Standing Water and Missing Data: The Murky Relationship between Flooding and Mosquito-Borne Diseases.

Mosquitoes need water to reproduce, and rainfall is correlated with mosquito-borne disease in humans.1 But as the authors of a recent review in Environmental Health Perspectives argue, this association is nonlinear and quite complex, especially following flooding events.2

In their analysis of previous studies, the authors found that flood events were generally associated with increased incidence of mosquito-borne diseases, such as malaria and dengue fever—sometimes after a brief dip in transmission. Yet the specifics varied considerably both by disease and by mosquito species and subspecies, which can differ across geographic locations. The authors note that flooding’s effects on the built environment and social infrastructure—such as housing damage and displaced populations—also influence the spread of vector-borne diseases. A nuanced understanding of local conditions may help public health experts and disaster responders reduce disease spread after flooding through better preparedness and targeted interventions, they conclude.

Residents of Machala, Ecuador, work to unclog a canal after massive flooding in February 2016. Malaria, Zika virus disease, dengue, and other mosquito-borne diseases are widespread in South America. However, relatively few studies have addressed flooding-related disease incidence in this area. Image: © Dany Krom.

Lead author Jenna Coalson, an assistant professor of epidemiology of vector-borne diseases at the University of Notre Dame, and colleagues at the University of Arizona, the University of Colorado, and the International Center for Research on Women, reviewed 131 studies to evaluate links between heavy rainfall, flooding, and incidence of mosquito-borne diseases. These studies spanned seven decades and six continents. The majority referenced events in Asia, Africa, and North America (47%, 25%, and 24% of studies, respectively) and covered 19 distinct diseases, including malaria, Rift Valley fever, West Nile virus disease, Zika virus disease, and six types of encephalitis.

Almost half the 131 studies focused on malaria, which is carried by Anopheles mosquitoes.3 Malaria thus represented the researchers’ best chance of finding clear evidence for a link between flooding and specific diseases carried by specific mosquitoes. Although the results did generally indicate a positive association, variations in methodology and study quality, as well as in local and event-related conditions, made it difficult to interpret the strength and timing of the link.

Forty-five studies looked at dengue fever. The findings from these studies indicated that disease incidence may decrease in the first month after a flood only to increase over the next 3 months. The reasons for this temporary depression are not fully clear but could be due, at least in part, to heavy rainfall disrupting mosquito development by flushing eggs, larvae, and pupae from breeding sites.4

Statistical evidence is limited for other mosquito-borne diseases, the authors write, although findings suggested that outbreaks of Murray Valley encephalitis, Ross River virus disease, Barmah Forest virus disease, Rift Valley fever, and Japanese encephalitis may also follow flooding. “One of the real takeaways is that it’s not as simple and straightforward of a relationship as you might expect,” Coalson says. “I think the big problem we discovered in this review is that the literature doesn’t really address this question of nonlinearity so well.”

Anna Stewart-Ibarra, scientific director of the Inter-American Institute for Global Change Research in Uruguay, who was not affiliated with the review, says another important revelation is the relative paucity of research on the subject in South America, where mosquito-borne diseases are widespread.5 “This issue affects most people in the Global South, where we have the greatest burden of disease but fewer studies,” Stewart-Ibarra says. “If we don’t have a good understanding of climate and health risks, then we can’t even begin to develop tools to inform decision makers.”

In coastal Ecuador, for example, knowledge of future flood risk and implications for mosquito-borne diseases could be leveraged to support impoverished residents of low-lying lands that already flood every year.6 “People who live in those areas have substandard housing; often the homes are built of materials that are totally permeable to mosquitoes,” Stewart-Ibarra says. “And they have less access to services from the public health sector. There are these intersecting risk factors that come together.”

Elke Hertig, a professor of regional climate change and health at the University of Augsburg in Germany, who also was not affiliated with the review, agrees that a better understanding of local phenomena can be leveraged to improve early warning and response systems. “The findings underline the high uncertainty and knowledge gaps with respect to the relationships between flooding and mosquito-borne diseases,” she says.

At least one thing is certain: The stakes will only get higher as climate change brings more flooding7 and warmer temperatures allow mosquitoes to expand their range.8,9,10 “Climate change is going to change the areas where we think these diseases [will] easily spread in human populations,” says Coalson. “How the temperature changes and how rainfall changes in different geographic settings might put new populations at risk in ways that they haven’t been historically.”