Oviposition dynamics of Aedes aegypti in Central Argentina.

Aedes (Stegomyia) aegypti (L.) (Diptera: Culicidae) is the vector of multiple arboviruses. To evaluate the association between environmental factors and the oviposition activity of Ae. aegypti in Argentina, data on the presence and abundance of eggs were collected using ovitraps, between September of 2018 and May of 2019, in the cities of Villa María, Río Cuarto and Salsipuedes (Córdoba province, Argentina). We analysed the relationships between oviposition and five environmental factors: Temperature, precipitation, vegetation cover, human population density and distance to sites with a potential high density of larval habitats, like cemeteries and trash dumps. Environmental factors' data were collected using satellite image products. The oviposition activity was randomly distributed in three cities. Using generalized linear mixed models, we show that the house where each ovitrap was placed was a source of variability in oviposition, suggesting the relevance of microsite factors and the importance of domestic control actions. Ae. aegypti oviposition was positively correlated with night-time temperature of the previous 3 weeks, and in a context-dependent manner, it was positively correlated with human population density, vegetation cover and precipitation. The consistency and magnitude of these relationships varied between cities, indicating that oviposition is related to a complex system of environmental variables.

Control of experimental Triatoma infestans populations: effect of pour-on cypermethrin applied to chickens under natural conditions in the Argentinean Chaco region.

Among peridomestic structures, chicken coops are sites of major importance for the domestic ecology of Triatoma infestans (Hemiptera: Reduviidae). The aim of this study was to evaluate in an experimental context the effects of a cypermethrin pour-on formulation applied to chickens on blood intake, moulting and mortality in T. infestans, under the natural climatic conditions of a region endemic for Chagas' disease. Experimental chicken huts were made of bricks and covered with plastic mosquito nets. Ninety fourth-instar nymphs were maintained in each hut. The study used a completely random design in which chickens in the experimental group were treated with a cypermethrin pour-on formulation. Five replicates (= huts) of the experimental and control groups were conducted. The number of live T. infestans, blood intake and moults to fifth-instar stage were recorded at 1, 5, 20, 35 and 45 days after the application of cypermethrin. Cumulative mortality was higher in nymphs exposed to treated chickens (> 71%) than in control nymphs (< 50%) (P < 0.01). Blood intake and moulting rate were lower in nymphs fed on treated chickens than in control nymphs (P < 0.05). Pour-on cypermethrin was able to cause significant mortality, although it did not eliminate the experimental population of T. infestans.

The association between the geographic distribution of Triatoma pseudomaculata and Triatoma wygodzinskyi (Hemiptera: Reduviidae) with environmental variables recorded by remote sensors.

In this study, predictive models of geographic distribution patterns of Triatoma pseudomaculata (Tps) and T. wygodzinskyi (Twy) were carried out. They were based on biophysical variables estimated from information provided by the satellite remote sensors AVHRR (Advanced Very High Resolution Radiometer) and MODIS (MODerate-resolution Imaging Spectroradiometer). Our goal was to analyze the potential geographic distribution of Tps and Twy and to assess the performance of three predictive models (one for each species and one for both species together) based on temperature, vapour pressure deficit, vegetation and altitude. The geographic distribution analysis shows that all models performed well (>85.7% of overall correct classification of presence and absence point data). The MODIS-based models showed lower correct classifications than the AVHRR-based models. The results strongly suggest that environmental information provided by remote sensors can be successfully used in studies on the geographic distribution of poorly understood Chagas disease vector species.

Infestation of rural houses by Triatoma infestans (Hemiptera: Reduviidae) in southern area of Gran Chaco in Argentina.

The impact of control activities against Triatoma infestans (Klug) (Hemiptera: Reduviidae) in South America has a marked contrast within and outside the Gran Chaco region. Development of a geographic information system, as part of an improvement in control program activities, allowed analysis of the spatial pattern of house infestations by T. infestans before and after house spraying with deltamethrin in the San Martin Department (an arid Chaco region of central Argentina). The overall peridomestic infestation index decreased from 48.2 to 28.2% after insecticide application. House infestation was spatially clustered in regions with low or high infestation levels that were located east and southwest of the department, respectively. This pattern was detected both before and after the insecticide application. Three environmental variables calculated from a temporal series of MODIS imagery (average of night temperature, maximum of day temperature, and temporal variation of vegetation index) were capable of correctly discriminating 96% of the places belonging to either high or low house infestation observed after the insecticide application.

Vectorial transmission of Trypanosoma cruzi: an experimental field study with susceptible and immunized hosts.

The dynamics of vectorial transmission of Trypanosoma cruzi and the level of host (guinea pigs) protection after immunization with attenuated parasites (TCC strain) was studied under natural climatic conditions in an endemic region of northern Argentina. The experimental design included two guinea pig corrals isolated by mosquito netting. One (controls) had 17 healthy and susceptible adult guinea pigs. The other had 19 guinea pigs immunized with attenuated T. cruzi TCC strain. Each corral was colonized in April 1988 with equal-sized populations of Triatoma infestans naturally infected by T. cruzi. To evaluate relevant variables in the natural transmission of Chagas' disease, corrals were sampled in both winter and late spring to assess vector populations, and to carry out parasitologic studies on both vertebrate and invertebrate hosts. In both corrals, vector density decreased in winter and reached a maximum in the hot season. The vector infection rate was very high (greater than 50%) throughout the experiment. Vector infectivity increased with temperature and vector age, but did not differ between the experimental and control corrals. The vector-host contact rate showed a close relationship with temperature, although a very high vector density decreased this rate, even with high ambient temperatures. Initial infections by T. cruzi occurred among guinea pigs only during the hot season. Vectorial transmission risk was estimated from the total number of bug bites per day, the proportion of infected bugs, and the daily incidence in the guinea pig population. During the hot season, this risk was 6.84 x 10(-4) in the control group and 1.82 x 10(-4) in the immunized group.

Density-dependent timing of defaecation by Triatoma infestans.

Defaecation timing of the Triatominae vectors of Trypanosoma cruzi directly affects the transmission probability of Chagas disease to mammal hosts. Experimental studies with fifth instar nymphs of Triatoma infestans showed that defaecation time was negatively affected by blood meal size and positively affected by starvation period and bug initial weight. Since blood meal size and starvation period are both density-dependent, low density domestic populations of T. infestans would represent a higher transmission risk than high density populations. As low density populations could occur in recently reinfested houses after control using insecticide, vigilance activities should be reinforced to protect the human population at the highest risk of disease transmission.

Dispersive flight by Triatoma sordida.

One hundred and thirty-two male and female Triatoma sordida were marked with fluorescent paint and released at an experimental site on the salt flats of central Argentina, distant from any inhabited dwellings. Flight by these bugs commenced just after sunset, and the bugs were then systematically searched for using portable ultraviolet lights. There was no apparent tendency for the bugs to fly in any particular direction, but those whose flight could be observed directly all seemed to fly in approximately straight lines at a height of 0.5-1 m. Of the 86 bugs that flew from the release point, 16 made trivial flights of less than 5 m, 4 landed at distances of 60-90 m away, while the other bugs were not recovered and appear to have flown further than 100 m. As expected from similar studies with T. infestans, the proportion of bugs initiating flight declined with increasing weight-length ratio, but T. sordida showed a much higher propensity for flight under these conditions than did T. infestans.

Seasonal changes in infectivity of domestic populations of Triatoma infestans.

This paper examines the infection rate of Trypanosoma cruzi in Triatoma infestans, the main vector of Chagas disease in Argentina and neighbouring countries. The study was carried out in 1986-1987 on 5 houses (ranchos) in the endemic area of central Argentina. Domestic T. infestans populations were sampled in each season with a constant capture effort (2.5 man-hours/house) using a chemical irritant. The rectal content of the bugs was examined for the presence of T. cruzi. The vector population density showed seasonal changes with highest values during the hot season (November-April). The percentage of infected bugs was higher in mid-spring (November) and autumn (April) than in winter (August) and early spring (October). The mean number of parasites (epimastigotes and trypomastigotes) per microliter of rectal material was very high during mid- and late spring (December). The percentage and number of metacyclic forms differed between seasons, reaching the highest values in late spring. The percentage of infected bugs in houses with children younger than 10 years old was higher than that in houses without children, during all the seasons. Late spring seemed to be the period when domestic populations of T. infestans had the highest vector potential.

Biosystematics of Old World Triatominae.

Morphometric analysis of Old World species of Linshcosteus and Triatoma showed the two genera to be well separated. However, although the analysis showed clear distinction between the five species of Linshcosteus, there was considerable overlap between the eight Old World species of Triatoma, including specimens of T. rubrofasciata from Old and New World populations. The results are interpreted as supporting the idea of a New World origin for the Old World species of Triatoma, with an independent Old World origin for Linshcosteus.

Population dynamics of Aedes albifasciatus (Diptera: Culicidae) south of Mar Chiquita Lake, central Argentina.

Spatial and temporal changes in the abundance of adult female Aedes (Ochlerotatus) albifasciatus (Macquart) were studied using CDC miniature light traps at 7 sites along an 80-km transect parallel to the southern shore of Mar Chiquita Lake (central Argentina) during the rainy seasons between 1994 and 1997. Abundance was greatest and most variable at sites near larval habitats. Rainfall and an index combining rainfall and temperature predicted adult female abundance near larval habitats 7-15 d in advance. The spatial distribution of population abundance seemed to be influenced strongly by local factors, because temporal change at sampling sites located >10 km apart was asynchronous.

Triatominae in furnariid nests of the Argentine Gran Chaco.

Triatomines (Hemiptera, Reduviidae) are bloodsucking insects involved in the transmission of Trypanosoma cruzi, the causative agent of Chagas disease, an important public health problem in Latin America. The triatomine species found in sylvatic habitats generally play a limited epidemiological role compared to domestic species, but they may act as a reinfestation source of dwellings after insecticide spraying and have to be carefully considered in control strategies of Chagas disease transmission. The objectives of this work were to carry out a survey of the sylvatic triatomine species colonizing Furnariidae nests in a typical area of the Chaco region of Argentina during the winter and to study the parasites and natural enemies associated with the collected triatomines. Sixty-three triatomine specimens were collected from Furnariidae nests (Coryphistera alaudina and Phacellodomus sibilatrix) randomly selected within the study area. Fifty-four were identified as Psammolestes coreodes, seven as Triatoma platensis, and two as Triatoma infestans. Specimens of T. infestans and T. platensis were found in one nest. The first finding of instar nymphs of T. infestans x T. platensis in a sylvatic habitat is reported. For the first time, sylvatic collected specimens of T. platensis were found infected by T. cruzi. Triatoma virus was found in one Ps. coreodes specimen.

Development of a geographical distribution model of Rhodnius pallescens Barber, 1932 using environmental data recorded by remote sensing.

Rhodnius pallescens, main vector of Trypanosoma cruzi in Panama and secondary vector in Colombia, Costa Rica and Nicaragua, represents an important epidemiological risk in those countries. It occupies sylvatic ecotopes, and because of this its distribution and abundance could be conditioned by environmental factors. In this work, we integrated environmental variables recorded by remote sensing and data of R. pallescens presence in the countries mentioned above in order to know the environmental variables with better capacity to describe the insects' distribution, which will help to entomological surveillance and control. Standard discriminant analysis (DA) was used to determine if there is a significant difference in the environmental variation between the presence and the absence sites of R. pallescens. Forward stepwise discriminant analysis (fDA) was used to determine the environmental variables which better discriminated between presence and absence sites, and to construct a predictive map of geographical distribution. Univariate analyses were used to determine the discriminatory power of individual variables. The model derived from DA showed 89% of sensitivity and 92.8% of specificity. Multivariate and univariate analyses showed the vapor pressure deficit minimum as the principal variable among the nine most important to describe the distribution of the species, which is in agreement with the R. pallescens stenohydric status. Map shows insects' distribution predicted by environmental variables, and moreover includes the distribution of most species belonging to Rhodnius genus, except R. domesticus, R. nasutus and R. neglectus. We interpreted these results as a reflection of the common evolution of the most Rhodnius species, except for the last ones that probably evolved isolated due to particular environmental conditions. In conclusion, this study showed that a reduced number of environmental variables can predict the distribution of R. pallescens and related species. This methodology can be very useful to make critical decisions for vectorial surveillance and control of Chagas disease vectors.

Predicting the spatial distribution of Ochlerotatus albifasciatus (Diptera: Culicidae) abundance with NOAA imagery.

Ochlerotatus albifasciatus is a vector of western equine encephalomyelitis in Argentina and a nuisance mosquito affecting beef and dairy production. The objective of this study was to analyze whether environmental proxy data derived from 1 km resolution NOAA-AVHRR images could be useful as a rapid tool for locating areas with higher potential for Oc. albifasciatus activity at a regional scale. Training sites for mosquito abundance categories were 3.3x3.3 km polygons over sampling sites. Abundance was classified into two categories according to a proposed threshold for economic losses. Data of channels 1, 2, 4 and 5 were used to calculate five biophysical variables: normalized differences vegetation index (NDVI), land surface temperature, total precipitable water, dew point and vapour saturation deficit. A discriminant analysis correctly classified 100% of the areas predicted to be above or below the economic threshold of 2500 mosquitoes per night of capture, respectively. Components of the NDVI, the total precipitable water and the dew point temperature contributed most to the function value. The results suggest that environmental data derived from AVHRR-NOAA could be useful for rapidly identifying adequate areas for mosquito development or activity.

Infestation of rural houses by Triatoma infestans in the region of Los Llanos (La Rioja, Argentina).

Vectorial transmission of Chagas disease has been strongly reduced in most parts of the Southern Cone countries of South America, except in the Gran Chaco region of Argentina, Bolivia, and Paraguay. Given periodical interruptions of the vector control programmes in the endemic region of the Gran Chaco of Argentina, the vectorial transmission of the disease has been increasing during the last years. From the beginning of 2004, the provincial Ministry of Health of La Rioja, Argentina, started a vector control programme to cover the rural houses of the Los Llanos area in the southwestern area of the Gran Chaco region. This article reports the result of a standardized entomological survey and insecticide application against Chagas disease vectors in the intra and peridomestic structures of the rural houses of Los Llanos. A total of 4062 houses were inspected, of which 46.8% were found to be infested by Triatoma infestans. Infestation by vector species other than T. infestans was less than 0.5%(T. eratyrusiformis and T. platensis). Intradomestic infestation was found in 27.2%, whereas peridomestic infestation was found in 39.3% of the houses. The lowest figure of intradomestic infestation was 6.6% (Department F Varela), and the highest value of intradomestic infestation was 45.1% (Department Independencia). In spite of the demonstrated success of vector control elsewhere, this study shows that the vector populations are susceptible to pyrethroid insecticides in the southern area of the Gran Chaco of Argentina, that there still are regions where rural houses show heavy infestation by T. infestans associated with big peridomestic structures and that the vectorial transmission of the Chagas disease will continue, unless a sustained and well organized vector control effort is installed in the region.

Population dynamics and control of Triatoma infestans.

Analysis of field populations of Triatoma infestans (Hemiptera: Reduviidae), after a 3-year study, shows that population growth rate is affected by both density-dependent and density-independent mortality. Although an equilibrium exists, apparently as a consequence of a density dependent-mechanism, population density fluctuates throughout the year because of the effect of monthly mean minimum temperature as a density-independent source of mortality. Simulation studies based on Moran curves shows that high population densities have an approximately constant extinction probability (around 0.20), independently of the season the population starts growing. However, at very low population densities, the extinction probability depends strongly on the season when the population begins to grow. Very low density populations beginning in winter or autumn have the highest extinction probability. The outcome of the simulation studies coincides with results observed in field populations affected by insecticide application at different seasons.

Recovery of Triatoma infestans populations after insecticide application: an experimental field study.

The capacity of populations of Triatoma infestans (Klug) (Hemiptera: Reduviidae) to survive and recover was assessed after application of insecticide (gamma-HCH at a rate of 0.5 g a.i./m2) at different seasons. T. infestans populations were maintained in experimental chicken houses under natural climatic conditions in a region of Argentina endemic for Chagas disease transmitted by these bugs. Based on previous studies of T. infestans populations in these habitats, each experimental group was set up with a total of 626 T. infestans, comprising 390 eggs, 204 nymphs of particular stages, fourteen male and eighteen female adults. The chicken houses were dismantled and rebuilt at monthly intervals to study the vector population changes over a period of 33 months. When the insecticide was applied during winter, spring or summer, populations of T. infestans recovered to untreated or precontrol levels during the next reproductive season (i.e. during the hot season, October-March). In contrast, populations treated during autumn (March) remained at very low densities for 2 years and then increased rapidly to surpass the untreated populations. All populations of the bugs fell to very low numbers (sometimes less than twenty individuals) after gamma-HCH applications, but none was driven to extinction. In all cases, the density of surviving populations was independent of their density before treatment. The fact that all treated populations recovered within 1-3 years, to at least the density of untreated populations, shows the high reproductive potential of T. infestans to recover from very low population densities. Moreover, the additive effect of climatic-induced mortality and insecticide-induced mortality is only apparent when insecticides are applied just before the onset of the cold winter months during which reproductive rates are at their lowest.

Population dynamics of Triatoma infestans under natural climatic conditions in the Argentine Chaco.

Using experimental chicken houses at a site in central Argentina where the bug Triatoma infestans (Klug) is endemic, nine populations of this vector of Chagas disease were monitored during a 34-month period. Bug populations with four chickens as hosts were consistently larger than those with two chickens as hosts. Age structure of the bug population followed a similar pattern irrespective of the initial age structure. Egg to adult mortality was consistently around 98.5% and there was no consistent evidence for density-dependent mortality. There was some evidence for density dependence in fecundity and recruitment rates, but these were heavily constrained by low temperatures during the winter months. Nymphal development rates correlated most strongly with mean minimum temperatures rather than with mean maximum temperatures. We conclude that vector control using insecticides against this species would be most effective at the onset of winter, when recovery of any surviving populations would be inhibited by low temperatures.

Daily pattern of flight activity of Aedes albifasciatus in central Argentina.

Aedes albifasciatus is an important common mosquito in Central Argentina. It is a confirmed vector of the Western Equine Encephalitis and is responsible for loss of milk production in dairy cattle during peak populations. This paper reports the flight activity pattern of Ae. albifasciatus for different seasons, in the southern coast of the Mar Chiquita Lake (central Argentina). Data were collected by sampling two sites between 1992 and 1993 with CDC traps and human-bait captures. Adult mosquito population density, estimated by CDC trapping and human-bait, were highly correlated. However, when compared to other species, the proportion of Ae. albifasciatus was higher in human-bait collections. Adult female populations were active only when temperature were higher than 6 degrees C. Two daily biting peaks were observed (dusk and dawn) during the spring, summer and autumn, and only one peak during winter (around 15:00 pm). Adult abundance was significantly correlated (R2 = 0.71; p < 0.01) with temperature and illumination.

Spatial pattern of abundance of the mosquito, Ochlerotatus albifasciatus, in relation to habitat characteristics.

Ochlerotatus albifasciatus (Macquart) (Diptera: Culicidae) is the main vector of the western equine encephalomyelitis (WEE) virus and potentially of other arboviruses in Argentina. Surges of adult population abundance during the rainy season are a nuisance, affecting milk and beef production. Larvae develop in short periods in shallow temporary ground pools on fresh or brackish water. Although adults seem to disperse long distances from larval habitats, little is known about their habitat preferences. This work studied factors affecting the spatial pattern of adult Oc. albifasciatus abundance. Adult mosquitoes were collected using CDC miniature light traps baited with CO2 at 28 sites located to the south of Mar Chiquita Lagoon, from November 1997 to April 1998. Each site was typified according to its predominating vegetation cover, potential breeding site occurrence, land slope and cattle density. The spatial and temporal patterns of abundance suggested that Oc. albifasciatus prefers prairies and natural grasslands subject to periodic flooding vs. woodland and farm land. A discriminant function based on the proximity to potential larval habitats, distance to woodland and land slope accurately classified 95% of the data categorized as having an average high (>500 mosquitoes) or low (<500 mosquitoes) abundance, and was validated using six sites located away from the study area. An analysis of the temporal variation of mosquito abundance highlighted the influence of the dynamics of the larval habitats on adult mosquito abundance.