Wolbachia Moves in: How Does the Bacteria fare in Mosquitoes in Urban Environments?

In a new study, Tiley et al. investigate how successful Wolbachia replacement is in different neighborhoods of Rio de Janeiro.
Wolbachia Moves in: How Does the Bacteria fare in Mosquitoes in Urban Environments?
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BioMed Central
BioMed Central BioMed Central

Climate and landscape features associated with successful Wolbachia replacement in the municipality of Rio de Janeiro, Brazil

Background Wolbachia replacement is a promising strategy for reducing arboviral transmission by Aedes aegypti. Wolbachia infection is refractory to arboviruses, including dengue and chikungunya, and Wolbachia spreads in mosquito populations by maternal transmission and cytoplasmic incompatibility, whereby mating between infected males and uninfected females produce nonviable offspring. Deployment programmes seek to establish stable, high Wolbachia prevalence through the release of Wolbachia-infected mosquitoes; however, prevalence varies spatially, particularly in complex urban environments where fine-scale heterogeneity in temperature and built environment may influence mosquito population dynamics. Here, we investigate how climate and landscape features are associated with heterogeneous Wolbachia replacement outcomes using wMel prevalence data from releases in the municipality of Rio de Janeiro, Brazil (2017–2019). Methods Across a 26-month period, 500 × 500 m spatial units were clustered into four distinct replacement trajectories: unsuccessful, low, moderate, and high success, defined using predicted Wolbachia prevalence time series generated from cell-level binomial regression models and grouped using k-means clustering. Associations were explored between each trajectory and features related to Ae. aegypti habitat suitability, climate sensitivity, flight barriers, and Wolbachia deployment barriers using a multinomial generalised additive model (GAM). We used forward selection based on Akaike Information Criterion, explicit consideration of spatial autocorrelation and screening for collinearity among covariates. Of the 23 covariates examined, four covariates were retained: proportion of tall buildings (% > 6 m in height), average air temperature (°C), average elevation (m above sea level), and average daily rainfall (mm). Results Results show high replacement success was mostly associated with areas with fewer tall buildings [β = −0.93 (95% CI −1.43, −0.44), P < 0.001] and higher temperatures [β = 1.11 (95% CI 0.59, 1.62), P < 0.001], while higher elevation and lower rainfall were associated with poorer outcomes. These associations may also reflect unmeasured socioeconomic or infrastructure variation, and context-dependent temperature effects on mosquito and Wolbachia fitness. Model-based predictions indicate non-linear dynamics, with areas of more tall buildings and higher temperatures showing divergent predicted outcomes towards either high success or failure rather than intermediate replacement. Residual spatial clustering suggests additional neighbourhood-scale processes not captured by the available covariates. Conclusions Outcomes in high-rise and topographically complex areas appear less likely to stabilise at intermediate replacement. Such areas may require higher release densities, targeted supplemental interventions, and more intensive monitoring, although these remain as hypotheses requiring empirical evaluation. Through iterative modelling, it is possible to support rapid hypothesis identification to inform field tests and operational refinement as more evidence becomes available. These findings reinforce the role of urban permeability, vertical structure, and local environmental conditions in shaping the outcomes of Wolbachia replacement programmes. Graphical Abstract

According to the WHO, each year almost 4 billion people are at risk of contracting mosquito-borne diseases, such as dengue fever. Other types of mosquito-borne diseases include malaria, Zika, yellow fever, West Nile virus, and chikungunya.

However, there are some potentially promising ways of containing and reducing the transmission of these diseases, such as by utilizing Wolbachia, a bacterium that can infect many different kinds of invertebrates. In fact, scientists estimate it can be found in 6 out of 10 of all insects. 

While they aren’t found in all types of mosquitoes, the bacteria can be introduced in a lab setting into Aedes aegypti, a species that carries dengue, yellow fever, and other mosquito-borne diseases. Scientists then release these mosquitoes into the wild, where they mate and pass it down to future generations.

But why should we care about these tiny microbes?

Because, as this Scientific American article puts it, “For our purposes, the most interesting characteristic of Wolbachia is that it appears to block the dengue virus from replicating in the tissues of mosquitoes.” It has also been shown to limit transmission of other mosquito-borne diseases, such as yellow fever and Zika.

However, in the grand scheme of things, this Wolbachia treatment is still relatively new and further studies are required to analyze some of the intricacies. For example, it’s not so simple as just releasing Wolbachia-infected mosquitoes into the environment and then letting nature take its course. In fact, the environment itself may impact how successful the Wolbachia replacement strategy is successful or not. This is especially noticeable in cities, as “complex urban environments where fine-scale heterogeneity in temperature and built environment may influence mosquito population dynamics.”

In a new Research Article entitled “Climate and landscape features associated with successful Wolbachia replacement in the municipality of Rio de Janeiro, Brazil” in Parasites & Vectors, available now as an Article in Press, Tiley et al. set out to “investigate how climate and landscape features are associated with heterogeneous Wolbachia replacement outcomes using wMel prevalence data from releases in the municipality of Rio de Janeiro, Brazil.”

To do this, the team broke the city down into 500-by-500-meter units and, across a 26-month timespan, analyzed how successful Wolbachia deployment was, exploring various associations, such as the temperature, elevation, rainfall, and building structure.

They found that the success of the Wolbachia introduction was related to warmer temperatures and fewer tall buildings, and negatively associated with high elevation and lower rates of rainfall. However, Tiley et al. did state that, “These associations may also reflect unmeasured socioeconomic or infrastructure variation.”

These results show that Wolbachia replacement strategies may struggle in drier urban environments with taller buildings. They speculate that when trying to introduce Wolbachia infected A. aegypti into these sorts of locations, researchers “may require higher release densities, targeted supplemental interventions, and more intensive monitoring,” though they note that these theories require further studies, pointing to potential directions for future research.

For the time being, strategies involving Wolbachia replacement remain works in progress, but they show great promise. Who knows. Maybe one day, as that Scientific American article puts it, we could defeat dengue.

Header image: Photo of Rio de Janeiro by chulhwan from PixaBay

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