How to Eradicate Dengue Virus from Sri Lanka

By a special correspondent

Sri Lanka is under the grip of a severe epidemic of Dengue fever and Dengue haemorrhagic fever.  The epidemic did not happen over a short period. Dengue has been in Sri Lanka since 1962. However, it was not a major public health problem. The disease was progressing at a very low rate. Since about 2004, the numbers of cases started going up every year and accelerated dramatically from about 2008.

The authorities have not been idle during this period.  A dengue control programme has been in operation throughout. In recent years, there has been a National Dengue Control programme involving most government departments, the police and even the armed forces, coordinated at the highest level.

As Graph 2 shows their efforts have not been in vain. The Breteau Index (BI) which is an index of the presence of mosquito breeding sites have continued to fall until 2012. It would be even lower now, 5 years later. But during the same period the dengue cases continued to rise (red line). However, the same strategy to control dengue is continued to be followed today because; one, there appears to be  no viable alternative and two, it is the recommended strategy; namely to reduce and eradicate aedes mosquito breeding sites.

Should we continue with this strategy?

As Graph 3 shows, every year, since 2012, the annual cases have been going up. Although there are periods of high and low incidence of cases, the pattern has been the same every year. In addition to the ongoing routine programme of dengue control, special 3, 5 or 7 day eradication programmes are also instituted periodically. We can use the pattern of dengue incidence during the past five years to test whether the special eradication programmes have any effect.

Graph 4 shows in blue, the pattern of average dengue cases during the years 2002 – 2015.  The red line is what happened in 2016. The dates on the horizontal axis (29^th Mar., 15^th Jun. etc.)  are the dates in 2016 on which the special programmes for eradication of breeding sites started.  The reader can judge if the special programmes have been successful or not in 2016,  from the way the red line has moved with the blue line in spite of the special programmes.

It is therefore time to institute an alternative method of control or even eradicating dengue from Sri Lanka. Before such a strategy is even considered, a certain mindset has to be eliminated from the public (and even officials). The mindset is that “the only way dengue can be controlled is by preventing breeding of aedes mosquitoes”.

On analysing the dengue cases since 2004, some features were observed.  The disease is more prevalent in some districts than others.

The more densely populated a district is, the more intensely will the epidemic spread in that district as seen in Graph 5. Also, the number of cases a district will experience during a quarter depends on how many cases there were during the previous quarter.

These are two features of an infectious disease. 

Knowing that dengue in Sri Lanka behaves as an infectious disease, we can adopt the measures that are traditionally adopted to control an infectious disease; when there is neither an effective treatment nor an effective vaccine. That measure or strategy is isolation.

 

Isolation works by preventing the transfer of the disease-causing organism from a sick individual to a healthy individual by whatever means. The transfer may be by touch, sneezing and coughing or by an insect.

In epidemiology, there is a concept named Effective Reproduction Rate (R). What this means is that in infectious disease epidemics, the behaviour or progress of the epidemic depends on, among other things, to how many healthy persons a sick individual will pass on  the disease. If the sick person infects only one other person (R=1),  the disease will continue to affect the community but will not become an epidemic. If the sick person (on average) passes on the disease to less than one person (R<1), the disease will slowly but surely disappear. On the other hand, if the sick person infects more than one healthy person (R>1), there will be an epidemic.

This in effect means that, in order to stop the epidemic (of dengue or any other disease), we do not have to isolate every case. Graph 6 shows  (in theory), what would happen if we reduced the average number of healthy people that a dengue patient infects.  As the graph shows, how soon we eradicate dengue will depend on how effective we are in isolating dengue patients. (50% isolation will eliminate dengue in 6 generations or 4 months).

This naturally raises the question; how do we isolate dengue patients? We do not actually “isolate” a dengue patients in the traditional sense. We isolate the dengue patient from mosquito bites.

One would notice that what is suggested is quite a radical departure from the accepted dogma. If we are to adopt the strategy proposed two things are essential. There must be very strong commitment at the highest level to the new strategy. Secondly, there is a need to mount an intensive health education campaign. Some basic misconceptions have to be corrected. For example, that dengue is acquired from mosquitoes. Dengue is actually acquired from a dengue patient, the mosquito only “transports” the virus. All campaign literature must display the image of a dengue virus and not a mosquito.

Undoubtedly there will be many who will object to the proposed solution. Some will say that what is proposed is not practicable. I should like to ask them, what is the aim of the present strategy, i.e. prevent mosquito breeding? The final aim of the present strategy is also to prevent mosquitoes biting dengue patients. There is no harm done if aedes mosquitoes bite healthy people, unless of course the mosquitoes had bitten a dengue patient previously. 

Only a little reflection is necessary to realise what a roundabout and wasteful method we employ to control dengue. Take Colombo district as an example. There are 2,000,000 people living in Colombo. Every month about 800 cases are reported from Colombo. When one infected mosquito bites a person  (2,000,000 – 800)  1,999,200 are bitten by non-infected mosquitoes. What this means is that in order to prevent one mosquito biting a dengue patient we have to prevent the birth of (2,000,000/800) 2,500 mosquitoes.

Another argument that might be advanced is that the cooperation of the public cannot be expected. Once again if we take Colombo district as an example, there are approximately 500,000 households in the district. What the authorities are now asking the public to do is keep these 500,000 premises free of mosquitoes. What I am asking is to look after 800 patients.  Which is easier?  A further difficulty is that people find it difficult to eliminate all breeding sites from houses and gardens because, some breeding sites are either inaccessible or hidden. Eg., on the roof, in tree holes and underground. Dengue -patients on the other hand are easily identified because of fever. 

Finally if evidence is required to prove that it is not mosquitoes that cause dengue epidemics, rather than individual dengue cases, the next graph is sufficient:

When there were plenty of mosquito breeding, premises index of 50 in 1966, there were less than 50 cases of dengue per 100,000. However when the premises index was brought down to about 2 by 2005, there were 300 dengue cases per 100,000 population.

Even in Singapore, the authorities have now realised that control of mosquito breeding alone is not sufficient to eradicate dengue.  From 2016 there is an additional bit of advice to people.

You should apply insect repellent in the following situations:

■ You are a confirmed or suspected dengue patient, having a fever. Fever in dengue patients coincide with the presence of dengue virus in their blood, which could be inadvertently transmitted to family members and neighbours via Aedes mosquitoes.

■ A family member or neighbour has contracted dengue.

■ Your neighbourhood has been declared a dengue cluster. Click here for the list of active clusters. Use of repellent can minimise chances of being bitten by an infective mosquito.

■ You are visiting a place that is a declared cluster or has high mosquito population”.

(http://www.dengue.gov.sg/subject.asp?id=160)

Who is the enemy?

It must not be forgotten that the enemy is the virus and not the mosquito. Although the number of cases per year has increased 50,000 times since 1962, is anyone for a moment suggesting that we have had a 50,000 times increase in the (aedes) mosquito population in the country since 1962?

The 50,000 times increase has been in the virus population. Since 1962 we have been recycling the virus in our bodies. With each dengue infection, we have increased the number of viruses in the country.

We Sri Lankans have several advantages over other countries.

• We are an island with only India close by.
• We do not have daily influx of people from other countries who might be bringing the dengue virus and even mosquitoes
• We are a small country with good communications that can handle a new project quickly.
• And most important we have an educated and intelligent population who can understand the dynamics of dengue transmission and act accordingly.

The advantage of what is proposed is that, no major changes to the present strategy are required. The present strategy can continue but with the additional element of “isolation” suggested, namely to screen dengue patients from mosquito bites. The most efficient method of testing the effectiveness of the strategy would be to pilot the scheme in one (relatively isolated) district for a short period say 3 months. If successful to extend island-wide.

(This is an abridged version of a larger document. The original document containing the development of the suggested strategy together with the sources of data, references etc. can be found in "How to Eradicate Dengue Virus from Sri Lanka - Detailed Report").

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