Scientists explain why lower transmission may increase dengue

February 5th, 2008 - 2:06 pm ICT by admin  


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

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

The researchers analysed dengue infection data from Thailand going back to 1981, and constructed a set of epidemiological models 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 due to extensive mosquito control efforts, lies in the way the body 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 a dengue strain they had already experienced, and that they also were less likely to be ill again with another strain for about a year after a first dengue infection. They say that this “cross-immunity” between strains wanes with time however, as immune system antibodies decrease in number.

According to them, as the cross-immunity fades, another strain of the virus gains the ability to take advantage of the remaining low level of antibodies to enter the cells, making infection more severe and resulting more often in DHF.

One of the models developed by Nagao and Koelle suggests that during the year of cross-immunity, the challenges of new strains of virus are defeated by antibodies from the first strain, without creating immunity to the newer strains.

According to Koelle, when transmission rates are higher, a person stands a better chance of being bitten by more than one strain during this period of cross-immunity. This results in the development of new antibodies to additional strains, and thus makes a person less likely to get a severe dengue infection after he 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-immunity, 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 hump” on the epidemiological curve.

She says that the chances of second and third infections leading to DHF would be dramatically reduced if transmissions were lowered 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 reduce the overall number of dengue hemorrhagic fever cases,” she said.

Given the unavailability of a reliable vaccine of all four strains of dengue, Koelle insists that tough mosquito control is still the best course of action.

The study has been published online in the Proceedings of the National Academy of Sciences (ANI)

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