With resistance to chloroquine and sulphadoxine-pyrimethamine well-established in many malaria endemic countries, artemisinin combination therapies (ACT) are now being introduced for first line treatment of Plasmodium falciparum malaria 12. Clinical trials of ACT demonstrate high efficacy 3 and no stable, naturally-occurring resistance has yet been reported. A potential further benefit is that the artemisinin derivative component of ACT has strong activity against gametocytes, the life stage of the parasite which is transmitted from humans to mosquitoes, at all but the most mature stages of their development 456. This contrasts with previous first line treatments such as chloroquine and sulphadoxine-pyrimethmine, which had only limited activity against early developing gametocytes 78. Furthermore ACT reduce the asexual parasite population, the source of new gametocytes, more swiftly 3. By reducing the infectiousness of treated, symptomatic patients, ACT may be used to reduce overall transmission intensity 9101112 and the spread of parasite strains resistant to the drug combined with the artemisinin derivative 1314. To assess the potential for these population level effects, it is important to have an accurate estimate of the size of ACT impact on the infectivity of individual, symptomatic P. falciparum infections.

Previous summary analyses of randomized ACT trials have shown a reduction in gametocyte prevalence at single timepoints during follow up but have focussed on clinical efficacy 615. Gametocyte density is important because it is correlated with transmission success to mosquitoes, although this relationship is variable 16. The best measures of transmission are the proportion of mosquitoes infected by feeding on a patient blood sample 161718, and the density of infection in those mosquitoes 161718. Here a pooled analysis of six trials is presented, comparing a range of ACT and non-artemisinin antimalarials with the focus on obtaining estimates of impact on gametocyte density over time and on infectivity of patients to mosquitoes. This includes to the best knowledge of the authors all published studies of mosquito infection by patients treated with ACT. The roles of treatment failure and pre-treatment gametocytaemia are investigated as these two factors are expected to influence the size of ACT impact on transmission as they are rolled out in malaria-endemic regions.

Data were available on 3,174 patients in the six trials, of whom 529 contributed data to the human-to-mosquito transmission results (Figure 1). Baseline age, asexual parasitaemia, gametocytaemia and numbers of mosquitoes fed per person were significantly different between the studies (p < 0.05), but treatment groups within studies were comparable (Table 1). Eighty six participants, (2.7%) were estimated to have >500 asexual parasites/μl on the initial Field's-stained slide but were subsequently found to fail this inclusion criterion after examination of the confirmatory Giemsa-stained slide. 111 (27.1%) gametocyte donors for transmission experiments similarly fit the criterion of gametocyte slide-positivity by the Field's-stained slide only. Those with zero or missing asexual parasitaemia on day 0 were excluded (n = 76), but the remainder were retained in the analysis.

These results demonstrate that gametocytaemia among ACT-treated patients is significantly lower than in individuals treated with non-artemisinin antimalarials during 28 days of follow up. ACT treatment is also associated with reduced human-to-mosquito transmission both among those with microscopically detectable gametocytes, and randomly selected patients. ACT appear to destroy a substantial proportion of immature, developing gametocytes while sequestered in the microvasculature, resulting in a significant reduction in the release of mature gametocytes into the peripheral blood. Their more rapid reduction of the asexual reservoir may also contribute.

Transmission to mosquitoes was measured mainly at day 7 when ACT impact on gametocytaemia was at one of its highest points. Although gametocyte density is not the only factor determining transmission to mosquitoes (maturity and sex ratio of gametocytes are also important 1727), the AUC analysis may give an estimate more representative of the overall impact during the follow up period. Longitudinal feeding studies would also be valuable as the infectiousness of gametocytes may change over the course of an infection 24.

Higher levels of parasitological treatment failure among non-artemisinin treated individuals did not account for any of the higher observed ACT impact on gametocytaemia and transmission to mosquitoes. However, parasitological failure did predict higher gametocytaemia and transmission to mosquitoes. Drug resistant parasite genotypes have been shown to enhance transmission by boosting gametocyte numbers, particularly in patients treated with monotherapies 1320. The late appearance of asexual parasites among treatment failures would mean that most of the resulting gametocytes would become patent and transmit after the 28 day follow-up period. Low rates of clinical symptoms among parasitological failures, at least during the period of follow up, suggests these individuals may not receive further treatment and so would subsequently be important contributors to transmission. ACT impact was slightly higher among treatment failures than successes, which may reflect a higher proportion of new parasite inocula as opposed to recrudescent infections in the ACT treatment failure group, which had not yet produced gametocytes 28. ACT were significantly less able to reduce gametocytaemia among individuals who were slide-positive for gametocytes on presentation, probably because they do not act against mature gametocytes 5. The pattern of gametocyte density over time among these patients was consistent with that of natural decay of mature gametocytes and limited subsequent release of developing gametocytes from sequestration. Duration of symptoms prior to treatment seeking has been shown to have a strong relationship with gametocyte prevalence at baseline 29. Interventions to improve the speed of treatment seeking could therefore enhance the impact of ACT on transmission. However, older asymptomatic infections also contribute to the presence of gametocytaemia at baseline 29 and therefore ACT impact may be reduced in high transmission settings or towards the end of a period of seasonal transmission.

Study 6 in Kenya 19 highlighted the importance of submicroscopic gametocytaemia in studies of human infectiousness. Taking this into account reduced the estimate of ACT impact on gametocytaemia and in particular transmission to mosquitoes. However, study 5 in the Gambia showed little evidence of transmission from submicroscopic carriers 21. The reason for this difference in findings is not clear. Differences in laboratory procedures are unlikely to be accountable because the infectiousness of gametocyte slide-positive individuals is comparable between the studies. The lower intensity of transmission in the Gambia, and concomitant seasonal fluctuations in gametocyte carriage are probably important 30. However, high levels of submicroscopic gametocytaemia have been found in other low transmission settings, although the infectiousness of these individuals was not tested 31. Further research is needed to determine the relationship between submicroscopic gametocytaemia, infectivity, and transmission setting.

The grouping of different treatments into non-artemisinin and ACT categories in these results is simplistic since individual treatment regimens within these groups sometimes had significantly different associations with the outcomes (Table 3). Therefore our estimates of ACT effect depend to some extent on the numbers of patients treated with each regimen in these studies. Some evidence exists that non-artemisinin antimalarials stimulate the post-treatment release of immature gametocytes to different extents 832, although these may not be sufficiently mature to infect mosquitoes 24, whilst other evidence suggests the gametocytaemia is no different from what would be expected in an untreated infection 3334.

While it is useful to have a pooled estimate of ACT impact from these studies, the heterogeneity in results should not be overlooked. The laboratory methods used in each of the studies were mostly very similar, but there were some variations which may have influenced the estimated ACT impact, such as the number of days between treatment and measurement of human-to-mosquito transmission, and different microscopists. For example, the SP-AS3 treatment group in study 1 did not show reduced oocyst density among blood-fed mosquitoes compared to non-artemisinin treatments, but only 12 patients in this group participated in feeding experiments. Of the 6 individuals successfully infecting mosquitoes at day 4, two were already harbouring gametocytes at day 0, and a third had very high day 4 gametocytaemia. These individuals almost certainly carried gametocytes on day 4 representative of a substantial pre-treatment gametocyte population, and may have been less infectious if transmission experiments were performed 7 or 14 days after treatment, as in the other studies analysed. Differences in baseline characteristics between studies such as transmission setting could also have caused differences in ACT effects.

These results are representative of ACT effects upon young symptomatic patients in a trial setting. In order to evaluate potential impact of ACT on population-level transmission, it will be crucial to know the proportion of infections which are asymptomatic and so untreated in different transmission settings, and their relative contribution to the infectious reservoir. Many other operational factors including the coverage of ACT relative to other antimalarials and levels of adherence to treatment regimens will also be important in determining such effects. Empirical observations on population gametocyte carriage and its evolutionary basis 1835 will aid this more comprehensive evaluation. Ongoing monitoring of the impact of ACT on transmission could be an important surveillance tool, as the early signs of spreading resistance to these regimens are likely to be enhanced transmission from treated individuals 24.

ACT reduce gametocytaemia and onward transmission to mosquitoes significantly compared to previous first-line non-artemisinin antimalarials, although transmission from ACT-treated patients is not ful