Development of a polymorphic strain of Plasmodium vivax in monkeys.

A strain of Plasmodium vivax from Thailand with a polymorphic repeat unit of the circumsporozoite protein was established in Saimiri sciureus boliviensis and 3 species of Aotus monkeys. All 11 attempts to transmit infection via sporozoite inoculation, 4 times to splenectomized S. sciureus boliviensis, 2 times to splenectomized Aotus nancymai, and 5 times to intact Saimiri monkeys, were successful. Anopheles freeborni, Anopheles stephensi, Anopheles dirus, and Anopheles gambiae mosquitoes were infected by feeding on parasitemic blood from a chimpanzee and an Aotus azarae boliviensis monkey. Our results indicate that this strain may be useful in antisporozoite vaccine trials.

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.

The susceptibility of the Indonesian I/CDC strain of Plasmodium vivax to chloroquine.

A strain of Plasmodium vivax from Indonesia was adapted to splenectomized Aotus and Saimiri monkeys and tested for its susceptibility to chloroquine. Animals were infected by intravenous inoculation of heparinized parasitized blood and subsequently treated with 8 or 15 mg (base) of chloroquine by oral intubation. Recrudescence of infection occurred in 4 of 4 Aotus and 5 of 6 Saimiri monkeys treated with 15 mg base of chloroquine, indicating a level of resistance between that of the standard Chesson strain of P. vivax and the recently reported resistant strains from Papua New Guinea.

Reinforcement of immunity in Saimiri monkeys following immunization with irradiated sporozoites of Plasmodium vivax.

To determine the duration of immunity to Plasmodium vivax following immunization, six Saimiri sciureus boliviensis monkeys were vaccinated with irradiated sporozoites of P. vivax and challenged multiple times with sporozoites. Over a period of almost four years, complete protection from repeated challenge with infective sporozoites was demonstrated in one monkey; protection in two monkeys was obtained on eight of nine occasions, in one monkey on seven of nine occasions, in one monkey on six or nine occasions, and in one monkey on four of eight occasions. Five of six monkeys were protected against infection during the last six challenges. Inoculation with blood-stage parasites at the end of the trial indicated that all animals were susceptible to infection. These results suggest that protection against sporozoite challenge may be strongly reinforced by subsequent exposure to viable sporozoites.

Plasmodium cynomolgi: immunization of a rhesus monkey with exoerythrocytic stages cultured in autologous hepatocytes.

To investigate the immune response to exoerythrocytic stages of malaria parasites, a rhesus monkey was immunized with autologous primary hepatocyte cultures infected with 7-day-old liver stage parasites of Plasmodium cynomolgi. A primary antibody response against EE stage antigens was obtained, and boosted after injection of homologous viable sporozoites. Antibodies directed against sporozoites and blood stages were also detected. The polyvalent immune response observed demonstrates the antigenicity of the liver stages and suggests their involvement in the general immune response against malaria.

Immunization of owl monkeys with the ring-infected erythrocyte surface antigen of Plasmodium falciparum.

Aotus nancymai were immunized with the 4-mer, 8-mer, and 11-mer repeat peptides of the ring-infected erythrocyte surface antigen molecule of Plasmodium falciparum conjugated to diphtheria toxoid with muramyl dipeptide (MDP) as adjuvant. Immunization failed to induce protective immunity against the Uganda Palo Alto strain of P. falciparum as judged by maximum levels of parasitemia of immunized monkeys relative to those of controls. The fused polypeptide FPAg632, when combined with MDP, also failed to induce protective immunity. However, the maximum level of parasitemia and serologic response to the 11-mer peptide were inversely correlated. The safety of the use of MDP was evident.

The Peruvian III strain of Plasmodium brasilianum in Saimiri sciureus boliviensis monkeys.

A strain of Plasmodium brasilianum was isolated from a naturally infected Saimiri monkey from Peru and subsequently passaged to 21 splenectomized Saimiri sciureus boliviensis monkeys. Nine of 12 attempts to transmit infection by sporozoite inoculation were successful with prepatent periods ranging from 23 to 41 days. Gametocytes were infective to Anopheles freeborni, Anopheles stephensi, Anopheles dirus, Anopheles maculatus, and Anopheles gambiae mosquitoes. The strain demonstrated a high level of resistance to cure with chloroquine.

Transmission of Plasmodium fragile to Saimiri monkeys.

Saimiri monkeys from Bolivia and Guyana were infected with the Nilgiri and Ceylon strains of Plasmodium fragile. Of 20 attempted sporozoite transmissions of the Ceylon strain involving 11 splenectomized Saimiri sciureus boliviensis, only 8 were successful, 2 by mosquito bite and 6 by intravenous injection of sporozoites dissected from salivary glands. Prepatent periods ranged from 18 to 30 days with a mean of 25.8 days.

Infection of chimpanzees with the Uganda I/CDC strain of Plasmodium malariae.

Nine splenectomized chimpanzees were infected with the Uganda I/CDC strain of Plasmodium malariae. Two had no history of previous malarial infection, whereas 6 had been infected with P. vivax and 1 with P. vivax and P. ovale. The animals with no previous infection had maximum parasitemias of 8,740 and 10,800/mm3. The other animals had maximum parasite counts of 930-75,700/mm3. Anopheles freeborni, An. stephensi, An. dirus, An. maculatus, An. quadrimaculatus, An. culicifacies, An. arabiensis, and An. gambiae were readily infected by feeding through membranes on heparinized blood from these animals.

The Uganda I/CDC strain of Plasmodium malariae in Aotus lemurinus griseimembra monkeys.

Two lines of the Uganda I/CDC strain of Plasmodium malariae were studied in splenectomized Aotus lemurinus griseimembra monkeys. A line initially adapted to these monkeys from an infected chimpanzee failed to produce high-level parasite counts or mosquito infection in 13 of this type of monkey during 16 linear passages. Another line, originally adapted from the chimpanzee to Aotus azarae boliviensis, after 7 linear passages in 3 different types of Aotus was then passaged to 14 splenectomized A. lemurinus griseimembra. Geometric mean maximum parasitemia in these monkeys was 18,400/mm3. Mosquito infections were readily obtained during the period just after the parasite count rose above 1,000/mm3. Anopheles freeborni, An. stephensi, An. dirus, and 2 strains of An. gambiae supported the development of the parasite to the presence of sporozoites in the salivary glands. Two attempts to transmit the strain to other splenectomized A. lemurinus griseimembra by sporozoite inoculation were unsuccessful.

Sporozoite-induced infections of the Salvador I strain of Plasmodium vivax in Saimiri sciureus boliviensis monkeys.

Twenty Saimiri sciureus boliviensis monkeys from Bolivia were inoculated intravenously with sporozoites of the Salvador I strain of Plasmodium vivax. All animals were splenectomized 7 days after inoculation. Inoculation of 100,000 sporozoites resulted in prepatent periods averaging 16.6 days; all monkeys developed high-level parasitemias with an average maximum of 103,000 per mm3. Inoculation of 10,000 sporozoites resulted in average prepatent periods of 19.4 days; one of the resulting infections produced a transient low-level parasitemia. Of 5 monkeys inoculated with 1,000 sporozoites, 4 had prepatent periods of from 24 to 35 days and 1 failed to demonstrate any parasitemia; 1 monkey supported a low-level transient parasitemia, whereas the other 3 monkeys had high-level parasitemias. It is proposed that by using a minimum inoculum of 10,000 sporozoites, the model system may be useful in the testing of anti-sporozoite vaccines directed against P. vivax.

Infection of Peruvian Aotus nancymai monkeys with different strains of Plasmodium falciparum, P. vivax, and P. malariae.

Aotus nancymai (karyotype I) monkeys from Peru were studied for their susceptibility to infection with Plasmodium falciparum, P. vivax, and P. malariae. Three strains of P. falciparum (Santa Lucia from El Salvador, Indochina I/CDC from Thailand, and Uganda Palo Alto) were inoculated into 38 monkeys. The results indicated that this species of Aotus monkey is highly susceptible to infection. The Uganda Palo Alto and the Santa Lucia strain parasites appear to be the most useful for immunologic and chemotherapeutic studies. Five strains of P. vivax (Chesson, ONG, Vietnam Palo Alto, Salvador I, and Honduran I/CDC) were inoculated into 28 monkeys. The Vietnam Palo Alto strain produced the highest level parasitemias ranging from 23,800 to 157,000/mm3. Mosquito infections were obtained with the ONG, Chesson, and Salvador I strains. Two out of 6 attempts to transmit P. vivax via sporozoite inoculation to splenectomized monkeys were successful with prepatent periods of 39 and 57 days. Five monkeys were infected with the Uganda I/CDC strain of P. malariae. Maximum parasitemias ranged from 10 to 5,390/mm3.

Transmission of the OS strain of Plasmodium inui to Saimiri sciureus boliviensis and Aotus azarae boliviensis monkeys by Anopheles dirus mosquitoes.

Eight Saimiri and 7 Aotus monkeys were exposed to infection with the OS strain of Plasmodium inui via the bites of from 2 to 7 Anopheles dirus mosquitoes. All Saimiri monkeys developed high-level infections of from 152,000 to 500,000/mm3 after prepatent periods of from 14 to 17 days. Only 1 Aotus monkey developed a patent infection after a period of 28 days. Feeding on these animals failed to result in infection of An. dirus mosquitoes.

Infection of chimpanzees with Nigerian I/CDC strain of Plasmodium ovale.

Seven splenectomized chimpanzees were infected with the Nigerian I/CDC strain of Plasmodium ovale. Two of the animals had no history of previous malarial infection whereas three had been infected with P. vivax, one with P. malariae, and one with P. vivax and P. malariae. The two animals with no previous malarial experience had maximum parasitemias of 88,700 and 127,000 per mm3 while the other animals had maximum parasitemias ranging from 10,100 to 60,600 per mm3. Anopheles freeborni, An. dirus, An. stephensi, and An. gambiae were readily infected via membrane feeding on heparinized blood obtained from these chimpanzees during the ascending phases of their primary attacks. The parasitemias in the chimpanzees with previous malarial experience were transient.

Chesson strain Plasmodium vivax in Saimiri sciureus boliviensis monkeys.

Nine Saimiri sciureus boliviensis monkeys were inoculated with sporozoites of Plasmodium vivax (Chesson strain) dissected from Anopheles stephensi mosquitoes infected by feeding on blood from infected chimpanzees. The animals were splenectomized 7 days after inoculation. Seven animals developed infections with prepatent periods ranging from 12 to 43 days (mean of 19.6 days). Parasitemias were low during the first 50 days. Maximum parasitemias in 5 animals in which the strain adapted ranged from 10,000 to 46,800 per mm3. Anopheles freeborni mosquitoes were infected by feeding on 4 of the monkeys.