Scientists have long known that epidemics of dengue fever wax and wane over a period of several years, but they've never been quite sure why.
With the incidence and range of the potentially deadly mosquito-borne illness increasing, understanding the factors that influence these epidemics has never been more important.
A new study by researchers at the University of Georgia suggests that a brief period of cross-immunity conferred by any one of the four viral strains, or serotypes, that cause dengue explains the timing of epidemics.
"We found that since about the mid 1980s, there's been a sequential replacement of the dominant serotype," said lead author Helen Wearing, a post-doctoral researcher at the UGA Institute of Ecology. "So, for example, one year serotype three is 60 percent of the cases and the next year serotype two is dominant and so on. Epidemics of individual serotypes recur every eight to 10 years, but, at the same time, if you look at all the data together, you see about an average three-year cycle with some seasonal component to it."
In addition to helping resolve a long-standing debate in public health, the study, published this week in the early online edition of the journal Proceedings of the National Academy of Sciences, gives researchers a framework that can be used to create models that predict dengue outbreaks in both space and time.
"It's a framework that highlights the key elements you need to take into account while developing a forecasting model," Wearing said. "Because if you were to build a forecasting model without understanding the trends in cross-immunity, you would not necessarily predict what we observe."
The researchers examined 30 years of data from the government of Thailand and from a Thai clinic that keeps what is widely regarded as the most comprehensive set of data on dengue. Southeast Asia has been a dengue hot spot since the 1950s, but the researchers note that their model applies to other regions where all four dengue strains circulate.
The researchers compared the data with results from mathematical models that explore - both independently and collectively - the role factors such as temporary cross-immunity and variation in serotype virulence play in epidemics. They found that cross-immunity alone is enough to create the patterns that are observed in nature.
The study is a major departure from other theories about what drives dengue epidemics. The conventional wisdom is that an amplification of the severity of disease caused by repeat infections with different strains, a phenomenon known as antibody-dependent enhancement, drives the boom and bust cycles of dengue epidemics.
"Temporary cross-immunity has been ignored by the epidemiological community," said study co-author Pejman Rohani, associate professor of ecology and UGA Biomedical and Health Sciences Institute researcher.
"It's discussed in the scientific literature," Wearing added, "but no one ever considers it an important factor in generating the epidemiological cycles."