Evolutionary ideas about genetically manipulated mosquitoes and malaria control

Transcription

1 32 Review Vol.19 No.1 January 23 Evolutionary ideas about genetically manipulated mosquitoes and malaria control Christophe Boëte and Jacob C. Koella Laboratoire de Parasitologie Evolutive, Centre National de la Recherche Scientifique, UMR 713, Université Pierre and Marie Curie, 7 Quai Saint Bernard, CC 237, Paris Cedex 5, France The release of mosquitoes that are genetically manipulated to destroy the malaria parasite Plasmodium falciparum is being considered as a possible method for malaria control. Hopes for this have been raised by the identification of genes involved in the mosquito s immune response and by advances in the tools required to transform mosquitoes. But, will such genes be able to spread in natural populations? What will their impact be on epidemiology of the disease? This article attempts to give some answers to these questions by reviewing some theoretical and empirical considerations underlying the evolutionary epidemiology of genetic manipulation and refractoriness. About 3 million clinical cases and over one million deaths every year make malaria one of our most serious and deadly diseases [1]. Yet, the situation can be expected to get worse, not only because of increasing resistance of the parasite and their vectors against the chemicals used to attack them [1], but also because of deteriorating socioeconomical conditions in many malaria-endemic areas [2], conditions which are deteriorating partly because malaria impedes development and economic growth [3]. New methods for malaria control are desperately needed. One objective is to block transmission by use of genetically modified mosquitoes that would resist infection by the malaria parasite [4]. This hope was originally based on the realization that the immune system of mosquitoes has potential to kill malaria parasites at several stages of their development. These defence mechanisms (reviewed in Ref. [5]), include the production of nitric oxide, which lyses ookinetes in the midgut wall, the production of antibacterial peptides, and the encapsulation and melanization of ookinetes and early oocysts. The molecular knowledge concerning the mosquito s immune response against malaria parasites is rapidly increasing, with studies concerning antimicrobial peptides, signalling pathways and pattern-recognition peptides [5,6]. In parallel with these studies are the genetic approaches that are beginning to localize and identify genes involved in the immune response [7,8]. Thus, several genes that determine the difference between a susceptible Corresponding author: Christophe Boëte (cboete@snv.jussieu.fr). line and one selected to be refractory against the malaria parasite Plasmodium cynomolgi have been localized in the mosquito genome [9,1]. Although the melanization response of this selected line is effective only against some genotypes of Plasmodium falciparum [11], we can hope that it is only a matter of time before genes allowing a more-general immune response are identified. In addition to the mosquito s natural defence mechanisms, scientists are increasingly focusing on monoclonal antibodies [12] or artificial peptides [13] that can interfere with the parasite s development within the mosquito. These genes or DNA sequences could then be inserted into the mosquito s genome with one of the transformation techniques that are being developed. To date, the most promising approach appears to be transforming mosquitoes with genes linked to transposons, several of which are currently available for transforming Anopheles mosquitoes [14]. Indeed, one of them (piggybac) has been used to transform Anopheles stephensi with an artificial peptide that makes the mosquitoes partially resistant against malaria infection [13]. Thanks to the success of modern molecular biology, molecular tools could soon allow us to transform mosquitoes to be refractory against malaria. But, even if we have the genes (or artificial molecules) that can code for refractoriness against all genotypes of the malaria parasite and even if we can stably transform mosquitoes with these genes, the success of genetic manipulation in a control programme is by no means certain. What are the conditions that allow transposons to drive genes encoding for refractoriness through a natural population of mosquitoes? How effective will these genes be under natural conditions? How will a possible spread of the genes affect transmission and epidemiology of malaria? Will the parasite be able to evolve a way around refractoriness? Although these questions have been neglected, studies on the evolutionary ecology of insect immunity and on the interactions between malaria parasites and mosquitoes allow some preliminary insights into possible answers. In our attempt to answer these questions, we focus on the natural immune system of mosquitoes. One reason for this is that the known artificial peptides are considerably less effective than the best immune responses. Consider, for example, the peptide SM1, which was chosen from a phage display library as the peptide that was most /3/$ - see front matter q 23 Elsevier Science Ltd. All rights reserved. PII: S (2)3-X

2 Review Vol.19 No.1 January effective in impairing the development of malaria parasites in the mosquito. In an ideal laboratory setting, transformation of mosquitoes with SM1 reduces the number of oocysts from an average of 93 to 18 [13], whereas the melanization response in selected mosquitoes lines is completely effective [11]. We emphasize, however, that a focus on artificial peptides would have to take into account the same evolutionary ideas and problems as a focus on naturally occurring genes, and that there is no reason to expect that qualitatively different processes will determine ultimate success; hence, using novel proteins for genetic manipulation, rather than genes involved in natural immunity, will not invalidate our discussion. The costs and benefits of refractoriness Whether the tandem formed by a transposon and a gene encoding for refractoriness can spread in a natural population depends on three crucial factors: (1) the evolutionary cost of refractoriness; (2) its benefit to the mosquito; and (3) the efficacy of the genetic drive mechanisms associated with the transposon [15 17]. If we consider first the natural situation, where no transposon drives the refractoriness gene through a population, the model in Box 1 suggests two main predictions: if the benefit of refractoriness is high enough, its frequency increases with the intensity of transmission, but refractoriness cannot spread if its cost exceeds a sharp threshold. Although quantitative estimates of the cost and benefits of refractoriness are scarce, several studies give at least some qualitative information. The benefits of refractoriness are associated with the ability of mosquitoes to resist infection by killing the malaria parasite, and to eliminate or at least decrease the negative impact of infection on their reproductive success. Although according to conventional wisdom, malaria does not harm mosquitoes, it is becoming clear that malaria does indeed have a detrimental effect on the reproductive success of the mosquito vector. Thus, laboratory studies have shown that malaria infection (more specifically, the early oocyst stage of the parasite) reduces the fecundity of mosquitoes [18 2] (Fig. 1a). This is, at least in part, a result of direct manipulation by the parasite [21,22], but could also be a result of the lack of motivation of oocyst-infected mosquitoes to bloodfeed [23,24]. Furthermore, although laboratory studies on malaria-induced mortality of mosquitoes give somewhat conflicting results, a recent meta-analysis shows that, in general, mosquitoes infected with malaria have a shorter lifespan than noninfected controls [25]. This is corroborated by a study in a natural population, where malaria sporozoites increased the mortality of their mosquito vectors [26] (Fig. 1b), probably because the sporozoites manipulate the mosquito to engage in risky biting more frequently [23,24,27]. Thus, overall, there seems to be a substantial benefit in resisting infection. Will this benefit be large enough to compensate for the evolutionary cost of the immune response? Immune responses are generally found to be costly, be it in vertebrates or in invertebrates. Hence, mounting an encapsulation response reduces competitive ability in Drosophila [28], mounting an antibacterial response reduces survival in bumblebees [29], and increased reproductive effort in damselflies [3] or in Drosophila [31] are associated with reduced immunocompetence and a reduction in the ability to clear a bacterial infection, respectively. Mosquitoes are no exception, and two kinds of costs have been observed. First, mounting an effective immune response requires the maintenance of specific cellular and biochemical machinery, which could require resources that would otherwise be used for other functions, such as growth or reproduction. A maintenance cost would be expressed as a genetic correlation between the effectiveness of the immune response and other traits that determine fitness. This has been observed in Aedes aegypti, where selection for early or late pupation brought with it a correlated response in the effectiveness of encapsulating negatively charged CM-25 Sephadex beads: the lines with the earliest pupation had the weakest encapsulation response [32] (Fig. 2a). Second, mosquitoes might be expected to pay for the act of activating the immune response. Such an activation cost has been observed for the antibacterial immune response [stimulating the immune response with lipopolysaccharides (LPS) leads to decreased fecundity [33]] (Fig. 2b) and for the encapsulation response [stimulating the immune response with negatively charged Sephadex beads leads to decreased fecundity (A. Schwartz and J.C. Koella, unpublished)] of Anopheles gambiae. As mentioned above, quantitative estimates of costs and benefits of refractoriness are lacking. The fact, however, that malaria parasites are very rarely melanized in natural populations (in one area of Tanzania, for example, parasites are melanized by,.5% of the infected mosquitoes [34]) suggests that the cost of being refractory is prohibitively high. Spread of refractoriness genes linked to a transposon Luckily, a high cost of refractoriness might not hinder its spread if the gene encoding for refractoriness is linked to a transposon. Indeed, basic population genetic models show that a transposon (together with the genes linked to it) will spread to fixation if the transposon creates sufficient genetic drive (i.e. if the transposon is transmitted to offspring with a probability higher than the 5% of mendelian segregation) [16,17]. The more detailed model described in Box 1 gives the same conclusion: despite a high cost of refractoriness, a segregation bias of 1 2% should be sufficient to drive refractoriness to fixation [15]. Unfortunately, the currently known transposon systems in mosquitoes appear to show mendelian segregation of the transgene insert and thus give no indication of genetic drive [14]. However, the lack of genetic drive mechanisms in mosquitoes might be a technical problem that can be overcome. Effect on disease Even if we can assume that genetic manipulation will eventually enable us to drive genes encoding for refractoriness to fixation, will they have a large effect on malaria epidemiology? This, of course, would be the goal of a control program, not the spread of one or several genes of

3 34 Review Vol.19 No.1 January 23 Box 1. Model of the spread of refractoriness Whereas several models have considered the spread of transposable elements in populations of mosquitoes [a,b], only one has combined population genetics with epidemiological ideas to discuss the spread of refractoriness [c]. In this model, epidemiological equations are used to calculate the mosquito fitness as a function of the cost and benefit of refractoriness. The population genetic equations then use the mosquito fitness to calculate the change of the frequency of the refractory allele, which in turn defines the prevalence of infection in the human population. The human prevalence feeds back onto the mosquito fitness, as the activation cost (i.e. the cost that is expressed when the immune system responds to an infection) and the benefit of refractoriness increase with the probability that the mosquito is infected. (More details of cost and benefit of refractoriness can be found in the main text.) Thus, the combination of epidemiological and population genetic Cost of refractoriness Maintenance cost Conditional cost Benefit of refractoriness Intensity of transmission (cost of refractoriness genes) (cost of immune response) (probability of infection) (effect of parasite) (probability of infection) (efficacy of refractoriness) Prevalence in human population Fig. I. The epidemiological feedback in a population genetic model and the spread of refractoriness. For equations and parameters, see Ref. [c]. equations gives a negative feedback that is not found in standard population genetic approaches (Fig. I). The predictions of the model can be summarized in three parts. First, when the gene for refractoriness is not linked to a transposon (i.e. in the absence of a genetic drive mechanism), the disease cannot be eradicated from the population due to the feedback of the parasite prevalence on the selection pressures (Fig. IIa). If the cost of refractoriness is below a threshold, the frequency of refractoriness increases with the intensity of transmission (before the spread of refractoriness); if the cost is above the threshold, refractoriness cannot invade a susceptible population. Second, if the gene for refractoriness is linked to a transposon (modeled by changing heterozygotes to refractory homozygotes with a given probability, which is defined here as the efficacy of the genetic drive [b]), it spreads to fixation if the efficacy of the genetic drive is sufficiently high. The required efficacy is rather low, even for high costs of refractoriness and of the transposon itself (Fig. IIb), and the conditions for the spread of refractoriness are almost independent of the rate of transmission; hence, refractoriness can spread equally well in areas of low or intense transmission. Third, even if the gene is fixed in a population with intermediate to intense transmission, its effect on the disease situation is negligible unless the efficacy refractoriness approaches 1% (Fig. IIc). References a Kiszewski, A.E. and Spielman, A. (1998) Spatially explicit model of transposon-based genetic drive mechanisms for displacing fluctuating populations of anopheline vector mosquitoes. J. Med. Entomol. 35, b Ribeiro, J.M.C. and Kidwell, M.G. (1994) Transposable elements as population drive mechanisms: specification of critical parameter values. J. Med. Entomol. 31, 1 15 c Boëte, C. and Koella, J.C. (22) A theoretical approach to predicting the success of genetic manipulation of malaria mosquitoes in malaria control. Malaria J. 1, 3 (a) (b) (c) Refractory (%) Cost of refractoriness.4 Efficacy of drive (%) Initial.2.1 transmission Cost of Cost of transposon refractoriness Human prevalence (%) Efficacy of resistance (%) Initial transmission Fig. II. (a) Spread of refractoriness in the absence of a transposon as a genetic drive mechanism. (b) Efficacy of the genetic drive mechanism (i.e. the segregation bias) that is required for the linkage group (refractoriness transposon) to spread to fixation. (c) Effect of fixation of a gene encoding for refractoriness on the prevalence of malaria. For details of parameters, see Ref. [c]. The costs of refractoriness and the transposon in (b) and (c) are expressed as the proportional reduction of the mosquito s fecundity. The initial intensity of transmission is the intensity that would be observed in the absence of the mosquito s immune response (R of standard epidemiological equations). refractoriness against the parasite in the mosquito population. Again, the model in Box 1 can help us to gain some insight. Its prediction is that, in areas of moderate to intense transmission, the prevalence of malaria in the human population decreases substantially only if the efficacy of the immune response in resisting infection is close to 1%. Indeed, this should come as no surprise, because the effect of refractoriness is essentially to reduce the number of susceptible mosquitoes, which has already been shown by Macdonald to be an inefficient control strategy that would have little effect on malaria epidemiology in areas of intense transmission [35]. Thus, the crucial factor determining the success of the control strategy is the efficacy of the immune response of

4 Review Vol.19 No.1 January (a) 8 (b) 2 Mean fecundity (%) Percentage with sporozoites Control Infected Control Infected Low High Oocyst burden Evening (before biting) Time of sample Morning (after biting) Fig. 1. The benefits of refractoriness: examples of reduced reproductive success in malaria-infected mosquitoes. (a) Effect (in the laboratory) of low (mean 4.4 ^.4 oocysts per midgut) and high (.75 oocysts per midgut) oocyst burdens of Plasmodium yoelii nigeriensis on the fecundity of Anopheles stephensi. A different control was used for comparison of the mosquitoes differing in oocyst burdens. The mean number of eggs per mosquito (bars) and the standard errors of the mean (vertical lines) are indicated. Modified from Ref. [18]. (b) Effect (in the field) of Plasmodium falciparum on the risk of feeding-associated mortality of Anopheles gambiae. A sample of mosquitoes was caught early in the evening (before they started to bite), and a second sample of the same population was caught the following morning (after they had completed their biting activity). The sporozoite prevalence (bars) and the standard errors of the percentage (vertical lines) are indicated. A decrease in prevalence implies that a higher proportion of sporozoite-infected than noninfected mosquitoes died during the observation period of one night (i.e. sporozoites increased mortality rate of mosquitoes). Modified from Ref. [26]. transformed mosquitoes. In principle, the genes encoding for refractoriness can be 1% effective, as demonstrated by the observation that all of the mosquitoes selected for refractoriness against Plasmodium cynomolgi completely melanize oocysts of the same parasite line [11], at least in the laboratory environment. The more important question, however, is whether the genes remain effective in natural conditions. How do environmental conditions and other non-genetic effects influence the mosquito s immune response? The answer to this question gives a less optimistic view. First, the immune response of mosquitoes is modified by environmental quality, as it is for most invertebrates [36]. If larvae are reared in crowded or undernourished conditions, the emerging adults have a weak encapsulation immune response. Thus, in one experiment using the strain of An. gambiae previously selected to be refractory [11], reducing food from 1 to 25% of a standard diet reduced the proportion of mosquitoes that melanized more than half of the surface of inoculated Sephadex beads from 75 to 36% [37] (Fig. 3a). Similarly, when adults are stressed by temperature [37] or by the lack of nutrition, be it blood or sugar [34,38], the encapsulation immune response is weakened (Fig. 3b). Second, the immune response of mosquitoes weakens with age [38,39] (Fig. 3c). Thus, in one natural population of An. gambiae, the efficacy of the immune response of bloodfed females decreases from close to 1% just after emergence to,75% four days later [34]. The epidemiological problem associated with the lower efficacy of immunity in the older mosquitoes is that, in the same area,, 25% of the mosquitoes become infected during each night of bloodfeeding [4]. At this bloodfeeding rate,, 5% of the mosquitoes will become infected when they are.5 days old (i.e. when the immune response is no longer completely effective). (a) Percentage of mosquitoes E1 E2 E3 L1 L2 L3 Selection line (b) Number of eggs Control Saline LPS Immune stimulation Fig. 2. The costs of refractoriness. (a) Maintenance cost: effect of selecting Aedes aegypti during ten generations for early (E1, E2, E3) or late (L1, L2, L3) pupation on the mosquito s ability to melanize an inoculated Sephadex bead. The percentage of mosquitoes that melanized a bead completely (green), that had an intermediate (i.e. patchy) response (yellow) and that were not able to mount a response (white) are shown for each of the six selection lines. Reproduced, with permission, from Ref. [32]. (b) Activation cost: effect of inoculating female Anopheles gambiae with lipopolysaccharide (LPS, to stimulate the antibacterial immune response) or saline on egg production. The vertical lines represent the standard errors of the mean. Modified from Ref. [33].

5 36 Review Vol.19 No.1 January 23 (a) Percentage of mosquitoes Larval food (% of standard) (b) Bloodfed Unfed Sugar concentration (%) (c) Age (days after emergence) Fig. 3. The effect of environmental parameters on the ability of mosquitoes to encapsulate and melanize negatively charged Sephadex beads. In (a) and (c), the percentage of mosquitoes that melanized.5% of the bead surface (bars) and the 95% confidence interval of the percentage (vertical lines) are indicated. (a) Effect of larval food (in percentage of a standard diet) on a refractory line of Anopheles gambiae. Modified from Ref. [37]. (b) Effect of sugar and bloodfeeding on four-day-old Aedes aegypti. The percentage of mosquitoes that completely melanized the bead (green), that had patchy melanization (i.e. left unmelanized areas on the bead) (yellow) or that had no visible melanization (white) are indicated [48]. (c) Effect of age after emergence on a refractory (green) and susceptible (white) line of An. gambiae. Modified from Ref. [38]. Thus, one could fear that, even if the genes for refractoriness are fixed in the population, the refractoriness will not have much influence on the prevalence of disease. Response of the parasite To make matters worse, one might fear that the parasite could respond to increasing refractoriness by evolving a mechanism to avoid or suppress the mosquito s immune response. Indeed, immunosuppression has evolved in several parasites and parasitoids of insects. The best examples of immunosuppression are found in several dipteran and hymenopteran parasitoid species, whose larvae develop within their insect host [41]. These parasitoids inject a poly-dna virus or a protein into the insect host [42,43], which shuts down the insect s immune system [44]. Other immunosuppressive systems include several mutualistic bacteria nematode complexes (e.g. Heterorhabditis Photorhabdus ) that parasitize insects. The nematode penetrates the insect, releases immunosuppressive substances into the haemocoel of the insect. It then releases its bacteria, which, uninhibited by the insect s immune response, replicate extensively to provide the nematode with nutrients [45,46]. Similar processes could be involved in the malaria mosquito interaction because infected mosquitoes suffer from decreased efficacy of the immune response during the late ookinete and early oocyst stages of the parasite [39] (Fig. 4), the period when the parasite is most sensitive to the mosquito s encapsulation immune response [11,47].It is not clear whether this phenomenon is a result of an active suppression by the parasite, and even less so whether its efficacy has a genetic basis. But, if it does, one could fear that any increase in the efficacy of the mosquito immune response due to genetic manipulation will be compensated by an increase in the efficacy of immunosuppression, thus reducing the efficacy of the immune response. Conclusion With the considerable advances in molecular genetics and biotechnology, it seems feasible that the technical tools will soon be available to create transgenic mosquitoes that resist infection by malaria and that can drive these genes (a) (b) (c) 1 Percentage of mosquitoes Infected Control Infected Control Infected Control Fig. 4. Melanization response of Aedes aegypti against inoculated beads. The proportion of infected mosquitoes and noninfected controls that completely melanized the bead (green), that had patchy melanization (i.e. left unmelanized areas on the bead) (yellow) or that had no visible melanization (white) are indicated. (a) Response against beads inoculated 24 hours after infection (or bloodfeeding) (i.e. when the parasite is in its late ookinete stage). (b) Inoculation after 48 hours (at an early oocyst stage). (c) Inoculation after 96 hours (at a later oocyst stage) [39].

6 Review Vol.19 No.1 January into natural populations. Whether these tools suffice to suppress malaria transmission, however, is far from clear. Indeed, we have outlined several potential obstacles to controlling malaria transmission and to reducing its prevalence in the human population: (1) the spread and efficacy of the genes in natural populations; and (2) the evolutionary response by the malaria parasite. Perhaps the most important conclusions are that refractoriness is unlikely to be determined by genes alone and that any environmental effects (whether these are due to growth conditions of the larvae, age of the adult or immunosuppression by the parasite) will render the control program ineffective where transmission is intense. While it might be possible to overcome these obstacles, it will require the current research on transgenic mosquitoes to be broadened to include the evolutionary ecology of refractoriness. Acknowledgements We thank Florence Rivenet for helpful comments on the manuscript. C.B. was supported by a Bourse Docteur Ingénieur from Centre National de la Recherche Scientifique. References 1 WHO, (1999) The World Health Report 1999, World Health Organization 2 Manfredi, C. (1999) Can the resurgence of malaria be partially attributed to structural adjustment programmes? Parassitologia 41, Sachs, J. and Malaney, P. (22) The economic and social burden of malaria. Nature 415, Aultmann, K.S. et al. (21) Genetically manipulated vectors of human disease: a practical overview. Trends Parasitol. 17, Dimopoulos, G. et al. (21) Innate immune defense against malaria infection in the mosquito. Curr. Opin. Immunol. 13, Barillas-Mury, C. et al. (2) Mosquito immune responses and malaria transmission: lessons from insect model systems and implications for vertebrate innate immunity and vaccine development. Insect Biochem. Mol. Biol. 3, Dimopoulos, G. et al. (2) Anopheles gambiae pilot gene discovery project: identification of mosquito innate immunity genes from expressed sequence tags generated from immune-competent cell lines. Proc. Natl Acad. Sci. USA 97, Oduol, F. et al. (2) Genes identified by an expression screen of the vector mosquito Anopheles gambiae display differential molecular immune response to malaria parasites and bacteria. Proc. Natl Acad. Sci. USA 97, Gorman, M.J. et al. (1997) Mapping a quantitative trait locus involved in melanotic encapsulation of foreign bodies in the malaria vector, Anopheles gambiae. Genetics 146, Zheng, L. et al. (1997) Quantitative trait loci for refractoriness of Anopheles gambiae to Plasmodium cynomolgi B.Science 276, Collins, F.H. et al. (1986) Genetic selection of a Plasmodium-refractory strain of the malaria vector Anopheles gambiae. Science 234, de Lara-Capurro, M. et al. (2) Virus-expressed, recombinant single-chain antibody blocks sporozoite infection of salivary glands in Plasmodiumgallinaceum-infected Aedes aegypti. Am. J. Trop. Med. Hyg. 62, Ito, J. et al. (22) Transgenic anopheline mosquitoes impaired in transmission of a malaria parasite. Nature 417, Catteruccia, F. et al. (2) Stable germline transformation of the malaria mosquito Anopheles stephensi. Nature 45, Boëte, C. and Koella, J.C. (22) A theoretical approach to predicting the success of genetic manipulation of malaria mosquitoes in malaria control. Malaria J. 1, 3 16 Kiszewski, A.E. and Spielman, A. (1998) Spatially explicit model of transposon-based genetic drive mechanisms for displacing fluctuating populations of anopheline vector mosquitoes. J. Med. Entomol. 35, Ribeiro, J.M.C. and Kidwell, M.G. (1994) Transposable elements as population drive mechanisms: specification of critical parameter values. J. Med. Entomol. 31, Hogg, J.C. and Hurd, H. (1995) Plasmodium yoelii nigeriensis: the effect of high and low intensity of infection upon the egg production and bloodmeal size of Anopheles stephensi during three gonotrophic cycles. Parasitology 111, Hogg, J.C. and Hurd, H. (1995) Malaria-induced reduction of fecundity during the first gonotrophic cycle of Anopheles stephensi mosquitoes. Med. Vet. Entomol. 9, Hogg, J.C. and Hurd, H. (1997) The effects of natural Plasmodium falciparum infection on the fecundity and mortality of Anopheles gambiae in north east Tanzania. Parasitology 114, Jahan, N. and Hurd, H. (1998) Effect of Plasmodium yoelii nigeriensis (Haemosporidia: Plasmodiidae) on Anopheles stephensi (Diptera: Culicidae) vitellogenesis. J. Med. Entomol. 35, Hopwood, J.A. et al. (21) Malaria-induced apoptosis in mosquito ovaries: a mechanism to control vector egg production. J. Exp. Biol. 24, Anderson, R.A. et al. (1999) The effect of Plasmodium yoelii nigeriensis infection on the feeding persistence of Anopheles stephensi Liston throughout the sporogonic cycle. Proc. R. Soc. Lond. Ser. B 266, Koella, J.C. et al. (22) Stage-specific manipulation of a mosquito s host-seeking behavior by the malaria parasite Plasmodium gallinaceum. Behav. Ecol. Sociobiol. in press 25 Ferguson, H.M. (22) Why is the impact of malaria parasites on mosquito survival still unresolved? Trends Parasitol. 18, Anderson, R.A. et al. (2) Plasmodium falciparum sporozoites increase feeding-associated mortality of their mosquito hosts Anopheles gambiae s.l. Parasitology 12, Koella, J.C. et al. (1998) The malaria parasite Plasmodium falciparum increases the frequency of multiple feeding of its mosquito vector Anopheles gambiae. Proc. R. Soc. Lond. Ser. B 265, Kraaijeveld, A.R. and Godfray, H.C. (1997) Trade-off between parasitoid resistance and larval competitive ability in Drosophila melanogaster. Nature 389, Moret, Y. and Schmid-Hempel, P. (2) Survival for immunity: the price of immune system activation for bumblebee workers. Science 29, Siva-Jothy, M.T. et al. (1998) Decreased immune response as a proximate cost of copulation and oviposition in a damselfly. Physiol. Entomol. 23, McKean, K.A. and Nunney, L. (21) Increased sexual activity reduces male immune function in Drosophila melanogaster. Proc. Natl Acad. Sci. USA 98, Koella, J.C. and Boëte, C. (22) A genetic correlation between age at pupation and melanisation immune response of the yellow fever mosquito Aedes aegypti. Evolution 56, Ahmed, A.M. et al. (22) The costs of mounting an immune response are reflected in the reproductive fitness of the mosquito Anopheles gambiae. Oikos 97, Schwartz, A. and Koella, J.C. (22) Melanization of Plasmodium falciparum and C-25 Sephadex beads by field caught Anopheles gambiae (Diptera: Culicidae) from Southern Tanzania. J. Med. Entomol. 39, Macdonald, G. (1957) The Epidemiology and Control of Malaria, Oxford University Press 36 Brey, P.T. (1994) The impact of stress on insect immunity. Bull. Inst. Pasteur 92, Suwanchaichinda, C. and Paskewitz, S.M. (1998) Effects of larval nutrition, adult body size, and adult temperature on the ability of Anopheles gambiae (Diptera: Culicidae) to melanize beads. J. Med. Entomol. 35, Chun,J.et al. (1995) Effect of mosquito age and reproductive status on melanization of sephadex beads in Plasmodium-refractory and -susceptible strains of Anopheles gambiae. J. Invertebr. Pathol. 66, Boëte, C. et al. (22) Reduced efficacy of the immune melanisation response in mosquitoes infected by malaria parasites. Parasitology 125, Lyimo, E.O. and Koella, J.C. (1992) Relationship between body size of adult Anopheles gambiae s.l. and infection with the malaria parasite Plasmodium falciparum. Parasitology 14,

7 38 Review Vol.19 No.1 January Strand, M.R. and Pech, L. (1995) Immunological basis for compatibility in parasitoid-host relationships. Annu. Rev. Entomol. 4, Beckage, N.E. (1998) Modulation of immune responses to parasitoids by polydnaviruses. Parasitology 116, S57 S64 43 Vinson, S.B. (199) How parasitoids deal with the immune system of their host: an overview. Arch. Insect Biochem. Physiol. 13, Rizki, R.M. and Rizki, T.M. (199) Parasitoid virus-like particle destroy Drosophila cellular immunity. Proc. Natl Acad. Sci. USA 87, Boemare, N. et al. (1997) Symbiosis and pathogenicity of nematodebacterium complexes. Symbiosis 22, Wang, Y.I. and Gaugler, R. (1999) Steinernema glaseri surface coat protein suppresses the immune response of Popillia japonica (Coleoptera: Scarabaeidae) larvae. Biological Control 14, Gouagna, L.C. et al. (1998) The early sporogonic cycle of Plasmodium falciparum in laboratory-infected Anopheles gambiae: an estimation of parasite efficacy. Trop. Med. Int. Health 3, Koella, J.C. and Sørensen, F.L. (22) Effect of adult nutrition on the melanization immune response of Anopheles stephensi (Diptera: Culicidae). Med. Vet. Entomol. 16, 1 5 World Federation of Parasitologists (WFP) news The 1th International Congress of Parasitology (ICOPA X) was held 4 9 August 22, in Vancouver, Canada. The World Federation of Parasitologists (WFP) board and council meetings were held at the same time, and it was decided that the next Congress (ICOPA XI) would be held at Glasgow, UK, in 26. WFP also elected and approved a new executive committee and board members for 22 26: President R.A. Khan (Canada); Past-President M. Suzuki (Japan); Vice-President D. Rollinson (UK); P. Dubinsky (Slovak Republic); A.M. Johnson (Australia); Treasurer, J. Andreassen (Denmark); and Secretary, N. Arizono (Japan). Other members-at-large include: H. Mehlhorn (Germany); M.Z. Alkan (Turkey); M.M. Hassan (Egypt); Y. Wattanagoon (Thailand); Soon-Hyung Lee (Korea); B.M. Christensen (USA); and M. Guadalupe Ortega-Pierres (Mexico). In the next four years, the WFP board will provide news of relevance on a timely basis. The WFP board is also considering possible amendments of the Constitution and bylaws of WFP, which you can find on the WFP home page ( Any comments or suggestions are most welcome. For more information, please contact N. Arizono at: arizonon@basic.kpu-m.ac.jp The President of the WFP (Rasul A. Khan) acknowledged in his address that our universe is currently facing a major catastrophe in the expansion and spread of diseases. People living in developing countries, especially Africa, Asia and Latin America, face increasing poverty, inadequate nutrition, poor sanitation and unsafe drinking water. Climatic change in some regions has reduced land available for cultivation and livestock production. Overexploitation of commercial fish stocks has led to their depletion and biodiversity, forests and coral reefs are declining rapidly. Vector-borne diseases, such as malaria and lymphatic filariasis, are increasing, and water- and food-borne infections, e.g. giardiasis and cryptosporidiasis, are now recognized as worldwide pathogens. The WFP, through its website, will provide information on current events relating to parasites which affect human and food production. It will also include news, views, dates of upcoming meetings, and agencies to contact for information. We invite all members and non-member organizations to express their views, participate in developing and fostering our website and, ultimately, the WFP. ICOPA XI The British Society for Parasitology (BSP) was delighted to receive the backing of the World Federation of Parasitologists to host ICOPA XI. The BSP, Glasgow and Strathclyde Universities, and the City of Glasgow are honoured to invite parasitologists from around the world to join us in Glasgow, UK, 6 11 August 26. David Rollinson, President of the British Society for Parasitology, asks that you send your ideas and suggestions for ICOPA XI to Mike Doenhoff (m.doenhoff@bangor.ac.uk) or Paul Hagan (p.hagan@bio.gla.ac.uk). The ICOPA XI website is being constructed, but in the meantime please visit We very much look forward to meeting with you in Glasgow in 26 and ask for your participation in making this The Best ICOPA Ever.