishment and distribution of that species.

Anopheles balabacensis balabacensis Identified as Vector of Simian Malaria in Malaysia

The mosquito Anopheles balabacensis balabacensis has been identified as a natural vector of at least two species of simian malaria in the monsoon forests of the northern Malay States. This mosquito is also a serious vector of human malaria from Viet Nam to northern Malaya. This is the first report of a mosquito which transmits both human and simian malaria in nature.

OBSERVATIONS ON THE INFECTIVITY OF TWO STRAINS OF PLASMODIUM VIVAX FROM VIETNAMESE REFUGEES TO AOTUS MONKEYS AND ANOPHELINE MOSQUITOES

Two strains of Plasmodium vivax (NAM and ONG) were isolated from Vietnamese refugees and established in splenectomized Aotus monkeys from Colombia and Bolivia. Mosquito infections were readily obtained from animals with no prior malarial experience or with a history of infection with P. falciparum only. Those animals with previous infections with P. vivax supported only low parasitemias, and the mosquito infections were minimal. Complete development of the sporogonic cycle was obtained with all species of mosquitoes tested. The most susceptible mosquito was An. dirus. Other mosquitoes readily infected with these strains were An. culicifacies, An. maculatus, An. gambiae, An. stephensi, and the two North American species, An. freeborni and An. quadrimaculatus. Transmission from one monkey to another was obtained via the bites of infected An. dirus, An. stephensi, and An. maculatus mosquitoes.

Studies on human malaria in Aotus monkeys. VII. Comparative infectivity of two strains of Plasmodium vivax to Anopheles freeborni, A. maculatus, and four strains of A. albimanus.

A strain of Plasmodium vivax from El Salvador was shown to be more infectious to the Apastepeque strain of Anopheles albimanus from El Salvador than was a strain of P. vivax from Colombia, S. A. The Apastepeque mosquito strain was also more susceptible to the Savadoran strain of P. vivax than were 2 other strains of this mosquito from El Salvador. A strain of A. albimanus from Panama was more susceptible to both of the parasite than were any of the Salvadoran strains of A. albimanus.

Infectivity of two strains of Plasmodium vivax to Anopheles albitarsis mosquitoes from Colombia.

Anopheles albitarsis obtained from Villavicencio, Colombia, were colonized in the laboratory using force-mating techniques. Laboratory reared mosquitoes were allowed to feed on Aotus monkeys infected with the Salvador II or the Rio Meta strains of Plasmodium vivax from El Salvador and Colombia, respectively. In comparison with other species, the An. albitarsis were less susceptible than Anopheles freeborni, Anopheles culicifacies and strains of Anopheles albimanus from El Salvador, Panama and Colombia and more susceptible than a strain of An. albimanus from Haiti.