By: Vijay Shankar
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[NEW DELHI] Indoor residual spraying (IRS) of DDT insecticide on to walls is ineffective against the sandfly which spreads the parasite that causes kala-azar, or leishmaniasis, a neglected tropical disease, says an Indo-UK study published July in PNAS.
The study is the first systematic analysis of DDT use in India’s IRS programme, according to scientists involved in the study from Liverpool School of Tropical Medicine (LSTM), Liverpool, and the Rajendra Memorial Research Institute of Medical Sciences, Patna, India.
Back in 2005, India, Nepal, and Bangladesh established a tripartite agreement to reduce kala-azar incidence to less than one per 10,000 populations at the sub-district level by 2015. IRS was part of the agreement to control sandfly populations and the Leishmania donovani parasite that causes kala-azar.
The parasite transmitted through sandfly bites migrates via the bloodstream to vital organs such as the liver and spleen. Left untreated infections are usually fatal.
Post-infection treatments using drugs such as amphotericin B can treat the disease, but vector control is the best way to prevent its occurrence. Previous research had shown that insecticides such as DDT, malathion, deltamethrin, and alpha-cypermethrin were effective against the vector.
However, the quality of IRS using DDT has not been optimal in controlling the sandfly population in India where kala-azar is endemic, particularly Bihar state. “We worked collaboratively with the government in India to use state-of-the-art techniques to help determine why the DDT IRS was less effective than anticipated,” Janet Hemingway, professor at LSTM and the lead author of the paper, tells SciDev.Net.
The researchers collected samples from houses in eight districts of Bihar to find that nearly 85 per cent of the walls were not sprayed adequately.
Earlier research had shown a widespread resistance in sandflies to DDT and this was attributed to uneven IRS or the intermediary decline in IRS practice during the 1970s, from when the sandfly population began to shoot up. But, “we are not able to say which of these is having the greater impact,” says Hemingway.Moreover, the 2001 Stockholm Convention calls for stopping the production and use of DDT as soon as practical, an agreement that India ratified in 2006. Taken together, the researchers suggest that it might be time for India — now the only country that manufactures DDT — to phase it out of kala-azar control.
In their study, the researchers also show that sandflies are susceptible to deltamethrin, a pyrethroid (a class of synthesised insecticides) at concentrations much lower than of DDT. “Both are effective if insects are susceptible — both could fail if resistance to them is [naturally] selected,” says Hemingway. She also notes that pyrethroids do not bio-accumulate in the food chain, like DDT does.
Since June, the government has been testing the alpha-cypermethrin SP, a pyrethroid, in four districts of Bihar — Muzaffarpur, Sitamarhi, Saran, and Arara by IRS. “This might be scaled up in many districts in 2016,” says William Starbuck, senior programme officer for leishmaniasis control, at the Bill and Melinda Gates Foundation.
This article has been produced by SciDev.Net's South Asia desk.
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