Scientists explain why lower transmission may increase dengue
5 Feb 2008, 1347 Hrs

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Washington, February 5 (ANI): A scientist duo has come up with an xplanation for why efforts to lower the transmission of dengue irus in Thailand have failed to reduce the life-threatening form f the infection.

It seemed that the number of dengue deaths had increased despite uch efforts, said Duke University Biology Professor Katia Koelle nd graduate student Yoshiro Nagao at Osaka University Graduate chool of Medicine.

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 orld'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 a ear after a first dengue infection. They say that this "cross-mmunity" between strains wanes with time however, as immune ystem 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 ow 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 uring the year of cross-immunity, the challenges of new strains f 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 ble to build a library of antibodies during that year of cross-mmunity, leaving them susceptible to subsequent strains down the oad.

In spite of these observations, Koelle says that Thai officials eed to keep up the heat on the disease-carriers to "get over the ump" on the epidemiological curve.

She says that the chances of second and third infections leading o DHF would be dramatically reduced if transmissions were owered still further.

"We're definitely at the point where we'd really like to see ecreases 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 (ANI)