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.

Risks associated with dispersive nocturnal flights of sylvatic Triatominae to artificial lights in a model house in the northeastern plains of Colombia.

BACKGROUND: Control initiatives and continuous surveillance of vector-borne transmission have proved to be effective measures for diminishing the incidence of Chagas disease in endemic countries. However, the active dispersal of infected sylvatic adult triatomines by flight represents one of the main obstacles to eliminating domestic transmission. METHODS: In order to determine the risk that active dispersal of sylvatic adult triatomines represents in Colombian northeastern plains, we quantified the distribution and abundance of triatomines in palm trees (primarily Attalea butyracea) using live bait traps. Directional light traps were used to estimate the frequency of sylvatic triatomine dispersal and their possible origin. Finally, the effect of environmental parameters and artificial light sources on the take-off of sylvatic Rhodnius prolixus was evaluated in field experiments. RESULTS: R. prolixus was found in 90 % of the palm trees that densely aggregated toward the northern portion of the study area. R. prolixus, and three other sylvatic triatomine species were found to actively disperse and were attracted to the directional light traps (Triatoma maculata, Panstrongylus geniculatus and Psammolestes arthuri). Temperature, relative humidity, wind speed and night luminosity did not affect the active dispersal of the triatomines which is higher the first two hours after sunset. Artificial lights from houses at 60 and 110 m played a key role in the directionality of the R. prolixus take-offs. Trypanosoma cruzi was isolated from R. prolixus, T. maculata and P. geniculatus and was genotyped as T. cruzi I, III and IV. CONCLUSIONS: Our results highlight the potential risk in Colombian northeastern plains of actively dispersing sylvatic triatomines and their role in the domestic introduction of Discrete Typing Units of T. cruzi associated to sylvatic foci of Chagas disease transmission.

The effect of convection on infrared detection by antennal warm cells in the bloodsucking bug Rhodnius prolixus.

Previous work revealed that bloodsucking bugs can discriminate between oscillating changes in infrared (IR) radiation and air temperature (T) using two types of warm cells located in peg-in-pit sensilla and tapered hairs (Zopf LM, Lazzari CR, Tichy H. J Neurophysiol 111: 1341-1349, 2014). These two stimuli are encoded and discriminated by the response quotient of the two warm cell types. IR radiation stimulates the warm cell in the peg-in-pit sensillum more strongly than that in the tapered hair. T stimuli evoke the reverse responses; they stimulate the latter more strongly than the former. In nature, IR and T cues are always present with certain radiation intensities and air temperatures, here referred to as background IR radiation and background T. In this article, we found that the response quotient permits the discrimination of IR and T oscillations even in the presence of different backgrounds. We show that the two warm cells respond well to IR oscillations if the background T operates by natural convection but poorly at forced convection, even if the background T is higher than at natural convection. Background IR radiation strongly affects the responses to T oscillations: the discharge rates of both warm cells are higher the higher the power of the IR background. We compared the warm cell responses with the T measured inside small model objects shaped like a cylinder, a cone, or a disc. The experiments indicate that passive thermal effects of the sense organs rather than intrinsic properties of the sensory cells are responsible for the observed results.

Differential effects of ambient temperature on warm cell responses to infrared radiation in the bloodsucking bug Rhodnius prolixus.

Thermoreceptors provide animals with background information about the thermal environment, which is at least indirectly a prerequisite for thermoregulation and assists bloodsucking insects in the search for their host. Recordings from peg-in-pit sensilla and tapered hairs on the antennae of the bug Rhodnius prolixus revealed two physiologically different types of warm cells. Both types responded more strongly to temperature pulses produced by switching between two air streams at different constant temperatures than to infrared radiation pulses employed in still air. In addition, both warm cells were better able to discriminate small changes in air temperature than in infrared radiation. As convective and radiant heat determines the discharge, it is impossible for a single warm cell to signal the nature of the stimulus unequivocally. Individual responses are ambiguous, not with regard to temperature change, but with regard to its source. We argue that the bugs use mechanical flow information to differentiate between pulses of convective and radiant heat. However, if pulses of radiant heat occur together with a constant temperature air stream, the mechanical cues would not allow avoiding ambiguity that convective heat introduces into radiant heat stimulation. In this situation, the warm cell in the tapered hairs produced stronger responses than those in the peg-in-pit sensilla. The reversal in the excitability of the two types of warm cells provides a criterion by which to distinguish the combination of convective and radiant heat from the stimuli presented alone.

The inheritance of radiation induced semi-sterility in Rhodnius prolixus.

Eggs from crosses of 40 adult male R. prolixus irradiated with 6K rad gamma-rays with normal females had a mean fertility of 23.9%, only 2 crosses being completely sterile. The 86 F1 progeny of both sexes, when outcrossed with normal mates, had a mean egg fertility of 12.6;, and 43 of these matings were completely sterile. Twenty-eight "F2" bugs reared from F1 X normal crosses were mated with normal partners and had a mean fertility of 44.6%, 6 of them being fully fertile, a reversal towards normal fertility. Cytogenetic examination of F1, "F2" and "F3" males showed that these changes in fertility correlated well with the degree of chromosomal abnormality found. The very high recovery rate of translocations in F1 generation males can be related to the holocentric chromosomes of these bugs which precludes the formation of dicentric chromosomes which are inviable in monocentric species. In F1 and "F2" males the majority of one bug was found with a ring of IV chromosome association and it is suggested that chromosome morphology, combined with a low chiasma frequency, favours chain association. Most chain multivalents showed linear orientation which may lead to duplication deficiencies and zygotic death. However, "parellel", "indifferent" and the more stable "convergent" modes of chain orientation were also all observed indicating that survival of some translocations in this species may be possible. The survival to the "F2" generation of chromosomal fragments confirmed the holocentric nature of triatomine chromosomes. It is suggested that semi-sterile males would prove more effective than releases of completely sterile males for reducing wild populations of R. prolixus, because of the delayed effects of sterilizing radiation consequent upon the holocentric structure of triatomine chromosomes.

PIP: The inheritance of radiation induced semisterility in the insect Rhodnius prolixus is reported. Eggs from crosses of 40 adult male R. prolixus irradiated with 6 K rad gamma-rays with normal females had a mean fertility of 23.9%, with only 2 crosses demonstrating complete sterility. 86 F1 progeny of both sexes were then outcrossed with normal mates. A mean egg fertility of 12.65 was seen with 43 completely sterile matings. 28 ''F2'' bugs were then mated with normal partners and had a mean fertility of 44.6%, 6 of them being fully fertile. Cytogenic studies were performed on F1 males, ''F2,'' and ''F3.'' The degree of chromosomal abnormality correlated with fertility level. All F1 males examined (49) showed chromosomal abnormalities in their spermatocytes so that in every male at least 1 interchange was evident. The majority of eggs from the 1st egg batch had only a single translocation while in later batches 2 and 3 translocations were found frequently. The high recovery rate of translocations in F1 generation males can be related to the holocentric chromosomes which are inviable in monocentric species. The majority of F1 and ''F2'' males had translocations associated as chains of 3 or 1 or as chains of 4. Only 1 bug was found with a ring of 4 chromosome association. Most chain multivalents showed linear orientation which may lead to duplication deficiencies and zygotic death. The survival of some translocations of this species is possible as suggested by the presence of ''parallel,'' ''indifferent,'' and the more stable ''convergent'' modes of chain orientation. The fertility of F1 males could also be related to their birth order. Males were significantly less fertile than their female counterparts. The control of this population of insects could most effectively be accomplished by the release of semisterile males as opposed to completely sterile males because of the delayed effects of sterilizing radiation consequent upon the aholocentric structure of triatomine chromasomes.