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PubMed1997

Plasmodium yoelii sporozoite infectivity varies as a function of sporozoite loads in Anopheles stephensi mosquitoes.

Mechanisms by which Plasmodium sporozoites survive and maintain their infectivity within the salivary glands of mosquitoes are unknown. In this study we establish a relationship between the number of sporozoites present in the salivary glands of individual mosquitoes (sporozoite load) and sporozoite infectiousness (or "quality") as measured by infections in BALB/c or ICR mice. When Plasmodium yoelii-infected Anopheles stephensi mosquitoes were each allowed to feed on a single mouse, we noted that sporozoites from mosquitoes with higher sporozoite loads were more infectious in 13 of 30 (43%) mice. In a second experiment, we inoculated mice with known numbers of sporozoites from individual mosquitoes. Eleven of 18 (61%) and 16 of 18 (89%) mice that received 25 and 100 sporozoites, respectively, became infected. For inoculations using 100 sporozoites, again we noted that sporozoites from mosquitoes with higher sporozoite loads were more infectious to mice. In a third and final experiment, the overall infectiousness of sporozoites from individual mosquitoes was evaluated first by allowing individual mosquitoes to feed on individual mice and then by intravenous inoculations of 100 sporozoites in a second mouse. There was a significant difference in host infections as a function of sporozoite loads in 14 of 19 (74%) mice. Analysis of the feeding times for infected versus noninfected mosquitoes did not show a significant difference between the 2 groups. The mean total feeding times for 50 infected and 45 noninfected An. stephensi mosquitoes were 306 (standard deviation [SD] = +/-230) and 441 (SD = +/-273) sec, respectively. Further, among infected An. stephensi mosquitoes there was no difference in probing times between cohorts that transmitted infectious sporozoites to mice and cohorts that failed to transmit infectious sporozoites. Our findings that sporozoite load influences sporozoite infectiousness or quality suggest that this may be an important factor in malaria parasite transmission.

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PubMed1997

Sporozoite loads of naturally infected Anopheles in Kilifi District, Kenya.

The number of salivary gland malaria sporozoites (sporozoite load) was determined by hemacytometer counts for 2,055 field-collected Anopheles mosquitoes from Kilifi District, Kenya. Of 48 gland-positive Anopheles gambiae s.l., sporozoite loads ranged from 125 to 79,875, with a geometric mean of 1,743 sporozoites per infected mosquito. About half of the infected mosquitoes had sporozoite loads < 1,000. Following hemacytometer examination of salivary gland samples, the same samples were subsequently tested for Plasmodium falciparum circumsporozoite (CS) protein by enzyme-linked immunosorbent assay (ELISA). The confirmation by ELISA of CS protein in 89.6% (43/48) of the salivary gland-positive samples compared to only 1.4% (28/2,007) of the dissection-negative mosquitoes indicated that dissection methods with hemacytometer counts of sporozoites were adequate for detecting even low numbers of sporozoites in field-collected mosquitoes. Detection of 17 or fewer sporozoites in blood meals of 7 freshly bloodfed An. gambiae s.l. provides a further indication that the actual number of sporozoites transmitted during bloodfeeding may be quite low.

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PubMed2026

Plasmodium falciparum sporozoite loads in Anopheles gambiae s.l. across agro-ecological zones of Benin: a cross-sectional field study.

BACKGROUND: Malaria transmission depends critically on the vectorial competence of Anopheles mosquitoes, which is partly reflected by the quantity of infectious Plasmodium falciparum forms harboured in the salivary glands. Whether agro-ecological heterogeneity and vector species identity modulate this parasite burden remains poorly characterized in Benin. METHODS: Anopheles gambiae s.l. were collected by Human Landing Catch across 12 localities spanning 7 agro-ecological zones in Benin during August-October 2022. Species were identified by SINE-PCR. P. falciparum infection was assessed by circumsporozoite protein (CSP) ELISA, and sporozoite load was quantified in positive head-thorax preparations by real-time PCR targeting the plasmepsin gene (NZYTech kit, ref. MD 02411). Statistical comparisons across agro-ecological zones (within each species) used the Kruskal-Wallis test, and the two-species comparison used the Mann-Whitney U test, in R v4.4.1. RESULTS: A total of 1,355 individuals were molecularly analyzed for species identification. Of 3,021 mosquitoes collected, these 1,355 yielded 858 An. coluzzii (63.32%), 489 An. gambiae (36.09%), and 8 An. arabiensis (0.59%). The overall sporozoite index was 5.98% (95% CI 4.8-7.4%), ranging from 0% in Cotonou and Parakou to 16.19% in Boukoumbé. No significant inter-zonal variation in sporozoite load was detected in either An. coluzzii (Kruskal-Wallis: H = 5.79, df = 3, p = 0.122) or An. gambiae (H = 3.25, df = 2, p = 0.197). However, An. coluzzii carried a significantly higher sporozoite burden than An. gambiae (Mann-Whitney U = 381, p < 0.001). CONCLUSIONS: Broad agro-ecological zone categories were not associated with detectable differences in individual P. falciparum sporozoite load in the principal malaria vectors of Benin. Anopheles coluzzii carried a significantly higher sporozoite burden than An. gambiae, indicating species-level differences in parasite permissiveness that merit further investigation.

Anopheles coluzzii↗

PubMedDate not supplied

Sporozoite load of mosquitoes infected with Plasmodium falciparum.

In the laboratory, mosquitoes given a second blood meal 5-11 d after an infective one have more sporozoites in their salivary glands than do those given a single infective blood meal only. The presence of specific anti-sporozoite antibody in the second blood meal does not reduce the number of sporozoites in salivary glands. On the contrary, the presence of the raised immunoglobulin levels--even non-specific ones--may result in higher gland infections. Oocyst maturation is extremely asynchronous in mosquitoes given a single blood meal, the maturation time being 10-22 d or more. The explanation for the increased density of sporozoites in salivary glands in mosquitoes having a second blood meal may be acceleration of oocyst maturation. Multiple blood meals are a normal event for infectious mosquitoes in nature, and therefore have no special epidemiological significance. However, in the laboratory a second blood meal could be a simple procedure for increasing the efficiency of sporozoite production.

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PubMed1992

Ingestion of Plasmodium falciparum sporozoites during transmission by anopheline mosquitoes.

We investigated the process of sporozoite transmission during blood feeding for Anopheles gambiae and An. stephensi experimentally infected with Plasmodium falciparum. When infective mosquitoes were fed 22-25 days postinfection on an anesthetized rat, sporozoites were detected in the midgut of 96.5% of 57 An. gambiae (geometric mean [GM] = 32.5, range 3-374) and in 96.2% of 26 An. stephensi (GM = 19.5, range 1-345). There were no significant differences between species either in salivary gland sporozoite loads or in the number of ingested sporozoites. There was a significant linear relationship between sporozoite loads and the numbers of ingested sporozoites for both An. gambiae (r = 0.38) and An. stephensi (r = 0.69). Subsequently, An. gambiae were tested for sporozoite transmission by allowing them to feed individually on a suspended capillary tube containing 10 microliters of blood. A total of 83.3% of 18 infective mosquitoes transmitted a GM of 5.9 (range 1-36) sporozoites. The same mosquitoes contained a GM of 23.4 (range 2-165) ingested sporozoites. The number of ingested sporozoites was related to sporozoite loads (r = 0.42) but not to the number of sporozoites ejected into capillary tubes. Ingested sporozoites remained in the midgut up to 10 hr after feeding. The comparable numbers of sporozoites ingested by infective mosquitoes in both experiments indicates that the actual number of sporozoites transmitted to the vertebrate host during blood feeding is significantly reduced by the blood ingestion process.(ABSTRACT TRUNCATED AT 250 WORDS)

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PubMed1991

Quantitation of Plasmodium falciparum sporozoites transmitted in vitro by experimentally infected Anopheles gambiae and Anopheles stephensi.

The frequency and numbers of Plasmodium falciparum sporozoites transmitted in vitro and corresponding sporozoite loads were determined for experimentally infected Anopheles gambiae and An. stephensi. Geometric mean (GM) sporozoite loads in three experiments ranged from 808 to 13, 905 for An. gambiae and from 6, 608 to 17, 702 for An. stephensi. A total of 44.1% of 68 infected An. gambiae and 49.2% of 63 infected An. stephensi transmitted sporozoites in vitro. The GM number of sporozoites transmitted was 4.5 for An. gambiae and 5.4 for An. stephensi. Overall, 86.9% of the mosquitoes transmitted from one to 25 sporozoites, and only 6.6% transmitted over 100 sporozoites (maximum = 369). Sporozoite loads were not a useful predictor of potential sporozoite transmission. Despite higher sporozoite loads, the numbers of sporozoites transmitted in vitro by the experimentally infected mosquitoes were similar to estimates obtained, using the same techniques, for naturally infected An. gambiae in western Kenya. The low but highly variable numbers of sporozoites transmitted in vitro by mosquitoes used in malaria vaccine challenge studies appears to be a reasonable simulation of natural sporozoite transmission.

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PubMed2003

Detection of malaria sporozoites by standard ELISA and VecTestTM dipstick assay in field-collected anopheline mosquitoes from a malaria endemic site in Ghana.

We compared the VecTestTM dipstick assay for detection of Plasmodium sporozoites in Anopheles vectors of malaria with standard circumsporozoite (CS) microplate ELISA for detection of Plasmodium falciparum circumsporozoite protein (PfCSP) in Anopheles mosquitoes. Mosquitoes were collected from a malaria endemic site (Kassena Nankana district) in northern Ghana. Of 2620 randomly sampled mosquitoes tested, the standard CS-ELISA gave a sporozoite rate of 10.8% compared with 11.2% by VecTestTM, which was not statistically different (P = 0.66). Visual reading of the CS-ELISA results gave a sporozoite rate of 13.4%, which was higher than the other tests (P > 0.05). To allow a more objective evaluation of the sensitivity of the dipstick, an additional 136 known CS-ELISA-positive specimens were analysed. The prevalence of the test (including the additional samples) was 14.6% and 14.7% for CS-ELISA and dipstick, respectively (P > 0.05). The estimated prevalence by visual assessment of the CS-ELISA results was 17.5%. The relative specificity and sensitivity of the VecTestTM dipstick and visually read ELISA were estimated based on the CS-ELISA as a gold standard. The specificities of the dipstick and visual ELISA were high, 98.0% and 96.6%, respectively. However, the sensitivities of the two assays were 88.8% for VecTest and 100% for visual ELISA (P < 0.01). Concordance between VecTest and CS-ELISA was good (kappa = 0.86). Similarly, there was a good concordance between the dipstick and the visually read ELISA (kappa = 0.88). Extrapolating from PfCSP controls (titrated quantities of P. falciparum sporozoites), mean sporozoite loads of CS-ELISA-positive An. gambiae (286 +/- 28.05) and An. funestus (236 +/- 19.32) were determined (P = 0.146). The visual dipstick grades showed high correlation with sporozoite load. The more intense the dipstick colour, the higher the mean sporozoite load (+ = 108, ++ = 207, +++ = 290, r = 0.99, r2 = 1). The VecTest dipstick offers practical advantages for field workers needing rapid and accurate means of detection of sporozoites in mosquitoes.

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PubMed1992

Sporozoite transmission by Anopheles freeborni and Anopheles gambiae experimentally infected with Plasmodium falciparum.

A micro-membrane feeding technique was used to evaluate sporozoite transmission for Anopheles freeborni and An. gambiae experimentally infected with Plasmodium falciparum. From cohorts of infected mosquitoes with equivalent sporozoite loads, 75.9% of 29 An. freeborni transmitted a geometric mean (GM) of 4.9 sporozoites and 80% of 30 An. gambiae transmitted a GM of 11.3 sporozoites. Ingested sporozoites, in the blood meal immediately after feeding, were detected in 86.2% of 29 An. freeborni (GM = 9.0) and in 70% of 30 An. gambiae (GM = 44.1). Overall, sporozoites were transmitted and/or ingested by 90% of both species. Most infective mosquitoes transmitted < 1% of the total sporozoites in the salivary glands, and only up to 30% of the variation in transmission, ingestion, or total sporozoite output was related to sporozoite loads. The demonstration that An. gambiae transmitted more than twice as many sporozoites as An. freeborni is the first indication that vector species of anopheline mosquitoes differ in their innate potential for sporozoite transmission.

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PubMed1990

Malaria sporozoite detection by dissection and ELISA to assess infectivity of afrotropical Anopheles (Diptera: Culicidae).

Malaria infection rates determined by dissection and Plasmodium falciparum enzyme-linked immunosorbent assay (ELISA) were compared for 26,935 Anopheles gambiae Giles sensu lato and 17,739 Anopheles funestus Giles collected during 20 mo in western Kenya. ELISA infection rates were about 43% higher than dissection sporozoite rates. In dissection-negative Anopheles, circumsporozoite (CS) protein was detected by ELISA in 5.2% of 10,017 salivary gland samples and in 12.2% of 237 thorax samples. The accuracy of dissection and ELISA techniques was compared by the following tests on a group of 352 field-collected Anopheles (held 10 d to ensure sporogonic development): salivary gland dissection, examination of Giemsa-stained dissection slides, ELISA tests on salivary gland and thorax body parts, and microscopic techniques for determining sporozoite loads. Respective infection rates were 9.9%, 10.8%, and 15.6% for dissection, stained slides, and ELISA. Sporozoite loads were associated significantly with ELISA absorbance values (r = 0.76). Compared with Giemsa-stained dissection slide results, the sensitivity of sporozoite detection was 92.1% for dissection compared with 78.9% for ELISA; specificity was 100.0% for dissection versus 92.0% for ELISA. Immunological detection of CS protein in head-thorax samples of Afrotropical vectors overestimated the proportion of infective Anopheles because the comparison of techniques indicated that 45.4% of the ELISA positive Anopheles did not contain salivary gland sporozoites.

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PubMed1987

Detection and anatomical localization of Plasmodium falciparum circumsporozoite protein and sporozoites in the afrotropical malaria vector Anopheles gambiae s.l.

Salivary glands from Anopheles gambiae s.l. collected in Burkina Faso, West Africa, were analyzed by both microscopic examination and immunoradiometric assay to determine the Plasmodium falciparum sporozoite rates. Using the same mosquito samples, the immunoassay revealed positive salivary glands with low sporozoite loads, which were frequently missed by microscopy. A closer agreement between both techniques was found using salivary glands with high sporozoite loads. We also found a number of mosquitoes with uninfected salivary glands which harbored the circumsporozoite antigen in their thoraces. In a particular village these mosquitoes represented 43.5% of all sporozoite antigen carrying specimens.

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PubMedDate not supplied

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.

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PubMed1989

Effect of human circumsporozoite antibodies in Plasmodium-infected Anopheles (Diptera: Culicidae).

Human circumsporozoite (CS) antibodies to Plasmodium falciparum were detected in blood meals from 45.0% of 1,547 field-collected Anopheles gambiae Giles sensu lato and Anopheles funestus Giles from western Kenya. Possible effects on malaria infections within the Anopheles host were investigated. Circumsporozoite antibodies were detected in blood meals up to 36 h after feeding. Antibodies crossing the midgut were detected experimentally in hemolymph from 4 to 36 h after feeding; human IgG also was present in hemolymph from fully gravid field-collected Anopheles. Ingestion of high-titer human CS antibodies or 2A10 monoclonal antibody to P. falciparum sporozoites by P. falciparum-infected An. gambiae, 10 d after feeding on an infected human, had no effect on oöcyst maturation, sporozoite rates, or sporozoite loads. Contact between CS antibodies and sporozoites in the hemocoel did not block sporozoite invasion of salivary glands. Human IgG antibodies were detected by an indirect fluorescent antibody technique on salivary gland sporozoites from 83.3% of 114 field-collected Anopheles. In 65.4% of 26 infections, antibodies persisted on sporozoites for at least three days. Thus, a high proportion of naturally infected An. gambiae s.l. and An. funestus in western Kenya transmit sporozoites that are bound with human IgG acquired during previous blood meals. The infectivity of such sporozoites needs to be determined in relation to natural transmission and to the potential use of malaria sporozoite vaccines.

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PubMed1994

Development of sporogonic cycle of Plasmodium vivax in experimentally infected Anopheles albimanus mosquitoes.

The sporogonic cycle of Plasmodium vivax was established and maintained under laboratory conditions in two different strains of Anopheles albimanus mosquitoes using as a parasite source blood from human patients or from Aotus monkeys infected with the VCC-2 P.vivax colombian isolate. Both the Tecojate strain isolate from Guatemala and the Cartagena strain from the colombian Pacific coast were susceptible to infections with P.vivax. A higher percentage of Cartagena mosquitoes was infected per trial, however the Tecojate strain developed higher sporozoite loads. Intravenous inoculation of Aotus monkeys with sporozoites obtained from both anopheline strains resulted in successful blood infections. Animals infected with sporozoites from the Tecojate strain presented a patent period of 21-32 days whereas parasitemia appeared between days 19-53 in monkeys infected with sporozites from Cartagena strain.

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PubMed2005

Comparative susceptibility of three species of Anopheles from Belize, Central America, to Plasmodium falciparum (NF-54).

In August of 2000, a comparative susceptibility study was conducted using 3 species of Anopheles mosquitoes from Belize, Central America, and a standard species used in laboratory infection studies, Anopheles stephensi. Test populations were fed human blood infected with cultured Plasmodium falciparum (NF-54 strain) parasites via a membrane feeder. The control species, An. stephensi, exhibited the highest infections, with 73.8% of dissected specimens positive for sporozoites in the salivary glands. The control species also showed heavier sporozoite loads; 74.0% of positive glands having greater than 200 sporozoites. Of species from Belize, Anopheles darlingi was the most susceptibile, e.g., 41.0% of salivary glands were positive, with more than 200 sporozoites per gland. Anopheles vestitipennis had a low salivary gland infection rate (9.3%) and a moderate number of sporozoites in glands (i.e., 85.7% containing 50-250 sporozoites). Anopheles albimanus was the least susceptible species to infection. No specimens of An. albimanus from the Golden Stream population developed sporozoites in the salivary glands, yet 20.7% of dissected specimens had positive midgut infections. The An. albimanus Buena Vista population showed similar results with only a 2.2% salivary gland infection rate and a 21.5% midgut infection rate. Oocysts in An. stephensi increased in size by 20% after day 10. Development peaked at day 12, with a mean oocyst diameter of 58 microm at onset of oocyst differentiation. Oocysts developed more slowly in An. vestitipennis until day 10. After day 10, there was a 53% increase in oocyst development over the previous 10 days. Oocyst differentiation was not observed until day 13 postfeed. As with An. vestitipennis, both populations of An. albimanus showed similar slow rates of oocyst development; however, no dramatic growth increase occurred after day 10. The oocysts in the Golden Stream population exhibited a cessation of growth after day 10, peaking at a mean of 30 microm. The Buena Vista population did not exhibit the same level of reduced oocyst development. A gradual increase in growth continued until days 13 and 14 (36.7 and 35.7 microm, respectively).

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PubMed1991

Anatomical dissemination of circumsporozoite protein in wild Afrotropical Anopheles affects malaria sporozoite rate determination by ELISA.

The head, thorax, wings, legs and abdomen of 320 wild-caught Anopheles gambiae Giles sensu lato and 115 An.funestus Giles were tested by an enzyme-linked immunosorbent assay (ELISA) for Plasmodium falciparum Welch to determine how anatomical dissemination of circumsporozoite (CS) protein could affect the estimation of malaria sporozoite rates by ELISA. Of fifty-three Anopheles with CS protein detected in any body part, positive reactions were observed for 58.5% of heads, 67.0% of thoraces, 39.6% of wings, 52.8% of legs and 60.4% of abdomens. Mean absorbance values (range 0-2.00) were highest in thorax samples (1.17), followed by heads (0.80), abdomens (0.67), wings (0.48) and legs (0.46). Circumsporozoite protein was present in the wings or legs, but not in the head or thorax, in 11.3% (6/53) of the infected Anopheles. The ELISA infection rate of 12.8% (41/320) for An.gambiae would have increased to 14.7% (47/320) by inclusion of six mosquitoes with CS protein in wings or legs alone. The slight overestimation of the proportion of infective mosquitoes due to disseminated CS protein would have little effect on estimates of relative infection rates by ELISA for field-collected Anopheles, with abdomens removed prior to testing. However, the widespread dissemination of CS protein indicates that sporozoite load estimates by ELISA, for mosquitoes without abdomens, may not provide adequate measurements of the numbers of sporozoites in the salivary glands. Operationally, careful processing of mosquito samples for the determination of infectivity rates by ELISA is necessary to prevent the mixing of wings or legs among samples representing individual mosquitoes.

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PubMed2004

Quantification of Plasmodium malariae infection in mosquito vectors.

Plasmodium malariae occurs in various tropical regions throughout the world and causes low, yet significant, levels of morbidity in human populations. One means of studying the ecology and frequency of this parasite is by measuring sporozoite loads in the salivary glands of infected mosquitoes. An effective, species-specific test that can be used to detect the presence of sporozoites in mosquitoes is the circumsporozoite ELISA. The aim of the present study was to standardize the circumsporozoite ELISA for P.malariae, by setting quantification parameters using, as antigen, either a synthetic peptide or extracts of whole sporozoites. The standard quantification curves produced indicated that the assay had a lower threshold of sensitivity of 250 sporozoites in a 50-microl sample, equivalent to about 1250 sporozoites in a mosquito.

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PubMed1995

Plasmodium gallinaceum: sporozoite invasion of Aedes aegypti salivary glands is inhibited by anti-gland antibodies and by lectins.

There is evidence which suggests that malaria sporozoites recognize mosquito salivary glands by specific receptor-ligand interactions. We are interested in identifying the putative salivary gland receptor(s) for sporozoite invasion. We used an in vivo bioassay for sporozoite invasion of salivary glands. In this assay, purified sporozoites from mature oocytes of Plasmodium gallinaceum were injected into Aedes aegypti mosquitoes and salivary glands were dissected at different time points after injection. One half of the maximum invasion of salivary glands by sporozoites occurred by 6 hr, and salivary gland sporozoite load did not increase further after 24 hr postinjection. This assay was used to determine the effect of experimental treatments with antibodies and lectins at 24 hr postinjection. We raised a rabbit polyclonal antiserum against female Ae. aegypti salivary glands which recognized tissue-specific determinants in the basal lamina of salivary glands. Purified IgG antibody fraction of the immune serum blocked sporozoite invasion in vivo. We tested a panel of 19 lectins and found 7 which bound to salivary glands. Of these 7, succinylated wheat germ agglutinin and wheat germ agglutinin completely blocked sporozoite invasion; Pisum sativum agglutinin and soybean agglutinin partially blocked; and concanavalin A, Dolichos biflorus agglutinin, and Phaseolus vulgaris erythroagglutinin did not block. Our results suggest that sporozoites interact with glycosylated salivary gland surface molecules which serve as receptors for invasion, and which may be in the salivary gland basal lamina. Because the putative sporozoite receptors contain immunogenic determinants, it is feasible to identify them by an immunological strategy.

Aedes↗

PubMed1998

Baseline immunity of the population and impact of insecticide-treated curtains on malaria infection.

It has been shown that insecticide-treated bed nets or curtains may reduce morbidity and mortality from malaria in hyper-holoendemic areas of sub-Saharan Africa. This protection could partially depend on the transitory imbalance between the anti-malaria immunity acquired by the population before the intervention and the lowered sporozoite load resulting from the anti-vector measure. To verify if the efficacy of the intervention is influenced by the baseline immune status of the population, we compared the protective effect of permethrin-impregnated curtains (PIC) against malaria infection among groups with different baseline levels of anti-malaria immunity. We analyzed the impact of PIC on the Plasmodium falciparum infection rate in two rural villages of Burkina Faso inhabited by three ethnic groups: the Fulani, Mossi, and Rimaibé. These have been previously shown to differ for several malariologic indices, with the Fulani being characterized by lower infection and disease rates and by higher immune response to P. falciparum with respect to the other ethnic groups. The PIC were distributed in June 1996 and their impact on malaria infection was evaluated in groups whose baseline levels of immunity to malaria differed because of their age and ethnic group. Age- and ethnic-dependent efficacy of the PIC was observed. Among the Mossi and Rimaibé, the impact (parasite rate reduction after PIC installation with respect to the pre-intervention surveys) was 18.8% and 18.5%, respectively. A more than two-fold general impact (42.8%) was recorded in the Fulani. The impact of the intervention on infection rates appears positively correlated with the levels of anti-malaria immunity. Since decreased transmission entails a reduction of immunity, the efficacy of the intervention in the long term cannot be taken for granted. The expected complementary role of a hypothetical vaccine is stressed by these results, which also emphasize the importance of the genetic background of the population in the evaluation and application of malaria control strategies.

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