As dengue spreads across Tamil Nadu, scientists suspect double trouble. A new vector and a new carrier, they feel, might be responsible for the outbreak in summer, which is considered a harsh condition for the virus to survive.
A group of scientists from the Indian Red Cross, Public Health Association of India and Citizen's Forum is drawing blood samples from pet animals to find out if they are harbouring the dengue virus.
Animals can be silent carriers of the virus, without showing any symptoms. Scientists also suspect that Aedes albopictus, a 'wild' variety of the 'domestic' Aedes ageypti mosquito, could have entered human settlements.
They may be biting these animals - and humans - establishing a new transmission route. While the most commonly spotted mosquito in the state is aegypti, since 2009, albopictus has been seen in human habitations.
Unlike ageypti, which survives only on human blood, the albopictus mosquito feeds on both animals and humans. "An infected albopictus can transmit the virus to both humans and animals. And this can result in a lot of cross-infection," said former director of public health Dr S Elango, who now works for the Public Health Association of India.
If the suspicion proves to be true, doctors in the state may have a bigger challenge on their hands. Dengue, a viral disease, is spread through the bites of infected female Aedes mosquitoes.
The virus circulates in the human blood for two to seven days when the patient experiences fever and severe body pain. During this time, other female Aedes mosquitoes biting the infected human can take up the virus and spread it to other human hosts during subsequent blood meals.
The virus is transferred from a female mosquito to her offspring via the eggs. So far, scientists have found Japanese encephalitis virus being harboured in water birds and pigs. Scientists at the Centre for Research in Medical Entomology (CRME) in Madurai say dengue virus in domestic animals is theoretically possible.
"Studies in Malaysia and Africa have shown that dengue virus can live in pets and domestic animals. But we have so far not seen it in animals in India," said CRME director Dr B K Tyagi. Investigations by Tyagi's team have revealed that the virus has been extremely virulent this season as there has been more than two subtypes.
The dengue virus has four subtypes and the team has already spotted two of them. "If there is more than one subtype, it normally increases the fatality rate. The body develops antigen to one subtype after infection.
If the same person gets infected with another subtype, the antigen can't protect and severity of the disease increases," he said. Storage of water in several residential areas of Tirunelveli district due to shortage of supply has led to increase in the mosquito population in the district, said director of public health Dr R T Porkai Pandian.
The 41 dengue deaths and outbreak in summer months have also triggered other scientific research projects. The Chennai-based National Institute of Epidemiology, for instance, is working in Tirunelveli to study the causes for high mortality.