Fighting dengue on multiple fronts

By Emilio Venezian  / 

Tue, Sep 04, 2018 - Page 8

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.

That by itself could be a cause for the differences in seasonality of outbreaks in the south and north of Taiwan as noted by King.

With the prospect of global warming, the measures adopted for the parts of Taiwan north of the Tropic of Cancer should include the possibility that the balance in transmission via the two vectors might change over time.

If the characteristics of Aedes albopictus in terms of mortality at all stages and reproductive rates are similar to those of Aedes aegypti, the same would apply to the south of Taiwan.

In any case, making better estimates of these parameters would lead to better planning of the use of insecticides in emergencies such as major epidemics.

Another area that is not discussed is prevention by biological warfare. There are chemicals and biological predators that can be used to control larvae and adults. The identification of such locally effective means of dealing with the mosquitoes — while imposing serious harm to people or economically important plants and animals — is another way to prevention that has been used in other nations and should not be neglected.

Details are important. It is not enough to know where a person was bitten. A person who develops dengue fever might have contracted the virus almost anywhere they were during the incubation period. Each of those places would be a potential focus.

If we can pinpoint that location, then we need to find out where the mosquitoes might have had a blood meal long enough before encountering the index case. Each of those places would also be a potential focus.

If I visited Happy Farm in Taichung’s Dali District (大里) during the incubation period, it might well be the focus.

However, if in the prior month my wife had dengue fever and one of my colleagues at the office had fever and aching joints, then maybe my home and my office should also be considered.

That requires the collection of much more information than hospitals, doctors or nurses typically collect from patients and it also requires reliable communication with health authorities.

One more area needs discussion. We need more information on how the insect vectors propagate.

The information I had 50 years ago was that an Aedes aegypti does not fly very far from the place where it emerges as an adult. Most of its mobility is helped by wind.

If that is so, having information about wind speed, duration and direction would be important in alerting possible new focal points to watch out for dengue fever and report promptly or when deciding how large an area should be treated with insecticide after the focus is established.

Emilio Venezian is a former visiting professor at Feng Chia University.