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Am. J. Trop. Med. Hyg., 69(2), 2003, pp. 188-194
Copyright © 2003 by The American Society of Tropical Medicine and Hygiene

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EXPERIMENTAL INFECTION OF THE OLIVE BABOON (PAPIO ANUBIS) WITH PLASMODIUM KNOWLESI: SEVERE DISEASE ACCOMPANIED BY CEREBRAL INVOLVEMENT

HASTINGS OZWARA, JAN A. M. LANGERMANS, JENNEBY MAAMUN, IDLE O. FARAH, DORCAS S. YOLE, JASON M. MWENDA, HORST WEILER, AND ALAN W. THOMAS
Institute of Primate Research, National Museums of Kenya, Nairobi, Kenya; Departments of Parasitology and Animal Science, Biomedical Primate Research Center, Rijswijk, The Netherlands


ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Experimental systems that model some of the complex interactions between parasite and host can be extremely valuable in identifying and developing new prophylactics and therapeutics against human diseases. Because primates have similar immune systems to humans, we have characterized a baboon model for understanding host response to Plasmodium knowlesi. Ten intact olive baboons (Papio anubis) of either sex were experimentally infected with P. knowlesi H strain erythrocytic parasites. The infection in these baboons was either acute or chronic. Animals with acute infection developed multiple system organ dysfunction and cerebral involvement. In chronically infected animals, only the spleen was moderately enlarged. The P. knowlesi parasitemia profile in baboons and rhesus monkeys was comparable. However, some clinical symptoms of the baboons and P. falciparum-infected humans were similar. These studies demonstrate for the first time that P. anubis is a suitable host for P. knowlesi for studying clinical symptoms and pathology.


INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The evaluation of new therapeutics and prophylactics for use in humans often requires testing in animal models that develop a disease comparable to that in humans. In certain basic and applied studies, primates are the only animal models susceptible to the human disease under study.1,2 Similarities in biologic mechanisms between humans and nonhuman primates underlie the value of nonhuman primates as the final system for evaluating the safety and efficacy of drugs and vaccines developed in studies with other laboratory animals and systems.3–5 Nonhuman primates are widely used in malaria drug and vaccine development5–7

The two major human malaria parasites (Plasmodium falciparum and P. vivax) have a very restricted host range,8 which limits research on parasite biology especially at the host-parasite interface. However, experimental systems have been used to model some of the complex interactions between parasite and host. There are three Plasmodium groups that are mainly used in experimental studies on host-parasite interactions, i.e., rodent, avian, and primate. Rodent malaria parasites are used to study parasite biology.9 However, these parasites are phylogenetically distant from human Plasmodium10 and do not easily allow investigations of natural host-parasite interactions. Although P. gallinaceum and P. lophurae, the most widely used avian malaria parasites are closely related to P. falciparum,11 their development in nucleated cells and the wide phylogenetic distance between birds and humans limits their applicability to study important questions on host-parasite interaction relevant to human malarias. Simian Plasmodium such as P. knowlesi have a comparable phylogeny and host-parasite relationships to human malaria parasites.10,12 The parasites are used to identify, develop, and evaluate vaccine and drug candidates7,13,14 and to characterize host responses.15–17

Plasmodium knowlesi malaria infection has been described in Macaca fascicularis (natural host), as well as experimentally induced in a number of other nonhuman primates such as M. mulatta, M. radiata, M. assamensis, Presbytis entellus, Callithrix jacchus, Aotus trivigatus, Saimiri sciureus, and baboons.8,12,18–22 In baboons, the infection has been induced in Papio cynocepahalus, P. doguera, P. jubileaus, and P. papio.12 The parasite also infects humans12,23 and clusters phylogenetically with P. vivax.24 In most experimental models, P. knowlesi infection is acute, whereas in their natural host M. fascicularis, it generally induces a chronic infection.6,12 The availability of both natural and artificial hosts combined with the close relationship between primates and man make P. knowlesi infection in nonhuman primates attractive to study host-parasite interaction in detail.

We have recently developed protocols for long term in vitro culture and genetic modification of P. knowlesi.25,26 These are powerful tools for understanding parasite biology, especially gene function. This has been facilitated further by the recent sequencing of its genome (http://www.sanger.ac.uk/Projects/Protozoa/) and that of P. falciparum.27 The function(s) of attractive drug and vaccine candidates can be determined using homologous P. knowlesi genes. However, versatile in vivo systems are required to determine host-parasite interaction of the genetically modified parasites. The baboon is attractive because it is a well characterized and frequently used primate in biomedical research.4 At the moment, the patterns and mechanisms of P. knowlesi infection in baboons are relatively unknown.

In this study, we determined the disease profile and pathology of the P. knowlesi H strain infection in the olive baboon as an experimental host system for understanding parasite biology of P. knowlesi, especially host-parasite interaction.


MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Parasites. Plasmodium knowlesi H strain23 parasites for inducing baboon infection were retrieved from liquid nitrogen and cultured overnight. The original parasite inoculum was clone Pk1(A+), previously cloned by micromanipulation and passaged in rhesus monkeys.28

Animals. Adult Papio anubis (weight range = 11–21 kg) of either sex and originating from the Kajiado district of Kenya were used. Prior to the experiment, all animals were screened and determined to be free of infection with Plasmodium by a Giemsa-stained thick blood smear film. Ten baboons were inoculated intravenously with 1 x 104 to 1 x 106 P. knowlesi blood stage parasites. As controls for clinical symptoms, four uninfected animals were housed under similar conditions. All the animals were fed on a standard diet for baboons and water was provided ad libitum. The Institutional Animal Care and Use and the Scientific Review Committees of the Institute of Primate Research approved the baboon experiments. All experiments were performed in a biocontainment facility.

For comparative purposes, historical data were collected from six rhesus monkeys that had been infected with 1 x 105 P. knowlesi H strain parasites of the same stock. These experiments were done at the Biomedical Primate Research Center after approval by the Institutional Animal Care and Use Committee (the DEC).

Animal observation and sampling. Baboon health was monitored on a daily basis. Animals were assessed for 1) general agility, 2) playing habit, and 3) appetite (by weighing leftover food). Agility and playing habits of the monkeys were assessed by an attendant familiar with normal behavior of the particular animals. Animals were weighed on a weekly basis. Parasitemia was determined microscopically on finger prick thin blood smears stained with Giemsa and plotted as the number of infected cells in 1 x 104 erythrocytes.

Clinical chemistry and hematology. Venous EDTA blood was obtained from infected baboons after sedation with ketamine. Blood was analyzed in a Coulter counter (Beckman-Coulter, Mijdrecht, The Netherlands) to determine hematocrit and hemoglobin changes. Serum was collected before and during peak infection, and used to analyze blood creatinine, bilirubin and urea. Commercial kits for Creatinine (Biotech Laboratories, Ipswich, United Kingdom), total bilirubin (Biotech Laboratories), and urea (Randox Laboratories, Antrim, United Kingdom) were used according to manufacture’s instructions.

Pathology. After the duty veterinarian had certified baboons with severe malaria as lethargic or comatose, they were humanely killed and autopsies performed immediately. Two animals died unexpectedly and were immediately presented for necropsy. Animals with chronic infection were killed at four weeks post-infection. Gross organ and pathophysiologic derangements were observed during necropsy. Brain, liver, lung, kidney, spleen, and lymph node specimens were collected and immediately fixed in formalin. Sections of 5-µm thickness were prepared from paraffin-embedded specimens, stained in hematoxylin and eosin and analyzed for pathologic changes. Erythrocyte aggregation in the brain was quantified by examining 100 small blood vessels and expressed as percentages, as described by Pongponratn and others.29


RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Parasitemia. All baboons inoculated with P. knowlesi H strain developed patent parasitemia by day five post-inoculation (Figure 1AGo). Seven animals had acute parasitemia that systematically increased to greater than 500 parasites per 1 x 104 erythrocytes, reaching as high as 4,950 parasites per 1 x 104 erythrocytes at the time of killing. All baboons with acute infection had become lethargic by day 12 post-infection (Table 1Go). The remaining three animals developed chronic parasitemia (Table 1Go) with peak levels of less than 300 parasites per 1 x 104 erythrocytes by day 16, which thereafter decreased to less than 50 parasites per 1 x 104 erythrocytes (Figure 1AGo). At low parasitemia, i.e., less than 300 parasites per 1 x 104 erythrocytes schizont parasite stages were rarely observed in the peripheral circulation, suggesting sequestration. At higher parasitemia, as observed in animals with severe malaria, the number of schizont stages in the peripheral circulation increased slightly, indicating reduced sequestration. The inoculum size and in vivo passage of parasites used to inoculate all the baboons did not determine the parasitemia and disease profile (Table 1Go).



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  		    FIGURE 1. Parasitemia profile of A, baboons and B, rhesus monkeys infected with Plasmodium knowlesi H strain. RBC = red blood cells.

 

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TABLE 1
Parameters of baboons infected with the Plasmodium knowlesi H strain
 
Historical data on patterns of parasitemia were collected from rhesus monkeys previously infected with the same parasites as the baboons. All rhesus monkeys had developed patent parasitemia by day six post-inoculation (Figure 1BGo). The parasitemia profile in three of the monkeys (Rh2T, Rh 4086, and Rh9154) was characterized as chronic (Figure 1BGo), with peak parasitemia less than 700 parasites per 1 x 104 erythrocytes and thereafter decreasing (Figure 1BGo). The other three animals (Rh C029, Rh 3015, and Rh 3337) developed acute parasitemia, i.e., levels greater than 700 parasites per 1 x 104 erythrocytes (Figure 1BGo).

Clinical symptoms. Onset of patent parasitemia was followed by loss of appetite in all animals as measured by decreased food intake (Figure 2AGo) and marginal weight losses. Baboons with acute parasitemia developed severe clinical symptoms and were characterized as having severe malaria. These symptoms included a significant increase in body temperature as the infection progressed (Figure 2CGo) and remaining in a sitting position in the cage (apathy) with raised fur. In addition, they progressively became lethargic, developed dyspnea, and produced dark colored urine suggesting cholestasis. There was reduced ocular tension and skin turgor indicating dehydration. Hematocrit and hemoglobin levels showed moderate to low decreases (Figures 2BGo and 2DGo). Analysis of serum for bilirubin, creatinine, and urea to determine liver and kidney functions showed that in animals with severe malaria, there was a four-fold increase in total bilirubin (Table 2Go) and an increase in creatinine and blood urea, suggestive of severe hemolysis and dysfunctional kidneys (Table 2Go). By day 12 post-inoculation, all the baboons with severe malaria were either lethargic or comatose.



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  		    FIGURE 2. Changes in A, appetite, B, hemoglobin, C, body temperature, and D, hematocrit in Plasmodium knowlesi–infected olive baboons with severe ({square}---{square}) and less severe ({blacksquare}—{blacksquare}) clinical symptoms. before = before infection; after = during peak infection.

 

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TABLE 2
Analysis of liver and kidney functions in baboons experimentally infected with the Plasmodium knowlesi H strain*
 
Animals with chronic parasitemia were classified as having mild malaria; they showed moderate to low level of the clinical symptoms observed in severely infected animals (Figure 2Go). However, they had a significant decrease in hematocrit and hemoglobin at peak parasitemia (Figure 2Go). There was a marginal increase in total bilirubin, creatinine and blood urea (Table 2Go). After day 16 post-inoculation, the behavior of these monkeys was similar to the uninfected and healthy control group.

Gross pathology. At necropsy, baboons with severe malaria were remarkably similar in the quality of gross appearance, varying only in the degree of manifestation of pathologic changes. As a general feature, all tissues, particularly the mesentery, were of yellow-tan appearance. Intramural vascular deposition of malaria pigment was a widespread finding. These animals presented with severe acute hemolytic anemia, severe diffuse pre-hepatic jaundice, hydropericardium, hydro-peritoneum and hydrothorax.

Baboons with mild malaria had no comparable alterations indicating severe acute hemolytic crisis. However, mucous membranes were pale with yellowish tinge but not extended to the mesentery.

In all infected baboons, the spleen was highly friable, hemorrhagic, and pronouncedly enlarged with cut surfaces bulging out. Lungs from animals with severe malaria were distended with patchy consolidations and edematous. In animals with mild malaria, lungs had patchy consolidation and diffuse hyperemia. The liver of animals with severe malaria was enlarged, firm, and hyperemic. The lobes were distinct with rounded edges and the gall bladder was distended. Animals with mild malaria had a slightly enlarged liver with whitish streaks. Kidneys showed diffuse hyperemia and adherence of capsule in animals with severe malaria while kidneys of animals with mild malaria were without alterations. The brain of animals with severe malaria was edematous, congested on the external surfaces, and blood vessels were prominent. No pathologic changes were observed in the brain of animals with mild infection.

Histopathology. Histopathology showed that in animals with severe malaria, as expected from the high peripheral parasitemias, blood vessels of all diameters down to the capillaries of all tissues studied were interspersed with parasitized erythrocytes.

The brains of animals with severe malaria showed pronounced edema, multifocal neuronal degenerations, and mild gliosis (Figures 3A and CGo), in addition to widespread intra-vascular schizont-infected erythrocytes in microvessels (Figure 3CGo). More than 70% of brain microvessels of these animals were filled with aggregates of infected and non-infected erythrocytes which might represent sequestration (Table 3Go). Infiltrations of lymphocytes and phagocytic cells between endothelial cells of brain blood vessels, as observed in murine cerebral malaria30 were not encountered in baboons. The brain of baboons with mild malaria was normal (Figure 3BGo).



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  		    FIGURE 3. A, Overview of the brain of a Plasmodium knowlesi-infected olive baboon with severe malaria showing cerebral edema, micro-vacuolization around nuclei (black arrows), and vacuolization around blood vessels (white arrows) (magnification x 200). B, Overview of the brain of a P. knowlesi-infected olive baboon with mild malaria. Black arrows = micro-vacuolization around nuclei; white arrows = vacuolization around blood vessels (magnification x 200). C, Parasitized (black arrows) and non-parasitized erythrocytes in a blood microvessel from the brain of P. knowlesi-infected olive baboon with severe malaria (magnification x 600). (Hematoxylin and eosin stained.)

 

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TABLE 3
Erythrocyte aggregation in blood microvessels of Plasmodium knowlesi–infected baboon brain
 
In baboons with severe malaria, alveolar septa were increased in diameter due to multifocal to diffuse mononuclear cell infiltration, segmental neutrophil infiltration and interstitial edema (Figure 4AGo). Pigment laden desquamated alveolar macrophages were commonly observed. In addition, multifocal neutrophilic granulocytes were found within alveolar lumina. Animals with mild infection showed low congestion of the lung.



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  		    FIGURE 4. A, Lungs of a Plasmodium knowlesi-infected olive baboon with severe malaria showing increased cellular infiltration resulting in thickened width of alveolar septa. B, Liver of a P. knowlesi-infected baboon with severe malaria showing hydropic swelling of hepatocytes, widespread pigment deposition (dark spots) and edema of the space of Disse. C, Kidney of P. knowlesi-infected baboon with severe malaria showing multifocal interstitial nephritis with infiltration of mononuclear cells. (Hematoxylin and eosin stained, magnification x 400.)

 
The liver of animals with severe malaria showed cloudy to hydropic swelling of hepatocytes (Figure 4BGo), centrilobular dissociation, and necrosis of hepatocytes. Spaces of Disse were increased in diameter. Presence of hypertrophic pigment laden Kupffer cells and pigment laden sinus lining endothelial cells was a common feature in animals with severe malaria but less pronounced in animals mild malaria.

Kidneys from animals with severe malaria were characterized by multifocal interstitial nephritis with infiltration of mononuclear cells (Figure 4CGo). Glomeruli often showed segmental or diffuse increase in volume of mesangial matrix. Atrophic glomeruli were an occasional finding. No lesions were observed in animals with mild malaria.

In all baboons, spleens were characterized by the presence of reactive germinal centers with mantle zones and showed pronounced pigment depositions. Compared with animals with mild malaria, thickening of lienal capsules and trabeculae appeared more pronounced in animals with severe malaria.


DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In this report, the clinical spectrum and pathology of experimental P. knowlesi infection in P. anubis is presented for the first time. The disease profile was either severe or mild. Baboons with severe malaria developed multiple system organ dysfunction with cerebral involvement.

In baboons with severe P. knowlesi infection, the brain showed considerable pathology including congestion, edema, neuronal degeneration, prominence of blood vessels, mild gliosis and aggregation of infected and uninfected erythrocytes in cerebral microvessels. The presence of aggregated erythrocytes in the brain of baboons with severe malaria suggests blockade of cerebral capillaries, which is associated with cerebral malaria in humans.31,32 Cerebral malaria is a serious neurologic condition that can lead to coma and death. It is defined as an altered consciousness in a patient who has malaria parasites in the blood and in whom no other cause of altered consciousness can be found.33 Blockade of brain blood microvessels in P. falciparum-infected humans and P. coatneyi- or P. fragile-infected macaques is mediated through sequestration of knob-forming, mature, parasite-infected erythrocytes.31,32,34,35 Although knob-formation has not been defined in P. knowlesi,36,37 sequestration of P. knowlesi-infected erythrocytes might be mediated by schizont-infected cell agglutination variant antigens.38–40 Further studies to elucidate cerebral phenomena in the brain of P. knowlesi-infected baboons are required.

In the brain microvessels of baboons with severe malaria, many parasitized erythrocytes were present, but no lymphocytes and phagocytic cells were observed in contact with parasite-infected erythrocytes. This is similar to human cerebral malaria.29,31,32 In contrast, numerous phagocytic cells are encountered in brain microvessels of rhesus monkeys infected with P. knowlesi and in rodent cerebral malaria.30,36 Together, our data suggest that the cerebral involvement in P. knowlesi-infected baboons resembles several aspects of human cerebral malaria.

Papio anubis developed either severe malaria or controlled the parasitemia, resulting in a mild infection. This is also seen in rhesus monkeys.36,37,41–44 The mechanisms that predispose P. knowlesi-infected monkeys to developing either severe or mild infection are unknown. In general, P. knowlesi produces a chronic self-regulating infection in the natural host M. fascicularis. However, Schmidt and others45 showed that the course of P. knowlesi infection can differ in M. fascicularis from different geographic origins. In our study, animals originated from the same area, excluding monkey origin as a factor involved in the different infection outcomes. Our study also shows that inoculum size, age, and sex were not indicative of infection outcome. In humans, basal cytokine levels at the time of infection and host genetic factors are most likely involved in determining P. falciparum infection outcome.46,47 The precise mechanism that predisposes a dual outcome during malaria warrants further investigation and P. knowlesi-infected baboons might be helpful in this.

Our report shows that P. anubis is fully susceptible to experimental P. knowlesi H strain infection since all infected animals developed patent parasitemia. The parasitemia profile observed in the baboons was comparable to those in rhesus monkeys following infection with the same parasites,26 indicating that the virulence of this strain is similar in both monkeys, although studies were not done in parallel. In contrast to rhesus monkeys, P. knowlesi-infected baboons develop clinical symptoms at onset of patent parasitemia. Plasmodium knowlesi-infected rhesus monkeys frequently show minor clinical symptoms until they suddenly collapse due to massive parasitemia6,12,37,41–43 (Langermans JAM and others, unpublished data). One possible explanation is that rhesus monkeys are relatively resistant to endotoxin-like characteristics mediated by malaria parasites.6,48,49 Humans infected with P. falciparum also frequently develop clinical symptoms at onset of parasitemia.8,33,46

Papio anubis was successfully infected with an inoculum size of 1 x 104 P. knowlesi parasites. This suggests that the infection can be initiated by mosquito bite since a single hepatocyte infected with Plasmodium could contain 1 x 104 merozoites. Sporozoite induced infection is necessary if the olive baboon is to be used to study P. knowlesi liver stage analyses. Moreover, we observed that P. knowlesi continued to produce gametocytes after four passages in baboons. The viability of the gametocytes has not yet been characterized.

Infection of olive baboons with P. knowlesi provides an additional malaria model that allows for in vivo analysis of mechanisms of host response during severe and mild malaria. The use of baboons to study P. knowlesi will specifically find relevance in facilities that are not home to other hosts of P. knowlesi. These include baboon source countries and primate research facilities with access to baboons. Baboons can also be used to analyze host-parasite interaction of transfected P. knowlesi (Ozwara H and others, unpublished data), which is an important tool for converting genome sequence information to medical use. Overall, our findings show that P. anubis infected with P. knowlesi show various clinical characteristics that are also seen in human malaria including cerebral involvement.

Received August 22, 2002. Accepted for publication June 2, 2003.

Acknowledgments: We thank Fred Nyundo, Eric Omollo, and Aloys Makokha for technical assistance. We also thank Clemens Kocken, Annette Beetsma and Annemarie van der Wel for sharing their rhesus monkey data.

Financial support: This study was funded by the European Union contracts (CT96-0125) for establishing the Primate Vaccine Evaluation Network (PVEN), the reference program (CT97-9104), and CT-99-10004 for developing attenuated malaria vaccines. Hastings Ozwara was funded by the Netherlands Foundation for the advancement of Tropical Research (WOTRO).

Authors’ addresses: Hastings Ozwara, Jenneby Maamun, Idle O. Farah, Dorcas S. Yole, and Jason M. Mwenda, Institute of Primate Research, National Museums of Kenya, PO Box 24481, Karen, Nairobi, Kenya. Jan A. M. Langermans and Alan W. Thomas, Department of Parasitology, Biomedical Primate Research Centre, PO Box 3306, 2280 GH, Rijswijk, The Netherlands, Telephone: 31-15-284-2640, Fax: 31-15-284-3986, E-mail: thomas{at}bprc.nl. Horst Weiler, Department of Animal Science, Biomedical Primate Research Centre, PO Box 3306, 2280 GH, Rijswijk, The Netherlands.


REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Bontrop RE, 2001. Non-human primates: essential partners in biomedical research. Immunol Rev 183: 5–9.[ISI][Medline]
  2. Kennedy RC, Shearer MH, Hildebrand W, 1997. Nonhuman primate models to evaluate vaccine safety and immunogenicity. Vaccine 15: 903–908.[ISI][Medline]
  3. 1988. Role of non-human primates in malaria vaccine development: memorandum from a WHO meeting. Bull World Health Organ 66: 719–728.[ISI][Medline]
  4. King FA, Yarbrough CJ, Anderson DC, Gordon TP, Gould KG, 1988. Primates. Science 240: 1475–1482.[Abstract/Free Full Text]
  5. Stowers AW, Miller LH, 2001. Are trials in New World monkeys on the critical path for blood-stage malaria vaccine development? Trends Parasitol 17: 415–419.[ISI][Medline]
  6. Butcher GA, 1996. Models for malaria: nature knows best. Parasitol Today 12: 378–382.
  7. Wengelnik K, Vidal V, Ancelin ML, Cathiard AM, Morgat JL, Kocken CH, Calas M, Herrera S, Thomas AW, Vial HJ, 2002. A class of potent antimalarials and their specific accumulation in infected erythrocytes. Science 295: 1311–1314.[Abstract/Free Full Text]
  8. Garnham PCC, 1966. Malaria Parasites and Other Haemosporidia. Oxford: Blackwell Scientific Publications.
  9. Cox FEG, 1988. Malaria: Principles and Practice of Malariology. Edinburgh: Churchill Livingstone 1988, 1503–1543.
  10. Escalante AA, Freeland DE, Collins WE, Lal AA, 1998. The evolution of primate malaria parasites based on the gene encoding cytochrome b from the linear mitochondrial genome. Proc Natl Acad Sci USA 95: 8124–8129.[Abstract/Free Full Text]
  11. Escalante AA, Ayala FJ, 1994. Phylogeny of the malarial genus Plasmodium, derived from rRNA gene sequences. Proc Natl Acad Sci U S A 91: 11373–11377.[Abstract/Free Full Text]
  12. Coatney GR, Collins WE, Warren M, Contacos PG, 1971. The Primate Malarias. Bethesda, MD: U.S. Department of Health, Education and Welfare, National Institutes of Health.
  13. Deans JA, Thomas AW, Alderson T, Cohen S, 1984. Biosynthesis of a putative protective Plasmodium knowlesi merozoite antigen. Mol Biochem Parasitol 11: 189–204.[ISI][Medline]
  14. Kocken CH, Dubbeld MA, Van Der Wel A, Pronk JT, Waters AP, Langermans JA, Thomas AW, 1999. High-level expression of Plasmodium vivax apical membrane antigen 1 (AMA-1) in Pichia pastoris: strong immunogenicity in Macaca mulatta immunized with P. vivax AMA-1 and adjuvant SBAS2. Infect Immun 67: 43–49.[Abstract/Free Full Text]
  15. Deans JA, Knight AM, Jean WC, Waters AP, Cohen S, Mitchell GH, 1988. Vaccination trials in rhesus monkeys with a minor, invariant, Plasmodium knowlesi 66 kD merozoite antigen. Parasite Immunol 10: 535–552.[ISI][Medline]
  16. Rogers WO, Baird JK, Kumar A, Tine JA, Weiss W, Aguiar JC, Gowda K, Gwadz R, Kumar S, Gold M, Hoffman SL, 2001. Multistage multiantigen heterologous prime boost vaccine for Plasmodium knowlesi malaria provides partial protection in rhesus macaques. Infect Immun 69: 5565–5572.[Abstract/Free Full Text]
  17. Gwadz RW, Koontz LC, 1984. Plasmodium knowlesi: persistence of transmission blocking immunity in monkeys immunized with gamete antigens. Infect Immun 44: 137–140.[Abstract/Free Full Text]
  18. Collins WE, Contacos PG, Chin W, 1978. Infection of the squirrel monkey Saimiri sciureus, with Plasmodium knowlesi. Trans R Soc Trop Med Hyg 72: 662–663.[ISI][Medline]
  19. Dutta GP, Singh PP, Banyal HS, 1978. Macaca assamensis as a new host for experimental Plasmodium knowlesi infection. Indian J Med Res 68: 923–926.[ISI][Medline]
  20. Dutta GP, Singh PP, Saibaba P, 1981. Presbytis entellus as a new host for experimental Plasmodium knowlesi infection. Indian J Med Res 73 (Suppl): 63–66.
  21. Dutta GP, Banyal HS, Kamboj KK, 1982. Bonnet monkey (Macaca radiata) as a suitable host for chronic non-fatal Plasmodium knowlesi infection. Indian J Med Res 76: 134–140.[ISI][Medline]
  22. Langhorne J, Cohen S, 1979. Plasmodium knowlesi in the marmoset (Callithrix jacchus). Parasitology 78: 67–76.[Medline]
  23. Chin W, Contacos PG, Coatney GR, Kimball HR, 1965. A naturally acquired quotidian-type malaria in man transferable to monkey. Science 149: 865.[Abstract/Free Full Text]
  24. Escalante AA, Barrio E, Ayala FJ, 1995. Evolutionary origin of human and primate malarias: evidence from the circumsporozoite protein gene. Mol Biol Evol 12: 616–626.[Abstract]
  25. van der Wel AM, Tomas AM, Kocken CH, Malhotra P, Janse CJ, Waters AP, Thomas AW, 1997. Transfection of the primate malaria parasite Plasmodium knowlesi using entirely heterologous constructs. J Exp Med 185: 1499–1503.[Abstract/Free Full Text]
  26. Kocken CH, Ozwara H, van Der Wel A, Beetsma AL, Mwenda JM, Thomas AW, 2002. Plasmodium knowlesi provides a rapid in vitro and in vivo transfection system that enables double-crossover gene knockout studies. Infect Immun 70: 655–660.[Abstract/Free Full Text]
  27. Gardner MJ, Hall N, Fung E, White O, Berriman M, Hyman RW, Carlton JM, Pain A, Nelson KE, Bowman S, Paulsen IT, James K, Eisen JA, Rutherford K, Salzberg SL, Craig A, Kyes S, Chan MS, Nene V, Shallom SJ, Suh B, Peterson J, Angiuoli S, Pertea M, Allen J, Selengut J, Haft D, Mather MW, Vaidya AB, Martin DM, Fairlamb AH, Fraunholz MJ, Roos DS, Ralph SA, McFadden GI, Cummings LM, Subramanian GM, Mungall C, Venter JC, Carucci DJ, Hoffman SL, Newbold C, Davis RW, Fraser CM, Barrell B, 2002. Genome sequence of the human malaria parasite Plasmodium falciparum. Nature 419: 498–511.[Medline]
  28. Barnwell JW, Howard RJ, Coon HG, Miller LH, 1983. Splenic requirement for antigenic variation and expression of the variant antigen on the erythrocyte membrane in cloned Plasmodium knowlesi malaria. Infect Immun 40: 985–994.[Abstract/Free Full Text]
  29. Pongponratn E, Riganti M, Punpoowong B, Aikawa M, 1991. Microvascular sequestration of parasitized erythrocytes in human falciparum malaria: a pathological study. Am J Trop Med Hyg 44: 168–175.
  30. Hearn J, Rayment N, Landon DN, Katz DR, de Souza JB, 2000. Immunopathology of cerebral malaria: morphological evidence of parasite sequestration in murine brain microvasculature. Infect Immun 68: 5364–5376.[Abstract/Free Full Text]
  31. Aikawa M, 1988. Human cerebral malaria. Am J Trop Med Hyg 39: 3–10.
  32. Aikawa M, Iseki M, Barnwell JW, Taylor D, Oo MM, Howard RJ, 1990. The pathology of human cerebral malaria. Am J Trop Med Hyg 43: 30–37.
  33. World Health Organization, Division of Control of Tropical Diseases, 1990. Severe and complicated malaria. Trans R Soc Trop Med Hyg 84 (Suppl 2): 1–65.
  34. Aikawa M, Brown AE, Smith CD, Tegoshi T, Howard RJ, Hasler TH, Ito Y, Collins WE, Webster HK, 1992. Plasmodium coatneyi-infected rhesus monkeys: a primate model for human cerebral malaria. Mem Inst Oswaldo Cruz 87 (Suppl 3): 443–447.
  35. Kawai S, Aikawa M, Kano S, Suzuki M, 1993. A primate model for severe human malaria with cerebral involvement: Plasmodium coatneyi-infected Macaca fuscata. Am J Trop Med Hyg 48: 630–636.
  36. Mahdi AA, Ahmad S, 1991. Pathogenesis of cerebral malaria. Indian J Exp Biol 29: 267–271.[ISI][Medline]
  37. Ibiwoye MO, Howard CV, Sibbons P, Hasan M, van Velzen D, 1993. Cerebral malaria in the rhesus monkey (Macaca mulatta): observations on host pathology. J Comp Pathol 108: 303–310.[ISI][Medline]
  38. Barnwell JW, Howard RJ, Miller LH, 1982. Altered expression of Plasmodium knowlesi variant antigen on the erythrocyte membrane in splenectomized rhesus monkeys. J Immunol 128: 224–226.[Abstract]
  39. Miller LH, Fremount HN, Luse SA, 1971. Deep vascular schizogony of Plasmodium knowlesi in Macaca mulatta. Distribution in organs and ultrastructure of parasitized red cells. Am J Trop Med Hyg 20: 816–824.
  40. al-Khedery B, Barnwell JW, Galinski MR, 1999. Antigenic variation in malaria: a 3' genomic alteration associated with the expression of a P. knowlesi variant antigen. Mol Cell 3: 131–141.[ISI][Medline]
  41. Abildgaard C, Harrison J, DeNardo S, Spangler W, Gribble D, 1975. Simian Plasmodium knowlesi malaria: studies of coagulation and pathology. Am J Trop Med Hyg 24: 764–768.
  42. Tatke M, Malik GB, 1990. Pulmonary pathology in severe malaria infection in health and protein deprivation. J Trop Med Hyg 93: 377–382.[ISI][Medline]
  43. Spangler WL, Gribble D, Abildgaard C, Harrison J, 1978. Plasmodium knowlesi malaria in the rhesus monkey. Vet Pathol 15: 83–91.[Abstract]
  44. Rosen S, Hano JE, Barry KG, 1968. Malarial nephropathy in the rhesus monkey. Arch Pathol 85: 36–44.[ISI][Medline]
  45. Schmidt LH, Fradkin R, Harrison J, Rossan RN, 1977. Differences in the virulence of Plasmodium knowlesi for Macaca irus (fascicularis) of Philippine and Malayan origins. Am J Trop Med Hyg 26: 612–622.
  46. Day NP, Hien TT, Schollaardt T, Loc PP, Chuong LV, Chau TT, Mai NT, Phu NH, Sinh DX, White NJ, Ho M, 1999. The prognostic and pathophysiologic role of pro- and antiinflammatory cytokines in severe malaria. J Infect Dis 180: 1288–1297.[ISI][Medline]
  47. Hill AV, 1998. The immunogenetics of human infectious diseases. Annu Rev Immunol 16: 593–617.[ISI][Medline]
  48. Tubbs H, 1980. Endotoxin in human and murine malaria. Trans R Soc Trop Med Hyg 74: 121–123.[ISI][Medline]
  49. Jakobsen PH, Baek L, Jepsen S, 1988. Demonstration of soluble Plasmodium falciparum antigens reactive with Limulus amoebocyte lysate and polymyxin B. Parasite Immunol 10: 593–606.[ISI][Medline]




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