Background
Residents of malaria endemic areas frequently harbour asexual blood stage parasites without developing symptoms or signs of acute febrile illness, implying that some degree of clinical tolerance to parasitaemia is acquired through repeated exposure to and experience with chronic blood stage infection. Epidemiological studies of this phenomenon have focused mainly on
Plasmodium falciparum, the dominant malaria species world wide, and attempted to quantify this complex clinical phenotype at a population level by estimating the peripheral parasite density at which body temperature exceeds a specific level or cut off value, i.e. the pyrogenic threshold [
1]) or the probability of acute febrile illness as a function of parasite density [
2‐
4]. In contrast, similar analyses of clinical tolerance to other major malaria species that infect humans,
Plasmodium vivax,
Plasmodium malariae and
Plasmodium ovale, are limited to one study of
P. vivax from Punjab [
5] and another of
P. ovale from Senegal [
6].
Understanding the differences in clinical tolerance to parasitaemia among various malaria species may be important in areas of the world where P. vivax and P. falciparum are co-endemic such as in Asia, the Pacific and South America. Anti-malarial drug resistance or deployment of vaccines that preferentially affects one species may alter innate and adaptive immunity and clinical tolerance to the other.
In highly endemic areas for
P. falciparum, the fever threshold expressed in terms of the density of parasitaemia in peripheral blood at which a given body temperature is exceeded declines progressively after the age of one year [
1,
2,
7]. Thus, children with high parasite densities tend to be asymptomatic compared with adults or adolescents with similar levels of peripheral parasitaemia. Nevertheless adults have a lower incidence of clinical malaria attacks than children. because
P. falciparum density is on average controlled at a lower level in adults than children
This is consistent with the idea that tolerance is the consequence of an immunological response with little memory, stimulated by toxins released during schizogony, and several possible mediators of tolerance have been proposed with this in mind, notably the anti-inflammatory molecule nitric oxide (NO) [
8] and antibodies to GPI [
1]. However recent studies in Papua New Guinea suggest that cytokine responses to GPI can better account for both immunological and epidemiological patterns [
9].
Variations in tolerance are only one possible explanation for differences in the operating characteristics of diagnostic thresholds of peripheral parasitaemia in
P. falciparum. Changes in pyrogenic threshold could also be explained in terms of differences in the ratio of circulating to sequestered parasites. Pyrogens or putative malaria toxins are released when sequestered
P. falciparum-infected erythrocytes burst during schizogony. Hence pyrogen concentrations may reflect more directly the density of sequestered parasites than that of trophozoites in the peripheral circulation. At present, approaches for assessing the number of sequestered
P. falciparum parasites in the living human host remain controversial [
10,
11], and there is no means of reliably quantifying circulating toxix(s) until their molecular nature is better understood. In contrast, the rate of schizogony and level of malaria toxin release should be approximately proportional to the peripheral blood parasite density in
P. vivax and
P. malariae infection since these species are not thought to sequester in deep vascular beds.
In population based studies, it is possible to estimate the apparent degree of clinical tolerance relative to the probability of an individual experiencing a given peripheral density. This approach may be a better way of assessing tolerance than using specific diagnostic cut-off values since the latter depends on the extent of incidental parasitaemia in a population as well as the pathogenic effect of a given parasite density. We have carried out an analysis of the relationships between the incidence of acute illness attributable to P. vivax and P. malariae and the densities of circulating parasites in an area of Papua New Guinea where these two malaria species as well as P. falciparum are highly endemic, over the period 1991–2003. The results for P. vivax and P. malariae are compared with those for P. falciparum, with consideration of the age dependence of apparent tolerance for each malaria species and implications for models of malaria pathogenesis and disease burden.
Discussion
Previous studies of malaria tolerance by age have generally concentrated on the age patterns in the attributable fractions (Figure
3a). By focusing on the diagnostic performance of different cut-offs and the identification of pyrogenic thresholds, such studies conclude that the age distribution of the pyrogenic threshold is similar to that of parasite densities, with high diagnostic cut-offs required in young children, who thus appear more tolerant. However the diagnostic performance of these cutoffs depend not only on pyrogenic thresholds, but also on age patterns of non-malaria fevers. The present analyses allow for age-variations in overall fever incidence in reporting incidence of attributable disease and suggests different age-patterns of tolerance.
Highly relevant to the mechanism of tolerance is the question of whether it is specific for a given species of
Plasmodium, or whether it is common to all malaria species. There may even be cross-tolerance with bacteria [
9], since malariatherapy studies found that Plasmodium infection can reduce the response to bacterial endotoxins [
28,
29].
Prior to our study there was little epidemiological evidence of whether tolerance to different
Plasmodium species follow similar dynamics. In the Punjab, the relationship between morbidity and parasite densities was found to be age-dependent in
P. falciparum, but not so in
P. vivax [
5]; however, unlike the situation in Papua New Guinea, this was observed in an area of relatively low malaria transmission where repeated infections with different malaria species are infrequent.
In Wosera, there are substantial differences between species in the prevalence and density of infections and in clinical incidence.
P. falciparum is clearly the most important cause of malaria morbidity (82.9%), followed by
P. vivax (15.1%), with
P. malariae accounts for only 2.1% of attributable cases. However, across most densities and age groups the incidence of disease at a given parasite density is similar for all three species, and much of the variation between the lines for different age groups in Figure
3b is in the less frequent density classes (i.e. the low density classes for
P. falciparum, and the higher ones for
P. vivax and
P. malariae), where sampling variation clearly plays a role. For all three species, the lowest attributable fractions at any given parasite density occur in the youngest age group (Figure
3a), but tolerance is achieved with similar age dynamics, even though infection with the different species occurs at different rates, and the age patterns of attributable morbidity are very different (Figures
3 and
2b).
The most important difference in age patterns of morbidity is the relatively high incidence of
P. vivax morbidity compared with
P. falciparum in the youngest age groups (Figure
2b). This contrasts with the higher prevalence of
P. falciparum in the same age groups (Figure
2a) and with the higher entomological inoculation rate of the latter parasite[
12]. This seems to mainly reflect better control of
P. falciparum densities in infants than of
P. vivax densities, and could reflect better protection for the former by maternal antibodies [
30] or/and active sensitization
in utero [
31]. Pregnant women in the Wosera are more likely to be infected with
P. falciparum than with
P. vivax [
32], which might lead to more acquired protection against high density parasitaemia in the former case. However it also appears to be the case that, at any given peripheral parasite density,
P. falciparum is less likely to cause disease in infants than it would in older age groups.
Systematic variation in the ratio of circulating to sequestered parasites with age has previously been suggested as explanation for patterns of age- and seasonal variation in apparent tolerance of
P. falciparum in infants[
33]. An important biologic difference between
P. falciparum and the other species is sequestration of late trophozoites in the former, which means that the density of
P. falciparum in peripheral blood is an indirect and possibly imprecise measure of the rate of pyrogen release at schizogony. Variation in the ratio of circulating to sequestered parasites presumably contributes imprecision to our analyses and also lends itself as a possible explanation of why there seems to be more age variation in levels of tolerance for
P. falciparum than for the other species (Figures
3), despite the greater sample size.
The interpretation of such variations also needs to take into consideration the logarithmic scales used on the axes of Figure
3b, so small differences, notably the rather higher incidence at given densities for
P. vivax, are not very obvious. Even if the mechanisms of tolerance are related, equivalence cannot be assumed in the pyrogenic potential of equal parasite counts of different species, with different biochemistry. The absence of sequestration in
P. vivax and
P. malariae means that the rate of schizogony relative to the circulating density must be much lower than for
P. falciparum, and the longer erythrocytic cycle of
P. malariae must also mean that it has an even lower rate of schizogony relative to the circulating density.
Variation in age-incidence patterns may also arise because of biases in the available data. Most obviously, not all episodes report to a health facility, so incidence estimates need to be adjusted for imperfect access if they are to be translated into disease burden. Age patterns in illness perception and help-seeking could bias the clinical incidence data, but there is no clear evidence that such biases are important in Wosera. For instance the effect of distance from health facility on help-seeking for febrile illnesses is independent of age group [
18]. The control surveys were based on sampling from a complete demographic database representative of the population and so do not represent a substantial source of bias.
Despite the effects of all these other factors, it appears that age variation in clinical incidence mainly arises because of differences in the ability to control parasitaemia, and both age- and species-variation in tolerance are secondary phenomena. Since tolerance may arise in tandem for the different parasite species, this suggests there may be cross-species mechanisms of tolerance, and leads to a similar empirical relationship between parasite density and incidence of attributable disease for all species.
This potentially provides a practical approach for burden of disease assessments in areas with high malaria endemicity, since it could provide a straightforward way of using representative community-based data to avoid the limitations of health management information systems. There is a clear need to evaluate the generalizability of this relationship to other settings, both to evaluate its practical utility for estimating disease burden from survey data, and for further understanding the biology of malaria tolerance.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
IM & TAS designed the study, conducted the analyses and wrote the initial draft of the paper. BG assisted in study design and with TAS and MA established the surveillance of clinical cases. LR, BK, and WK led field work and organised cross-sectional surveys. PZ, JK, MA were responsible for conduct of the 1998/99 and 2001–03 surveys. All authors participated in writing of the manuscript and approved the final version.