INTRODUCTION
Plasmodium simium is a parasite of black howler monkeys (Alouatta fusca) and wooly spider monkeys (Brachyteles arachnoides) in a small area of southeast Brazil. It has many biological features similar, if not identical to, Plasmodium vivax. Biological and genetic studies indicate that the parasite has very recently (during the past 500 years) evolved in and adapted to the large Atelid New World primates and could be considered a strain or even a subspecies of P. vivax that by molecular phylogenetics cannot be readily separated from strains of P. vivax.1 The circumsporozoite (CS) gene of P. simium has been shown to be identical to that of type I or VK210 of P. vivax.2 Similarly, the rRNA genes are conserved between the two species. Morphologically, the asexual and gametocyte stages are remarkably like that of P. vivax in the Saimiri monkey host. In mosquitoes, the sporogonic development requires several days longer for completion than do strains of P. vivax with which it has been compared.
The testing of vaccines against P. vivax has used S. boliviensis because of their susceptibility to sporozoite and trophozoite infection with Salvador I and certain other strains of P. vivax.3–10 However, the percentage infection with sporozoites via bite has been low, and it has been necessary to intravenously inject relatively large numbers of sporozoites harvested from the salivary glands of infected mosquitoes to induce predictable infection in these monkeys. Plasmodium simium, having undergone natural adaptation in New World monkeys, is therefore being studied as a second model for the testing of antisporozoite, transmission-blocking, and blood-stage vaccines against P. vivax. Sporozoite vaccines and vaccines against other stages of the parasite that are effective against P. simium in S. boliviensis could/should also be effective against P. vivax in the same model system and quite possibly in humans. As will be shown, monkeys challenged via mosquito bite are readily susceptible to infection with P. simium.
Deane and Okumura had reported that splenectomized Saimiri sciureus were susceptible to infection with P. simium.11 Subsequently, we reported the infection of Aotus lemurinus griseimembra, Saimiri sciureus, Aotus nancymai, Aotus vociferans, Aotus azarae boliviensis, and Saimiri boliviensis with this parasite.1,12–14 Five of the latter hosts were infected, four by intravenous injection of parasitized erythrocytes and one via the bites of an infected Anopheles stephensi mosquito. Two of the animals had been previously infected with P. vivax and two had been previously infected with P. falciparum.
Because of the very close, if not identical, genetic relationship between P. vivax and P. simium,15,16 it is argued that P. simium and S. boliviensis would be a suitable model for the testing of certain candidate vaccines directed against P. vivax. High density of the asexual parasite count, ready infection of mosquitoes, and ease of transmission by mosquitoes would support the use of P. simium in vaccine trials for P. vivax. In the studies reported here, splenectomized S. boliviensis were infected with P. simium to determine the effect of previous infection with P. vivax as well as other species of Plasmodium on prepatent period and the subsequent course of parasitemia after trophozoite and sporozoite challenge.
MATERIALS AND METHODS
Animals.
Saimiri boliviensis monkeys were laboratory born. Animals were fed a diet of animal chow, fruits, and vegetables considered suitable for their maintenance in captivity. Splenectomy was performed under sterile conditions by a qualified veterinarian using standard procedures.
Monkeys were infected by the intravenous inoculation of infected erythrocytes, either freshly collected from a donor animal or that had been stored frozen. For mosquito infection or for transmission by bite, mosquitoes were allowed to feed directly on a tranquilized animal. Alternatively, for sporozoite transmission, the salivary glands of infected mosquitoes were dissected and the sporozoites then injected intravenously into the monkeys. Beginning 1 day after injection of parasitized erythrocytes or 7 days after sporozoite inoculation, thick and thin blood films were prepared by the method of Earle and Perez,17 stained with Giemsa, and examined microscopically. Parasite counts were recorded per microliter of blood.
Parasites.
The strain of P. simium was provided by Dr. L. M. Deane in blood from a naturally infected howler monkey and was received in our laboratory in 1966. The strain has been maintained by cryopreservation and by passage in susceptible hosts.12 The strains of P. vivax to infect the Saimiri boliviensis monkeys prior to their use in this study were Chesson, Thai III, Thai IV, North Korean, Indonesian XIX, Mauritania I, PNG, and NMRU I from Asia and Africa, and Salvador I and Brazil I from Central and South America. These P. vivax strains have been maintained as cryopreserved infected erythrocytes and by passage in monkeys.
Mosquitoes.
In the DPD/CDC insectary, Anopheles stephensi (from India), Anopheles dirus (from Thailand), Anopheles freeborni (from California), and Anopheles gambiae (from The Gambia) were maintained continuously. The procedures for feeding, handling, and dissection of the mosquitoes have been reported previously.18 After dissection, the salivary glands of mosquitoes were examined and the density of sporozoites present graded 1+ (1–10 sporozoites), 2+ (11–100 sporozoites), 3+ (101–1,000 sporozoites), or 4+ (> 1,000 sporozoites). On other occasions, infected salivary glands were harvested in 20% fetal bovine serum/saline, the number of sporozoites present determined using a Neubauer Cell Counting Chamber, and the sporozoite suspension injected intravenously into the femoral vein of the recipient monkey.
RESULTS
Thirty-one monkeys were infected by the injection of parasitized erythrocytes; for 23 of these animals, the inoculum had been stored frozen (Table 1). Animals previously infected were treated with appropriate antimalarial drugs and determined to be free of parasites prior to reinfection. Prepatent periods ranged from 1 to 15 days. Maximum parasite counts ranged from 5,220 to 296,000/μL. The mean maximum parasite count was 77,580/μL. Three monkeys (SS-0173, SI-2144, and SI-0301) had never been infected, SI-0751 and SS-0451 had been previously infected with P. falciparum, and SS-0051 had been previously infected with Plasmodium malariae. The maximum parasite counts for these animals were 62,123, 62,460, 296,000, 212,000, 280,000, and 264,000/μL, respectively, with a mean maximum parasite count of 161,886/μL. All the other monkeys had been previously infected with P. vivax. In addition, some of these animals had also been infected with other species of Plasmodium.
Attempts were made to infect different species of mosquitoes by feeding on S. boliviensis monkeys infected with P. simium. Mosquitoes were fed during the ascending phase of the asexual parasitemia, which usually occurred during the first 7 to 10 days after trophozoite injection or after the first appearance of parasites in the peripheral blood after sporozoite inoculation. A total of 142 lots of mosquitoes were infected, the most frequently being An. freeborni (Table 2). After being held in incubators at 25°C for 12 or more days, mosquitoes were used for transmission studies.
Twenty-one monkeys were infected via sporozoites, 19 via mosquito bites and 2 by intravenous injection of sporozoites dissected from the salivary glands (Table 3). Prepatent periods ranged from 14 to 24 days with a median of 15 days, with maximum parasite counts ranging from 1,980 to 80,000/μL. The mean maximum parasite count was 29,234/μL. Only one monkey (SI-2214) had not been previously infected, and in this animal, the maximum parasite count was 13,050/μL. It was apparent that trophozoite-induced infections often reached maximum levels much higher than those obtained after sporozoite injection.
A comparison was made between the maximum parasite counts for those monkeys that had been previously infected with Old World strains of P. vivax (Asia and Western Pacific) versus those that had been infected with strains from the New World (Central and South America) (Figure 1). The mean maximum parasite count for 17 monkeys with previous Old World experience was 26,337/μL versus 56,362/μL for 22 monkeys with New World experience. There was a statistical difference by pooled t test (P = 0.0106), suggesting there might be a closer antigenic relationship between P. simium and Old World P. vivax than New World P. vivax strains.
DISCUSSION
The selection of potential new model systems rely on the availability of animals previously devoted to vaccine and drug trials where they have been exposed to infections with heterologous malaria parasites. Splenectomized monkeys were injected with trophozoites and sporozoites and examined for maximum parasitemia and prepatent periods as they became available for study. Forty-nine of the 52 animals infected with P. simium had been infected previously for other studies. Previous infection did not prevent the development of relatively high maximum parasite counts in many of these splenectomized animals. The potential usefulness of the model became evident.
First was the relatively high density of the asexual parasite count even in animals with previous P. vivax infection. The observation that parasite counts were reduced if this experience was to Old World strains of P. vivax was intriguing. It suggested that P. simium may have evolved from one of the Old World lines of P. vivax. Second, successful transmission via bites was much higher than had been obtained with P. vivax, and the prepatent periods were much shorter than previously observed.19,20 Thus, this model system could be expected to be more suitable for testing antisporozoite and liver-stage vaccines, particularly after challenge via mosquito bite. Third, mosquito infection by feeding on infected S. boliviensis monkeys was readily demonstrated. Thus, reliance on other primates such as chimpanzees, as has been the case with P. vivax, as a source of infected mosquitoes may not be necessary.
It is proposed that P. vivax antisporozoite, liver-stage, and even blood-stage vaccines could be tested and developed using the P. simium/S. boliviensus model system. Obviously, immunization and challenge studies are better conducted in intact rather than in splenectomized animals. Protocols for the testing antisporozoite and liver-stage vaccines against P. vivax have been developed in which the animals are immunized and challenged while intact and only splenectomized after the liver-stage schizonts are about to or have ruptured. Further passage in intact S. boliviensis monkeys will be needed to obtain predictable high-density parasite counts for the testing of blood-stage vaccines.
This study presents another alternative nonhuman primate model system for the testing of vaccines directed against P. vivax. The first system is the use of Aotus or Saimiri monkeys and various strains of P. vivax. The second is the use of P. cynomolgi and the rhesus monkey. However, although this parasite is the closest related simian species to P. vivax,21 vaccines directed against P. vivax likely could not be tested in this system because of the very significant level of diversity (~20%) between the two species. Here, we propose the use of P. simium and S. boliviensis monkeys. In this case, specific vaccines directed against P. vivax would be evaluated in the model system. The animals are relatively small (< 1 kg) and breed well in captivity.The strain of P. simium currently available, however, has not been shown to be infectious to humans.1
Inocula, previous malarial experience, and maximum parasite count for 31 splenectomized Saimiri boliviensis monkeys infected with parasitized erythrocytes of Plasmodium simium
| Previous malaria | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Monkey no. | Inoc. 106 | Frozen | Prepatent period | Pv* | Pv# | Pf | Pk | Pb | Pc | Pm | Maximum Parasitemia (parasites/μL) |
| Pv*, Old World Plasmodium vivax; Pv#, New World P. vivax; Pf, P. falciparum; Pk, P. knowlesi; Pb, P. brasilianum; Pc, P. cynomolgi; Pm, P. malariae. | |||||||||||
| SS-0173 | 140 | Yes | 9 days | 62,124 | |||||||
| SI-2144 | 12.0 | No | 1 day | 62,460 | |||||||
| SI-0301 | 0.01 | No | 1 day | 296,000 | |||||||
| SI-0751 | 6.3 | Yes | 2 days | X | 212,000 | ||||||
| SS-0451 | 0.7 | No | 2 days | X | 280,000 | ||||||
| SS-0051 | 100 | Yes | 9 days | X | 264,000 | ||||||
| SI-0153 | 12.0 | Yes | 1 day | X | 7,380 | ||||||
| SI-0112 | 12.0 | Yes | 2 days | X | 9,540 | ||||||
| SI-0151 | 24.0 | Yes | 2 days | X | 22,860 | ||||||
| SI-0152 | 12.0 | Yes | 5 days | X | 24,120 | ||||||
| SS-0073 | 70.0 | Yes | 9 days | X | 38,316 | ||||||
| SI-2181 | 2.1 | Yes | 2 days | X | X | 5,580 | |||||
| SI-2153 | 11.0 | Yes | 15 days | X | X | 14,040 | |||||
| SI-1709 | 18.0 | Yes | 5 days | X | X | 48,000 | |||||
| SI-0762 | 3.9 | No | 3 days | X | X | 79,200 | |||||
| SI-2157 | 18.0 | Yes | 4 days | X | X | 60,000 | |||||
| SI-2173 | 2.1 | Yes | 2 days | X | X | 140,000 | |||||
| SI-0095 | 9.5 | Yes | 4 days | X | X | 5,220 | |||||
| SI-2231 | 8.6 | No | 1 day | X | 39,000 | ||||||
| SI-2232 | 8.6 | No | 1 day | X | 45,000 | ||||||
| SI-2581 | 2.0 | No | 1 day | X | 47,880 | ||||||
| SI-2187 | 12.0 | Yes | 4 days | X | 84,000 | ||||||
| SI-2162 | 12.0 | No | 1 day | X | 85,680 | ||||||
| SS-0068 | 70.0 | Yes | 9 days | X | 96,000 | ||||||
| SI-2209 | 12.0 | Yes | 2 days | X | 136,000 | ||||||
| SI-0277 | 9.5 | Yes | 4 days | X | 156,000 | ||||||
| SI-2185 | 12.0 | Yes | 2 days | X | 192,000 | ||||||
| SI-2221 | 27.0 | Yes | 15 days | X | X | 172,000 | |||||
| SI-0707 | 6.3 | Yes | 1 day | X | X | 12,420 | |||||
| SI-0274 | 9.5 | Yes | 4 days | X | X | 25,820 | |||||
| SI-0730 | 6.3 | Yes | 1 day | X | X | X | X | 108,000 | |||
Infection of different species of mosquitoes with Plasmodium simium by feeding on infected Saimiri boliviensis monkeys
| Species of Anopheles | Positive lots | Positive mosquitoes/mosquitoes dissected | Mean percent infection | Oocysts/mosquito gut | Oocysts/+mosquito gut |
|---|---|---|---|---|---|
| freeborni | 71 | 614/1825 | 34.1 | 1.24 | 3.63 |
| stephensi | 27 | 160/620 | 25.4 | 0.74 | 2.91 |
| gambiae | 16 | 75/410 | 20.6 | 0.86 | 4.17 |
| quadrimaculatus | 19 | 118/515 | 23.4 | 0.78 | 3.34 |
| dirus | 7 | 26/122 | 25.5 | 0.56 | 2.20 |
| farauti | 2 | 3/54 | 5.6 | 0.12 | 2.00 |
Vector mosquitoes, prepatent periods, and maximum parasite counts for 21 splenectomized Saimiri boliviensis monkeys infected with sporozoites of Plasmodium simium
| Previous malaria | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Monkey no. | Mosquito species | Inoc.a | Prepatent period | Pv* | Pv# | Pf | Pfr | Pb | Pc | Pm | Pi | Maximum parasite count (parasites/μL) |
| Pv*, Old World Plasmodium vivax; Pv#, New World P. vivax; Pf, P. falciparum; Pfr, P. fragile; Pb, P. brasilianum; Pc, P. cynomolgi; Pm, P. malariae; Pi, P. inui. a Accumulated salivary gland ratings of mosquitoes fed; For SI-2126 and SI-1702, total number of sporozoites injected. | ||||||||||||
| SI-2214 | An. Gambiae | 72+ | 14 days | 13,050 | ||||||||
| SI-2216 | An. Stephensi | 111+ | 14 days | X | 80,000 | |||||||
| SI-0017 | An. gambiae | 28+ | 15 days | X | X | 18,000 | ||||||
| SI-1029 | An. stephensi | 15+ | 18 days | X | X | 26,640 | ||||||
| SI-2552 | An. freeborni | 20+ | 14 days | X | X | 27,000 | ||||||
| SI-2538 | An. freeborni | 28+ | 14 days | X | X | 34,560 | ||||||
| SI-0773 | An. gambiae | 12+ | 22 days | X | X | 36,540 | ||||||
| SI-1027 | An. stephensi | 18+ | 16 days | X | X | 80,000 | ||||||
| SI-2588 | An. freeborni | 88+ | 15 days | X | 1,980 | |||||||
| SI-0788 | An. stephensi | 104+ | 20 days | X | 51,040 | |||||||
| SI-2198 | An. dirus | 3+ | 24 days | X | X | 10,350 | ||||||
| SI-2230 | An. stephensi | 56+ | 16 days | X | X | 15,120 | ||||||
| SI-2222 | An. gambiae | 26+ | 10 days | X | X | 21,420 | ||||||
| SI-2202 | An. stephensi | 10+ | 19 days | X | X | X | 19,800 | |||||
| SI-1031 | An. gambiae | 11+ | 16 days | X | X | X | 18,810 | |||||
| SI-0269 | An. gambiae | 11+ | 15 days | X | X | X | X | 72,000 | ||||
| SI-0820 | An. gambiae | 8+ | 18 days | X | X | X | 80,000 | |||||
| SI-0140 | An. stephensi | 56+ | 15 days | X | X | 60,000 | ||||||
| SI-2101 | An. stephensi | 96+ | 21 days | X | X | 58,680 | ||||||
| SI-2126 | An. freeborni | 37,500 | 14 days | X | X | 84,000 | ||||||
| SI-1702 | An. freeborni | 21,500 | 14 days | X | X | X | 20,700 | |||||
Maximum parasite counts of Plasmodium simium in Saimiri boliviensis monkeys previously infected with Old World and New World strains of Plasmodium vivax.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 73, 3; 10.4269/ajtmh.2005.73.644
Maximum parasite counts of Plasmodium simium in Saimiri boliviensis monkeys previously infected with Old World and New World strains of Plasmodium vivax.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 73, 3; 10.4269/ajtmh.2005.73.644
Maximum parasite counts of Plasmodium simium in Saimiri boliviensis monkeys previously infected with Old World and New World strains of Plasmodium vivax.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 73, 3; 10.4269/ajtmh.2005.73.644
Address correspondence to William E. Collins, Centers for Disease Control and Prevention, Mail Stop F-36, 4770 Buford Highway, Chamblee, GA 30341. E-mail: wec1@cdc.gov
Authors’ addresses: William E. Collins, JoAnn S. Sullivan, Douglas Nace, and John W. Barnwell, Division of Parasitic Diseases, Centers for Disease Control and Prevention, Mail Stop F-36, 4770 Buford Highway, Chamblee, GA 30341. G. Gale Galland and Allison Williams, Scientific Resources Program, Centers for Disease Control and Prevention, Atlanta, GA 30333. Tyrone Williams, Atlanta Research and Education Foundation, Atlanta, GA 30033.
Financial support: This work was supported in part by an interagency agreement between the United States Agency for International Development, Malaria Vaccine Development Program, and The Centers for Disease Control and Prevention, project no. 936-6001, and the Atlanta Research and Education Foundation of the Veterans Affairs Medical Center of Atlanta.
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