Are asymptomatic dengue “patients” a source of dengue infection to others?

A proposal has been made that the way to control the current dengue epidemic is by “isolating” known dengue patients from the bite of aedes mosquitoes. The rationale being that the only source of the dengue virus is a dengue patient. If that source is eliminated by preventing aedes mosquitoes from biting dengue patients, the epidemic could be brought under control, with the demise of the already infected aedes mosquitoes. See www.eradicatedengue.blogspot.com.

It is known that some people do develop dengue infection without showing any symptoms i.e. asymptomatic patients. A quite reasonable doubt has been raised that the asymptomatic patients would be a continuing source of the dengue virus and the above mentioned strategy would not work. 

The sequence of events for a dengue infection

Before responding to this possible drawback to the suggested strategy, it is necessary to briefly remind readers the sequence of events for a dengue infection. Before a person catches dengue, he or she has to be bitten by an dengue infected aedes mosquito. An aedes mosquito becomes infected by biting a dengue patient. During the illness period of a dengue patient, which usually lasts for about a week, (unless there are complications), there is a “window” of about 5 days or 7 at the most, when the patient becomes infectious to a mosquito. This period begins on the day before the patient becomes ill and lasts for about the first 5 days of illness. The reason why the patient is infectious only during this period of illness is because the dengue virus is present in sufficient numbers in the blood of the patient only during these days. The virus is not present in the blood of the patient during the incubation period or after the fever has subsided. The patient is infectious to aedes mosquitoes if the mosquito can collect the virus from the blood of the patient by biting. If there is no virus in the blood of the patient, the patient is not infectious to the mosquito A sure sign that a dengue patient is infectious (i.e. has virus in the blood) is the presence of fever. If fever is absent the patient is most unlikely to be infectious. By definition an asymptomatic “patient” has no fever.

Asymptomatic “patients”

One other misunderstanding that need to be clarified is the way that asymptomatic patients are counted.  The manner by which asymptomatic patients are counted is as follows. 

A group of people are followed for a period, say one year. At the beginning of the year, blood is collected from all members of the group, who are of course all well at this stage. Their blood is then tested for the presence of antibody against dengue virus, indicating previous dengue infection. Then the group is closely watched for symptoms and sometimes their temperature is also measured daily. If symptoms and/or fever appears, they are tested for dengue. If positive then these positive persons  are counted as a dengue case.

At the end of the year blood is again taken from everybody. Everybody’s blood is then  tested for the second time for evidence of antibody for dengue infection. This second test, if positive now, but negative at the beginning of the study, will mean that the person has had dengue during the study period but did not show symptoms. These people would be classified as asymptomatic cases. Those who had symptoms are of course known and would be classified as (symptomatic) cases.

A good  example is a study conducted in Borella (Sri Lanka) during 2008 -2010 by a team  led by Dr Hasitha Thisera from the Dengue Control Unit.  A total of 799 children under 12 years were followed up for 16 months.  During this period there was a total of 681 fever episodes. Out of this total, 27 were due to dengue as proved by blood tests taken at the time of illness including tests for the presence of dengue virus. There were 40 children who were negative for dengue antibody at the start of the study but were positive at the end of the study, but did not have fever at any time during the period of study. These 40 children are the asymptomatic cases. This then gives a ratio for Asymptomatic : Symptomatic of 40/27 or 1.48. What this means is that for every child with proven dengue infection there were 1.5 times as many children who also had dengue infection but did not have symptoms.

Now the question is did these 40 children pose a threat as a source of dengue virus to aedes mosquitoes who bit them at the time they were infected. The answer is very unlikely. If they did not have fever, they did not have a viraemia (i.e. dengue virus in the blood), for a mosquito to be infected at time of biting.

If a mosquito bites a patient on the day before he develops fever due to dengue, the mosquito will become infectious. There is nothing we can do about these events (i.e the bites on the day before the patient gets fever). However, it is not necessary to isolate or prevent mosquito bites of all infectious people. It will be impossible to do so., It is not necessary to isolate every infectious person for the strategy to succeed. A 50% success rate will eradicate dengue from Sri Lanka in 6 months as explained in the blog. (See "How long do we have to isolate dengue patients? Is 4 months enough?").

Dr Lal Jayasinghe
laljayasinghe@hotmail.com

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