Dengue: Who is winning the war… Virus or Sri Lanka?

We are only wasting time and money trying to control dengue by preventing mosquito breeding any more.  We need to resort to protecting infected people from mosquito bites.

We have generally described the intensity of dengue infection by the number of cases reported during a period, a week, a month or a year. For example, we say that during 2016 there were 54,000 dengue cases reported. What this means is that there were that many new cases of dengue in that year. This figure does not give an indication of how many people were carrying the dengue virus at any time. 
We can estimate the actual number of people who were harbouring the dengue virus on an average day in 2016. Let us assume that the number of new infections were reported evenly throughout the year. This is not strictly true because we know that during some months more cases are reported than during other months. However in order to compare years this is a fair assumption. We also know that the dengue virus lives in a patient’s body for an average of 7 days. 

Therefore, on average the number of people who are harbouring the virus on any day is:

Yearly Total / 365 x 7

In the case of the year 2016 it is 54,000 / 365 x 7 = 1,036

We can use this method to estimate the “size” of the dengue infection from year to year.

The next question we need to pay attention to is the nature of the  virus

We generally consider a virus to be an insignificant and delicate bit of microscopic matter. But there is another way of looking at viruses. Let’s take the dengue virus for example. It is true that Individually the dengue virus is small and insignificant. However if we consider all of the dengue viruses present in Sri Lanka at any time as one entity then it is a very powerful creature.

This is not an unusual way of looking at it. If we take a beehive for example or an ants nest or termite nest, some would consider them to be one organism. The same concept can be extended to the dengue virus. The difference is that unlike in a beehive for example, the individual “members” of the organism are spread widely apart in different people all over the country (or even the world).  When looked at like this, we can consider the dengue virus as an alien that has invaded Sri Lanka. It first appeared in 1962. Therefore we can say that dengue invaded Sri Lanka in 1962. 

A virus is also very smart

The virus particle consists of only what can be considered the brain of the organism. It has no “body” as such. It has no mouth or stomach or lungs. It uses other living things to grow and multiply. It does not have to eat and drink like other creatures to grow. It uses the metabolic processes of living cells to multiply using the resources of the cell. Naturally other creatures don’t like this and would try to get rid of the virus and mount an attack. If the host organism succeeds the virus would die. If the host organism loses it would die and the virus would also die with it. Therefore in either case the virus would die. Therefore, the virus has to find a means of getting to another organism if it wants to live. 

Take the influenza virus for example. It causes the patient to sneeze and cough.  When the patient does this, the virus gets transported to another person. In the case of dengue, the virus uses a mosquito for the purpose. Maybe the dengue virus causes dengue patients to be more attractive to mosquitoes than other people.

Dengue Monster

Now getting back to dengue and Sri Lanka, when the dengue virus appeared in 1962 it would have been very small in size and occupied a single person or a mosquito. Gradually it spread to other people. The table below shows the “size” of the virus for different years calculated by the above method.

The diagram below is an attempt to show graphically the effect of this increase in size of the dengue “monster” on Sri Lanka. It must be remembered that the dengue monster does not stay still. It moves from one group of people to another group gradually. When it loses its grip on one set of people it has got in to another set.In seven days the monster occupies a completely new set of people. 

Another thing that happens is that the mosquito that takes the virus from an infected person, does not limit the number of people it infects to one person. It will infect all the people it bites. When a mosquito bites to get a blood meal, it does not collect the blood from one person. It keeps stinging person after person in what is called “probing” until it has collected enough blood.

It must also be remembered that once a mosquito is infected it remains infectious for life. Therefore it will infect people for the rest of its life which is generally a month. So imagine the efficiency of the dengue virus in the method of its spread. What this further means is that not many mosquitoes are required for the dengue monster to remain the same size if not actually grow in size.

The chart below shows the same thing in a different way. It  shows the size of the dengue monster from 1992 to 2017. 

How to beat dengue

By looking at the dengue epidemic in this manner we can easily understand how we can beat dengue. If we take the case of 1992, on average there were only 13 persons carrying the dengue virus in the whole of Sri Lanka. Of course, it was not the same 13 people for the whole year. As one person got better (or died) a different person took his/her place and the 13 people affected moved along. Now if we looked at the later years, the number of people carrying the virus got bigger and bigger and by 2017, on average, 2,961 people carry the virus at any moment. It must be remembered that the virus can only live in a human or mosquito. If we stopped the virus from hopping from one human to another the dengue monster cannot grow bigger.  In 1992 if we protected the 13 infected people from the bite of mosquitoes the dengue monster would have died out. 

The other significant feature that comes out is how the ability for mosquitoes to find an infected person has changed over the years. In 1992 there were only 13 people that the mosquito could get the virus from. This is the reason why the infection did not spread. This situation remained so until 2000 when the number of people harbouring the virus jumped to 100 and it was easier for a mosquito to accidentally “bite” a person with the virus. From then on it got easier and easier. In 1992 there were many more aedes mosquitoes around because there was no intensive control programme. But the number of people with the virus were rare. The situation is reversed now. There are much less mosquitoes around but there are many more infected people. As the graph shows the rate of increase gets steeper with each passing year.

We have now arrived at a stage when we cannot reduce the breeding of mosquitoes any more by eliminating accessible mosquito breeding sites. We have reached a stage that can be described as the stage with irreducible minimum Aedes mosquito numbers. We are only wasting time and money trying to control dengue by preventing mosquito breeding any more.  We need to resort to protecting infected people from mosquito bites. 

It is easier than it looks at first sight. We know that in 2017, on average, there are only about 3,000 people with the virus in them at any time. Not only that, a large number of the 3,000 are in hospital. Now, that people are told to get tested quickly for dengue if they are ill, we will also know who are harbouring dengue virus but not in hospital and we can take appropriate measures to protect them as well. 

What is required to control dengue is common sense and political will.

Dr Lal Jayasinghe


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