Can you please summarize your findings for us?
Dengue is a common mosquito-borne viral disease. In non-tropical urban cities with substantial water sanitary control, mosquitoes are active only in summer and there are few mosquitoes around humans.
We investigated what conditions can be important for a dengue epidemic to occur in an urban city such as Tokyo, using a mathematical model that consists of high-risk-area and low-risk-area parts.
The high-risk area is supposed to be the area where number of mosquitoes per person is high, e.g., parks. The low-risk area is supposed to be the area where it is low, e.g., business districts and residential areas.
Assuming that mosquitoes are active for only 90 days in a year, we assess five potential countermeasures: (1) restriction of the travel between the two areas, (2) insecticide application, (3) call on citizens to use mosquito repellents, (4) reducing carrying capacity of mosquitoes and (5) isolation of infected people.
Measures 2–5 are more or less effective. However, Measure 1 can have the opposite effect, depending on the mobility (traveling) rates. When the mobility rate between the two areas is small, the population in the high-risk area becomes a kind of core population, which can promote the epidemic in the entire population.
In other words, when the mobility rate is sufficiently small, the number of mosquitoes per person in the high-risk area increases, and the epidemic starts earlier in the high-risk area than in the low-risk one, while the number of infected people increases slowly.
Why did you choose to model these particular countermeasures?
Since the vaccine of dengue is not provided in Japan in general, we need to choose the countermeasures to avoid contact with infected mosquitoes.
In 2014, we experienced a dengue epidemic in Tokyo, where most of the authors live. In Tokyo, mosquitoes are active only few months in a year, and water sanitation system is well-ordered.
Thus, a dengue epidemic rarely occurs. Since the vaccine of dengue is not provided in Japan in general, we need to choose the countermeasures to avoid contact with infected mosquitoes.
Assessing these countermeasures may contribute to evaluating effects of the measures that were actually taken for the epidemic in Tokyo 2014.
Why is this such a surprising result?
It is intuitive to think that the epidemic can be controlled by the restriction of travel between the high-risk and low-risk areas. For example, when the dengue epidemic occurred in Tokyo 2014, use of parks was actually restricted, and our study supports this countermeasure since complete closure of parks is effective in controlling the epidemic.
However, our results suggest that depending on the strictness of the restriction, the epidemic can start earlier than in a non-restricted case. This seems counter-intuitive and is a pitfall of this countermeasure.
Should this result change how we approach controlling epidemics?
The result of the restriction of travel implies that we may have to reconsider how strictly the entry to parks should be controlled.
The result of the restriction of travel implies that we may have to reconsider how strictly the entry to parks should be controlled. Results of other measures also have implication for controlling epidemics.
In non-tropical urban cities where mosquitoes are active only for a few months in a year, some countermeasures that seem ineffective in tropical and sub-tropical dengue-epidemic regions can be practically effective. Our results help us choose effective measures considering cost performance, local economic conditions, living environment and life style.
How applicable are these conclusions to management of other diseases?
Some conclusions are applicable to controlling other mosquito-borne viral diseases such as malaria and zika fever. Especially in non-tropical urban cities, the conclusions will give some suggestion for controlling the epidemic of such a disease in its early stage.
Hiroki Masui, Itsuki Kakitani, Shumpei Ujiyama & Kazue Kudo
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