Species Of Malarial Parasites Research Articles

Interspecific interactions tested: two species of malarial parasite in a West African lizard.

Plasmodium giganteum and P. agamae, parasites of the rainbow lizard, Agama agama, in West Africa were studied to determine the nature of any interspecific interactions between the two malaria species. The plasmodia are distributed in A. agama throughout the mesic zone of Africa; P. agamae is sometimes found as a solitary malaria species in populations of the lizard, but P. gigateum has not been found alone. In 3170 lizards from Sierra Leone the prevalence of lizard malaria at 22 sites varied considerably (8-90% of lizards were infected), but the ratio of the two species was similar among sites (52-91% P. agamae). Larger lizards were more often infected. Mixed infections occurred 2-5 times more often than expected by chance. Parasite density within individual hosts, or parasitemia, was similar for each species when alone or in mixed infection. Natural infections followed in laboratory lizards stayed at constant levels for as long as 211 days. The two species use different classes of host cells (P. giganteum in immature cells and P. agamae in mature erythrocytes) and may have different periods of peak transmission. Analysis of the data does not support a neutral relationship between P. giganteum and P. agamae, nor ongoing competition for resources or heterologous immunity. The data best support facilitation in which P. agamae alters the host in a way that allows more successful establishment of P. giganteum.

Sequences similar to genes for two mitochondrial proteins and portions of ribosomal RNA in tandemly arrayed 6-kilobase-pair DNA of a malarial parasite

Erythrocytic stages of mammalian malarial parasites contain acristate mitochondria whose functions are not well understood. Moreover, little is known about the genome of these organelles. We have previously reported that all species of malarial parasites examined contain highly conserved, tandemly arrayed DNA with a unit length of about 6.0 kb that is transcribed into discrete RNA molecules in erythrocytic stages. We now report the complete DNA sequence of the 5984-bp repeating unit of Plasmodium yoelii, a rodent parasite. Two slightly overlapping regions transcribed into large RNA molecules were found to have significant DNA and protein sequence similarity with mitochondrion-coded proteins, cytochrome c oxidase subunit I and cytochrome b. Significant sequence similarity with other mitochondrial protein genes could not be detected. Ribosomal RNA (rRNA)-like genes were not detected in this sequence either. However, two regions, 82 and 50 nucleotides long, specified by different strands, were found to have extensive similarity with the highly conserved central loop of the peptidyl transferase domain of the large rRNA of Escherichia coli, mitochondria, and chloroplasts. Compensatory nucleotide substitutions were present in these regions, so that the predicted secondary structure was not affected. Functional utilization of these regions, if it exists, could argue for a trans-associative origin of rRNA. In organization, size and sequence, the tandem arrays of 6.0 kb malarial DNA appear to be a very unusual form of mitochondrial DNA.

The sex ratio of Plasmodium gametocytes.

Sex ratio theory usually predicts an equilibrium sex ratio and equal proportions of males and females in a population, including the progenitors of the reproductive cells of protozoans. This proposal was tested with three species of malarial parasites of lizards, Plasmodium mexicanum of the western fence lizard, and P. agamae and P. giganteum of the African rainbow lizard, using single samples from naturally infected lizards, repeated samples from free-ranging lizards (P. mexicanum only), and repeated samples from laboratory maintained animals. Macrogametocytes were usually more abundant than microgametocytes, and were slightly larger, revealing a typically greater investment of resources by the progenitors of female reproductive cells. However, the proportion of microgametocytes varied among the three species and among infections within each species of Plasmodium. The sex ratio of gametocytes often remained constant within infections followed over time even if the absolute number of gametocytes was changing. However, the equilibrium sex ratio of gametocytes varied among those infections that had an unchanging microgametocyte proportion. Thus, although an equilibrium sex ratio apparently occurs for most infections, there appears to be no characteristic proportion of microgametocytes for any of the species. Potential explanations for this conflict with theory are presented.

Mechanisms of immunity to malaria and the possibilities of a blood-stage vaccine: a critical appraisal.

Resistance developed by the immune system in response to blood-stage malaria is complex in nature, involving humoral and non-antibody effector mechanisms. Different species of malarial parasites may vary in their ability to elicit, or their susceptibility to, those immune effectors. This complexity is enhanced by the different results obtained in vaccinated as opposed to drug-controlled infections. It is therefore important that some attempt be made to unravel these interactions. This is particularly so when we have to decide on methods for assessing the potentiality of antigens to induce protective immunity. In this review the limitations of some in vitro assays of immunity, as well as those of various host-parasite models, are discussed. The relative importance of cell-mediated and humoral immunity in laboratory models and natural infections is also considered in the context of vaccine development.

DNA hybridization assays for detection of malarial sporozoites in mosquitoes.

An Important aspect of malaria epldemlology Involves accurate tnct-IrnInation of mosquito vectors, and determination of the proportIon of Infective mosquitoes (known as the sporozolte rate). The traditional way to calculate the sporozolte rate requires dlssectlng the sallvary glands of lndlvldual anophellnes, and examining them mlcroscoplcally for sporozoltes. In many endemic areas the sporozolte rate IS very low (less than 0. I %), so large numbers of fresh mosquitoes must be dissected. Moreover, even by painstaking microscopy, It IS vlrtually lmposslble to Identify the species of malarial parasite In the mosquito. In the past few years alternative assays to detect sporozoltes have been developed, using monoclonal antibodies against the circumsporozolte protein (CSP)‘. The advantage of these tests IS their capacity for mass screening which can be performed on dried or frozen mosquitoes. Moreover the CSP antibodies are species specific, and are now available for several species of malaria. Detection of a specific genetic sequence by the DNA hybrldlzatlon assay IS another method for very sensetlve detectjon of mlcroorganlsms (Boxes I ,2). The sensitlvlty of the test depends on the copy number of the sequence to be detected, and on the speclflc activity of the probe. Recently, methods of in vitro amplification of DNA sequences have been developed, thus greatly enhancing the detection of low copy number sequences. Of the human malarias, only in P. fa/ciporum has a sequence been found sufficiently repetitive and specific to be diagnostically useful. However, it IS very likely that other species have similar repeats In their genomes, and that group-specific sequences within the plasmodiae will be revealed. The diagnostic probe for P. folciporum has recently been identified24 and evaluateds. It contains a sequence of 2 I base pairs that is repeated several thousand times tn the malaria genome3’6. This probe IS sufficiently sensitive to detect around IO pg of P. fo/ciporum DNA roughly equivalent to 500 parasites. At this level, testing mosquitoes for sporozoites is no different in principle to testing a patient’s blood for erythrocytlc stages. Using this probe, the same sensttivity and specificity is M. Holmbergand H. Wigzell

Tandemly arranged gene clusters of malarial parasites that are highly conserved and transcribed

A molecular clone containing a 5.8 kb Eco RI fragment was isolated from a genomic library of the rodent malarial parasite Plasmodium yoelii. The P. yoelii genome contains about 150 copies of this sequence, making up almost 3% of the DNA. These sequences are tandemly arrayed in head-to-tail configurations with the unit length of the repeat being 5.8 kb. Several poly(A +) RNAs of P. yoelii ranging from 1.6 to 0.3 kb are recognized by the 5.8 kb clone. Five additional species of malarial parasites ( P. chabaudi, P. berghei, P. falciparum, P. knowlesi, and P. cynomolgi) contain tandemly repeated arrays of sequences having the same unit length of 5.8 kb, which readily hybridize to the sequence cloned from P. yoelii.

Ultrastructure of Human Erythrocytes Infected with Plasmodium Ovale

Ultrastructural changes of the human erythrocytes infected with Plasmodium ovale were studied, and differences of P. ovale from other human malarial parasites were discussed. Four characteristic morphological alterations were observed on the host cells: caveola-vesicle complex, excrescence, nodule, and cytoplasmic cleft. Caveola-vesicle complexes consisted of caveolae surrounded by vesicles in an alveolar fashion and were formed along the host cell plasmalemma. Similar complexes have been reported previously in P. vivax but not in P. falciparum and P. malariae. This complex probably corresponds to a Schüffner's dot. Excrescences similar to those observed by others on the host cell membrane in P. falciparum and P. malariae were observed also in P. ovale. The excrescences in P. ovale, as in P. falciparum, were limited to the erythrocytes infected by asexual parasites, whereas the excrescences in P. malariae have been demonstrated on erythrocytes with asexual parasites and gametocytes. Nodules were observed on the erythrocytes infected with asexual parasites of P. ovale. Structures such as the nodule have not been described previously in the erythrocytes infected with any other species of malarial parasite. Clefts within the cytoplasm of the host erythrocytes were present in P. ovale, as in all other malarial parasites.

Electron Microscopy of Knobs in Plasmodium falciparum-Infected Erythrocytes

Erythrocytes infected by certain species of malarial parasites show two types of alterations of erythrocyte membranes such as electron-dense protrusions called knobs and caveola-vesicle complexes (Aikawa et al., 1975, Am. J. Path. 79: 285; Aikawa, 1977, Bull. WHO 55:2). The knobs are found on the membrane of erythrocytes infected with falciparum-type malaria parasites. These knobs form focal junctions with the endothelial cell membranes or with the knobs of other erythrocytes, resulting in the sequestration of these infected erythrocytes along the vascular endothelium (Trager et al., 1966, Bull. WHO 35: 883; Luse and Miller, 1971, Am. J. Trop. Med. Hyg. 20: 655; Aikawa et al., 1972, Z. Zellforsch. 124: 72; Udeinya et al., 1981, Science 213: 555). The erythrocyte membrane covering the knobs is immunologically different from the rest of the erythrocyte membrane (Kilejian et al., 1977, Exp. Parasitol. 42: 157; Langreth et al., 1979, J. Exp. Med. 150:1241; Chulay et al., 1981, Am. J. Trop. Med. Hyg. 30: 12). The erythrocytes infected with small trophozoites show few knobs, but they increase in number as the parasite grows within the erythrocyte. Because the morphology of these knobs is still not fully understood, we have attempted to characterize them by transmission and scanning electron microscopy together with freeze-fracture and etch techniques. Plasmodiumfalciparum (Camp strain) in Aotus erythrocytes and P. falciparum (Thailand and Liberian strains) in human erythrocytes were obtained from culture (Udeinya et al., 1981, Science 213: 555). The infected erythrocytes were fixed in 2% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.4) containing 4% sucrose, washed in 0.1 M cacodylate buffer and postfixed for 1 hr in 1% osmium tetroxide. After fixation, the samples were centrifuged and then processed for thin section transmission and scanning electron microscopy. Freeze-fracturing and etching were performed on P. falciparum in human RBC's. Fracturing was carried out at -120 C under a vacuum of 2 X 10-7 Torr. Etching was performed at -100 C, under a vacuum of 2 X 10-7 Torr for 2 min. The surfaces obtained were replicated with platinum and carbon. Replicas were cleaned as described previously (Aikawa et al., 1981, J. Cell Biol. 91: 55). Transmission electron microscopy was performed with a JEOL 100CX electron microscope. Scanning electron microscopy was performed with a JEOL 100CX electron microscope with a scanning unit at 20 kV. Thin-section electron microscopy showed many knobs on the erythrocytes infected with P. falciparum (Fig. 1). Each knob is a cone-shaped, electron-dense structure measuring 30 to 40 nm in height and 9 to 100 nm in width and is covered by the erythrocyte membrane. The base is not sharply demarcated, but gradually merges into the erythrocyte cytoplasm. Scanning electron microscopy showed numerous cone-shaped knobs evenly distributed over the entire erythrocyte surface (Fig. 2). The distribution pattern of the knobs was more easily detectable by scanning electron microscopy than by thin-section transmission electron microscopy. Therefore, scanning electron microscopy appears to be a useful and easy method for the study of knob distribution. Freeze-fracture and etching demonstrated that the knobs were protruded structures and were evenly distributed over the erythrocyte membrane. The P face of the infected, human RBC membrane showed evenly distributed intramembrane particles (IMP) and no aggregation or depletion of IMP could be observed over the erythrocyte membrane covering the knobs (Fig.

Properties of DNA from some malarial parasites.

The base compositions of DNA from some species of malarial parasite have been estimated by measurement of density in CsCl gradients and of thermal denaturation temperatures. Species from avian (Plasmodium gallinaceum) and rodent (P. bergheiandP. vinckei) hosts – often used as model systems in malarial studies – contain single DNA components with base compositions of about 20% G + C. By contrast, species from primate hosts (P. knowlesiandP. falciparum) contain major DNA components of about 40% G + C and minor ones of about 20% G + C. These differences could be reflexions of markedly different genomes within these two groups of the genusPlasmodium.Apart from differences in base composition, the physico-chemical properties of the main components of DNA from primate and rodent malarial parasites are very similar to those of DNA from mammalian cells. DNA from avian malarial parasites appears to have some unusual properties.One of us (P.I.T.) received financial assistance from the World Health Organization. We thank Dr F. Hawking and Dr J. Williamson for many helpful discussions, Dr W. H. G. Richards, Dr A. Voller and Dr D. C. Warhurst for the provision of some of the strains and Miss Jane Dunnett, Mrs A. C. Gutteridge and Mr T. Scott-Finnigan for technical assistance.

Plasmodium vinckei: Production of chloroquine-resistant strain

To date only two species of malarial parasites, Plasmodium falciparum in man and P. berghei in mice, have demonstrated the ability to develop a high degree of resistance to chloroquine. A strain of P. vinckei which is resistant to the maximum tolerated dose (MTD) of chloroquine in mice has been selected from a pyrimethamine-resistant parent strain. Groups of five or more mice were treated with varying doses of drug for 3–4 days starting the day after parasite inoculation. Subpassages were made from mice treated with the highest dose of drug that allowed for some development of parasites. Resistance developed gradually during 37 successive chloroquine-treated passages over a period of about 44 weeks. In subsequent passages, the parasite was resistant to the MTD of chloroquine for mice (200 mg/kg/day). The dose of chloroquine which suppressed the parent strain by 99.9% on Day 7 was 5 mg/kg/day for 4 days. Sixty percent of mice treated with 10 mg/kg/day of chloroquine and all mice treated with doses of 20 mg/kg/day and greater were cured. Associated with resistance to chloroquine were the parasite's failure to produce detectable pigment in treated mice and the appearance of multiple vacuoles. The resistant strain in treated mice is characterized by slower developing, less fatal parasitemias while in untreated mice fulminating infections often result in death. This new species of chloroquine-resistant malaria may be of value as a laboratory model for the study of acquired drug resistance.

The Role of the Pineal in Mediating Photoperiodic Control of Growth and Division Synchrony and Capillary Sequestration of Plasmodium berghei in Mice

Control over growth and division synchrony as well as vascular capture and release of late growth forms is shown to be mediated by the pineal body. Vascular trapping of these late growth forms seems to be most intense in the liver, spleen, and bone marrow. Since we observed these older parasites adhering to the vascular wall, vascular trapping is probably by a mechanism other than phagocytosis. Certain types of photoperiodic rhythm have been shown to influence the growth of Plasmodium berghei, a malarial parasite of rodents. Two effects of the photoperiodic rhythm have so far been observed. The first is the regulation of growth and division synchrony, and the second appears to be the tendency of segmenting parasites to sequester or to be sequestered in certain parts of the vascular system (Arnold, Lalli, and Martin, 1969). Growth and division of malarial parasites of many species have been entrained to 1-, 2-, and 3-day patterns, primarily by light rhythms. Each species of host and parasite has special characteristics. Infection of mice by P. berghei is a particularly interesting example of synchrony because the generation time of the parasite is 24 hr, and thus fits a diurnal pattern (Arnold et al., loc. cit.). One of the more interesting aspects of the problem of synchrony in P. berghei is that light rhythms require specific periods of light and dark exposure for effective augmentation of growth and division synchrony. As we have previously shown only certain intensities and spectra of light produce this synchronizing effect on parasite growth. In addition to the process of growth and division synchrony, the photoperiodic rhythm Received for publication 19 July 1968. * Supported by U. S. Army Contract No. DA-49193-MD-2545 from the U. S. Army Research and Development Command Office of the Surgeon General. This paper is contribution No. 405 from the Army Research Program on Malaria. appears to modify the sequestration of segmenting parasites in the vascular system. This phenomenon is frequently observed with several species of malarial parasites in addition to P. berghei in mice. A considerable amount of morbidity and mortality of the host, for instance, is produced by capillary blockade caused by the sequestration of mature schizonts of P. falciparum in man (Rigdon and Fletcher, 1945; Edington, 1967). For the most part, vascular sequestration of the parasite involves its older dividing asexual forms (Alger, 1963). This process of sequestration has been called margination by some authors because these older parasites appear to stick to the walls of the capillaries. It seems likely that once an erythrocytic form of the parasite is phagocytized, it is destroyed. Therefore, the capture and release of living parasites by the vascular endothelium would require a mechanism other than that of phagocytosis since these parasites are not destroyed. The capture and release of parasites by the vascular system is accentuated by the photoperiodic mechanisms. For example, we showed in a previous study that the segmenting forms in a synchronous infection were largely absent from the circulating blood at 3 AM when the light intensity was under 100 foot candles (ft-c). Normally, at 3 AM we would expect a maximum number of mature schizonts. When the light intensity was raised to 1,000 ft-c, sequestration seemed to be complete again. In this present study, we hope to

Fine Structure of a Reptilian Malarial Parasite

The erythrocytic stages of a reptilian malarial parasite, Plasmodium floridense, were studied for the first time with the electron microscope. The fine structure of the erythrocytic stages of this reptilian parasite was compared with the erythrocytic and exoerythrocytic stages of avian parasites and the erythrocytic stages of mammalian parasites. The overall structure and developmental sequence of P. floridense are essentially similar to those of avian and mammalian malarial parasites. The merozoite possesses all of the organelles which have been described in the merozoites of avian and mammalian parasites. However, we also noticed large less-electron-opaque rounded bodies of unknown nature located near the paired organelles in the merozoite of P. floridense. The dedifferentiation phase is also observed in P. floridense in that certain organelles of the merozoite are lost, such as an anterior end structure (conoid), paired organelles, dense bodies, spherical body and the thick inner membrane and a layer of microtubules of the pellicular complex. During the growth phase, the cytostome of P. floridense ingests host cell cytoplasm and forms food vacuoles. Nuclear division is also similar to that of avian and mammalian malarial parasites. The redifferentiation phase is initiated by the reappearance of the thick inner membrane, paired organelles and anterior end structure (conoid), as we have observed in other species of malarial parasites. The budding of the merozoites from a schizont occurs at the areas which are covered by the thick inner membrane. By the migration of various organelles into the budding areas, several merozoites are formed from a schizont as we have observed among avian and mammalian malarial parasites. A reptilian malarial parasite, Plasmodium floridense, was first described in Sceloporus undulatus and Anolis carolinensis by Thompson and Huff (1944) and the infection of P. floridense has been extensively studied by Goodwin (1951) and Jordan (1964). Though there are considerable studies on the fine structure of the erythrocytic and exoerythrocytic stages of avian (Rudzinska and Trager, 1961; Ristic and Kreier, 1964; Aikawa, 1966; Aikawa et al., 1967; Meyer and Oliveira Musacchio, 1965; Hepler et al., 1966) and mammalian (Rudzinska and Trager, 1959, 1968; Rudzinska et al., 1965; Ladda et al., 1966, Aikawa et al., 1966b) malarial parasites, no report has been made on reptilian malarial parasites. We have, therefore, undertaken a study of the reptilian malarial parasite, P. floridense, and investigated the possibility of morphological and developmental differences in ultrastructure between the reptilian parasite and avian and mammalian parasites. MATERIALS AND METHODS Sceloporus undulatus infected with P. floridense was obtained in Clinch County, Georgia, in the vicinity of the Okefenokee Swamp. Blood from these lizards with 1 to 5% parasitemia was fixed Received for publication 7 June 1968. * Contribution No. 399 from the Army Research Program on Malaria. in 1.25% glutaraldehyde solution with 0.05 M phosphate buffer at pH 7.3 and containing 4% sucrose for 1 hr at room temperature. The blood was centrifuged for 10 min at 200 g. The resulting pellets were washed several times in 0.05 M phosphate buffer solution for at least 1 hr. Following the wash, the pellets were postfixed in 1% Os04 for 1 hr. The pellets were dehydrated in an ascending ethanol series and in propylene oxide and embedded in Epon 812. Thin sections were cut on a Porter-Blum MT-2 ultramicrotome with a Dupont diamond knife. The sections thus obtained were mounted on 300-mesh copper grids and stained with 1% uranyl acetate and lead citrate. The material was examined with a Siemens Elmiskop 1A. OBSERVATIONS AND DISCUSSION The merozoites of the erythrocytic stages of Plasmodium floridense are similar to those of certain avian (Aikawa, 1966; Aikawa et al., 1967; Hepler et al., 1966) and mammalian (Aikawa et al., 1966b; Aikawa, 1968) parasites. These include P. fallax, P. lophurae, P. cathemerium, P. gallinaceum, P. elongatum, P. berghei, P. knowlesi, and P. cynomolgi. The organelles of the merozoites include an ante ior end structure (conoid*) with polar rings, pa red organelles, dense bodies, a mitochondrion, a spherical body, a nucleus, ribosomes, endoplasmic reticulum, a cytostome, a pellic* Our use of the term conoid differs from the original use of Gustafson et al. We use the term to refer to the truncated anterior end.

In Vitro Studies on Erythrocytic Stages of Plasmodia; Medium Improvement and Results with Seven Species of Malarial Parasites

In Vitro Studies on Erythrocytic Stages of Plasmodia; Medium Improvement and Results with Seven Species of Malarial Parasites

Cross Reactions in Human and Simian Malaria

THE DETECTION of antibody in humans infected by various species of malarial parasites has presented a problem for more than 50 years. Of all the tests, the complement-fixation reaction has provided the most information, but even with this test the chief difficulty has been the lack of a standardized specific antigen of sufficient sensitivity. During the course of complement-fixation studies of malaria, certain investigators have noted cross reactions. In 1927, Kingsbury 1 reported that Plasmodium falciparum antigen reacted to about the same degree with P vivax antisera as with P falciparum antisera. However, the same amount of cross reaction did not occur when P vivax antigen was used. In this instance, the P vivax antigen detected the presence of antibody in 67% of the P vivax cases but in only 31% of the P falciparum cases. Some years later Eaton and Coggeshall, 2 using antigen prepared from the parasite,

Staining of malarial parasites by the fluorescent antibody technic.

SummaryTwo species of malarial parasites have been stained by the fluorescent antibody technic. P. cynomolgi bastianellii was stained with antibodies obtained from convalescent Rhesus monkeys experimentally infected with the parasite. The erythrocytic stage of P. gallinaceum was stained with antibodies produced in rabbits by injections of the sporozoite stage of the organism.The authors thank Doctors William F. Cantrell and Don E. Eyles for their help and suggestions.

On the position of malarial parasites with relation to erythrocytes

Eight species of malarial parasites were studied in the living condition by phase contrast microscopy using direct observation and cinemicrography. All of the evidence obtained indicates that the normal parasites lie within the host erythrocyte.

Chromosome Studies in the GenusPlasmodium

SUMMARY. A study has been made of the number and morphology of the chromosomes of five species of malarial parasites as seen in living material with phase contrast microscopy. Plasmodium knowlesi, of monkeys, is essentially similar to other primate malarial parasites previously studied, having two chromosomes of unequal length. P. lophurae, from ducks, P. relictum from pigeons and sparrows, and P. floridense from a lizard all have two small chromosomes of equal size. These facts, with ether data, indicate that the genus may consist of two species complexes. P. berghei, studied in white mice and rats, has a karyotype similar to the avian and saurian malarial parasites. The suggestion is made that P. berghei, on a basis of chromosome pattern and other characteristics, is probably more closely related to the avian and saurian species of Plasmodium than to other mammalian species.

IMMUNITY AGAINST ANIMAL PARASITES

THIS most readable and interesting book is divided into three parts. The first discusses the general principles underlying natural resistance and acquired immunity to parasitic infections. The factors influencing natural resistance to parasitic infection are numerous and no one of them applies to all examples. The importance of the genetic constitution of the host is clearly shown in the differing racial resistance to malaria ; negroes-even the very young before natural infection can confuse the issue-sometimes resist all efforts to infect them with P. vivax, whereas white persons are uniformly susceptible. Age also plays its part in resistance, and a complete chapter is devoted to the discussion of the various ways in which age influences immunity to parasites. Sex appears relatively unimportant although resistance to a few diseases is apparently influenced by this factor. Intercurrent infections usually result in depression of resistance but in some instances resistance to one parasite may be increased by simultaneous infection by another, as is demonstrated by the diminished pathogenicity of certain trypanosomes in mice infected with spirochaetes compared to the same trypanosomal infection alone, and by the behaviour of different species of malarial parasites simultaneously infecting man. The character of the diet also influences profoundly certain parasitic infections; and although the most conspicuous effects are seen upon parasites residing in the alimentary canal, yet there is evidence of dietary influences on some somatic infections. The various organs and tissues also play their own special part in resistance to parasitic infection ; the action of the reticulo endothelial system is well recognized, but the effect of special defensive factors associated with other cells of the host and the influence of various invasive factors of the parasites themselves are not so commonly taken into account. Immunity against Animal Parasites By Prof. James T. Culbertson. Pp. x + 274. (New York: Columbia University Press; London: Oxford University Press, 1941.) 36s. 6d. net.

THE AGGLUTINATION OF PLASMODIUM KNOWLESI BY IMMUNE SERUM

A specific agglutination of Plasmodium knowlesi detectable both by macroscopic and by microscopic methods is described. Agglutinins for Plasmodium knowlesi appear in the sera of monkeys between 15 and 45 days after the onset of the infection and become progressively stronger as the malarial infection gradually subsides. Agglutinins persist in the sera of chronically infected animals for a year or longer. The sera of animals which have been repeatedly superinfected agglutinate parasites at dilutions as high as 1:1,000. Sera from normal monkeys, from monkeys acutely ill with malaria, and from monkeys chronically infected with a different species of malarial parasite (Plasmodium inui) do not agglutinate Plasmodium knowlesi. Immune serum agglutinates mature intracellular or extracellular parasites but does not agglutinate unparasitized cells or cells containing immature parasites. The relation of these observations to the mechanism of active and passive immunity in monkey malaria is discussed.