Tyrone Williams

EXPERIMENTAL INFECTION OF ANOPHELES FARAUTI WITH DIFFERENT SPECIES OF PLASMODIUM

Studies were conducted to determine the susceptibility of Anopheles farauti to different species and strains of Plasmodium. Mosquitoes were infected by feeding on animals or cultures infected with different strains of P. vivax, P. falciparum, P. ovale, P. coatneyi, P. gonderi, P. simiovale, P. knowlesi, and P. brasilianum. Infections of P. vivax and P. coatneyi were transmitted via sporozoites from An. farauti to monkeys. Comparative infection studies indicated that An. farauti was less susceptible to infection than An. stephensi, An. gambiae, An. freeborni, and An. dirus with the Salvador I strain of P. vivax, but more susceptible than An. stephensi and An. gambiae to infection with the coindigenous Indonesian XIX strain.

Protective Efficacy of a Plasmodium vivax Circumsporozoite Protein-Based Vaccine in Aotus nancymaae Is Associated with Antibodies to the Repeat Region

We have previously reported that Vivax Malaria Protein 001 (VMP001), a vaccine candidate based on the circumsporozoite protein of Plasmodium vivax, is immunogenic in mice and rhesus monkeys in the presence of various adjuvants. In the present study, we evaluated the immunogenicity and efficacy of VMP001 formulated with a TLR9 agonist in a water-in-oil emulsion. Following immunization, the vaccine efficacy was assessed by challenging Aotus nancymaae monkeys with P. vivax sporozoites. Monkeys from both the low- and high-dose vaccine groups generated strong humoral immune responses to the vaccine (peak median titers of 291,622), and its subunits (peak median titers to the N-term, central repeat and C-term regions of 22,188; 66,120 and 179,947, respectively). 66.7% of vaccinated monkeys demonstrated sterile protection following challenge. Protection was associated with antibodies directed against the central repeat region. The protected monkeys had a median anti-repeat titer of 97,841 compared to 14,822 in the non-protected monkeys. This is the first report demonstrating P. vivax CSP vaccine-induced protection of Aotus monkeys challenged with P. vivax sporozoites.

Additional Observations on the Sporozoite Transmission of Plasmodium knowlesi to Monkeys

Saimiri boliviensis monkeys were infected by the intravenous injection of 50 sporozoites of the H strain of Plasmodium knowlesi dissected from the salivary glands of Anopheles dirus mosquitoes; prepatent periods were 11, 12, 13, 13, 13, and 16 days. Sporozoites of P. knowlesi stored frozen for 7 days, 53 days, 20 mo, 7 yr and 7 mo, and 11 yr and 5 mo induced infections in Macaca mulatta monkeys with prepatent periods of 7, 6, 8, 10, and 7 days, respectively. After frozen storage for 11 yr and 5 mo, infections were induced in S. boliviensis with prepatent periods of 10–13 days.

The Development of Exoerythrocytic Stages of Plasmodium inui shortti in New World Monkeys

Attempts are being made to adapt Old World monkey malarial parasites to New World monkeys for vaccine and molecular studies. Several of these (Plasmodium cynomolgi Berok, Plasmodium fragile, and Plasmodium knowlesi) grow readily but have failed to produce infective gametocytes. Plasmodium gonderi and Plasmodium fieldi develop in the liver after sporozoite inoculation but have failed to establish infection in the erythrocyte. Anopheles dirus mosquitoes infected with Plasmodium inui shortti by feeding on infected macaques transmitted the infection to Saimiri boliviensis monkeys. Infective gametocytes were produced, and sporozoite transmission from Saimiri to Saimiri monkey was obtained. Exoerythrocytic stages have also been observed in the liver tissue of Saimiri monkeys. The availability of the complete transmission cycle provides an additional resource for immunologic and vaccine studies.

Transmission of Different Strains of Plasmodium cynomolgi to Aotus nancymaae Monkeys and Relapse

Abstract Forty-four splenectomized Aotus nancymaae monkeys were infected with 6 different strains of Plasmodium cynomolgi, 11 via trophozoites and 33 via sporozoites. Sporozoites from Anopheles dirus, Anopheles freeborni, Anopheles gambiae, Anopheles maculatus, and Anopheles stephensi resulted in prepatent periods ranging from 9 to 39 days (median of 15 days). Importantly, relapse was demonstrated in 5 of 5 sporozoite-induced infections with the Rossan strain following treatment with chloroquine.

ISOLATES OF PLASMODIUM INUI ADAPTED TO MACACA MULATTA MONKEYS AND LABORATORY-REARED ANOPHELINE MOSQUITOES FOR EXPERIMENTAL STUDY

Plasmodium inui is a parasite of macaques and other nonhuman primates in Asia that is studied as a model for the human malaria parasite P. malariae. Presented here are descriptions of the isolation, passage histories into Macaca mulatta monkeys, and infectivity to different Anopheles spp. mosquitoes of 18 different isolates of this parasite.

Observations on the Vietnam Palo Alto Strain of Plasmodium vivax in Two Species of Aotus Monkeys

Thirty-three splenectomized Aotus lemurinus griseimembra monkeys with no previous experience with malaria were infected with the Vietnam Palo Alto strain of Plasmodium vivax. The median maximum parasite count was 280,000/μl. Nine splenectomized monkeys with previous infection with Plasmodium falciparum had median maximum parasite counts of 120,000/μl. Splenectomized Aotus nancymai monkeys supported infections at a lower level. Transmission via the bites of Anopheles dirus mosquitoes was obtained in a splenectomized A. lemurinus griseimembra, with a prepatent period of 31 days. It is estimated that between 1.5 × 108 and 1.6 × 109 parasites can be removed from an infected animal for molecular or diagnostic antigenic studies.

STUDIES ON SPOROZOITE-INDUCED AND CHRONIC INFECTIONS WITH PLASMODIUM FRAGILE IN MACACA MULATTA AND NEW WORLD MONKEYS

Plasmodium fragile continues to be investigated because of its biologic similarities to the human malaria parasite, Plasmodium falciparum. Two strains of P. fragile are available for study; one strain is able to infect mosquitoes, whereas the other strain is transmissible only by blood inoculation. The Sri Lanka strain of P. fragile was transmitted to Macaca mulatta, Macaca fascicularis, Aotus lemurinus griseimembra, Aotus nancymaae, Aotus vociferans, and Saimiri boliviensis monkeys via sporozoites that developed to maturity only in Anopheles dirus mosquitoes. The prepatent periods ranged from 12 to 35 days for macaques and from 15 to 30 days for New World monkeys after intravenous injection of sporozoites. Eight rhesus monkeys were infected with the Nilgiri strain and followed for 482 days. Parasitemia in 6 animals persisted at relatively high density through the period of observation. Erythrocyte, hematocrit, and hemoglobin values reached their lowest levels 3 wk after infection and slowly recovered; however, the values did not approach preinfection levels as long as parasitemia persisted in the monkeys. The mean corpuscular volume and corpuscular hemoglobin concentration reached their peak and lowest values, respectively, at day 38 and then returned to the preinfection level. The mean corpuscular hemoglobin value decreased to its lowest level at day 87 and then returned to preinfection level.

STUDIES ON TWO STRAINS OF PLASMODIUM CYNOMOLGI IN NEW WORLD AND OLD WORLD MONKEYS AND MOSQUITOES

Infections that cause the Gombak and Smithsonian strains of Plasmodium cynomolgi were induced in Macaca mulatta, Aotus lemurinus griseimembra, Aotus nancymai, and Saimiri boliviensis monkeys. Transmission of the Gombak strain to Aotus spp. monkeys was obtained by the injection of sporozoites dissected from the salivary glands of experimentally infected Anopheles dirus and by the bites of infected An. dirus and Anopheles farauti mosquitoes. Two S. boliviensis monkeys were infected via the injection of sporozoites dissected from An. dirus. Prepatent periods in New World monkeys ranged from 14 to 44 days, with a median of 18 days. The Smithsonian strain was transmitted via sporozoites to 1 A. lemurinus griseimembra and 9 A. nancymai monkeys. Prepatent periods ranged from 12 to 31 days.

Mosquito Infection Studies with Aotus Monkeys and Humans Infected with the Chesson Strain of Plasmodiun vivax

Oocyst counts were compared between mosquitoes that fed on humans versus mosquitoes that fed on Aotus monkeys, both of which were infected with the Chesson strain of Plasmodium vivax. Oocyst counts obtained from mosquitoes fed on humans were almost 10-fold higher in number. Mosquitoes were more likely to be infected and with a higher rate of infection when they fed on monkeys before the peak in the asexual parasite count. Mosquitoes that fed on humans were more likely to be more heavily infected when fed after the peak in the asexual count. Of several species of owl monkeys, Aotus vociferans was infected at a higher frequency. On the basis of oocyst counts, Anopheles dirus were the most susceptible and An. maculatus were the least susceptible of the mosquito species tested.