Shiloh
Editor, Senior Moderator
Source: http://www.eht-forum.org/news.html?fileId=news100304022318&from=home&id=0
Thursday 04 March 2010
Questions loom over dengue control
Call to re-think vector management points to tentative steps in fight against dengue
Countries battling dengue fever epidemics have few weapons at their disposal to rein in the mosquito vector, Aedes aegypti, and stop the disease from spreading. But vector control programmes fraught with poor results are less likely to succeed when health authorities take a ?one-size-fits-all approach?, say infectious-disease scientists this week in the Bulletin of the World Health Organization.
?The public health response should extend beyond larviciding or focal spraying,? write Natarajan Arunachalam, of the Indian Council of Medical Research in Madurai, and colleagues. ?A change of paradigm in vector management seems essential.?
Eyeing a growing threat from large outbreaks of the disease and the spread of the dengue virus in new parts of the world, experts admit that little is clear over which strategy is best suited to meet the challenge of controlling it.
In their study the team led by Arunachalam analysed data collected from urban neighbourhoods in six Asian countries, looking into a host of factors that affect transmission of the dengue virus ? ecological, biological, and social. Citing problems with comparing results of earlier research they cast a wide geographic net and aimed to pinpoint how vector management can benefit from better knowledge of where Ae. aegypti prefer to breed, and what helps them to thrive.
One of their main conclusions is that for vector control programmes to be more effective, ?productive container types have to be identified and targeted... with special attention to those that are outdoors, unused, uncovered and in the shade.?
In all six areas included in the study, mosquitoes were breeding and developing into the adult stage most efficiently in unprotected water containers kept outdoors in shaded areas. But governments in all but one of the countries tended to deal with these vectors by spraying insecticides and treating water with larvicides, without checking containers or educating people about the risk to their health.
The authors argue that instead of relying on these measures, people in affected communities and their leaders should participate actively in control programmes. Their interviews with more than 42,000 people in these Asian cities suggested that most had ?very good? knowledge of the disease and how it spreads. ?Two groups of factors were protective against high vector densities: people?s knowledge and awareness of dengue and vector control activities,? they write.
Axel Kroeger, from the WHO Special Programme for Research Training in Tropical Diseases in Switzerland, believes the study makes a case for the need to involve different stakeholders in integrated vector management. This approach, backed by the WHO, is based on the premise that tackling dengue or other infectious diseases is not the sole responsibility of the health sector.
?In vertical interventions [those that focus on a particular disease] there is always the tendency of ?one fit for all? [approaches],? Kroeger points out. ?But if the situation is so complex as in our case much more integrated approaches have to [be] looked for.?
Paul Reiter, from the Institut Pasteur in France, agrees with the authors? call for a re-think of how dengue vectors should be controlled. But he does not see integrated vector management as the answer. ?I have been preaching change of paradigm for the past 20 years,? he says. ?The fact is that there isn?t really any successful dengue control [strategy].?
Looking back on vector management
Efforts to control Ae. aegypti began early in the 20th century, when it was confirmed that the mosquitoes transmit the virus that causes yellow fever. But now the world faces a different problem, Reiter tells EHTF News. The mosquito has found new breeding grounds in tyres and other plastic containers, and a huge number of people are at risk for dengue fever as nearly all the world?s large cities are in the tropics. ?It?s a really daunting problem,? he says.
Reiter tracks the history of mosquito-control efforts, starting with a time when this relied on improving sanitation. By the mid-1940s, attention turned to destroying breeding sites. When DDT came along an eradication campaign in the Americas claimed to have eliminated the mosquito from 22 countries, he says ? but the numbers picked up again after resistance to the pesticide emerged and concerns over its toxic effects led to a phase-out. Then came the fogging machines, spewing out aerosolised insecticides on the streets; but it turned out they had ?absolutely no impact on the adult population [of mosquitoes] at all.?
With the arrival of community-based control in the 1980s, public health scientists enlisted the help of social scientists and psychologists to help people adopt protective behaviours against the mosquito. But doing this was more difficult than anyone expected, according to Reiter, and proved successful only in Cuba and Singapore. But in Singapore, even after the government spending 30 million Euros, dengue is still rising, he points out.
Reiter believes that by turning to integrated control strategies, the public health community is falling back on every intervention that could help at the expense of ?realistic? options. He points to COMBI, a social mobilisation and communication strategy for dengue control, as the latest initiative put forward under this approach. But there is no evidence it is working, he says.
According to Kroeger, COMBI works very well if one knows what they expect of a community. ?This is the problem in dengue,? he says, ?that we are not that sure what to recommend.?
The way forward
Kroeger points to multi-country studies, supported by the WHO, which show that targeting water containers known to produce the largest number of vectors could be a useful control strategy. ?We found that the targeted approach is as effective as the blanket approach but much cheaper and easier to do.?
Also underway are studies into the effectiveness of insecticide-treated window curtains and water-container covers, notes Kroeger. ?First results are very promising.?
Reiter takes a different view of what could work, saying the only strategy that has been successful so far is the use of DDT. He puts down its success to the behaviour of Ae. aegypti: the female mosquito ?doesn?t lay all its eggs in one basket?, he explains, but in different containers. This means that by spraying enough breeding sites, the probability of a mosquito hitting one of them is high.
Amid debate over the risks and benefits of using DDT, the WHO is currently allowing use of the pesticide to control malaria in some parts of the world. But Kroeger is sceptical of this as a way to curb dengue, noting that ?indoor residual spraying is better against malaria vectors than against dengue vectors.? He also points out that better insecticides than DDT are available now.
Reiter mentions that new technologies could also help to manage dengue vectors ? but such tools are unlikely to prove a viable solution any time soon. Although dengue vaccines are in the pipeline, they are a long way away from being approved, he explains. A method his team is working on, to produce sterilised or genetically modified mosquitoes designed to drive down the vector population, is still being developed and has yet to be tested.
Reference and link
1.
Arunachalam N, Tana S, Espino F, Kittayapong P, Abeyewickreme W, Wai KT, et al. Eco-bio-social determinants of dengue vector breeding: a multicountry study in urban and periurban Asia. Bull World Health Org 2010, 88:173?84. doi: 10.2471/BLT.09.067892
WHO information on dengue and dengue haemorragic fever
Thursday 04 March 2010
Questions loom over dengue control
Call to re-think vector management points to tentative steps in fight against dengue
Countries battling dengue fever epidemics have few weapons at their disposal to rein in the mosquito vector, Aedes aegypti, and stop the disease from spreading. But vector control programmes fraught with poor results are less likely to succeed when health authorities take a ?one-size-fits-all approach?, say infectious-disease scientists this week in the Bulletin of the World Health Organization.
?The public health response should extend beyond larviciding or focal spraying,? write Natarajan Arunachalam, of the Indian Council of Medical Research in Madurai, and colleagues. ?A change of paradigm in vector management seems essential.?
Eyeing a growing threat from large outbreaks of the disease and the spread of the dengue virus in new parts of the world, experts admit that little is clear over which strategy is best suited to meet the challenge of controlling it.
In their study the team led by Arunachalam analysed data collected from urban neighbourhoods in six Asian countries, looking into a host of factors that affect transmission of the dengue virus ? ecological, biological, and social. Citing problems with comparing results of earlier research they cast a wide geographic net and aimed to pinpoint how vector management can benefit from better knowledge of where Ae. aegypti prefer to breed, and what helps them to thrive.
One of their main conclusions is that for vector control programmes to be more effective, ?productive container types have to be identified and targeted... with special attention to those that are outdoors, unused, uncovered and in the shade.?
In all six areas included in the study, mosquitoes were breeding and developing into the adult stage most efficiently in unprotected water containers kept outdoors in shaded areas. But governments in all but one of the countries tended to deal with these vectors by spraying insecticides and treating water with larvicides, without checking containers or educating people about the risk to their health.
The authors argue that instead of relying on these measures, people in affected communities and their leaders should participate actively in control programmes. Their interviews with more than 42,000 people in these Asian cities suggested that most had ?very good? knowledge of the disease and how it spreads. ?Two groups of factors were protective against high vector densities: people?s knowledge and awareness of dengue and vector control activities,? they write.
Axel Kroeger, from the WHO Special Programme for Research Training in Tropical Diseases in Switzerland, believes the study makes a case for the need to involve different stakeholders in integrated vector management. This approach, backed by the WHO, is based on the premise that tackling dengue or other infectious diseases is not the sole responsibility of the health sector.
?In vertical interventions [those that focus on a particular disease] there is always the tendency of ?one fit for all? [approaches],? Kroeger points out. ?But if the situation is so complex as in our case much more integrated approaches have to [be] looked for.?
Paul Reiter, from the Institut Pasteur in France, agrees with the authors? call for a re-think of how dengue vectors should be controlled. But he does not see integrated vector management as the answer. ?I have been preaching change of paradigm for the past 20 years,? he says. ?The fact is that there isn?t really any successful dengue control [strategy].?
Looking back on vector management
Efforts to control Ae. aegypti began early in the 20th century, when it was confirmed that the mosquitoes transmit the virus that causes yellow fever. But now the world faces a different problem, Reiter tells EHTF News. The mosquito has found new breeding grounds in tyres and other plastic containers, and a huge number of people are at risk for dengue fever as nearly all the world?s large cities are in the tropics. ?It?s a really daunting problem,? he says.
Reiter tracks the history of mosquito-control efforts, starting with a time when this relied on improving sanitation. By the mid-1940s, attention turned to destroying breeding sites. When DDT came along an eradication campaign in the Americas claimed to have eliminated the mosquito from 22 countries, he says ? but the numbers picked up again after resistance to the pesticide emerged and concerns over its toxic effects led to a phase-out. Then came the fogging machines, spewing out aerosolised insecticides on the streets; but it turned out they had ?absolutely no impact on the adult population [of mosquitoes] at all.?
With the arrival of community-based control in the 1980s, public health scientists enlisted the help of social scientists and psychologists to help people adopt protective behaviours against the mosquito. But doing this was more difficult than anyone expected, according to Reiter, and proved successful only in Cuba and Singapore. But in Singapore, even after the government spending 30 million Euros, dengue is still rising, he points out.
Reiter believes that by turning to integrated control strategies, the public health community is falling back on every intervention that could help at the expense of ?realistic? options. He points to COMBI, a social mobilisation and communication strategy for dengue control, as the latest initiative put forward under this approach. But there is no evidence it is working, he says.
According to Kroeger, COMBI works very well if one knows what they expect of a community. ?This is the problem in dengue,? he says, ?that we are not that sure what to recommend.?
The way forward
Kroeger points to multi-country studies, supported by the WHO, which show that targeting water containers known to produce the largest number of vectors could be a useful control strategy. ?We found that the targeted approach is as effective as the blanket approach but much cheaper and easier to do.?
Also underway are studies into the effectiveness of insecticide-treated window curtains and water-container covers, notes Kroeger. ?First results are very promising.?
Reiter takes a different view of what could work, saying the only strategy that has been successful so far is the use of DDT. He puts down its success to the behaviour of Ae. aegypti: the female mosquito ?doesn?t lay all its eggs in one basket?, he explains, but in different containers. This means that by spraying enough breeding sites, the probability of a mosquito hitting one of them is high.
Amid debate over the risks and benefits of using DDT, the WHO is currently allowing use of the pesticide to control malaria in some parts of the world. But Kroeger is sceptical of this as a way to curb dengue, noting that ?indoor residual spraying is better against malaria vectors than against dengue vectors.? He also points out that better insecticides than DDT are available now.
Reiter mentions that new technologies could also help to manage dengue vectors ? but such tools are unlikely to prove a viable solution any time soon. Although dengue vaccines are in the pipeline, they are a long way away from being approved, he explains. A method his team is working on, to produce sterilised or genetically modified mosquitoes designed to drive down the vector population, is still being developed and has yet to be tested.
Reference and link
1.
Arunachalam N, Tana S, Espino F, Kittayapong P, Abeyewickreme W, Wai KT, et al. Eco-bio-social determinants of dengue vector breeding: a multicountry study in urban and periurban Asia. Bull World Health Org 2010, 88:173?84. doi: 10.2471/BLT.09.067892
WHO information on dengue and dengue haemorragic fever