Zweiter Bericht über die Thätigkeit der Malariaexpedition

Increasing incidence of malaria in children despite insecticide-treated bed nets and prompt anti-malarial therapy in Tororo, Uganda

BACKGROUND

The burden of malaria has decreased in parts of Africa following the scaling up of control interventions. However, similar data are limited from high transmission settings.

METHODS

A cohort of 100 children, aged six weeks to 10 months of age, were enrolled in an area of high malaria transmission intensity and followed through 48 months of age. Children were given a long-lasting insecticide-treated bed net (LLIN) at enrolment and received all care, including monthly blood smears and treatment with artemisinin-based combination therapy (ACT) for uncomplicated malaria, at a dedicated clinic. The incidence of malaria was estimated by passive surveillance and associations between malaria incidence and age, calendar time and season were measured using generalized estimating equations.

RESULTS

Reported compliance with LLINs was 98% based on monthly routine evaluations. A total of 1,633 episodes of malaria were observed, with a median incidence of 5.3 per person-year (PPY). There were only six cases of complicated malaria, all single convulsions. Malaria incidence peaked at 6.5 PPY at 23 months of age before declining to 3.5 PPY at 48 months. After adjusting for age and season, the risk of malaria increased by 52% from 2008 to 2011 (RR 1.52, 95% CI 1.10-2.09). Asymptomatic parasitaemia was uncommon (monthly prevalence <10%) and rarely observed prior to 24 months of age.

CONCLUSIONS

In Tororo, despite provision of LLINs and prompt treatment with ACT, the incidence of malaria is very high and appears to be rising. Additional malaria control interventions in high transmission settings are likely needed.

TRIAL REGISTRATION

Current Controlled Trials Identifier NCT00527800.

Functional and immunological characterization of a Duffy binding-like alpha domain from Plasmodium falciparum erythrocyte membrane protein 1 that mediates rosetting

The Duffy binding-like (DBL) domains are common adhesion modules present in Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) variants, which are responsible for immune evasion and cytoadherence. Knowledge about how immune responses are acquired against polymorphic DBL domains of PfEMP1 can aid in the development of vaccines for malaria. A recombinant DBLalpha domain, encoded by R29 var1, which binds complement receptor 1 to mediate rosetting by the P. falciparum laboratory strain R29, was expressed in Escherichia coli, renatured by oxidative refolding to its native form, and purified to homogeneity. Antibody levels in 704 plasmas obtained from residents of areas of different levels of malaria endemicity in Orissa (India) and Manhiça (Mozambique) were assessed by enzyme-linked immunosorbent assay. The refolded DBLalpha domain was pure, homogeneous, and functional in that it bound human erythrocytes with specificity and was capable of inhibiting rosetting. The proportion of individuals who had measurable anti-DBLalpha immunoglobulin G responses was low in areas of low malaria endemicity in Orissa (6.7%) but high in areas of high endemicity in Orissa (87.5%) and Manhiça (74.5%). Seroprevalence and antibody levels against the recombinant protein increased with the age of inhabitants from areas with high transmission rates (P < 0.001). Half of the children in these areas had seroconverted by the age of 5 years. These findings suggest that in spite of the extreme polymorphism of PfEMP1 DBLalpha domains, the acquisition of specific antibodies is rapid and age related and reflects the reduced risk of malaria in areas with high transmission rates. Further studies are required to elucidate the role of these antibodies in protection from malaria.

The epidemiology of malaria in adults in a rural area of southern Mozambique

Background

Epidemiological studies of malaria in adults who live in malaria endemic areas are scarce. More attention to the natural history of malaria affecting adults is needed to understand the dynamics of malaria infection and its interaction with the immune system. The present study was undertaken to investigate the clinical, parasitological and haematological status of adults exposed to malaria, and to characterize parasites in these individuals who progressively acquire protective immunity.

Methods

A cross-sectional survey of 249 adults was conducted in a malaria endemic area of Mozambique. Clinical, parasitological and haematological status of the study population was recorded. Sub-microscopic infections and multiplicity of infections were investigated using polymerase chain reaction (PCR) and restriction fragment length polymorphism of Plasmodium falciparum merozoite surface protein 2 (msp2).

Results

Prevalence of P. falciparum infection by microscopy (14%) and PCR (42%) decreased progressively during adulthood, in parallel with an increase in the prevalence of sub-microscopic infections. Anaemia was only related to parasitaemia as detected by PCR. Multiplicity of infection decreased with age and was higher in subjects with high P. falciparum densities, highlighting density-dependent constraints upon the PCR technique.

Conclusion

Adults of Manhiça progressively develop non-sterile, protective immunity against P. falciparum malaria. The method of parasite detection has a significant effect on the observed natural history of malaria infections. A more sensitive definition of malaria in adults should be formulated, considering symptoms such as diarrhoea, shivering and headache, combined with the presence of parasitaemia.

Host age as a determinant of naturally acquired immunity to Plasmodium falciparum

The usual course of infection by Plasmodium falciparum among adults who lack a history of exposure to endemic malaria is fulminant. The infection in adults living with hyper- to holoendemic malaria is chronic and benign. Naturally acquired immunity to falciparum malaria is the basis of this difference. Confusion surrounds an essential question regarding this process: What is its rate of onset? Opinions vary because of disagreement over the relationships between exposure to infection, antigenic polymorphism and naturally acquired immunity. In this review, Kevin Baird discusses these relationships against a backdrop of host age as a determinant of naturally acquired immunity to falciparum malaria.

Human resistance to Plasmodium falciparum increases during puberty and is predicted by dehydroepiandrosterone sulfate levels

Immunity to Plasmodium falciparum develops slowly in areas of endemicity, and this is often ascribed to poorly immunogenic or highly variant parasite antigens. However, among populations newly exposed to malaria, adults acquire immunity more rapidly than children. We examined the relationship between pubertal development and resistance to P. falciparum. During two transmission seasons in western Kenya, we treated the same cohort of young males to eradicate P. falciparum and then obtained blood smears each week for 4 months. We determined pubertal development by Tanner staging and by levels of dehydroepiandrosterone sulfate (DHEAS) and testosterone in plasma. In multivariate and age-stratified analyses, we examined the effect of pubertal development on resistance to malaria. In both seasons (n = 248 and 144 volunteers, respectively), older males were less susceptible than younger males. Age-related decreases in the frequency and density of parasitemia were greatest during puberty (15- to 20-year-olds). DHEAS and testosterone were significant independent predictors of resistance to P. falciparum parasitemia, even after accounting for the effect of age. Fifteen- to 20-year-old males with high DHEAS levels had a 72% lower mean parasite density (P<0.01) than individuals with low DHEAS levels. Similarly, 21- to 35-year-old males with high DHEAS levels had a 92% lower mean parasite density (P<0.001) and 48% lower frequency of parasitemia (P<0.05) than individuals with low DHEAS levels. These data suggest that the long period needed to attain full immunity could be explained as a consequence of host development rather than as the requirement to recognize variant or poorly immunogenic parasite antigens.

The dynamics of drug resistance in Plasmodium falciparum

Drug resistance of Plasmodium falciparum is not a recent phenomenon, but it became a major problem when the parasite became resistant to chloroquine, the cheapest and initially the most effective antimalarial compound that could be used for treatment and suppression. In some areas this problem is compounded by resistance to the first line of alternative drugs, and rapid loss of sensitivity to the next line. The dynamics of drug resistance are regulated mainly by drug related selection pressure and intensity of malaria transmission. Mass drug administration in its various forms, and insufficient treatment are obviously the most important motors of selection.

Malaria of the orang-utan (Pongo pygmaeus) in Borneo

The primary objective of this project was to study the life cycle and ecology of Plasmodium pitheci, a malaria parasite of the orang-utan. The field work was based on the orang-utan rehabilitation centre in the Sepilok Forest Reserve of eastern Sabah. Two visits were made to Sepilok, the first in February and March, 1972, and the second (by W.P.) in January 1974. On the first visit two species of "surrogate host" were taken to Sabah, i.e. chimpanzees and Aotus monkeys for experimental work. The arboreal habitat of the orang-utan in the dipterocarp forests of eastern Sabah is described. In the Sepilok Forest Reserve dwell gibbons and leaf-monkeys, in addition to a small population of semi-domesticated and wild, free-ranging orang-utans of various ages. Although numerous species of anopheline mosquitoes have been collected in eastern Sabah, longitudinal studies are not available. Anopheles balabacensis was caught both attracted to orang-utans and to man at Sepilok. This species which is the main vector of human malaria in the north of Borneo, is suspected also of transmitting orang-utan malaria in this part of Sabah. Repeated blood examinations have been made on a number of orang-utans in the centre since 1966 and a high prevalence of infection was recorded with Plasmodium pitheci. In 1966 10 out of 19 animals had demonstrable parasitaemia. Detailed case histories are presented to show the course of parasitaemia in several orang-utans. Infections of P. pitheci were found to run a very chronic course. During the 1972 expedition a second, previously undescribed malaria parasite of the orang-utan was discovered, and was named P. silvaticum. The new parasite was successfully transmitted both by blood inoculation and, later, by sporozoite inoculation, into splenectomized chimpanzees. Although both species of malaria parasite may cause transitory signs of illness, orang-utans in general appear to be little discomforted by the infection. The animals do however suffer from other infectious diseases such as amoebic and balantidial dysentery, and melioidosis is a serious natural hazard which may have accounted for several deaths of wild orang-utans. An unidentified, intraerythrocytic structure that appeared in the blood of one chimpanzee, which had been inoculated with blood from an orang-utan, may have contributed to its death. Detailed descriptions and illustrations of P. pitheci and P. silvaticum are given. All stages of the life cycle of P. silvaticum are known (the tissue stages having been described in the liver of a "surrogate host", the chimpanzee) but only the blood and sporogonic stages of P. pitheci have been seen. This species was not infective to a chimpanzee, although there is an earlier report of a transient infection in this host by other workers. In the blood both parasites showed a tertian periodicity. From the appearance of the tissue schizonts on the seventh day it was estimated that the complete pre-erythrocytic cycle of P. silvaticum in the chimpanzee would occupy 8 days. P...