FAQ: How does Dengue spread? How can it be prevented?

With at least one death reported almost on a daily basis in the country because of Dengue fever, the disease has become a dangerous epidemic.

Entemologists say that it's not only during the monsoon season that the Dengue virus spreads. Even during non-monsoon times the virus spreads through vertical transmission, and that has made the disease all the more dangerous.

B K Tyagi, director of the Madurai-based Centre for Research in Medical Entomology, talks with Rediff.com’s Shobha Warrier about how dengue fever can be controlled.

When was it first found that dengue virus is transmitted vertically too?

Dengue virus was detected in India in 1950s and the biological way of transmission was detected then itself. It was only much later that scientists found that the virus was transmitted vertically also by mosquitoes; though vertical transmission is not a common way of transmission.

What exactly is vertical transmission or ovarian transmission and how different is it from biological transmission of a mosquito biting and transmitting the virus to a human being?

Biological transmission is when a mosquito takes the blood as well as the pathogen while biting someone with the virus. The pathogen completes the life cycle inside the mosquito. Then it comes to the salivary glands, and after 6-7 days, when the mosquito bites, the pathogen is transmitted to a new human being. This is the most common way of transmission.

Vertical transmission is of evolutionary significance in which pathogen or virus can go to other parts of the mosquito’s body during its journey inside after it sucks the blood of an infected human being. From stomach to the stomach walls and through the body cavity and blood to different parts of the body. This way, it also goes to the ovaries. Inside the ovaries, the eggs that are developing get infected.

When the eggs are laid by the female mosquito, the virus also comes out of the body of the mosquito through the eggs. There may be several of infected eggs. The virus remains as the eggs develop into larvae, pupae and finally several mosquitoes come out carrying the virus, ready to bite human beings. These mosquitoes carry the viruses and they need not even bite an infected person to get the virus; they are born with the virus.

You said, evolution wise, vertical transmission was very important. Why do you say so?

In vertical transmission, the virus has to cross many barriers inside the body of the mosquito. After the mosquito bites and the virus enters the body, the first barrier is the stomach wall followed by the ovarian wall and the egg wall to finally get into the egg. This cannot be achieved unless there is a reduction of physiological amicability between the virus and the mosquito body. This phenomenon might not have developed in a day or two; it would have taken hundreds and thousands of years. This is a strategy on the part of the virus to survive.

Would this not make the virus stronger?

Naturally. Crossing all these barriers make the virus stronger and wiser! They learn to survive, divide and multiply.

Which is more dangerous for human beings?

You cannot pinpoint any one as more dangerous. Both have to be looked at from different perspectives. The most common and natural way of transmission of dengue virus is through mosquito bites. That means for effective transmission of the virus, a mosquito has to bite a human being. When it bites, the load of virus is high and millions of viruses can go into the body of the mosquito. So, it can get infected and inject the virus to other bodies more strongly.

On the other hand, in vertical transmission, the virus load is quite low and it has to pass through many barriers and every time it passes through a barrier, many of the particles in the virus will be destroyed. By the time a person gets the virus, it is genetically not that potential enough to infect.

So, in vertical transmission, number of particles is low compared to biological transmission. But how vertical transmission becomes dangerous is when transmission is low in nature -- that is during non-monsoon times -- you as a human being tend to feel there is no pathogen in nature, and then the virus erupts out. You wonder from where did the virus come out? This is very important. If there is a place where the virus in nature can flourish, it can suddenly cause havoc surprising people.

Is it true that Aedes Aegypti can dominate over Anopheles mosquito?

Both are different species of mosquitoes and both have different places to breed. But both attack the same human beings and both will be seen in the same area. So, there will be competition for survival. This is natural.

So, what precautions should people or authorities take?

The most important thing people should be aware of is that they should not allow mosquitoes to breed inside or around their houses. There should not be any stagnant fresh water. It is in fresh water that Aedes Aegypti, the Dengue and Chikun Gunya spreading mosquitoes, breed. If there is stagnant fresh water in pots, overhead tanks, etc, they should be covered not allowing mosquitoes to breed. If there is fresh water kept openly, it has to be changed at least once in three days whether there is larvae in it or not. It has to be a habit for everyone.

Next, people should learn to use mosquito nets while sleeping. If the windows have wire meshes, it will prevent mosquitoes from entering the house.

Communities should also see to that there is no stagnant water or solid waste anywhere in the locality which can be a breeding ground for mosquitoes. There should be proper disposal of solid waste and garbage. People should be responsible for their locality.

There is no vaccine available for Dengue or Chikun Gunya. However, if someone gets sick, he should see a physician, test blood and get hospitalised, and the chances of his recovery will be good.

Image: An Aedes Aegypti mosquito feeding on a human host