Age and seasonal variation in the transition rates and detectability of Plasmodium falciparum malaria

The effect of acquired immunity on the duration of Plasmodium falciparum infections is unclear, although this is an important term in models of malaria transmission. It is problematical to determine the duration of infections because of the difficulty of distinguishing persisting infections from new ones, and because parasite densities are often transiently below the limit of detection. We recently developed a dynamic model for infection incidence, clearance and detection of multiple genotype P. falciparum infections and fitted it to a panel dataset from a longitudinal study in Northern Ghana. We now extend this model to allow for seasonal and age variation in infection rates and also age dependence in clearance and in detectability of infections. These models indicate that there is seasonal variation in the infection rate, and age dependence in detectability. The best fitting models had no age dependence in infection or clearance rates, suggesting that acquired immunity mainly affects detectability.

[Prevalence, genetic diversity and multiplicity of Plasmodium falciparum infection in school children in central Cote d'Ivoire]

A study was carried out in the village of Taabo, located in the vicinity of a large man-made lake in central Côte d'Ivoire. The objectives were (i) to determine the level of prevalence, genetic diversity and multiplicity of Plasmodiumfakiparum infection in schoolchildren and (ii) to compare the diagnostic performance of light microscopy and polymerase chain reaction (PCR). A total of 424 schoolchildren ranging in age from 5 to 15 years underwent diagnostic testing using both light microscopy of blood smears and PCR. Multiplicity of P. falciparum infection was investigated in 196 children (46.2%). The prevalence of malaria was 54.7% based on light microscopy and 83.9% based on PCR. Genotyping based on polymorphism in the length of the restriction fragment of the gene encoding the merozoite surface protein-2 (msp2) showed that 86.5% of cases involved multiple infection with a geometric mean of 3.87 genotypes per positive child. There was a strong positive correlation between multipcity of infection and parasite density in the 56-year old age group. A total of 50 genotypes including six observed for the first time were identified and classified into families with similar-sized sequence groups: 26 x FC27 (52%) and 24 x 3D7 (48%). In comparison with PCR, the sensitivity and specificity of light microscopy for diagnosis of P. falciparum was 81.3% and 88.2% respectively. Data are discussed in the light of similar studies carried out in sub-Saharan Africa and elsewhere. These findings can serve as a basis for monitoring the longterm effect of major water resource management projects on the prevalence, genetic diversity and multiplicity of P. falciparum infection.

A prospective study of Plasmodium falciparum multiplicity of infection and morbidity in Tanzanian children

Several studies suggest that in individuals with substantial previous exposure to malaria, co-infection with multiple clones of Plasmodium falciparum can protect against subsequent clinical malaria attacks. Other studies, mainly of individuals with little previous exposure, found the converse relationship. To test whether acquisition of such cross-protection tracks the acquisition of clinical immunity in general, 610 Tanzanian children aged 0-6 years were enrolled in a nine-month prospective study of the risk of morbidity in relation to parasitological status and merozoite surface protein 2 genotypes on enrolment. Prevalence of parasitaemia and multiplicity of infection increased with age. In the first year of life, the incidence of clinical malaria was almost three times higher in children with parasites at baseline than in those without. In older children, baseline P. falciparum infections appeared to protect against both parasitaemic and non-parasitaemic fever episodes. In children aged less than three years, baseline multiple infection tended to be associated with higher prospective risk of clinical malaria than single infection while in children aged more than three years the converse was found, but these effects were not statistically significant. These results provide further evidence that relationships between asymptomatic malaria infections and clinical malaria change with cumulative exposure.

Species-specific field testing of Entamoeba spp. in an area of high endemicity

Entamoeba histolytica has been separated in recent years into 2 morphologically identical species: the apathogenic E. dispar and the pathogenic E. histolytica, only the latter being pathogenic. Although various laboratory techniques allow discrimination between the 2 species there is a lack of field data about the suitability of available diagnostic tests for use in epidemiological studies and few epidemiological studies using species-specific diagnosis have been performed at community level in endemic areas, especially in sub-Saharan Africa. We conducted a repeated cross-sectional study of 967 schoolchildren in central Côte d'Ivoire to compare and evaluate light microscopy, 2 different antigen detection assays, and one polymerase chain reaction (PCR) assay. Microscopy and a non-specific antigen capture Entamoeba enzyme-linked immunosorbent assay (ELISA) were used for the primary screening of all children (time t0). The prevalence of the E. histolytica/E. dispar species complex at t0 was 18.8% by single microscopical examination and 31.4% using the non-specific ELISA. Approximately 2 months after the initial screening, fresh stool specimens were collected on 2 consecutive days (t1 and t2) from (i) all the children who were positive by microscopy at t0 (n = 182) and (ii) 155 randomly selected children who were negative at the primary screening. These samples were tested with a second antigen detection ELISA specific for E. histolytica (n = 238) and with a species-specific PCR assay (n = 193). The second and third examinations (t1 and t2) revealed an additional 43 infections with the species complex E. histolytica/E. dispar, so that the cumulative microscopical prevalence for t1 and t2 was 27.7%. The overall prevalence of E. histolytica by species-specific ELISA antigen detection was low (0.83%), while the prevalence of E. dispar was 15%. When analysing only microscopically positive samples by PCR (n = 129), the ratio E. histolytica: E. dispar was very low (1:46), suggesting that the vast majority of Entamoeba infections in this area were apathogenic. Both species-specific tests performed well but the ELISA was easier to use for large-scale field screening.

Molecular epidemiology of Plasmodium falciparum infections among asymptomatic inhabitants of a holoendemic malarious area in northern Ghana

Age dependence of malaria infection was assessed in an age-stratified cluster sample of 308 individuals from Kassena-Nankana District of northern Ghana during June and July 2000. Overall prevalence of Plasmodium falciparum by microscopy was 70%, with the maximum among 5-9 year olds. Parasite density was highest (geometric mean 1922/microl blood) in 1-2 year olds. Eighty-two per cent of samples were positive by polymerase chain reaction (PCR), and restriction fragment length polymorphism typing of the P. falciparum msp2 revealed a mean msp2 multiplicity of 3.4 (range: 1-8) genotypes per PCR positive sample. Multiplicity increased with age until 5-9 years and then started to reduce again into adulthood. About 49.3% of infections belonged to the msp2 FC27 allelic family and 50.7% to the 3D7 family. On the day of the survey, only 3.6% of the participants had fever (axillary temperature >or= 37.5 degrees C) and 2.3% had fever associated with parasitaemia. The correlation between parasite density and msp2 multiplicity was 0.42; highest among infants, and decreased with age to a minimum among 5-9 year olds. Contrasting with results from Tanzania, this correlation increased with age in adolescents and adults. Parasite multiplicity is very high in this community, and the patterns of age dependence are similar to those in other holoendemic sites in Africa, validating the use of the age-multiplicity relationship as an indicator of malaria endemicity.

Prospective risk of morbidity in relation to multiplicity of infection with Plasmodium falciparum in São Tomé

The prospective risk of acute morbidity was analysed in relation to multiplicity of Plasmodium falciparum infection in 491 individuals in a peri-urban community in São Tomé. In an initial cross-sectional survey, 40.5% of individuals were recorded by microscopy as infected with P. falciparum, and by PCR 60.5%, with the maximum prevalence in children aged 5-10 years. PCR-RFLP typing of the msp-2 gene of P. falciparum found a mean of 2.4 parasite genotypes per infected person, with little age dependence in this multiplicity and a total of 43 different msp-2 alleles identified. None of these were unique for São Tomé. Study participants were encouraged to report to a project worker whenever they suffered a febrile illness. During the 3 months following the parasitological survey the recorded incidence rates decreased with increasing baseline msp-2 multiplicity, both for P. falciparum-positive episodes and for fever without parasitaemia. While this is consistent with suggestions that multiple P. falciparum infections may protect against super-infecting parasites, confounding by patterns of health service usage is an alternative explanation. The incidence of clinical malaria episodes was only a little higher in children than in adults. This weak age-dependence in clinical immunity might be a consequence of a cohort effect resulting from resurgence of the disease after the breakdown of malaria control programs in the 1980s.

Limited polymorphism in Plasmodium falciparum sexual-stage antigens

In areas highly endemic for malaria, individuals are frequently found to be infected simultaneously with multiple Plasmodium falciparum clones. This raises the question of whether all parasite clones produce gametocytes equally or whether gametocytogenesis is suppressed in some clones. In order to assess this in epidemiological studies, polymorphic genes specifically expressed in gametocytes could be analyzed by both amplification of genomic DNA from blood samples and by reverse transcribed polymerase chain reaction amplifying expressed gametocyte-specific genes only. Here we report the analysis of diversity in the three gametocyte-specific genes Pfs16, Pfs48/45, and Pfs230. In addition to the previously published data, limited polymorphism was found in the coding sequences of Pfs16 and Pfs48/45. Larger polymorphism was identified in Pfs230, which might allow the development of a discriminating PCR-based genotyping scheme for transmission studies. However, the limited polymorphism in Pfs16 and Pfs48/45 renders these molecules poorly useful for such studies.

Plasmodium falciparum: expression of gametocyte-specific genes in monolayer cultures and malaria-positive blood samples

In order to facilitate molecular epidemiological studies on the transmission of the malaria parasite Plasmodium falciparum a sensitive assay for gametocyte detection based on RT-PCR was developed. The transcription of the sexual stage-specific genes Pfs16, Pfs48/45, Pfs230, and Pfs25, as well as the sexual stage- and sporozoite-specific S 18S rRNA, was detected by RT-PCR. S 18S rRNA was present in seven of nine P. falciparum-positive blood samples, despite the lack of microscopic detection of gametocytes and a parasitemia below 0.1%. Expression of the other four gametocyte-specific genes was detected less frequently in malaria-positive blood samples. These findings indicate that RT-PCR of S 18S rRNA is a highly sensitive method for gametocyte detection and, furthermore, that gametocytes are present in the peripheral blood of most malaria carriers, even if the parasitemia is below 0.1%. To determinate the expression pattern of sexual stage-specific genes in more detail, RT-PCR was performed at consecutive time points of highly synchronized monolayer cell cultures. Transcripts of all examined genes except Pfs25 were detected directly after invasion of merozoites of the strain NF54 in red blood cells.

Genomic distribution and functional characterisation of two distinct and conserved Plasmodium falciparum var gene 5' flanking sequences

Approximately 50 highly diverse var genes distributed throughout the haploid genome of the malaria parasite Plasmodium falciparum code for PfEMP1 variants located on the surface of infected erythrocytes. PfEMP1 is involved in cytoadherence of parasitised red blood cells and undergoes antigenic variation through differential expression of var genes. Members of the var gene family are located in chromosome-internal positions on chromosomes 4, 7, 8 and 12, and in subtelomeric regions of all chromosomes. Here we show that there are two distinct and conserved types of 5' upstream regions (var17-type and 5B1-type) of var genes, and suggest that most subtelomeric var genes are flanked by a var17-type 5' upstream sequence. In contrast, 5B1-type 5' upstream are localised to chromosomes that have been shown to contain var genes within chromosome-internal regions. Transcriptional analysis using RT-PCR revealed that var genes flanked by either type of 5' upstream sequence are transcribed in in vitro cultured trophozoite stage parasites. In addition, we have shown that the 5' flanking sequences of four different var genes are able to drive transient expression of the cat reporter gene. Our results suggest that at least the minimal regulatory sequences required for transcription of var genes are conserved among both subgroups of the var gene family. Furthermore, these sequences provide new markers for the investigation of the chromosomal organisation of var genes.

Analysis of Plasmodium falciparum infections in a village community in Northern Nigeria: determination of msp2 genotypes and parasite-specific IgG responses

The genetic diversity of P. falciparum and multiplicity of infection has been studied in a village in Northern Nigeria at the end of the rainy season, when transmission is high. We analysed blood samples from 104 individuals aged 5-70 years by polymerase chain reaction (PCR) amplifying the gene for the merozoite surface protein MSP2 followed by genotyping based on restriction fragment length polymorphism (RFLP). 94.2% of all samples were parasite positive by PCR and over 80% of those had multiple infections. The age distribution of the average number of parasite clones present in P. falciparum infections showed an initial increase, then reached a peak multiplicity in children 8-10 years of age, and afterwards decreased significantly with age. Mean multiplicity in those 8-10-year-old children was 5.4 clones per carrier. Peak multiplicity and parasite diversity in Nigerian individuals is compared to findings from other study sites in Africa and PNG. The prevalence of IgG antibodies against the circumsporozoite protein (CSP), an indicator for malaria exposure, was over 85% in all age groups showing a high exposure of villagers to P. falciparum. OD values in ELISA were positively correlated with age. There was no correlation between the level of IgG against CSP and the multiplicity of P. falciparum infections determined by PCR of msp2. These results imply that in highly endemic areas multiplicity of infection is not directly correlated with exposure to P. falciparum.

Consequences of multiple infection with Plasmodium falciparum in an area of high endemicity

Most Plasmodium falciparum infections occur in partially immune hosts in highly endemic areas. In such situations, many hosts are simultaneously infected with multiple parasite genotypes, which must lead to intense competition between different parasite populations. We here summarise a series of studies of multiple infection, mostly using polymerase chain reaction-restriction fragment polymorphism (PCR-RFLP) genotyping of the highly polymorphic msp-2 gene. These indicate that chronic infections, characteristic of the partially immune host, appear to protect against super-infecting parasites. This protection is not seen in infants. A consequence is that selection for fast-growing (virulent) parasites, occurs mainly in the youngest, immunologically naïve, hosts. The normal situation for P. falciparum is one in which the host is partially immune, and competition between parasite genotypes in this situation is not expected to result in selection for virulence.

Rapid DNA extraction for molecular epidemiological studies of malaria

DNA isolation from blood samples collected in molecular epidemiological studies is crucial for the quality and reproducibility of data. Blood samples from two malaria endemic sites have been prepared by four different DNA isolation methods with subsequent PCR amplification of the msp2 locus of Plasmodium falciparum. We tested a rapid boiling method; the guanadine isothiocyanate DNA extraction; QIAmp blood kit; and the ISOCODE STIX PCR template preparation dipstick, and analysed the numbers of concurrent infections/sample. The rapid boiling method and the ISOCODE STIX provided overall the best sensitivity combined with ease of handling. The possibility to store and ship the ISOCODE STIX at ambient temperature adds further advantage to this method.

Malaria infection and morbidity in infants in relation to genetic polymorphisms in Tanzania

To investigate the effects of host polymorphisms on malaria morbidity and infection, the frequency distributions of TNF alpha promotor gene and sickle cell trait were studied in infants in an area highly endemic for Plasmodium falciparum transmission in Tanzania. Differences in parasite prevalence, density, febrile episodes with and without parasitaemia, PCV levels and the frequencies of different MSP-2 parasite infections were assessed by genotype. The frequency of the TNF alpha promotor allele 2 was 0.09, and the trait was in Hardy-Weinberg equilibrium. There were no differences in malariametric indices between infants with the normal TNF alpha promoter gene and those who were heterozygous for this trait. Infants heterozygous for the TNF alpha promotor gene appeared to have fewer febrile episodes when they were free of parasites. The two infants homozygous for the TNF alpha promoter allele 2 had both a much higher incidence of fever, independently of parasitaemia, than the average for the other genotypes. The frequency of the sickle cell allele S was 0.06 and the trait was also in Hardy-Weinberg equilibrium. Infants heterozygous for the sickle cell trait had significantly lower parasite densities, but similar prevalence, and MSP-2 infections compared to infants with normal haemoglobin. PCV levels, and the incidence of febrile episodes both with and without parasitaemia were also similar. In contrast to the sickle cell trait, the TNF alpha promotor polymorphism appeared not to have any protective effect on malaria in this study, and its importance in other unspecified fever-causing diseases in this population needs further investigation.

Dynamics of multiple Plasmodium falciparum infections in infants in a highly endemic area of Tanzania

The force of infection and recovery rate for malaria in infants in a highly endemic area of Tanzania were analysed using polymerase chain reaction-restriction fragment length polymorphism genotyping of the Plasmodium falciparum msp2 locus in 99 paired blood samples. Overall, new genotypes were acquired at a rate of 0.064 per day, and the average duration of infections was estimated to be 23 d. The highest recovery rates were in children under 4 months of age. The higher susceptibility of infants to clinical malaria in comparison with older children, in areas of very high transmission, may be largely a consequence of the short duration of infections which precludes the establishment of concomitant immunity. The high turnover of infections also implies that infection prevalence and multiplicity approach an equilibrium even in very young children, and calls into question the use of infant conversion rates as a measure of transmission intensity.

Effect of insecticide-treated bed nets on the dynamics of multiple Plasmodium falciparum infections

The rates of acquisition and loss of individual genotypes belonging to the FC27 family of the Plasmodium falciparum merozoite surface protein 2 (msp2) gene were studied in 120 children aged 5 months to 2.5 years, in a randomized controlled trial of insecticide-treated bed nets (ITNs) in Kiberege village, Tanzania. Analysis of longitudinal changes in positivity for individual alleles in samples collected at intervals of one month indicated that the average duration of infections, allowing for undetected parasite genotypes, was 73 d in those aged < 18 months and 160 d in children aged > or = 18 months, consistent with a shift from acute to chronic infection with age. Overall, 51% of genotypes infecting the host were estimated to be detected by polymerase chain reaction-restriction fragment length polymorphism analysis in any one sample of 0.5 microL of packed peripheral blood cells. In children less than 18 months old this sensitivity was 61% (SE = 6%) compared with 41% (SE = 6%) in older children. Conversely, the rate of appearance of new parasite genotypes was higher in children < 18 months of age than in older children, but this partly reflected the difference in sensitivity. The overall incidence of new infections was estimated to be reduced by 17% in ITN users. There was no statistically significant difference between users and non-users in observed infection multiplicity, sensitivity, recovery rate, or estimated infection rates for individual alleles. This suggests that, in areas of high P. falciparum endemicity, ITNs have little effect on the establishment of chronic malaria infection.

Premunition in Plasmodium falciparum infection: insights from the epidemiology of multiple infections

Epidemiological studies of multiple clone infections by Plasmodium falciparum in highly endemic areas have demonstrated age dependence in both the multiplicity of infection and the relationships between this multiplicity and the risk of acute illness. We hypothesize that, in infants, host defence against blood-stage infections with P. falciparum relies mainly on fever and cytokine activities, and the infections are of short duration. In older children, a high multiplicity of infection is characteristic of low-level chronic parasitaemia. This appears to confer cross-protection against newly inoculated parasites, via partially genotype-specific responses which are short-term, lasting little longer than the infections themselves. This has important implications for our understanding of immunity against P. falciparum, its ecological niche, and the epidemiological impact of interventions against it.

Multiple Plasmodium falciparum infections in Tanzanian infants

Paired blood samples from 99 Tanzanian infants were analysed to examine the infection dynamics of Plasmodium falciparum during the first year of life. Infecting parasites were genotyped by polymerase chain reaction amplification of the polymorphic gene for the merozoite surface protein 2 and subsequent analysis according to the resulting restriction fragment length polymorphism pattern. The same samples served as controls in a parallel case-control study for which an additional blood sample was taken from each child during a fever episode. The relationship of the number of concurrent infections (multiplicity) with age and morbidity was analysed and results were compared to those of a similar study on older children between 2 and 7 years of age, carried out in the same village at the same time. The mean of 2 infecting genotypes per positive blood sample from community surveys was low compared to that in older children, and there was no significant age-dependency of multiplicity within the first year of life. Multiplicity of infection in fever cases was also independent of age. In infants, multiplicity was positively associated with parasite density and risk of clinical malaria, in contrast to the situation in older children (> 2 years). The findings help in the understanding of infection dynamics, premunition, and development of semi-immunity in malaria.

Age dependence of the multiplicity of Plasmodium falciparum infections and of other malariological indices in an area of high endemicity

The relationship between age and various malariological indices in the Kilombero valley of Tanzania were examined by compiling data from 6 different community studies carried out between 1989 and 1996. The rate of acquisition of Plasmodium falciparum infection was highest in children 1-5 years of age, while recovery rates were lowest between the first birthday and early adolescence. As a result, peak prevalence was reached in 3-5 years old children. However, the prevalence of clinical malaria (estimated from the excess risk of axillary temperatures > or = 37.5 degrees C attributable to parasitaemia) was highest in children under one year of age. The peak in multiplicity of infection (identified by polymerase chain reaction-restriction fragment length polymorphism of the msp2 locus) occurred in 3-7 years old children. There was a significant correlation between parasite density and multiplicity of infection in infants and young children (1-2 years of age) but not in older individuals.

Effect of iron supplementation and malaria prophylaxis in infants on Plasmodium falciparum genotypes and multiplicity of infection

During a randomized placebo-controlled trial of chemoprophylaxis against Plasmodium falciparum malaria and iron supplementation, in infants living under conditions of intense transmission, all samples of P. falciparum obtained from children aged 5 and 8 months were genotyped by polymerase chain reaction-restriction fragment length polymorphism analysis for the msp2 locus. One hundred and six blood samples were analysed for the number of concurrent infections (multiplicity), and the allelic family of each msp2 genotype was determined. Mean multiplicity of infection was, overall, 2.76 infections/child, and it was significantly reduced in infants receiving chemoprophylaxis. This finding might help to explain the rebound effect in morbidity observed after prophylaxis was ended. Iron supplementation did not affect multiplicity of infection. In infants receiving placebo only, or placebo and iron supplementation, a significant positive association was observed between the number of infections and parasite densities (Spearman's p = 0.25, P-0.047). This association was lost in the group receiving chemoprophylaxis alone, or in combination with iron. This study showed a significant association of FC27-like msp2 alleles with prospective risk of clinical malaria in children (relative risk = 1.487, P = 0.013). Such an association was also found for the present risk of clinical malaria in infants receiving prophylaxis (odds ratio = 3.84, P = 0.026), which might imply that chemoprophylaxis may impair the development of premunition.

Genotypes of merozoite surface protein 2 of Plasmodium falciparum in Tanzania

The merozoite surface protein 2 (MSP2) of Plasmodium falciparum is extremely polymorphic: 82 different msp2 alleles were found in 4 studies of molecular epidemiology conducted in Tanzania. This diversity renders msp2 suitable as a marker gene for the genotyping of P. falciparum infections. Amplification of msp2 by the polymerase chain reaction (PCR), and subsequent restriction digests of the PCR product (PCR-restriction fragment length polymorphism genotyping), has proved to be an informative tool for enumerating multiple concurrent infections in a blood sample, and distinguishing individual alleles. Depending on the specific questions asked in a genotyping study, analytical techniques of different degrees of complexity are employed. The restriction fragments resulting from a single HinfI digest generally allow the enumeration of multiple concurrent infections and the determination of their allelic families. When a restriction pattern is too complex to be resolved, owing to the high number of concurrent infections, or due to the appearance of previously undescribed alleles, one or more additional digests (DdeI, RsaI, ScrfI) may be necessary. To determine individual alleles unequivocally, in particular in longitudinal studies, when several consecutive samples need to be compared with each other, a more detailed analysis involving all 3 additional digests is applied. The methodological experience and results gained in 4 epidemiological field studies involving msp2 genotyping are summarized. We also provide the HinfI restriction patterns and some nucleotide sequences of the alleles found so far in our studies in Tanzania.

Effect of insecticide-treated bed nets on haemoglobin values, prevalence and multiplicity of infection with Plasmodium falciparum in a randomized controlled trial in Tanzania

A randomized controlled trial of insecticide-treated bed nets (ITNs) was conducted in an area of high malaria transmission in Tanzania in order to assess the effects of ITNs on infection and anaemia. One hundred and twenty-two children, aged 5 to 24 months, were randomly allocated to 2 groups, one of which received ITNs. Outcome measures were assessed in 6 consecutive months with monthly cross-sectional surveys. These measures were haemoglobin values, Plasmodium falciparum prevalence and density, and multiplicity of infection determined by polymerase chain reaction-restriction fragment length polymorphism analysis (PCR-RFLP) of the msp2 locus. There was a significant increase in mean heamoglobin values and a significant decrease of 16.4% in microscopically determined P. falciparum prevalence in children in the ITN group six months after the start of the trial. Both effects were more pronounced in younger children. However, no significant difference was observed in parasite density or multiplicity of infection among infected children. Comparison with PCR results indicated that microscopically subpatent parasitaemia was more frequently found in children in the ITN group. This, together with the observed similar multiplicity in the 2 groups, suggests that infections are maintained despite ITN use, owing to the chronicity of infections. This study shows that ITNs reduce the risk of anaemia in highly exposed young children. The virtually unchanged multiplicity of infection indicates that the potentially protective concomitant immunity is not compromised.

Distinction of recrudescences from new infections by PCR-RFLP analysis in a comparative trial of CGP 56 697 and chloroquine in Tanzanian children

OBJECTIVE

To test the efficacy of a new compound drug (CGP 56697) against acute, uncomplicated falciparum malaria.

METHOD

Reappearing parasites were analysed by PCR-RFLP within a randomized controlled trial. 130 patients received chloroquine and 130 patients were treated with CGP 56697. Samples from 96 patients with parasitological failure were tested by PCR-RFLP for MSP2 of Plasmodium falciparum. Seven days after treatment 32 patients of the chloroquine control group with reappearing parasites were tested by PCR and one infection was unequivocally determined as a new infection. After 7 days, in the CGP 56697 group, 6 samples were tested in which one new infection was identified. Similar observations were made one and three weeks later in both groups.

RESULTS

Although a high multiplicity of infections on admission was observed, there was no significant correlation between multiplicity and either recrudescence or new infection. Patients in both treatment groups with subsequent recrudescent parasites had higher initial mean parasite densities than patients who cleared. Those of the patients with recrudescent parasites who were treated with CGP 56697 had higher initial parasite densities than those treated with chloroquine. The rate of re-infection increased with time as expected in holoendemic areas and appeared to be higher in chloroquine patients. Generally, CGP 56697 showed a superior clearance rate, successfully cleared higher parasite densities and suppressed new infections over a longer period of time.

CONCLUSION

The PCR analysis confirmed that reinfections beyond day 7 are significant in areas highly endemic for malaria and showed the necessity of excluding these when estimating 14 day clearance rates. Provided new infections are excluded, the 28-day clearance rate can also be used to determine the efficacy of antimalarial drugs in highly endemic areas, and adds to our knowledge of drug resistance and dynamics of infections in people living in such areas.

Biogeographic analysis of the Uhu and LOA elements in the Hawaiian Drosophila

Two transposable elements have been isolated from the Hawaiian Drosophila, the Uhu and LOA elements. The Uhu element has been shown to be present in a group of closely related species, the planitibia subgroup of the picture-winged Drosophila. This study examines the distribution of the Uhu element in several other subgroups of the picture-winged Drosophila, as well as the modified-mouthparts and antopocerus groups of the Hawaiian Drosophila. The LOA element has a much more limited distribution, having only been found in representatives of the planitibia and grimshawi subgroups of the picture-winged Drosophila. For both the Uhu and LOA elements there is an inverse correlation between the copy number of the element and the age of the island on which the species is endemic, i.e., species endemic to the Island of Hawaii, the youngest island, have the highest copy number, while species endemic to Kauai, the oldest island, have the lowest copy number of the element. The correlation suggests there is a relationship between speciation and the activity of transposable elements.

Sequence diversity and molecular evolution of the merozoite surface antigen 2 of Plasmodium falciparum

Eleven new alleles of the Plasmodium falciparum merozoite surface antigen 2 (MSA2) from Papua New Guinea were analyzed by direct sequencing of polymerase chain reaction (PCR) products. We have used the sequence information to trace the molecular evolution of MSA2. The repeats of ten alleles belonging to the 3D7 allelic family differed considerably in size, nucleotide sequence, and repeat copy number. In the repeat region of these new alleles, codon usage was extremely biased with an exclusive use of NNT codons. Another new allele sequenced belonged to the FC27 family and confirmed the family-specific conserved structure of 96 and 36 bp repeats. In order to assess sequence microheterogeneity within samples defined as the same genotype by restriction fragment length polymorphism (RFLP), we have analyzed single-strand conformation polymorphism (SSCP) of different samples of the most frequent allele (D10 of the FC27 family) in the study population. No sequence heterogeneity could be detected within the repeat region. Based on analysis of the repeat regions in both allelic families, we discuss the hypothesis of a different evolutionary strategy being represented by each of the allelic families. Kew words: Merozoite surface antigen 2 - Nucleotide sequence comparisons - Molecular evolution

Analysis of multiple Plasmodium falciparum infections in Tanzanian children during the phase III trial of the malaria vaccine SPf66

In the first phase III efficacy trial of the malaria vaccine SPf66 in Africa, MOIs in SPf66- and placebo-vaccinated children were analyzed by polymerase chain reaction-restriction fragment length polymorphism of the Plasmodium falciparum merozoite surface antigen 2 (MSA2). MOIs were significantly reduced in asymptomatic vaccine recipients compared with those in asymptomatic placebo recipients; however, no differences were observed among symptomatic children in the vaccine and control groups. These results show that immunization with SPf66 modulates the course of naturally occurring infections, as reflected by reduced MOIs. In placebo recipients, however, there was a significant negative correlation between numbers of infecting genotypes, as identified by MSA2, and morbidity. Asymptomatic placebo recipients had an average of 5 concurrent infections, whereas children with clinical cases had an average of 3.4 infections. These data provide further evidence that premunition from concurrent infections is important in immunity against clinical malaria. No such effect of multiple infections was found in the vaccinated group.

Evidence of HLA class II association with antibody response against the malaria vaccine SPF66 in a naturally exposed population

The antibody response against the malaria vaccine SPf66 and against circumsporozoite (CS) protein has been tested in immune adults from a malaria endemic area in Papua New Guinea. All individuals were genotyped for the HLA class II DQB1 and DRB1 loci, and the humoral response was analyzed with respect to the identified class II alleles. At each locus, only three alleles were frequent, namely DRB1*11, *15, and *16, and DQB1*0301, *0502, and *0601. Antibodies against SPf66 and CS protein were found in 84% and 79% of the individuals, respectively. A strong negative association was detected between the humoral response against SPf66 and DRB1*15 and DQB1*0601. A positive association of the response was observed with DRB1*11 and DQB1*0301. After analysis with a multiple regression model in which all alleles were included simultaneously, only DRB1*15 remained significantly associated with low antibody responses. This suggests that nonresponders may be expected after immunization with SPf66 in certain populations.

Plasmodium falciparum: malaria morbidity is associated with specific merozoite surface antigen 2 genotypes

Plasmodium falciparum merozoite surface antigen 2 (MSA2) is considered a vaccine candidate in a subunit vaccine against blood stage malaria. In order to test if a specific genotype of the highly polymorphic MSA2 is associated with disease, we conducted a case-control study in a malaria endemic area of Papua New Guinea involving 227 individuals, mostly children under the age of 10 years. All cases and controls were genotyped by polymerase chain reaction for their respective MSA2 genotypes. We report that at the time of the study parasites carrying the FC27-like genotype were twice as likely to be found in symptomatic malaria cases than in asymptomatic controls. Mixed genotype infections were significantly less frequent in symptomatic malaria infections. One individual allele (WOS10) of the FC27 family was found only in cases. This may be a form of P. falciparum infrequently encountered and may cause morbidity due to lack of immunity to it. This study provides evidence that MSA2 is involved in the morbidity of malaria and supports the inclusion of MSA2 in a subunit vaccine.

The use of the polymerase chain reaction for more sensitive detection of Plasmodium falciparum

The prevalence of Plasmodium falciparum in children and adults living in a malaria-endemic area in Papua New Guinea was determined by microscopy and by polymerase chain reaction (PCR). The sensitivity of detecting P. falciparum infections increased two-fold with PCR. Undetected infections by microscopy were more frequent in adults (including adolescents) than in children. Detecting this subpatent parasitaemia by PCR resulted in an equal P. falciparum prevalence in children and adults; in children the parasitaemia rate increased from 32% to 48% and in adults from 23% to 47%. In more than 50% of all blood samples positive for P. vivax and P. malariae an underlying P. falciparum infection remained undetected by microscopy. The introduction of PCR has opened up new possibilities in malaria diagnosis and research.

Humoral and cell-mediated immunity to the Plasmodium falciparum ring-infected erythrocyte surface antigen in an adult population exposed to highly endemic malaria

A parasitological and immunological survey was carried out in an area in Papua New Guinea highly endemic for malaria. Two hundred fourteen adult individuals were selected for studies to assess their immune responses against the malaria vaccine candidate ring-infected erythrocyte surface antigen (RESA). Total immunoglobulin G (IgG) antibodies directed against RESA as well as specific IgG1, IgG2, and IgG3 antibodies were determined. Humoral responses directed against RESA were frequent in all IgG subclasses. Only IgG3 responses were found to be age dependent. Total anti-RESA IgG antibodies were not correlated with protection against malaria as measured by parasite prevalence, parasite density, or health center attendance. In contrast, cytophilic antibodies (IgG1 and IgG3) were associated with reduced Plasmodium falciparum prevalence and reduced health center attendance. T-cell proliferation in general was low and very infrequent. No correlation between humoral and cellular immune responses could be found. Parasite density, parasite prevalence, and health center visits tended to be reduced in individuals with good humoral and cell-mediated immune responses.

Plasmodium falciparum: extensive polymorphism in merozoite surface antigen 2 alleles in an area with endemic malaria in Papua New Guinea

Plasmodium falciparum: Extensive polymorphism in merozoite surface antigen 2 alleles in an area with endemic malaria in Papua New Guinea. Experimental Parasitology 79, 106-116. The prevalence of Plasmodium falciparum in 304 individuals from two villages in Papua New Guinea has been determined by PCR amplification of the gene encoding the merozoite surface antigen 2 (MSA2). Forty-seven percent of the blood samples were positive for P. falciparum. The MSA2 alleles of this parasite population were characterized by PCR-RFLP genotyping. In 144 P. falciparum infections 38 different MSA2 alleles were found. The most common allele (22%) was a variant of FC27. Further alleles, found in the study area, were IC1, KF1916, and MAD71. In addition to these previously described alleles, 33 novel variant forms of MSA2 were detected, most of which were represented at very low frequency in the study population. MSA2 genotyping of a local P. falciparum population revealed an unexpected amount of genetic heterogeneity. The diversity is mostly due to variation in the repeat region resulting in length polymorphism that can be easily detected by PCR-RFLP.

Assessment of the humoral and cell-mediated immunity against the Plasmodium falciparum vaccine candidates circumsporozoite protein and SPf66 in adults living in highly endemic malarious areas of Papua New Guinea

To assess natural immunity against the circumsporozoite (CS) protein and the synthetic vaccine SPf66, immunologic studies were carried out in a highly endemic malarious area of Papua New Guinea. Antibody prevalence, antibody titers, and T cell proliferation against both antigens were measured in 214 adults. Immunologic data were analyzed with respect to longitudinal malariologic and morbidity data. Evidence of genetic traits such as glucose-6-phosphate dehydrogenase deficiency and ovalocytosis was analyzed. Antibody prevalence was high, with 79% and 84% for CS protein and SPf66, respectively, while T cell proliferation was infrequent and low, with 14% and 12% responders, respectively. Anti-CS protein antibodies increased with age but showed no association to malaria indices or morbidity. No protective value was observed with T cell responses or with humoral response to SPf66. These results provide a first description of naturally developed immunity against SPf66 and suggest further studies in to fully understand the mechanism of immunity against this antigen.

A non-LTR retrotransposon from the Hawaiian Drosophila: the LOA element

An interspersed sequence has been isolated from the genome of D. silvestris, a species endemic to the Hawaiian Islands. The LOA element is 7.7 kb long and its 3' end consists of (TAA)n tandem repeats. Five different LOA elements were isolated, of which three were truncated at their 5' ends. Large deletions within the elements were frequent. A consensus sequence of the LOA element has been constructed using the nucleotide sequence of three elements. Two overlapping open reading frames (ORF) are present in the LOA element. In ORF1 two 'cys' motifs characteristic for gag proteins are found. The protein translated from ORF2 has similarities with retroviral pol genes. A protein databank search revealed 22% to 25% identity with the reverse transcriptase domains of retrotransposons. This region also shows the pattern of invariant amino acid residues which are most conserved in retroviral reverse transcriptases. In ORF2 an integrase specific 'cys' motif and a conserved sequence of retroviral proteases were identified. Structural similarities with LINE-like elements suggest that the LOA element represents a new non-LTR retrotransposon.

Cytological localization and organization of dispersed middle repetitive DNA sequences of Drosophila subobscura

To study the middle repetitive fraction of the Drosophila subobscura genome, 26 phage clones containing repetitive sequences were examined by Southern DNA blot analysis and by in situ hybridization to polytene chromosomes. These results led to a classification of the clones according to five different types of hybridization patterns. Two types, each containing seven clones, are characterized by hybridization at 100 to 300 sites dispersed over the euchromatic parts of the chromosome band. One of these two classes also showed strong labelling of the chromocentre. The remaining types of hybridization pattern lacked a prominent band but showed hybridization either to the euchromatic regions or to the chromocentre or both. Chromosome A (= X) was the preferred location of prominently labelled bands and it also showed an excess of labelling by some clones. Some of the cloned dispersed sequences were localized cytologically on chromosomes of larvae from crosses between different strains of D. subobscura and between two closely related species, in order to detect heterozygosity at hybridization sites. Comparisons of the chromosomal distribution of labelling sites showed differences in number and location, indicating the possibility of transposition events.