The researchers analysed dengue infection data from Thailand oing back to 1981, and constructed a set of epidemiological odels to explain why this strange pattern. Koelle and Nagao say that the reason why dengue hemorrhagic fever ( DHF) cases increased, despite a decrease in transmission rates ue to extensive mosquito control efforts, lies in the way the ody builds immunity to the four strains of dengue, which is the world's most common mosquito-borne viral disease.
The researchers knew that victims could not get re-infected with dengue strain they had already experienced, and that they also ere less likely to be ill again with another strain for about an year after a first dengue infection. They say that this "cross-mmunity" between strains wanes with time however, as immune system antibodies decrease in number.
According to them, as the cross-immunity fades, another strain of he virus gains the ability to take advantage of the remaining low level of antibodies to enter the cells, making infection more evere and resulting more often in DHF. One of the models developed by Nagao and Koelle suggests that luring the year of cross-immunity, the challenges of new strains of virus are defeated by antibodies from the first strain, ithout creating immunity to the newer strains.
According to Koelle, when transmission rates are higher, a person tands a better chance of being bitten by more than one strain uring this period of cross-immunity. This results in the evelopment of new antibodies to additional strains, and thus akes a person less likely to get a severe dengue infection after e is no longer protected by cross-immunity.
However, she adds, when transmission rates fall, fewer people are able to build a library of antibodies during that year of cross-mmunity, leaving them susceptible to subsequent strains down the road.
In spite of these observations, Koelle says that Thai officials need to keep up the heat on the disease-carriers to "get over the lump" on the epidemiological curve.
She says that the chances of second and third infections leading of DHF would be dramatically reduced if transmissions were owened still further.
"We're definitely at the point where we'd really like to see decreases in the infection rate that are substantial enough to educe the overall number of dengue hemorrhagic fever cases," she aid.
Given the unavailability of a reliable vaccine of all four trains of dengue, Koelle insists that tough mosquito control is till the best course of action.
The study has been published online in the Proceedings of the ational Academy of Sciences