How long do we have to isolate dengue patients? Is 4 months enough?

The period of time that we need to isolate patients depend on how efficient we are in isolating patients. Remember that mosquitoes do not infect mosquitoes. Patients infect mosquitoes. So if we isolated all patients, dengue will disappear immediately; that is as soon as the existing patients recover. Of course, even if we isolated all patients, the already infected mosquitoes will continue to give the infection to new people and this will continue as long as the already infected mosquitoes live; that is a month. And the patients infected by the last infected mosquito on the last day of its life will recover in a week. No more dengue!

R describes the generation of secondary cases in a population that has both cases and susceptible people, as in Sri Lanka.

When  R = 1  each infectious case gives rise to one new case
When  R > 1  each infectious case gives rise to more than one new case
When  R < 1  each infectious case gives rise to less than one new case

If we are not 100% efficient in isolating patients, we can still eradicate dengue if we are CONSISTENTLY isolating a percentage of patients. This is where R comes in. Remember, if R<1 is maintained, the disease has to disappear eventually. How the four months was estimated was by using the following theoretical diagram.

A patient has to be isolated for 5-7 days. But, it is not the same set of patients that we isolate everyday. Some patients will enter the set that needs isolating and some will leave the set everyday as they have been isolated for 7 days or at least 5 days. Therefore, the set of patients will vary from day to day and place to place. What we need to do is try and isolate as many of the patients as possible, and not just to try to achieve 50%. The more we isolate, the quicker the disease will disappear. What is important is to be CONSISTENT. That is, NOT TO RELAX our efforts.

The 4 months is based on a generation being 20 days. Six generations = 6x20 days = 120 days.

What is a Dengue Generation?


Viraemia = the dengue virus is present in the blood of the patient.

Extrinsic incubation period = the number of days required by the mosquito after acquiring the virus before it can spread the infection.

Intrinsic Incubation period = The number of days that must pass by the patient after being bitten by an infected mosquito before becoming ill.

We can start the clock from the first day of viraemia of the first human case to the first day of viraemia in the second human case. This would give rise to a generation time of 20 days.

(There is a small flaw in this calculations, because a generation is a bit longer than 20 days but not a lot longer, maybe 30 days. It  would take some space to explain the reasons and is due to the infected mosquito living for 30 days).

So, even if an average mosquito lives for as long as 30 days, we can still eradicate dengue in six months if we CONSISTENTLY isolate 50% of the patients. 

Dr Lal Jayasinghe


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