Daily variation of the response to heat in Rhodnius prolixus: the roles of light and temperature as synchronisers

Developmental study of the Proboscis Extension Response to heat in Rhodnius prolixus along the life cycle

Triatominae are blood-sucking insects that localise their hosts using a range of sensory signals to find food, and among them, the heat emitted by the hosts. Heat is one of the main short-range cues in vertebrate hosts, able to trigger alone the Proboscis Extension Response (PER) that precedes the bite. Previous studies demonstrated that heat responsiveness of fifth-instar nymphs is maximum to moderate temperatures (30-35°C) compatible with those of their vertebrate host's body surface. This study investigated whether this thermal preference for biting is maintained along the life cycle of R. prolixus, from the first larval instar to male and female adults. The results showed that PER rates were at maximum around 30-35°C and decreased for a warmer temperature. The same thermal preference was maintained all along the life cycle, despite the increase in the size of the antennae linked to the growth. Interestingly, a decreased thermal responsiveness was stated in males as compared with larval instars and females. This decrease might reveal a lower motivation for host-seeking and might have an impact on males's vectorial competence.

Neuropeptides in the regulation of Rhodnius prolixus physiology

In the kissing bug Rhodnius prolixus, events such as diuresis, antidiuresis, development and reproduction are triggered by blood feeding. Hence, these events can be accurately timed, facilitating physiological experiments. This, combined with its relatively big size, makes R. prolixus an excellent model in insect neuroendocrinological studies. The importance of R. prolixus as a Chagas' disease vector as much as an insect model has motivated the sequencing of its genome in recent years, facilitating genetic and molecular studies. Most crucial physiological processes are regulated by the neuroendocrine system, composed of neuropeptides and their receptors. The identification and characterization of neuropeptides and their receptors could be the first step to find targets for new insecticides. The sequences of 41 neuropeptide precursor genes and the receptors for most of them were identified in the R. prolixus genome. Functional information about many of these molecules was obtained, whereas many neuroendocrine systems are still unstudied in this model species. This review addresses the knowledge available to date regarding the structure, distribution, expression and physiological effects of neuropeptides in R. prolixus, and points to future directions in this research field.

The role of light in Chagas disease infection risk in Colombia

Background

Chagas disease is the most important vector-borne disease in Latin America and Rhodnius prolixus is the main vector in Colombia. Control strategies in this region have shown poor outcomes due to the insect’s ability to disperse between the sylvatic and the domestic habitat. Because insect migration to houses is responsible to sustain contact rates between vectors and humans, understanding the risk factors that promote migration could be important in designing control strategies. In this respect, it has been reported that adult triatomines have the ability to move over long ranges at night attracted by artificial light. Thus, light bulbs could be playing a critical role in house invasion. The main objective of this study is to understand the role of artificial light, or simply light, in house infestation by R. prolixus.

Methods

To investigate the role of light, we combined fieldwork in the village of Chavinave, Casanare, Colombia and a mathematical model of Rhodnius prolixus dynamics. The model allowed us to simulate insect mobility and distribution in the village based on field results. We created 11 scenarios representing different amounts of light in the village (from 0 to 100 %, with increments of 10 %) with 100 simulations each for a time of 1000 days (2.7 years) and compare the results between the scenarios.

Results

None of the Gomez-Nuñez traps were positive at any stage of the study, suggesting that insects do not colonize houses. The model predicts that with current village connections the proportion of houses that have visiting insects should be around 98 %. Additionally we showed that an increase in light allows for insect spreading and migration to previously un-infested areas.

Conclusions

Increments in light could increase the contact rates between vectors and humans; a two-fold increase in human cases for a 30 % increase in the use and visibility of light on this particular village was estimated with the model.

Circadian modulation of learning abilities in a disease vector insect, Rhodnius prolixus

Despite the drastic consequences it may have on the transmission of parasites, the ability of disease vectors to learn and retain information have just begun to be characterized. The kissing-bug Rhodnius prolixus, vectors of the Chagas disease, is an excellent model, particularly because conditioning the proboscis extension response (PER) constitutes a valuable paradigm to study their cognitive abilities under carefully controlled conditions. Another characteristic of these bugs is the temporal organisation of their different activities in a bimodal endogenous daily rhythm. This offers the opportunity to address the implication of the circadian system in learning and memory. Using aversive conditioning of the PER, we have tested whether the ability of kissing-bugs to learn and remember information varies during the day. We found that bugs perform well during the night, but not during the day: their ability to acquire information -but not that to retrieve it- is modulated in time. When keeping bugs under constant conditions to analyse the origin of this rhythm, the rhythm continues to freerun, showing its endogenous and truly circadian nature. These results are the first ones to evince the implication of the circadian system in the learning abilities of disease vectors and one of the few in insects in general.