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Chikungunya more prevalent in women
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An investigation of a 2012 chikungunya outbreak in Bangladesh showed that women were disproportionately affected by the virus.
Researchers used an integrative approach to better understand disease transmission and determined that women were more likely to develop infection because of the amount of time they spend at home.
“Typically, when there is an outbreak, we study who is sick and try to understand why,” Henrik Salje, PhD, a post-doctoral fellow in the department of epidemiology at Johns Hopkins Bloomberg School of Public Health, said in a press release. “In this case, we not only studied those who became infected with chikungunya, but also those who avoided illness. This allowed us to determine what factors may impact who comes down with a disease and who does not — and to help us determine the best way to intervene.”
Salje and colleagues were deployed to Palpara in November 2012 to investigate the chikungunya outbreak. They visited every household in the village to interview individuals who experienced chikungunya symptoms and their household members. The researchers also recorded the GPS location of the homes to help them track outbreak patterns.
Overall, 1,933 participants from 460 households were included in the study. Among them, 364 (18%) reported having a fever with severe joint pain or rash consistent with chikungunya between May 29 and Dec. 1, 2012. IgM ELISA testing confirmed the infection in a subset of 175 patients.
A transmission model was used to determine disease risk factors. According to the model, women were 1.5 (95% CI, 1.2-1.8) times more likely to be infected with chikungunya than men. The probability of chikungunya transmission was 12% (95% CI, 8-17) between household members, which dropped to 0.3% (95% CI, 0.2-0.5) among those living 50 m away, and 0.2% (95% CI, 0.1-0.2) among those living 100 m away.
To determine how these factors contributed to the epidemic, the researchers reconstructed 200 transmission trees consistent with the information they gathered. They found that 27% (95% CI, 23-31) of all transmission events occurred within the same household and 58% (95% CI, 51-65) of infections occurred in households within 200 m.
The researchers then used data from a separate study examining movement habits among a wider Bangladesh population to determine whether spending more time at home increased the risk for infection. They found that women were 1.5 times (95% CI, 1.4-1.6) more likely to be in or around their homes between the hours of 8:00 a.m. and 8:00 p.m. compared with men (66% vs. 45%).
“It appears that mosquitoes are very lazy,” Salje said in the release. “They bite someone in a household and get infected with a virus and then hang around to bite someone else in the same home or very nearby. The extra time women spend in and around their home means they are at increased risk of getting sick.”
In other results, daily use of anti-mosquito coils designed to prevent transmission did not have an impact on infections (RR = 1; 95% CI, 0.8-1.2).
“We don’t yet have a very good toolbox for fighting [chikungunya],” Salje said. “But once we do, this research tells us how we could trigger a response and tailor our interventions — particularly in rural communities — to those at greatest risk, and those people are the ones who spend the most time in and around their homes.” – by Stephanie Viguers
Disclosures: The researchers report no relevant financial disclosures.
Perspective
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J. Glenn Morris, Jr.
The research reported by Salje and colleagues uses sophisticated modeling techniques and careful data collection to “take apart” transmission of Chikungunya, a mosquito-borne virus. Their findings underscore the importance of the local (household, neighborhood) environment in predicting transmission; as one example, differences in infection rate between men and women appear to be attributable to subtle differences in factors such as the time spent within the household (women spend more time at home).
What kicks this up to a higher level of relevance is its potential applicability to Zika — which is a virus transmitted by the same mosquitoes as chikungunya. We still do not know that much about Zika, and approaches to its control have tended to be “one size fits all.” The beautiful modeling done with Chikungunya by Salje and colleagues suggests that we need to go more slowly, doing good science and looking carefully at the myriad of factors that control where the mosquito goes, how it gets infected, and who it is likely to bite. Increasingly, it appears that Zika is here to stay, at least in portions of the southern United States, and massive spraying of insecticides may not be that acceptable environmentally, or that effective. We need to do some careful science, and see if we can be a little smarter in the ways we approach control of transmission of this newly emergent virus, with its disturbing propensity for causing serious fetal injury.
Disclosure: Morris reports receiving grant funding from the NIH and the European Commission.
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