An international research team led by the University of Pittsburgh Graduate School of Public Health has shown that epidemics of dengue, which is caused by a mosquito-borne virus, across southeast Asia appear to be linked to the abnormally high temperatures brought by the El Niño weather phenomenon.
Now, as the most intense El Niño in nearly two decades is emerging in the Pacific, the finding – reported in a recent issue of the Proceedings of the National Academy of Sciences (PNAS) – may be a harbinger of a spike in cases of the dangerous hemorrhagic fever throughout southeast Asian countries early next year.
“Large dengue epidemics occur unexpectedly, which can overburden the health care systems,” said lead author Dr. Willem G. van Panhuis, assistant professor of epidemiology at Pitt Public Health. “Our analysis shows that elevated temperatures can create the ideal circumstance for large-scale dengue epidemics to spread across a wide region. The ability to predict and prepare for these epidemics should lead to more effective disease surveillance and control efforts.”
Dengue virus is transmitted by mosquitoes in the tropics and subtropics. It causes an estimated 390 million infections each year. In many countries, reported cases of dengue wax and wane during the rainy season following a repeating annual cycle. So far, it has been difficult to predict when these epidemics will become unusually large, spreading beyond country borders.
The research team collected and analyzed 18 years of monthly dengue surveillance reports on a total of 3.5 million reported cases in 273 provinces in eight countries in southeast Asia. By bringing the data together from several countries, the scientists were able to see patterns – or synchronicity – in dengue transmission across the entire region.
In 1997 and 1998, dengue transmission was very high, matching up perfectly with high temperatures that allowed mosquitoes to reproduce faster and spread dengue virus more efficiently. These high temperatures were caused by an exceptionally strong El Niño season, which occurs when rising sea water temperatures in the eastern Pacific move westward. This phenomenon occurs about every five years, with one of the largest episodes expected in the coming months.
This study also found that urban areas act as dengue epidemic “pacemakers” because of their constant supply of new people who are susceptible to dengue. In addition, traveling waves of large epidemics were found to emerge from west Thailand, central Laos and the southern Philippines.
The international team involved scientists from 18 institutions around the world, including the Ministries of Health in each study country. Dr. Donald S. Burke, dean of Pitt Public Health and UPMC Jonas Salk Professor of Global Health, was the principal investigator of the study, and Dr. Derek A.T. Cummings, of the University of Florida and the Johns Hopkins Bloomberg School of Public Health, was senior author.
This research was funded by Bill & Melinda Gates Foundation grant 49276 and the National Institutes of Health National Institute of General Medical Sciences grant 5U54GM088491.
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