The mosquito catchers

By Ashutosh Mishra
Published: Tuesday 30 June 2009

Scientists go door to door to bring the pests in, PCR test tells them which ones are dangerous

OVER the last few years, people from some villages of Orissa got used to a team of researchers knocking on their doors at five in the morning. The team was on a mission to collect mosquitoes to study the insects' role in the transmission of malaria in the area.

The team would enter houses and use sucking tubes to collect mosquitoes. Mosquitoes outside the houses were collected in the evening using light traps which produce ultraviolet light to attract mosquitoes. Orissa's Boudh, Gajapati, Phulbani, Cuttak and Keonjhar were the districts roped in for the project as they are endemic to malaria. Three Anopheles mosquito species are dominant here A annularis, A philippinensis and A pallidus.

In one such field work, the team from the Regional Medical Research Centre (rmrc) and the Institute of Life Sciences in Bhubaneswar, collected 186 mosquitoes. In the lab, the researchers used a newly-developed technique called the multiplex polymerase chain reaction (pcr) to identify the mosquitoes and assess the threat they could pose.

Positive ID
Down to Earth Step 1 Sucking tubes were used to collect mosquitoes. One end of the tube was placed over the mosquito on the wall while it was sucked in through the other end 
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  • Mosquitoes outside the houses were collected in the evening using light traps hung on trees

  • Step 2 The collected mosquitoes were dissected
    Down to Earth
    Step 3 Different parts of a dissected mosquito were collected
    a - the saliva for the Plasmodium parasite
    b - the stomach for human blood
    c - the mosquito's own DNA sample
    Down to Earth
    Down to Earth Step 4 The samples were analyzed by the multiplex PCR device. The results confirmed if the mosquito had fed on a human and if it carried the disease
    The pcr technology uses bits of dna to identify the organism from which it has been taken, by comparing it to standard sequences of the organism's dna already available. The team had three variables to identify the species of the mosquito, whether it had fed on a human host and whether the malaria pathogen (Plasmodium falciparum) was present in its salivary glands. If the mosquito sample being analyzed showed the presence of genetic material from human blood (in its stomach contents) as well as from the protozoan (in its salivary glands), along with its own dna, it meant the mosquito fed on human host and was a potential vector of the parasite. For comparison, separate markers from human blood and the parasite's dna were added.

    The common method to identify the mosquito involves catching a female about to lay eggs, right after its blood meal. The growing larvae are studied which takes 12 days. The presence of the protozoan is ascertained by visually examining cells from the mosquito's salivary glands; the methods are cumbersome. Not only does one multiplex pcr test take care of all these parameters, it is less time-consuming and more accurate, said S antanu Kar, director of rmrc and a team member.

    Out of the 186 mosquitoes, 91 were A annularis, two of which were positive for P falciparum and one fed on a human. The 56 A philippinensis and 39 A pallidus did not carry the parasite. One A philippinensis fed on a human.

    "Any malaria control strategy has to consider aspects like distribution and identification of the vectors and their host preferences besides their susceptibility to insecticides. Now all this will be easy," said Rupenangshu Hazra, a team member. Details of the technology are to be published in the coming issue of Transactions of the Royal Society of Tropical Medicine and Hygiene.

    Kar said government employees should be trained to collect mosquitoes from every nook and corner of the state. The multiplex pcr tests done on the collected samples could be used to carry out surveys on mosquito populations throughout the country.

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