The research, conducted by an Argentine team from the Universidad de Buenos Aires and CONICET as part of what they call the Pampa del Indio Project, tracked T. cruzi infection in T. sordida from 2007 to 2016. The researchers had reason for suspicion. Across the Gran Chaco, where T. sordida thrives once the main vector is gone, reported infection prevalence varies wildly, from under one percent to as high as 46 percent in some Bolivian and Paraguayan settings. And the standard diagnostic tool, direct optical microscopy of the insect’s gut contents, is known to miss a substantial fraction of infections. The team hypothesized that once T. infestans was suppressed, infection in T. sordida would rise above the low levels, zero to six percent, they had previously reported.

What they found instead was remarkable stability. Out of 10,728 T. sordida collected from 528 premises over nearly a decade, 1,946 insects were tested by optical microscopy or a molecular technique called vkDNA-PCR, which targets the parasite’s kinetoplast DNA. The raw, or naive, prevalence of infection was just 1.44 percent. On average, each infested house harbored about 1.12 infected bugs, and across the entire surveillance period fewer than two percent of examined insects carried the parasite, with no more than roughly five percent of houses harboring infected specimens at any time. Infection never spiked, never crept upward, and never filled the gap left by the main vector.

The technical twist lies in the occupancy modeling. Because neither diagnostic method is perfect, the team applied single-season occupancy models, a statistical framework borrowed from ecology that jointly estimates the true probability of infection and the probability of detecting it. The models confirmed what earlier comparisons had hinted: optical microscopy detected at most 73 percent of truly infected insects, meaning it missed nearly one in every four infections. The molecular assay, by contrast, was assumed to catch essentially all of them. This method gap matters, because microscopy alone returned a prevalence of 0.65 percent while vkDNA-PCR returned 3.42 percent among tested insects, a difference that was statistically significant and that explains why older surveillance reports based on microscopy may have understated risk.

The fine structure of infection within the landscape tells its own story. Adjusted prevalence was highest in domiciles, reaching 4.72 percent, intermediate in peridomestic kitchens, storerooms and granaries at 3.30 percent, and lower in chicken-associated sites, corrals and other outbuildings. Adult bugs were far more likely to be infected than nymphs, a pattern seen across triatomine species and attributed to cumulative exposure to infected bloodmeals over a longer life and larger blood meals in later stages. Intriguingly, males showed more than twice the infection rate of females, 3.81 percent versus 1.34 percent, a finding the researchers find hard to explain given that male T. sordida live shorter lives in the laboratory and no known sex-based susceptibility differences exist. They speculate that males may feed more often or roam more widely, but the question remains open.

Spatial analysis added another layer. Global tests found no overall clustering of infection, but a local statistic pinpointed a significant hotspot in the northwestern corner of the municipality, spanning 24 houses within a radius of roughly 200 to 1,300 meters. Notably, the geography of infection shifted over time. At baseline and in the early post-intervention phase, infected bugs concentrated in that northwestern zone; by the late phase, the handful of infections appeared in the central and eastern sections instead. Baseline prevalence, adjusted to 2.65 percent, was about double the prevalence in both post-intervention phases, and the difference between baseline and pooled post-intervention data was statistically significant.

Molecular epidemiology offers a plausible explanation for that spatial and temporal shift. Trypanosoma cruzi is a genetically diverse parasite with several discrete typing units, or DTUs, that occupy different transmission cycles. In the Argentine Chaco, hybrid lineages such as TcV and TcVI dominate the domestic cycle, while TcI prevails in sylvatic settings, often linked to opossums. In Pampa del Indio, TcI predominated among infected T. sordida throughout the study, and hybrid lineages were detected only at baseline and early post-intervention, when domestic transmission by T. infestans still simmered. Once that domestic cycle was suppressed, the remaining infections in T. sordida appear to have flowed entirely from the sylvatic cycle, with molecular fingerprinting previously linking the TcI parasites in these bugs to local white-eared opossums, Didelphis albiventris, captured near the hotspot.

The blood-feeding habits of T. sordida likely dampen its threat. Chickens and other poultry, which are refractory to T. cruzi infection, rank among its favorite hosts in peridomestic environments across the region, and infected bugs in Pampa del Indio were most often collected in poultry-associated sites. Combined with the species’ strong flight dispersal and opportunistic feeding, this avian-leaning diet helps explain why a bug that invades houses freely nevertheless carries so little parasite. The picture contrasts sharply with scenarios in Brazil, Paraguay, Bolivia and Colombia, where native secondary vectors, after the main vectors were removed, exhibited high infection levels and genuinely jeopardized control gains.

The authors are careful not to declare victory. Infection prevalence is only one component of transmission risk, which also depends on bug abundance, feeding behavior and the degree of contact with humans. The T. sordida complex itself contains several cryptic species that morphological identification cannot reliably separate, adding uncertainty, though recent work suggests the relevant species share similar feeding and defecation behavior. Intense deforestation and land-use change during the study period may also have reshaped sylvatic populations and their house-invasion dynamics in ways the data cannot yet resolve. Because T. cruzi transmission is enzootic, permanently circulating in wild mammals and their insects, infected T. sordida continued to appear even a decade into control.

The practical message is twofold. First, molecular diagnostics should be standard in vector surveillance, because microscopy systematically understates infection and can mislead risk assessments across the Gran Chaco. Second, successful elimination of a primary vector does not end the story; it reshapes the ecosystem of risk. The Pampa del Indio experience shows that with sustained, adaptive surveillance, a secondary invader can be monitored effectively and kept from becoming a new engine of transmission. But it also shows that vigilance cannot lapse, because the sylvatic cycle persists indefinitely at the edges of human settlement, ready to send the occasional infected bug across the threshold.

Subject of Research: Long-term variation of Trypanosoma cruzi infection in the secondary Chagas disease vector Triatoma sordida following Triatoma infestans elimination in rural Argentina

Article Title: The Pampa del Indio Project: long-term variations of Trypanosoma cruzi infection in Triatoma sordida, a non-target invasive vector

Article References: The Pampa del Indio Project: long-term variations of Trypanosoma cruzi infection in Triatoma sordida, a non-target invasive vector. (n.d.). https://doi.org/10.3389/fpara.2026.1953889

Image Credits: AI Generated

DOI: 10.3389/fpara.2026.1953889

Keywords: Chagas disease, Trypanosoma cruzi, Triatoma sordida, Triatoma infestans, Gran Chaco, vector control, occupancy models, molecular diagnostics, discrete typing units, Argentina, entomological surveillance, Didelphis albiventris

The research, led by Julieta Nattero of the Universidad de Buenos Aires and CONICET together with colleagues including Delfina Trezza-Neumayer, Federico G. Fiad, Ricardo E. Gürtler, and María S. Gaspe, set out to answer four linked questions. Does the frequency of the melanic form change along a gradient running from city to countryside? Do melanic bugs cluster in space? Do the two color forms differ in traits linked to flight and dispersal? And which features of the surrounding landscape best predict where melanic individuals turn up in and around human dwellings? The answers carry practical weight, because understanding what drives house reinfestation is critical for the final push against a disease that still burdens millions of people in Latin America.

To do this, the team assembled an unusually detailed dataset. They collected 901 adult Triatoma infestans from 67 infested houses spanning urban, peri-urban, and rural environments in Avia Terai. Rather than relying on subjective visual judgments of color, the researchers quantified dorsal coloration using red, green, and blue pixel metrics taken from two body regions: the forewing and the connexivum, the flattened lateral margin of the abdomen. They then used K-means clustering, an unsupervised machine learning method, to sort individuals into chromatic groups objectively. This quantitative approach matters because color variation in insects can be subtle, and consistent classification is the foundation for every downstream analysis.

The first striking result is rarity combined with strict environmental restriction. Melanic individuals made up just 16.98 percent of the bugs analyzed, and they were found only in rural environments, never in peri-urban or urban houses. That pattern suggests the dark morphotype is tied to particular ecological conditions that exist in the countryside but not in more built-up settings. Whether those conditions involve temperature, humidity, background substrates, or something else remains an open question, but the urban-to-rural gradient clearly acts as a filter on this phenotypic variation.

Color, however, is only one part of the story. The team also examined whether melanic and non-melanic bugs differ in traits related to dispersal, a key question because flying adults are thought to be important agents of reinfestation after insecticide campaigns. Using geometric morphometrics on the forewings, a technique that captures size and shape variation independent of overall body size, the researchers found that wing size and shape were broadly similar between the two morphotypes. The single exception was male wing shape, which differed between color forms. Flight muscle development, assessed directly, did not differ between morphotypes or between sexes, indicating that darker bugs are not inherently better or worse fliers by this measure.

One dispersal-related trait did stand out. The researchers calculated a dispersal index based on the ratio between the membranous and stiff portions of the forewing, a morphological feature that in other insects correlates with flight capacity. Non-melanic individuals had higher values of this index than melanic ones. Taken together with the similar flight muscle development, the result paints a nuanced picture: the color morphotypes are not simply fast and slow dispersers, but subtle differences in wing architecture hint that the two forms may move through the landscape in different ways, with consequences for how quickly they can colonize newly sprayed houses.

The spatial analysis delivered perhaps the most consequential finding. Using Moran’s I, a standard statistic for spatial autocorrelation, and Local Indicators of Spatial Association, or LISA, which pinpoint where clusters of similar values occur, the team detected significant positive spatial autocorrelation in melanic frequency at distances of 2,500 meters, a pattern that persisted out to 5,000 meters. In practical terms, houses with melanic bugs are not distributed at random across the map. Instead, they form localized hotspots where the dark morphotype occurs at unusually high frequency, and these hotspots are large enough to span several kilometers. Such spatial structure implies that the processes producing melanic bugs, whether local population persistence, restricted gene flow, or shared environmental conditions, operate at landscape scales rather than house by house.

What landscape features explain those hotspots? To find out, the researchers calculated landscape metrics within eight concentric buffers around each dwelling, ranging from 200 to 3,200 meters in radius. This multi-scale design allowed them to identify the scale of effect, the spatial extent at which the surrounding environment most strongly influences the phenomenon of interest. They then modeled melanic frequency using negative binomial generalized linear models combined with multi-model inference based on Akaike’s Information Criterion corrected for small samples, a rigorous framework for comparing competing explanations while accounting for overdispersed count data.

The models converged on a clear answer. Landscape metrics measured at buffers of 800 to 1,000 meters best predicted the frequency of melanic individuals, and higher values of those metrics, associated with increased landscape connectivity, corresponded to more melanic bugs. In other words, dark-colored Triatoma infestans are most frequent around houses embedded in well-connected rural landscapes, where patches of suitable habitat are linked in ways that may facilitate bug movement and population persistence. The identification of this specific scale of effect gives vector control programs a concrete spatial radius to consider when assessing reinfestation risk around treated houses.

The broader significance of the study lies in its integration of phenotypic, spatial, and landscape perspectives. Chromatic dimorphism in Triatoma infestans was first reported in sylvatic populations, bugs living in wild habitats away from houses, and later documented in domestic and peridomestic settings, but its ecological meaning has remained murky. By showing that the melanic morphotype is spatially aggregated, environmentally restricted, and landscape-dependent, the new work suggests that color variation is not mere noise. It appears to reflect genuine ecological differentiation tied to the settings where bug populations persist and from which they reinvade treated houses. If melanic bugs mark populations with particular spatial dynamics, then their presence could serve as a phenotypic signal of reinfestation risk, helping surveillance teams prioritize villages and households for follow-up inspection and spraying.

The study also underscores a methodological lesson for disease ecology more generally. Phenotypic variation in disease vectors is often studied at the level of individual insects or single houses, yet the patterns documented in Avia Terai emerge only when the surrounding landscape and spatial structure are taken into account. As the authors conclude, understanding phenotypic variation in Triatoma infestans requires integrating landscape context and spatial structure, because the occurrence of the melanic morphotype is associated with particular ecological settings and spatial dynamics that influence persistence and reinfestation. For a vector that continues to infest houses despite decades of control efforts, every clue about where and why certain bug populations thrive is a clue about how to stop them, and in the dark wings of these rural hotspots, researchers have found a marker written in color that points toward the landscapes where the battle against Chagas disease is hardest to win.

Subject of Research: Spatial distribution and landscape associations of melanic Triatoma infestans, the Chagas disease vector, along an urban-to-rural gradient in Argentina

Article Title: Spatial aggregation and landscape associations of melanic Triatoma infestans in an urban-to-rural gradient

Article References: Nattero, J., Trezza-Neumayer, D., Fiad, F. G., Gürtler, R. E., & Gaspe, M. S. (2026). Spatial aggregation and landscape associations of melanic Triatoma infestans in an urban-to-rural gradient. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07712-7

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07712-7

Keywords: Triatoma infestans, Chagas disease, melanism, chromatic dimorphism, spatial autocorrelation, landscape connectivity, geometric morphometrics, vector control, Argentina, Parasites & Vectors, reinfestation, flight traits