Malaria mortality rates in South Asia and in Africa: implications for malaria control

Malaria mortality in human populations varies greatly under different circumstances. The intense malaria transmission conditions found in many parts of tropical Africa, the much lower malaria inoculation rates currently sustained in areas of southern Asia, and the epidemic outbreaks of malaria occasionally seen on both continents, present highly contrasting patterns of malaria-related mortality. Here Harsha Alles, Kamini Mendis and Richard Carter examine malaria-related mortality under different circumstances and discuss implications for the management of malaria in these settings. They emphasize the power of rapid case treatment to save lives at risk under virtually all circumstances of malaria transmission.

Association of high plasma TNF-alpha levels and TNF-alpha/IL-10 ratios with TNF2 allele in severe P. falciparum malaria patients in Sri Lanka

Plasma levels of pro- and anti-inflammatory cytokines of Plasmodium falciparum-infected patients with severe malaria (SM; n = 62) and uncomplicated malaria (UM; n = 69) from Sri Lanka were assessed. SM patients had significantly higher levels of TNF-alpha (P < 0·01), IL-6 (P < 0·01), and IL-10 (P < 0·05) compared to the UM patients. Plasma IL-2 levels of these patients were undetectable. TNF-alpha levels of a third group of patients with uncomplicated P. falciparum malaria, who were recruited during their fever episodes (UMF; n = 14) were significantly higher than those of the UM patients (P < 0·001) and comparable to SM patients. Plasma IFN-gamma levels of SM patients were higher compared to UM patients, but was not statistically significant. Body temperature in both SM and UMF groups were significantly higher compared to UM group, whereas percentages of parasitemia in all three groups were comparable. Analysis of plasma TNF-alpha levels and the ratio of TNF-alpha/IL-10 in UM (n = 34) and SM (n = 34) patients carrying TNF1 and TNF2 allelic types showed that SM patients carrying TNF2 had significantly higher TNF-alpha levels as well as TNF-alpha/IL-10 ratio compared to UM patients carrying TNF1, UM patients carrying TNF2 and SM patients carrying TNF1 (P < 0·05). These results suggest that the high circulating TNF-alpha levels and the inadequate IL-10 response in the SM patients carrying TNF2 allele could have contributed to the development of severe falciparum malarial disease.

ABO-blood-group types and protection against severe, Plasmodium falciparum malaria

Although the ABO blood group of the human host has been reported to influence malarial infection, there have been few clinical observations on this effect. A hospital-based, comparative study was therefore performed to investigate the relationship between blood-group type and severe disease i nPlasmodium falciparum malaria. Overall, 243 cases of malaria (163 uncomplicated and 80 severe) and 65 patients with severe, non-malarial infections were studied. In terms of ABO-blood-group composition, the patients with severe malaria were significantly different from the patients with the uncomplicated disease (P<0.001) and also from a population control described previously (P<0.0001). The patients with uncomplicated malaria or severe but non-malarial disease were, however, similar to the population control. The cases of severe malaria were significantly less likely to be of blood group O (P=0.0003), and significantly more likely to be of group AB (P<0.0001), than the patients with nonsevere malaria. It appears that individuals who are of blood-group O are relatively resistant to the severe disease caused by P. falciparum infection.

Clinical disease and pathogenesis in malaria

This is the report of a meeting held in Ahungalla, Sri Lanka, 16-19 January 1994, under the sponsorship of the Rockefeller Foundation, Health Sciences Division. The meeting was initiated jointly by the Rockefeller Foundation and the TDR Special Programme of the World Health Organization in order to bring together scientists with a wide spectrum of experience relating to malarial disease and pathogenesis. The objective was to generate interdisciplinary discussion ranging from the clinical pictures of malarial infections and their impact in different parts of the world, to current investigations on mechanisms of pathogenesis and clinical immunity and the genetic determinants in human and parasite populations affecting the nature of the disease.

Cognitive performance at school entry of children living in malaria-endemic areas of Sri Lanka

In a cross-sectional study, carried out in January 1997 at the beginning of the school year, the impact of repeated attacks of malarial infection on the cognitive performance of children at school entry in moderate malaria-endemic areas of Sri Lanka was investigated. The cognitive performance of 325 schoolchildren in grade 1 (mostly aged 5-6 years) in 2 districts of Sri Lanka which are endemic for malaria (Anuradhapura and Moneragala) was assessed by an entry performance test developed by the National Institute of Education, Sri Lanka. The indices assessed included writing, language and mathematical skills. There was no difference in any of the cognitive performance indices between children from Anuradhapura and Moneragala districts. The scores of most of the indices decreased as the number of malaria infections experienced by a child increased and the ability to identify letters was significantly impaired by the number of malaria infections a child had experienced after controlling for socio-economic and nutritional status. These findings suggests that repeated attacks of malaria in children can have an adverse impact on their development.

The impact of repeated malaria attacks on the school performance of children

The impact of repeated malarial infections on the school performance of children was investigated in 571 school children 6-14 years of age in a malaria-endemic area in southern Sri Lanka where both Plasmodium falciparum and P. vivax infections are prevalent. Malaria infections confirmed by microscopy were monitored over a six-year period. School performance was assessed by two specially designed, school grade-specific, test papers for Sinhala language and mathematics. The scores for Sinhala language and mathematics for each school term test for the year 1997 were obtained. Malarial infections were a major predictor of children's performance in language and mathematics after controlling for parent's education, monthly family income, and house type. The education of the father predicted language scores but not mathematics scores. A child who experienced more than five attacks of malaria scored approximately 15% less than a child who experienced less than three attacks of malaria. The data suggest that repeated attacks of malaria have an adverse impact on the school performance of children.

Quantifying genetic and nongenetic contributions to malarial infection in a Sri Lankan population

Explaining the causes of variation in the severity of malarial disease remains a major challenge in the treatment and control of malaria. Many factors are known to contribute to this variation, including parasite genetics, host genetics, acquired immunity, and exposure levels. However, the relative importance of each of these to the overall burden of malarial disease in human populations has not been assessed. Here, we have partitioned variation in the incidence of malarial infection and the clinical intensity of malarial disease in a rural population in Sri Lanka into its component causes by pedigree analysis of longitudinal data. We found that human genetics, housing, and predisposing systematic effects (e. g., sex, age, occupation, history of infections, village) each explained approximately 15% of the variation in the frequency of malarial infection. For clinical intensity of illness, 20% of the variation was explained by repeatable differences between patients, about half of which was attributable to host genetics. The other half was attributable to semipermanent differences among patients, most of which could be explained by known predisposing factors. Three percent of variation in clinical intensity was explained by housing, and an additional 7% was explained by current influences relating to infection status (e.g., parasitemia, parasite species). Genetic control of Plasmodium falciparum infections appeared to modulate the frequency and intensity of infections, whereas genetic control of Plasmodium vivax infections appeared to confer absolute susceptibility or refractoriness but not intensity of disease. Overall, the data show consistent, repeatable differences among hosts in their susceptibility to clinical disease, about half of which are attributable to host genes.

Spatial targeting of interventions against malaria

Malaria transmission is strongly associated with location. This association has two main features. First, the disease is focused around specific mosquito breeding sites and can normally be transmitted only within certain distances from them: in Africa these are typically between a few hundred metres and a kilometre and rarely exceed 2-3 kilometres. Second, there is a marked clustering of persons with malaria parasites and clinical symptoms at particular sites, usually households. In localities of low endemicity the level of malaria risk or case incidence may vary widely between households because the specific characteristics of houses and their locations affect contact between humans and vectors. Where endemicity is high, differences in human/vector contact rates between different households may have less effect on malaria case incidences. This is because superinfection and exposure-acquired immunity blur the proportional relationship between inoculation rates and case incidences. Accurate information on the distribution of malaria on the ground permits interventions to be targeted towards the foci of transmission and the locations and households of high malaria risk within them. Such targeting greatly increases the effectiveness of control measures. On the other hand, the inadvertent exclusion of these locations causes potentially effective control measures to fail. The computerized mapping and management of location data in geographical information systems should greatly assist the targeting of interventions against malaria at the focal and household levels, leading to improved effectiveness and cost-effectiveness of control.

TNFalpha*2 marks high risk of severe disease during Plasmodium falciparum malaria and other infections in Sri Lankans

We have investigated the association between alleles of the genes for tumour necrosis factor-alpha (TNF-alpha) and TNF-beta and severity of disease during malarial (Plasmodium falciparum) and other infections in the Sri Lankan population. Patients were categorized as having either (i) uncomplicated malaria, (ii) severe and complicated malaria, or (iii) severe and complicated infection in which a diagnosis of malaria had been excluded. For all the patients, as well as for a group of matched healthy controls, TNF-alpha and TNF-beta allelic types were identified using the polymerase chain reaction (PCR) and allele-specific oligonucleotide probes and restriction enzyme digestion. The odds in favour of carrying the TNFalpha*2 allele, mainly of the heterozygous genotype (TNFalpha*1,*2), were two to three times greater among individuals with severe disease, of either malarial or other infectious origin, relative to healthy controls or to those with uncomplicated malarial infections. No significant risk was associated with either of the alleles of TNF-beta.

Malaria risk factors in an endemic region of Sri Lanka, and the impact and cost implications of risk factor-based interventions

In an 18-month study of malaria in a population of 1,875 residents in 423 houses in an endemic area in southern Sri Lanka, the risk of malaria was found to be 2.5-fold higher in residents of poorly constructed houses than in those living in houses of good construction type. In residents of poorly constructed houses but not in others, the risk was even greater when the house was located near a source of water that could act as a potential breeding place for malaria vector mosquitoes (P = 0.0001). Based on previous findings that confirmed that house construction type was itself a risk determinant, and not merely a marker of other behavioral factors, we have estimated the potential impact of two feasible interventions to reduce the risk of malaria: 1) the imposition of a buffer zone of 200 meters around bodies of water from which houses of poor construction were excluded, which was estimated to lead to a 21% reduction of the malaria incidence in the overall population and a 43% reduction in the relocated community; and 2) the conversion of houses of poor construction type located in the buffer zone to those of a good construction type, which was estimated to lead to a 36% reduction in the incidence rates in the whole population and a 76% reduction in the residents of houses whose construction type was improved. Taking into consideration the cost to the Government of malaria prevention, we estimated the worth of a Government's investment in improving house construction type. The investment in housing was estimated to be offset in 7.2 years by savings to the Government on malaria costs alone, and beyond this period, to bring a return on the Government's investment by way of savings to the malaria control program.

Demonstration of anti-disease immunity to Plasmodium vivax malaria in Sri Lanka using a quantitative method to assess clinical disease

Clinical immunity to malaria was studied by quantifying the intensity of symptoms as well as by measurement of several hematologic indicators of pathology (the erythrocyte sedimentation rate [ESR], serum bilirubin, reticulocyte count, plasma tumor necrosis factor-alpha [TNF-alpha], and blood glucose levels) in 39 Plasmodium vivax malaria patients exposed to endemic malaria in southern Sri Lanka, and for comparison in 43 nonimmune patients who were residents of nonmalarious regions of the country. The intensity of 11 symptoms was scored numerically in all patients using a questionnaire. This clinical score was validated by introducing internal controls to the questionnaire, and by correlating it with the underlying pathology. Both the intensity of clinical disease as well as the degree of underlying pathology were found to be significantly lower in endemic area patients (mean clinical score = 8.8, median ESR = 8 mm) compared with the nonendemic area patients (mean clinical score = 19.0, median ESR 31.5 mm). Endemic area patients also had lower parasite densities (mean = 0.06%) than those from the nonendemic area (0.12%) (P < 0.05). However, at any parasite density, both clinical disease and pathology were significantly less in the endemic area patients (P < 0.001, for both clinical score and ESR), indicating that the clinical immunity seen in the endemic area patients was a true tolerance of parasites. Although plasma TNF-alpha levels were elevated in both groups of patients, they were significantly higher in the nonendemic area patients than in patients from the endemic area (P < 0.01). Furthermore, at comparable levels of plasma TNF-alpha, nonendemic area patients had both a higher intensity of clinical disease and an underlying pathology than those from the endemic area, suggesting that if TNF-alpha is indeed a mediator of clinical disease, the endemic area patients may be tolerant to its effects. Hypoglycemia was not observed in any of these P. vivax patients despite some with high levels of plasma TNF-alpha.

Optimizing the malaria data recording system through a study of case detection and treatment in Sri Lanka

The potential of using malaria incidence data routinely collected from endemic regions for disease control and research has increased with the availability of advanced computer-based technologies, but will depend on the quality of the data itself. We report here an investigation into the relevance of malaria statistics provided by the routine data collection system in Moneragala, a rural malaria-endemic region in Sri Lanka. All patients (n = 321) treated for malaria in 2 clusters of health care centres (HCCs) of both the private and the public sector in the administrative regions of Moneragala and Buttala Divisional Secretariat (D.S.). Divisions were studied in December 1995/ January 1996. The catchment area of these HCCs included a population resident in 53 Grama Niladhari (GN) areas, the smallest administrative units of the country. Almost equal numbers of malaria patients were detected and treated at Government and private health care institutions, and in 70% of them treatment was based on a diagnosis confirmed by microscopy. The routine data recording system, however, included only statistics from the Government sector, and only of patients whose diagnosis was microscopically confirmed. In compiling data, the origin of a case of malaria is attributed to the D.S. Division in which the institution (at which the patient was treated) was located, rather than the area in which the patient was resident, which was inaccurate because 90% of malaria patients sought health care at institutions located closest to their residence, thus crossing administrative boundaries. It also led to a loss of resolution of spatial data because patients' addresses recorded at the Government HCCs to the village-level are replaced in the statistics by the D.S. Division, which is a coarse spatial unit. Modifications to the system for malaria case recording needed to correct these anomalies are defined here. If implemented, these could result in major improvements to the quality of data, a valuable resource for the future of malaria control. The paper reiterates the call for the use of a standard spatial unit within a country to facilitate exchange of data among health and other sectors for the control of tropical diseases.

The ParaSight-F dipstick test as a routine diagnostic tool for malaria in Sri Lanka

Blood from 1053 persons who presented for treatment at outpatient clinics of government health institutions in Sri Lanka, and 250 who took part in a blood survey for malaria, was examined by thick blood film microscopy under routine field conditions, and by the ParaSight-F dipstick method. All the samples were also examined microscopically under laboratory conditions when 4 times the number of microscope fields were examined. Compared with this reference standard, the sensitivity and specificity of the ParaSight-F test were 90.2% and 99.1%, and those of microscopy in the field were 92.4% and 98.4% respectively, there being no statistically significant difference between the 2 methods. The ParaSight-F test reading correlated significantly and positively with the intensity of clinical disease of patients but not with their peripheral parasitaemia, indicating that it may be a more accurate measure of the true parasite load than microscopy, which detects only parasites which are in the peripheral blood and not those which are sequestered in deep organs. The ParaSight-F test, however, failed to detect Plasmodium falciparum infections with only gametocytes in the blood (19.6% of the infected blood samples in this study). The time taken for a patient to revert to negativity by the ParaSight-F test was also significantly longer, up to 14 d. This would make the test unsuitable for checking the response to antimalarial treatment within 14 d. In an endemic area it would therefore fail to detect drug resistant populations of parasites.

Cloning and characterisation of a gene from Plasmodium vivax and P. knowlesi: homology with valine-tRNA synthetase

We have previously described a lambdagt11 clone detected by immune screening with a monoclonal antibody (mAb) A12. This mAb is capable of completely blocking Plasmodium vivax transmission in the mosquito vector. An epitope recognised by A12 was mapped to six amino acids (aa) within the translated sequence of this clone. Here, we describe the complete sequence of the gene within which we mapped this epitope. Surprisingly, the translated sequence of the full-length open reading frame shows homology with that of valine-tRNA synthetases (Val-tRS) from other organisms. DNA cross-hybridisation with several of these species was observed by Southern blot. In addition, the corresponding gene has been obtained from the closely related simian malaria parasite, P. knowlesi. The two aa sequences show 66% identity and yet are very divergent from other Val-tRS sequences, apart from conserved blocks related to functional activity. Multiple sequence alignments reflect this dichotomy, as do predicted differences in antigenicity.

Features of recrudescent chloroquine-resistant Plasmodium falciparum infections confer a survival advantage on parasites and have implications for disease control

This paper reports on the features of recrudescent infections of chloroquine-resistant Plasmodium falciparum (CQRPf) malaria from a study in vivo of patients from a malaria endemic (n = 527) and non-endemic (n = 129) region of Sri Lanka where the incidence of RI resistance was 30% and 55%, respectively. In both groups of patients, the recrudescent infections which emerged after treatment of the primary infection with chloroquine (CQ) and primaquine had significantly lower peripheral parasitaemia (0.036% and 0.108% in endemic and non-endemic patients, respectively) compared to their primary infections (mean parasitaemia 0.13% and 0.49%; P = 0.021 and 0.002, respectively). The recrudescences of CQ resistant infections also gave rise to clinical disease of markedly reduced severity (average clinical scores of 10.1 and 8.2) compared to their primary infections (average clinical scores of 12.4 and 12.3; P = 0.003 and 0.001, respectively, in endemic and non-endemic patients). CQ resistant recrudescent infections therefore had a lower probability of being diagnosed and treated. In endemic patients, a higher proportion of CQRPf infections (57%) had gametocytaemia compared to the chloroquine sensitive ones (29%) (P = 0.014, chi 2 = 5.96) and were significantly more infective to mosquitoes (P = 0.047). these findings imply that, in areas where CQ resistance is prevalent, the continued use of the drug may confer a survival and propagation advantage on resistant parasites and favour the rapid expansion of their reservoir. In support of this, we also present epidemiological evidence showing that, in endemic areas, the proportion of P. falciparum patients carrying gametocytes has increased significantly since the emergence of chloroquine resistance. These findings are relevant to the management of drug resistance and malaria control in countries where P.falciparum is only partially resistant to CQ.

A malaria parasite toxin associated with Plasmodium vivax paroxysms

We have previously demonstrated a correlation between clinical paroxysms in Plasmodium vivax malarial infections and the appearance in patients' plasma of factors that kill blood stage parasites (gametocytes). This activity was, as previously shown, dependent on the presence in paroxysm plasma of tumour necrosis factor-alpha (TNF-alpha), which acts in conjunction with other 'complementary' factors. Here we have identified a parasite component which is essential for this activity and functions as a 'complementary' factor together with TNF, and a third component of unknown origin. The P. vivax parasite component present in paroxysm plasma can be substituted with a blood-stage schizont extract of either P. vivax or P. falciparum. This was demonstrated by restoring the parasite-killing activity to post-paroxysm plasma (from which it was absent) with the addition of the extracts together with TNF. The active materials in these extracts, however, are different from the natural components in P. vivax paroxysm plasma, i.e. while the schizont extracts are immunologically cross-reactive between species, the activity of the natural P. vivax toxin(s) in patients' plasma is neutralized only by the homologous antisera. Plasmodium falciparum infections have neither distinct paroxysms nor parasite-killing activity in plasma. The pronounced paroxysms of P. vivax infections may thus be due in part of a species-specific toxin(s).

Transmission blocking immunity in Plasmodium vivax malaria: antibodies raised against a peptide block parasite development in the mosquito vector

One approach towards the development of a vaccine against malaria is to immunize against the parasite sexual stages that mediate transmission of the parasite from man to mosquito. Antibodies against these stages, ingested with the blood meal, inhibit the parasite development in the mosquito vector, constituting "transmission blocking immunity." Most epitopes involved in transmission-blocking immunity depend on the tertiary conformational structure of surface antigens. However, one of the transmission-blocking monoclonal antibodies we have raised against Plasmodium vivax reacts with a linear epitope on both asexual stages and gametes. This monoclonal antibody (A12) is capable of totally blocking development of the parasite in the mosquito host when tested in membrane feeding assays with gametocytes from P. vivax-infected patients. Immune screening of a P. vivax lambda gt11 genomic expression library with A12 led to the isolation of a clone to which was mapped the six-amino acid epitope recognized by A12. Antisera raised in mice against a 12-mer synthetic peptide containing this epitope coupled to bovine serum albumin not only had high titers of antipeptide antibodies as measured by enzyme-linked immunosorbent assay, but in addition recognized the same 24- and 57-kD parasite components as A12 on Western blots and reacted with the parasite by immunofluorescence. When tested in membrane feeding assays, these antibodies have significant suppressive effects on parasite development in the mosquito.

Plasmodium vivax merozoite surface protein 1 C-terminal recombinant proteins in baculovirus

Recombinant proteins derived from the Plasmodium vivax merozoite surface protein 1 have been produced in the baculovirus expression system. These proteins correspond approximately to the Plasmodium vivax analogs of the 42-kDa or 19-kDa C-terminal processing products previously described for Plasmodium falciparum. Each was produced in two versions, either as a membrane-bound entity located on the cell surface and probably carrying a glycosylphosphatidylinositol addition, or as a secreted entity lacking a membrane anchor. Many native conformational epitopes appear to be accurately reproduced in these molecules. Both the 42-kDa and 19-kDa analogs can be N-glycosylated in the baculovirus system and the N-glycosylation appears to be necessary for efficient secretion of both the 42-kDa and 19-kDa recombinant proteins.

The role of cytokines in Plasmodium vivax malaria

The cytokine tumor necrosis factor and other as yet unidentified factor(s) which together mediate the killing of intraerythrocytic malaria parasites are transiently elevated in sera during paroxysms in human Plasmodium vivax infections in non-immunes. These factors which included TNF and parasite killing factor(s) are associated with the clinical disease in malaria to the extent that their transient presence in infection sera coincided with paroxysms, the the most pronounced clinical disturbances of P. vivax malaria and secondly because their levels were markedly lower in paroxysm sera of semi-immune patients who were resident of an endemic area. Further, a close parallel was obtained between serum TNF levels and changes in body temperature that occur during a P. vivax paroxysm in non-immune patients, suggesting a causative role for TNF in the fever in malaria. P. vivax rarely if ever cause complicated clinical syndromes. Nevertheless serum TNF levels reached in acutely ill P. vivax patients were as high as in patients suffering from cerebral complications of P. falciparum malaria as reported in studies from the Gambia. Cytokine profiles and other changes accompanying clinical disease in P. vivax and P. falciparum malaria are compared in this paper with a view to discussing the potential role of cytokines in the causation of disease in malaria.

Transient increase in circulating gamma/delta T cells during Plasmodium vivax malarial paroxysms

The percentage of peripheral blood mononuclear cells (PBMC) bearing the CD3+ phenotype and the alpha/beta and gamma/delta T cell receptors (TCR) in PBMC were examined in Plasmodium vivax malaria patients and convalescents. The cells were labeled with monoclonal antibodies, stained with either fluorescence or phycoerythrin, and examined by ultraviolet (UV) microscopy. A highly significant increase in both the proportion and the absolute numbers of gamma/delta T cells (p < 0.005 and < 0.001, respectively, Student's t test) was observed in nonimmune P. vivax patients during clinical paroxysms compared to nonmalarial controls. These T cells, which normally constitute not more than 3-5% of PBMC, constituted < or = to 30% of PBMC during paroxysms in these nonimmune patients in whom the clinical symptoms were severe. A less significant increase of gamma/delta T cells were also observed in these nonimmune patients during infection, between paroxysms and during convalescence. In contrast, in an age-matched group of semi-immune patients resident in a malaria-endemic region of the country, in whom the clinical disease was comparatively mild, there was no increase in gamma/delta T cells either during infection, even during paroxysms, or convalescence. The severity of disease symptoms in patients as measured by a clinical score correlated positively with the proportion of gamma/delta T cells in peripheral blood (r = 0.53, p < 0.01), the most significant correlation being found between the prevalence and severity of gastrointestinal symptoms, nausea, anorexia, and vomiting, and the proportion of gamma/delta T cells (r = 0.49, p = 0.002). These findings suggest that gamma/delta T cells have a role to play in the pathogenesis of malaria, possibly in the general constitutional disturbances and particularly in gastrointestinal pathology in malaria.

Cytokine-mediated inactivation of malarial gametocytes is dependent on the presence of white blood cells and involves reactive nitrogen intermediates

Supernatants of human peripheral blood mononuclear cells (PBMC) incubated for 24 hr in the presence of extracts of freeze-thawed blood stage parasites of Plasmodium vivax or P. falciparum mediate inactivation of gametocytes of either species when incubated in vitro with whole human blood cells. Cultured P. falciparum gametocytes incubated with these malaria extract-stimulated PBMC supernatants in the presence of human blood from which white blood cells (WBC) had been removed were not inactivated. Thus the effects of the PBMC supernatants on gametocyte infectivity were dependent upon the presence of WBC. The suppressive effects mediated in the presence of WBC could be partially reversed in the presence of concentrations of 1 mM or higher of the L-arginine analogue NGL-monomethyl arginine acetate (L-NMMA). Our results indicate that the effects of WBC in inactivating gametocytes are due, at least in part, to a mechanism involving an L-arginine-dependent pathway. Previous studies have shown that the mediators of gametocyte inactivation in the stimulated PBMC supernatants comprised tumour necrosis factor (TNF) acting in conjunction with unidentified, but essential, 'complementary factors'. In the present study we show that these mediators, TNF and complementary factors, affect gametocytes indirectly through their interaction with WBC.

Plasmodium vivax:recombination between potential allelic types of the merozoite surface protein MSP1 in parasites isolated from patients

The merozoite surface protein MSP1, which is one of the most promising candidates for a malaria vaccine directed against erythrocytic stages, has been shown to be polymorphic in different malarial species. Characterization of the Plasmodium vivax MSP1 gene (Pv200) in two strains (Belem and Salvador-1) revealed the existence of several polymorphic regions. One of these regions has been examined here in primary parasite isolates obtained from patients in Sri Lanka. Oligonucleotide primers hybridizing to conserved parts of the gene on either side of a polymorphic region were used to amplify DNA from 22 isolates. Sequence analysis of the amplified portion of the MSP1 gene in five patients showed the existence of three types of polymorphic regions. Two were almost identical either to that of the Belem or to that of the Salvador-1 strain. The third polymorphic type appeared to have resulted from recombination between the two others. This recombination event took place inside a repeated part of the sequence.

Infectivity of Plasmodium vivax and P. falciparum to Anopheles tessellatus; relationship between oocyst and sporozoite development

The assessment of malarial infectivity, for example in the evaluation of transmission blocking immunity, is generally based on counting oocysts in mosquitoes fed on infected blood. Ultimate transmission of the disease may, however, depend on the sporozoite load in the mosquito and its relationship to the size of the inoculum introduced to man. We conducted a laboratory study on Anopheles tessellatus infected with 108 different natural isolates of Plasmodium vivax from patients and 24 of P. falciparum to determine the relationship between oocyst numbers, sporozoite loads, and the effect of these on mosquito mortality. It was found that the P. vivax parasite density was positively correlated with the proportion of mosquitoes infected by a given feed at both the midgut and gland stages of parasite development (correlation coefficient [r] = 0.77, P < 0.001 and r = 0.6, P < 0.05 respectively). A significant positive linear correlation was observed between the number of oocysts and sporozoites in P. vivax (r = 0.5; P < 0.05); the proportions of mosquitoes infected with oocysts and sporozoites were also similarly related, although in general about 15% of mosquitoes infected with oocysts failed to develop salivary gland infections with sporozoites. The number of mosquitoes infected with P. falciparum parasites was too low for statistical analysis. Infection with either species of parasite did not appear to affect mosquito survival, nor was parasite density in the mosquito correlated with mosquito mortality.

Tumour necrosis factor-dependent parasite-killing effects during paroxysms in non-immune Plasmodium vivax malaria patients

Plasmodium vivax malaria infections in non-immune individuals manifest as periodic clinical episodes of fever with chills and rigors known as paroxysms. We have demonstrated that in non-immune patients the period of paroxysm is associated with the transient presence of plasma factors which kill gametocytes, the intra-erythrocytic sexual stages of the malaria parasite which transmit the infection from humans to mosquito, rendering them non-infectious to mosquitoes. Gametocyte killing in paroxysm plasma is mediated by tumour necrosis factor (TNF) acting in conjunction with other essential serum factor(s). Plasma TNF levels were elevated during a paroxysm. In semi-immune individuals from a P. vivax-endemic area clinical symptoms of malaria are mild and the parasite killing factors are not induced during paroxysm. Serum TNF levels were correspondingly lower in endemic patients during a paroxysm. Human peripheral blood mononuclear cells (PBMC) can be stimulated in vitro by extracts of P. vivax blood stage parasites to produce TNF and associated parasite killing factor(s), thus simulating in vitro the events that occur during a paroxysm, this being the release of parasite exo-antigens by rupturing schizonts and the subsequent induction of PBMC to produce TNF and other parasite-killing factors. We were able to show that convalescent serum from P. vivax semi-immune individuals block the induction of TNF and parasite-killing factors by malaria antigens in vitro, presumably through antibodies that neutralize parasite exo-antigens. Thus, individuals living in malaria-endemic areas appear to acquire clinical immunity to malaria by avoiding their induction during infection; we have shown that one such mechanism is the neutralization of parasite exo-antigens that induce the production of parasite killing factors.

Transmission blocking immunity to human Plasmodium vivax malaria in an endemic population in Kataragama, Sri Lanka

Serum effects on gametocyte infectivity, that is, transmission blocking/enhancing immunity, were measured in the sera of 196 acute Plasmodium vivax patients who were residents of a malaria region in Kataragama, southern Sri Lanka. Direct mosquito feedings were also performed on 170 of these patients. Sera of about 48% of patients suppressed gametocyte infectivity significantly (by more than 75%) and of a smaller proportion (12%) had pronounced infectivity enhancing effects. Transmission immunity did not increase with age of patients, rather, immunity tended to be higher in younger patients. Data suggest that immunity levels are boosted by reinfections only if they occur within a period of 4 months from the previous infection, i.e., that immune memory for boosting does not last beyond 4 months. Enhancing effects in the sera of patients correlated with the absence of gametocytes at the time of investigation suggesting that enhancement occurs early during the course of a blood infection, and blocking later, when serum antibodies reach higher levels. The blocking and enhancing effects of serum appears to depend not only on the antibody concentration in serum, but also on the intrinsic infectivity of the parasite isolate against which it is tested: thus, infectivity enhancing effects were potentiated by low intrinsic infectivities of the parasite isolate. The direct infectivity of patients to mosquitoes correlated with transmission immunity indicating that transmission immunity is an influential factor determining infectivity of malaria patients.

Dynamics of fever and serum levels of tumor necrosis factor are closely associated during clinical paroxysms in Plasmodium vivax malaria

Paroxysms are sharp episodes of high fever accompanied by chills and rigors that occur periodically, once in every 48 hr in Plasmodium vivax infections. We have measured the changing levels of serum tumor necrosis factor (TNF) during paroxysms in non-immune patients infected with P. vivax malaria. The changes in TNF levels closely paralleled the rise and fall in temperature during the paroxysms but tended to precede them by 30-60 min. These observations suggest that the rise and fall in temperature during P. vivax paroxysm may be directly related to the periodic changes in TNF levels induced during these infections. The peak TNF levels reached during P. vivax infections were much higher than even those which have been recorded during severe and fatal P. falciparum infections in which TNF has been postulated to contribute to the severe manifestations of this disease.

Serological relationship of tumor necrosis factor-inducing exoantigens of Plasmodium falciparum and Plasmodium vivax

Exoantigens of Plasmodium vivax-parasitized erythrocytes stimulated macrophages to secrete tumor necrosis factor, and antisera raised against the exoantigens inhibited this secretion. The antisera also inhibited the activity of Plasmodium falciparum and Plasmodium yoelii exoantigens, and conversely, antisera against the latter cross-reacted with the exoantigens of P. vivax.

Anti-circumsporozoite protein antibodies measure age related exposure to malaria in Kataragama, Sri Lanka

Antibodies to two peptides DDAAD and (NANP)40 representing the repetitive sequence of circumsporozoite antigens (CS protein) of P. vivax and P. falciparum respectively were measured in a cohort of 149 and 107 individuals respectively at four, 6 monthly blood surveys performed on residents of Kataragama, a P. vivax malaria endemic region in southern Sri Lanka. The prevalence of antibodies to the CS protein of both species was relatively low being less than 20% to either peptide in the population as a whole, this being consistent with the low entomological inoculation rates in the area. A marked age related prevalence pattern was evident, with the prevalence of antibodies increasing with age to reach between 25 to 30% in the 25-50 year age group in both P. vivax and P. falciparum. The population had had a life long exposure to P. vivax malaria but not to P. falciparum, an epidemic of which occurred in this region a few months prior to the beginning of this study. Nevertheless, the age-related prevalence of these antibodies was identical with respect to the two species. This suggests that the age-related prevalence pattern reflected differences in inoculation rates between the age groups due to differences in exposure to inoculation rather than an age acquired response resulting from a cumulative experience over several years. An analysis of antibody prevalence in individuals showed first, that sporozoite inoculations must have been clustered rather than homogeneously distributed in the population and secondly, that sero-conversion did not correlate with malaria infections in these individuals.

Infectious reservoir of Plasmodium vivax and Plasmodium falciparum malaria in an endemic region of Sri Lanka

The infectious reservoir of Plasmodium vivax and P. falciparum in a malaria endemic region in Sri Lanka was defined in a population of 3,625 by directly feeding mosquitoes on a sample of infected individuals during a period of 17 months. The malaria case incidence in this population was concurrently monitored. P. vivax gametocyte densities were highest in the youngest age groups, and decreased steadily with increasing age. However, the infectivity per gametocyte appeared to be lower in the younger age groups than in the older ones. There was no significant correlation between the age of patients and their gametocyte densities for P. falciparum, to which this population was only recently exposed, nor was there a discernible trend in the infectivity per gametocyte in different age groups. The average infectivity of patients was lowest in the youngest (0-5 years) and the oldest (greater than 50) age groups. The contribution made by P. vivax patients in the different age groups to the reservoir of infection was estimated. Patients in the 6-25 year age groups made the largest contribution to the reservoir, followed by those in the 26-50 year age group. Patients in the youngest and the oldest age groups contributed least to the infectious reservoir. When population sizes in the different age groups were taken into consideration, the age groups between 6 and 50 years contributed almost equally to approximately 87% of the infectious reservoir. The reservoir of P. falciparum malaria was very small, being confined to 9% of the patients, and this appears to be a characteristic of epidemic malaria, as was the case with P. falciparum.

Clustering of malaria infections within an endemic population: risk of malaria associated with the type of housing construction

The occurrence of malaria infections due to Plasmodium vivax and P. falciparum was monitored in a population of 3,023 people living in six contiguous villages in Kataragama, an area of endemic malaria in southern Sri Lanka, over a period of 17 months. The annual incidence of malaria in this population during the study period was 25.8%. Malaria attacks were clustered, occurring more frequently than expected in certain individuals and housing groups and less frequently than expected in others. In one of these villages, the distribution of cases was examined in relation to locality and to the type of house construction. There was a strong association between the malaria incidence and house construction, independent of location. The risk of getting malaria was greater for inhabitants of the poorest type of house construction (incomplete, mud, or cadjan (palm) walls, and cadjan thatched roofs) compared to houses with complete brick and plaster walls and tiled roofs. Houses that were better constructed had a significantly lower malaria incidence rate (10.5%) than those that were poorly constructed (21.2%; P less than 0.01, by Student's t-test). There was also a significantly higher number of indoor resting mosquitoes collected from the poorly constructed houses than from those better constructed; the average (geometric mean) of mosquito densities found in houses of better versus poor construction were 0.97 and 1.89 per collection in the dry season, and 1.95 and 3.42 per collection in the wet season, respectively (P less than 0.05 in both seasons). This indicated that the higher malaria risk associated with poorly constructed houses was at least partly due to higher human-mosquito contact among their inhabitants.

Immune responses against sexual stages of Plasmodium vivax during human malarial infections in Sri Lanka

During natural infections of P. vivax malaria a variety of immune responses to the infection affect infectivity of the parasites to mosquitoes. Sexual stage antigens present in the blood stage parasites induce antibodies which may either enhance or suppress the infectivity of the sexual parasites to mosquitoes. Subsequent infections of P. vivax do not, unless occurring within less than 4 months, boost this response indicating a very short immune memory for the relevant antigens. Blood infection also results in the release of cytokines and other non-antibody factors which together can mediate death of the blood stage sexual parasites. These factors are associated with paroxysm in non-immune individuals. In individuals from an endemic area with age-acquired anti-disease immunity clinical symptoms are mild and the parasite killing factors are not induced.

Antigenic polymorphism in malaria: is it an important mechanism for immune evasion?

Malarial infections do not readily evoke an effective protective immunity against re-infection. Possible reasons for this include the ability of the parasites to interfere with the host's immune response and to evade the response in an immune host, by, for example, exploiting antigenic polymorphism or variation. Antigenic polymorphism undoubtedly exists in malaria parasite populations but does this polymorphism actually contribute to immune evasion by the parasite? Here, Kamini Mendis and colleagues examine the evidence for this and its implications for future malaria vaccines.

Cytokines kill malaria parasites during infection crisis: extracellular complementary factors are essential

Malaria infection crisis, at which the parasitemia drops precipitously and the parasite loses infectivity to the mosquito vector, occurs in many natural malaria systems, and has not been explained. We demonstrate that in a simian malaria parasite (Plasmodium cynomolgi in its natural host, the toque monkey), the loss of infectivity during crisis is due to the death of circulating intraerythrocytic gametocytes mediated by crisis serum. These parasite-killing effects in crisis serum are due to the presence in the serum of cytokines tumor necrosis factor and interferon gamma, which are produced by the host as a result of the malaria infection. The killing activity of each cytokine is absolutely dependent upon the presence of additional, as yet unidentified factor(s) in the crisis serum.

A mathematical model for Plasmodium vivax malaria transmission: estimation of the impact of transmission-blocking immunity in an endemic area

We have developed a multi-state mathematical model to describe the transmission of Plasmodium vivax malaria; the model accommodates variable transmission-blocking/enhancing immunity during the course of a blood infection, a short memory for boosting immunity, and relapses. Using the model, we simulated the incidence of human malaria, sporozoite rates in the vector population, and the level of transmission-blocking immunity for the infected population over a period of time. Field data from an epidemiological study conducted in Kataragama in the south of Sri Lanka were used to test the results obtained. The incidence of malaria during the study period was simulated satisfactorily. The impact of naturally-acquired transmission-blocking immunity on malaria transmission under different vectorial capacities was also simulated. The results show that at low vectorial capacities, e.g., just above the threshold for transmission, the effect of transmission-blocking immunity is very significant; however, the effect is lower at higher vectorial capacities.

Plasmodium cynomolgi: serum-mediated blocking and enhancement of infectivity to mosquitoes during infections in the natural host, Macaca sinica

The infectivity of Plasmodium cynomolgi in its natural host, the toque monkey, Macaca sinica, to Anopheles tessellatus mosquitoes was studied in relation to the evolution of anti-sexual-stage immunity in the host during the course of a blood-induced infection. The effects of serum on the infectivity of gametocytes and the intrinsic infectivity of gametocytes to mosquitoes on each day were assessed in membrane feeding experiments. Mosquitoes were also directly fed on the animal on each day. Our results demonstrate that during the very early patent period, before the peak of gametocytemia, the infection serum enhanced the infectivity of gametocytes up to two to three times above their infectivity in normal monkey serum. Subsequently, serum drawn post-peak of parasitemia ceased to enhance, and began to suppress, infectivity. After 2-3 months, long after parasitemias ceased patency, the serum no longer suppressed and between 3 and 4 months the serum again tended to enhance gamete infectivity before losing any significant effect. Serum infectivity enhancing effects were consistent with low indirect immunofluorescence test antibody titers against blood stage parasites first during the very early days of a blood infection before reaching blocking levels, and again during convalescence when antibodies were declining. The serum infectivity blocking effects on gametocytes were seen at the peak of antibody titers from about Days 9 to 23 of an infection. From 78 to 95% of the total infectivity of the parasite to mosquitoes during an infection occurred when infectivity enhancing activity was present in the serum. Hence, the infectivity of the parasite to mosquitoes was largely dependent on infectivity enhancing antibodies in host serum.

Anti-parasite effects of cytokines in malaria

Cytokines induced during natural malaria infections, e.g., at crisis of a blood infection of Plasmodium cynomolgi, and during clinical paroxysms in human Plasmodium vivax infections, mediate killing of intra-erythrocytic blood stage malaria parasites. These cytokines, TNF and IFN-gamma, require additional, yet unidentified complementary factors that are present in "crisis" and "paroxysm" serum to kill intra-erythrocytic blood stage parasites. In contrast, cytokines, (mainly IFN-gamma) are able to effect killing of intra-hepatic stages of the parasite by themselves independent of serum complementary factors, suggesting that the mechanisms of killing may be different with respect to the two parasite stages. Cytokines also appear to be critical intermediates in mechanisms of clinical disease in malaria. Serum cytokine (TNF) levels and killing effects on blood stage malaria parasites were lower in patients who were exposed to endemic P. vivax malaria who had partial clinical immunity, than in non-immune patients. Evidence suggest that individuals acquire natural immunity to the disease by avoiding the induction of high levels of cytokines and complementary factors.

Human immune responses against sexual stages of malaria parasites: considerations for malaria vaccines

Studies on the natural immune responses to the sexual stages of malaria parasites have been reviewed in the context of human malaria transmission-blocking vaccines. Antibodies against the sexual stages of the malaria parasite, gametocytes and gametes, are readily evoked by natural malaria infections. These antibodies that suppress infectivity at high concentrations can, at low concentrations, enhance the development of the parasite in the mosquito; however, because enhancing antibodies are prevalent during natural malaria infections, it is likely that a vaccine would rapidly boost these antibodies to blocking levels. The immunogenicity of sexual stage antigens appears to be constrained in the human host, probably due to T epitope polymorphism and MHC restriction in humans. These constraints apply mainly to those antigens that are sensitive targets of host immunity such as the gamete surface antigens and not to internal gamete antigens, indicating that antigenic polymorphism may have evolved in response to immune selection pressure. Evidence for immunosuppression of the host by exposure to endemic malaria is presented and its consequences on vaccine development are discussed.

Target antigens of transmission blocking immunity of Plasmodium vivax malaria. Characterization and polymorphism in natural parasite isolates

A panel of 20 anti-Plasmodium vivax female gamete mAb has been established and was characterized with respect to their transmission-blocking properties in membrane-feeding experiments and their target Ag identified. Seven mAb suppressed the infectivity of P. vivax parasites to Anopheles tesselatus mosquitoes. The m.w. of the Ag recognized by these mAb were ascertained by SDS-PAGE and Western blots. Three sets of polypeptides of low Mr--20, 24, and a doublet of 37/42 kDa--have been defined as target Ag of transmission-blocking antibodies of P. vivax. All epitopes of these target Ag were found to be dependent on the tertiary conformational structure of the Ag. Polymorphism of target Ag of transmission-blocking immunity was investigated in over 30 natural isolates of P. vivax in Sri Lanka based on the reactivity of a mAb with an isolate as assessed by the indirect immunofluorescent test with the use of live extracellular female gametes, and in Western blots with the use of extracted gametes. The functional consequences of antigenic polymorphism on immunity was investigated in transmission-blocking assays by using membrane-feeding experiments. A majority of target Ag of transmission-blocking immunity were found to be polymorphic, exhibiting size as well as epitope polymorphism. Results indicate that failure of a mAb to affect the infectivity of a parasite isolate of P. vivax to mosquitoes can be caused by polymorphism of the target Ag among isolates.

Human T cell proliferative responses to Plasmodium vivax antigens: evidence of immunosuppression following prolonged exposure to endemic malaria

Human T cell proliferative responses, of 33 adult Sri Lankans convalescing from Plasmodium vivax infections, to several P. vivax antigens (i.e. a soluble extract of asexual erythrocytic stage parasites and two cloned antigens that are potential vaccine candidates PV200 and GAM-1) were assessed. The peripheral blood mononuclear cell proliferative responses to the soluble extract of P. vivax, as assessed by studying both the proportion of responders and the degree of the response, were significantly lower in a group of individuals resident in a malaria endemic area in Sri Lanka than in another group that did not have a life-long exposure to malaria but had acquired the disease on a visit to an endemic region. Individuals of both groups responded equally well to mitogen. The responses to a non-malarial antigen such as purified protein derivative of tuberculin were only marginally lower in residents of the malaria-endemic region. These findings suggest that exposure to endemic P. vivax malaria leads to a specific immunosuppression to P. vivax antigens. Immunosuppression of a much lower degree was evident to a non-malarial antigen.

Target antigens of transmission blocking immunity of Plasmodium vivax malaria. Characterization and polymorphism in natural parasite isolates

A panel of 20 anti-Plasmodium vivax female gamete mAb has been established and was characterized with respect to their transmission-blocking properties in membrane-feeding experiments and their target Ag identified. Seven mAb suppressed the infectivity of P. vivax parasites to Anopheles tesselatus mosquitoes. The m.w. of the Ag recognized by these mAb were ascertained by SDS-PAGE and Western blots. Three sets of polypeptides of low Mr--20, 24, and a doublet of 37/42 kDa--have been defined as target Ag of transmission-blocking antibodies of P. vivax. All epitopes of these target Ag were found to be dependent on the tertiary conformational structure of the Ag. Polymorphism of target Ag of transmission-blocking immunity was investigated in over 30 natural isolates of P. vivax in Sri Lanka based on the reactivity of a mAb with an isolate as assessed by the indirect immunofluorescent test with the use of live extracellular female gametes, and in Western blots with the use of extracted gametes. The functional consequences of antigenic polymorphism on immunity was investigated in transmission-blocking assays by using membrane-feeding experiments. A majority of target Ag of transmission-blocking immunity were found to be polymorphic, exhibiting size as well as epitope polymorphism. Results indicate that failure of a mAb to affect the infectivity of a parasite isolate of P. vivax to mosquitoes can be caused by polymorphism of the target Ag among isolates.

Characteristics of malaria transmission in Kataragama, Sri Lanka: a focus for immuno-epidemiological studies

Parasitological and entomological parameters of malaria transmission were monitored for 17 months in 3,625 residents in a Plasmodium vivax malaria endemic region in southern Sri Lanka; the study area consisted of 7 contiguous villages where routine national malaria control operations were being conducted. Malaria was monitored in every resident; fever patients were screened and 4 periodical mass blood surveys were conducted. An annual malaria incidence rate of 23.1% was reported during the period: 9.3% was due to P. vivax and 13.8% was due to P. falciparum; there had been a recent epidemic of the latter in this region, whereas the P. falciparum incidence rate in the previous 10 years had been negligible. There was a wide seasonal fluctuation in the malaria incidence, with the peak incidence closely following the monsoon rains. The prevalence of malaria due to both species detected at the 4 mass blood surveys ranged from 0.98% (at low transmission) to 2.35% (at peak transmission periods). Adults and children developed acute clinical manifestations of malaria. Entomological measurements confirmed a low degree of endemicity with estimated inoculation rates of 0.0029 and 0.0109 (infectious bites/man/night) for P. vivax and P. falciparum, respectively. Several anopheline species contributed to the transmission, and the overall man biting rates (MBR) showed a marked seasonal variation. Malaria at Kataragama, typical of endemic areas of Sri Lanka, thus presents characteristics of "unstable" transmission. Malaria was clustered in the population. There was a low clinical tolerance to P. falciparum malaria, to which most had only been at risk, compared to P. vivax, to which most had had a life-long exposure.