This study reports results of a large-scale epidemiologic study of antibody responses to Aedes saliva and entomologic data for Ae. aegypti exposure. The results showed that IgG responses to Ae. aegypti saliva were detected in many persons living in an urban area in Bolivia where Ae. aegypti is found. Despite disparate OD values, the IgG response to saliva was age dependent and identified most responders in the youngest age group. The specific antibody response decreased with age and stabilize in persons > 35 years of age. This study demonstrated a positive association between the IgG response to saliva and the level of exposure to Ae. aegypti as measured by the two entomologic parameters (immediate exposure to Ae. aegypti adults and long-lasting breeding site). Logistic regression confirmed that these associations were not dependent on age or sex. Age was not a confounding factor for the association between IgG response to saliva and level of human exposure to the mosquito vector.
The influence of age on the development of the antibody response to saliva has described. Levels of IgE and IgG4 against Aedes
saliva were higher in the youngest children exposed to Ae. aegypti
The same results were obtained for children exposed to An. gambiae
However, no study has investigated the antibody response to saliva in children and adults. Our results showed that the IgG response to saliva was higher in children. Adults and children in our study lived in the same households, and their exposure to Ae. aegypti
could be assumed to be similar. Three hypotheses might explain these results. The first hypothesis is that the antibody response to saliva correlated with the number of bites received, which implies that children are bitten more than adults in this area. The second hypothesis is that children react more strongly to Aedes
bites than adults. The third hypothesis is that adults might show desensitization to saliva proteins and become immune tolerant to saliva antigens after long-term exposure.
With regard to the first hypothesis, it is generally accepted that Ae. aegypti
is aggressive during the day and shows peaks early in the morning and at the start of the evening. At these times, children and adults probably had the same exposure to mosquitoes. Nevertheless, we cannot exclude the possibility that children may be more attractive to Aedes
than adults. A study that analyzed blood meals of female Ae. aegypti
by DNA fingerprinting showed that young adults are bitten more often than children.29
Another study using the same method showed an association between biting rate and age of women (women > 15 years of age received more bites).30
With regard to the second hypothesis, the immune system of children would be more sensitive to antigenic stimulation than that of an adult. Aedes
saliva is highly allergenic and induces a strong specific antibody response, which could explain why children could show development of stronger antibody responses to saliva than adults.22
With regard to the third hypothesis, potential natural desensitization that occurs in adults over time may be a factor.31
A shift to production of IgG4 and IgE could be driven by chronic antigenic stimulation.32
The present work evaluated only IgG responses to Ae. aegypti
saliva because previous studies have demonstrated that the IgG response is a useful biomarker for exposure to mosquito bites.16,33,34
Further investigations are therefore needed to establish whether the antibody response to saliva is age dependent.
The major result of this study is that the strength of the IgG response against Ae. aegypti saliva is positively associated with exposure to the vector, as confirmed by logistic regression analysis. On the basis of immature stage counts, two entomologic parameters were defined to evaluate exposure: immediate exposure to Ae. aegypti adults and long-lasting breeding sites. The population was then divided into five classes according to both parameters of exposure. We demonstrated that the percentage of IgG responders differed significantly between exposure groups. This difference was pronounced between the highest exposure and lowest exposure groups. It suggests that the evaluation of the IgG response to saliva might distinguish high-level exposure to Aedes bites. However, our study did not show a linear progression of antibody response to saliva according to exposure.
The reference entomologic methods measuring exposure in this study failed to distinguish such a progression. This lack of discrimination between low and high exposure represents a limitation, but this study clearly showed an increase in IgG response against Ae. aegypti saliva linked to both entomologic exposure parameters. Logistic regression analysis suggests that this association is significant for the long-lasting breeding site parameter. Other factors (human genetic background, concomitant infection, nutritional status) might have an effect. Nevertheless, this study suggests that the antibody response to saliva could be a useful complementary tool in the evaluation of human exposure to Aedes. In this study, no correlation between IgG response to saliva and dengue transmission was observed, probably because the rate of dengue seroconversion (IgM) was low. Additional studies should be carried out to address this specific point and define whether antibody responses to saliva could be used to assess the risk of dengue transmission.
We cannot exclude the possibility of cross-reactivity with other arthropod salivary proteins. Previous results evaluating the cross-reactivity between different Aedes
, and Culex
species have identified species-shared and species-specific antigens.35
Preliminary data on rabbits experimentally exposed to single species of mosquitoes have shown minor cross-reactivity between Ae. aegypti, An. gambiae
, and Culex quinquesfasciatus
(Mouchet F, unpublished data). Saliva composition depends on age, feeding, and infection.36
Thus, an adequate biomarker for exposure needs to be based on Aedes
-specific immunogenic proteins or peptides, as has been developed for An. gambiae
The sialome of Ae. aegypti
is currently being investigated by using an immune-proteomic approach to define antigenic candidates for a specific, sensitive, and reproducible biomarker of exposure to Ae. aegypti
The present study is a first step toward being able to use human IgG responses to Aedes salivary proteins as a biomarker of individual exposure to bites. This procedure could provide a reliable measurement of human–vector contact in epidemic settings where Ae. aegypti-borne diseases are emerging or re-emerging. Further studies need to be conducted to design a sensitive biomarker for Aedes exposure. The present study indicates that use of antibodies to saliva could lead to development of a useful diagnostic tool. In addition, such an indicator could be also useful for monitoring the efficacy of vector control strategies.