INVASION OF THE AEDES
Shift in mosquito population
Malaria-Causing Vectors Replaced By Those Causing Dengue & Chikungunya


Chennai: There might have been a 49% decrease in the cases of malaria since 2005, but the directorate of public health forecasts an increase in the cases of either dengue or chikungunya this monsoon. It has even devised strategies and plans for civic bodies to tackle the breeding of vectors to prevent their spread.
“The forecast is based on some scientific data we have at hand,” says director of public health Dr S Elango. “In the last few months, our entomologists have seen a drift in the mosquito population across the state. The malaria-causing anopheles, which formed nearly 80% of the diseasespreading vectors, have been replaced by another genre, aedes, which causes chikungunya or dengue,” he said. The cases of malaria in TN recorded an all-time low in 2009.
In 2005, the state recorded nearly 40,000 cases of malaria, which came down to about 28,000 in 2006. “That year
changed the monsoon-disease dynamics even in the city. It was a year that saw maximum number of chikungunya cases,” points out Chennai Corporation health officer Dr P Kuganandam. Chennai, which records about 80% of the state’s malaria cases, has also recorded a drastic decline.
In 2006, there were 64,802 cases of chikungunya in Tamil Nadu, while dengue cases fell from 1,150 to 477. In 2007, dengue rose to 700 while malaria was down to 22,000 cases. Last year, the number of dengue cases was 650 and malaria 21,000. In 2009, until June, 483 malaria cases were recorded as against 326 dengue and 440 chikungunya cases. In July alone, the state recorded 114 chikungunya cases and 37 dengue cases. The data for malaria is yet to be assessed. “But it is surely not higher than 50,” said an analyst at the directorate of public health.
Entomologists have noticed stephensi being increasingly replaced by aedes aegypti in the most common places they were earlier sighted. “We don’t know why this happened,” says state’s chief entomologist Dr S Sridharan. “It is possibly like how the American variety of the cockroach called periplaneta americana, the brown ones that we see commonly, replaced the native Indian striped grey variety we see no more.” The department, he said, will be studying the sudden population shift with the assistance of the Institute of Vector Control and Zoonoses in Hosur.
BEWARE of the BITE
There are approximately 3,500 species of mosquitoes, grouped into 41 genera. They are vectors causing several diseases, mostly after rains, that include malaria, dengue, chikungunya, Japanese encephalitis and filariasis. The female mosquitoes draw blood, which has the protein required by them to lay their eggs AEDES: Commonly spotted species are Aedes aegypti and Aedes albopictus. The flight range for this vector is 100 m from the breeding site and hence the disease it causes, be it dengue or chikungunya, is always seen in clusters
They are day biting mosquitoes, active in late morning and early evening
They are anthroprophilic, which means they feed only on human blood. But they change hosts frequently. One meal often comes from 4-5 persons
They breed in artificial containers like pots of water, holes in trees, coconut shells and all other places that have no soil content
In the case of dengue, the virus is even passed on to the progeny
ANOPHELES:
Commonly spotted species are Anopheles stephensi and Anopheles culicifacies. The flight range is up to half a kilometer from the breeding site — the reason why malarial outbreaks happen in a wider region
They are night biting mosquitos, active in early mornings and late evenings
While stephensi predominantly goes for the human blood, cuclifaces draws blood from humans as well as animals
The vector pierces into the skin with its stylus and sucks out the blood
Stephensi breeds in clear water, wells and irrigation tanks where there is no organic pollution, while culicifacies can survive in mildly saline water

Aedes aegypti


Anopheles


What is malaria?
It is caused by a parasite called plasmodium, which is transmitted through the bite of infected female anopheles.
Parasites multiply in the liver and infect red blood cells. Fever, headache and vomiting usually appear 10-15 days after the bite. Can become life-threatening


What is dengue?
It is transmitted by the bite of an aedes infected with one of the four dengue viruses. Symptoms range from mild to incapacitating high fever, with severe headache and pain in muscles and joints. Dengue haemorrhagic fever is potentially lethal, affecting mainly children


What is chikungunya?
It is a mosquito-borne viral disease characterised by abrupt onset of fever and debilitating joint pain. Recovery takes a few days or weeks, but joint pain may persist for several months or even years