Marked differences in gSG6-P1 specific seroprevalence and antibody levels were observed between the uphill and valley bottom populations. The IgG levels of to gSG6-P1 in the valley bottom population were two-fold higher than observed in the uphill population, suggesting that, the valley bottom population is exposed to higher vector densities than the uphill population. Seasonally, antibody levels of gSG6 P1 in the rainy season were higher than the levels observed in the dry season; however this was not the case in the seroprevalence. It has been previously shown that the levels of specific IgG response to this peptide reflect recent exposure; antibody levels tend to be higher in actively exposed individuals with a concomitant decline in the absence of exposure . The differences in the seroprevalence and Ab levels to gSG6-P1 may be attributed to the differences in vector densities between the two sites; majority of breeding habitats in the hilly highlands are confined to the valley bottoms because the hillside gradients provide efficient drainage . Githeko et al. found that, the majority of the adult Anopheles females An. gambiae s.l. 98% and An. funestus 99%], were found up to 500 m from the breeding habitats which were clustered in the valley bottom. Very few vectors are found at mid hill (1%) and at the hill top (1%). It is well known that the topography of the highlands in western Kenya play a major role in malaria prevalence as this difference between the two sites has been reported by other studies [33, 34]. In respect to this evidence it may be indicated that the gSG6-P1 biomarker is robust and sensitive enough to distinguish between the vector densities of exposure in a population which is only 5 km apart.
Regression analysis revealed that, the odds of having detectable anti-MSP-119 Ab response was significantly higher for gSG6-P1 seropositive individuals. This implies that, the risk of exposure to malaria parasite is higher in individuals presenting with anti -gSG6 P1 Ab; this was consistent both in the uphill and valley residents. With the known differences in vector exposure between the two sites, it is conservable to say that, higher exposure to vector results in higher risk of parasite transmission. This conclusion is consistent with an established view of malaria epidemiology; that the risk of receiving a mosquito bite as well as susceptibility to infection are highly heterogeneous and that 20% of people receive 80% of all bites and infections [42, 43].
Indeed it was exactly the case when we compared anti -gSG6-P1 IgG responses in school children from three different malaria transmission intensity zones. Marani, Kakamega and Kombewa, where their respective EIR are as follows: 0.4, 16.6 and 31.1 . In the same order we observed parasite rates of 4%, 19.7% and 44.6%. The observed gSG6-P1 seroprevalence significantly declined with level of endemicity (Kombewa (54%), Kakamega (34.1%) and Marani (28.1%) which paralleled the parasites rates (Figure 4). However the gSG6-P1 seroprevalence rates relatively higher being indicative of higher sensitivity.
At the population level, we have observed that gSG6-P1 seroprevalence may correlate with the risk of pathogen transmission, in agreement with previous results showing that high anti-saliva IgG levels were predictive indicators of malaria morbidity . Anti-Anopheles dirus salivary proteins Ab occurred also predominantly in patients with acute P. falciparum or Plasmodium vivax malaria compared to individuals from non-endemic areas . In South Americas, the presence of anti-Anopheles saliva Ab has been described in malaria-endemic areas. Adult volunteers from communities in the state of Rondônia, Brazil, were tested for Ab response against Anopheles darlingi salivary gland sonicates (SGS). Individuals infected with P. vivax presented higher levels of anti-SGS Ab than did non-infected individuals. This potential biomarker appeared thus useful as an epidemiological tool for discriminating between infected and non-infected individuals with a high likelihood ratio .
As expected, MSP-119 seroprevalence was strongly associated with age at individual sites and in general. The longevity and the cumulative nature of anti-MSP-119 Ab responses is well known. However the age trends observed for gSG6-P1 responses in the same cohort was intriguing. The overall correlation between anti- gSG6-P1 Ab seroprevalence and age was very weak, particularly so in the valley bottom population, however these trends were considerable in the uphill population (Figure 3). The lack of correlation in the age trends of gSG6-P1seroprevalence in the overall data as well as the valley bottom residents in comparison to MSP-119 may be informative that anti - gSG6-P1 Ab is not cumulative. It is noteworthy that age trends of gSG6-P1is influenced by the differences in the transmission intensity. In hyperendemic areas of Burkina Faso, Rizzo and others found that children had higher responses to whole salivary gSG6 proteins while adults had diminished Ab responses, suggesting desensitization of the immune response to the salivary proteins . The current study confirms the non cumulative nature of Ab response gSG6-P1 peptide and thus its robustness in measuring transient exposure (or seasonal) in a hypoendemic population as observed in our uphill residents and not only restricted to children in hyperendemic areas. It will add to the advantages of gSG6-P1 as it will be more useful even under low malaria transmission period as envisaged in the pre elimination and elimination phase of malaria.
MSP-119 Ab responses appeared largely higher than anti- gSG6-P1 responses in terms of Ab level. The differences further lends credence to the observation that MSP1 is cumulative or perhaps more immunogenic than gSG6-P1. Furthermore, the amount of gSG6 proteins injected in the blood as well as the time of contact with immuno-competent cells is relatively shorter than MSP119 which is a blood stage antigen and multiples severally during the erythrocytic cycle of the parasite.
The gSG6-P1 is a synthetic peptide specially designed to enhance its sensitivity and immunogenicity , and it has reportedly been very sensitive [25, 28] and highly immunogenic developing immune responses even in travelers only transiently exposed to mosquito bites.