Tue, Sep 04, 2018 - Page 8 News List

Fighting dengue on multiple fronts

By Emilio Venezian

I agree with the overall conclusion of King Chwan-Chuen (金傳春) (“Prevention key to halting dengue,” Aug. 30, page 8). However, there are parts where questions seem appropriate and parts in which she does not delve enough.

King’s description makes it clear that transmission of the disease requires, as far as we know, an insect vector and that the virus must come into contact with people.

This suggests that disease prevention could be achieved in three basic ways: elimination of the vector, elimination of the virus in areas where the vectors are present, or interruption of the link between the vector and humans. Or possibly by combining the three ways.

She does not mention vaccination as a possible strategy and I agree with her.

In 1972, when I was studying the prevention of diseases transmitted by the Aedes aegypti in the Americas, there was a strong suspicion that dengue hemorrhagic fever (DHF), a much more serious condition than dengue fever, might be causing infections with more than one strain of the dengue virus, which is usually economically significant, but not nearly as life threatening.

I felt that any attempt to try a vaccine in populations for which dengue was a risk would entail unacceptably high risks. The evidence now seems to favor the view that the risk is appreciable. We need to understand the relation between dengue fever and DHF before we can view vaccination as a viable solution.

Another issue that was not mentioned is the possibility of hosts other than humans in the cycle of the disease. I do not know if any of the Taiwanese fauna might play a role in the transmission of the disease.

However, we do know that at least some species of rhesus and macaque monkeys develop dengue fever when exposed to the virus.

Information on other possible hosts is needed if we are to break the cycle, so a sensible approach requires determining through blood tests or experiments whether wild or domestic animals are susceptible.

If such hosts do exist, we need to develop strategies to prevent contact between them and the insect vectors.

King’s statement that “northern and central Taiwan experience small and medium-sized outbreaks, and another species of dengue-transmitting mosquito — Aedes albopictus — can be found in Taipei, New Taipei City and Taichung” along with her recommendation that “Taipei and New Taipei City should become an Aedes albopictus mosquito reduction zone, using different mosquito indices to routinely evaluate the effectiveness of different preventive measures,” suggest that Aedes albopictus is the sole, or at least the overwhelming, path of transmission and is likely to retain that position.

The first observation might be correct, but the size of epidemics is not necessarily caused by a difference in vectors: It could also be caused by differences in temperature or rainfall.

The implication that people need not worry about Aedes aegypti north of Chiayi is questionable. For one, the latitudes at which the two species exist in the continental US include the latitude where Tokyo lies, well beyond the northernmost point of Taiwan.

A second reason is that with global warming, it is not a safe bet that the northern end of the range in Taiwan, if indeed it exists, will not move northward.

There is also a possible third reason that needs to be considered. I cannot speak about the ecology of Aedes albopictus, since it was not within the scope of our assignment, but we did look into the ecology of Aedes aegypti. The latter is very sensitive to variations in temperature and tends to die off quickly during periods of peaks or troughs in temperature, although the availability of water and shade moderate the mortality of larvae and adults.

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