Questions and answers: South Asia earthquake and tsunami
Health risks: stagnant water
What are the health risks related to stagnant water?
Pools of standing or slow-flowing water provide a breeding ground for many insects, including mosquitoes that can transmit diseases. These mosquitoes are known as vectors. Different species of mosquitoes transmit different diseases, and they will also breed in different types of water collections. In the context of the tsunami disaster, the key water-associated vector-borne diseases are malaria and dengue, with possibly and more speculatively Japanese encephalitis in very local conditions.
Are there different health risks linked to fresh and brackish (salty) water?
All malaria transmitting mosquitoes belong to the genus Anopheles which breeds exclusively in fresh, clean water, with the exception of Anopheles sundaicus, which breeds in brackish water. The original distribution of Anopheles. sundaicus is linked to its natural habitats of mangrove forests and coastal lagoons. It starts in eastern India (but not as far south as Tamil Nadu) then along the coasts of Bangladesh, Myanmar, Thailand, Malaysia, Indonesia and Viet Nam. Organically polluted water does not support the breeding of the malaria vector; rather it helps propagate Culex quinquefasciatus, in some parts a vector of lymphatic filariasis, and a main nuisance mosquito. In the context of this disaster lymphatic filiarisis is however not a disease of immediate relevance due to its long incubation period.
Where is the risk of malaria most acute and what should be done?
There is no malaria in the Maldives and along the coastline of Southern Thailand; malaria in South India is limited to urban areas; thus the main risk areas are Sri Lanka, Myanmar and Indonesia. In Sri Lanka, risks will increase when brackish water is washed out by monsoon rains and the floods recede: An. culicifacies will start breeding in the remaining pools. In Myanmar and in northern Sumatra the evolution of malaria risks in relation to floods and flood recession is less clear.
Important measures include:
- Personal protection, using insecticide treated mosquito nets, and impregnation with insecticides of all material for temporary shelter (such as tents); spraying with residual insecticides will only be feasible where housing structures are still intact.
- Making available mosquito repellents to affected populations to re-inforce personal protection using nets or other impregnated materials
- Access to anti-malaria drugs linked to effective monitoring of the start of outbreaks
Where is the risk of dengue fever most acute and what should be done?
The viruses that cause dengue fever and dengue haemorrhagic fever are transmitted by the mosquito Aedes aegypti, and in Asia to a lesser extent by Ae. albopictus. Aedes aegypti breeds mainly in water collections in artificial containers in the environment of human settlements, but not in groundwater pools and puddles, nor in swamps or other large natural bodies of water. Effective prevention and control of epidemic dengue requires control of vectors. No vaccine exists for dengue..
Prevention of mosquito-breeding in drinking-water containers, by covering them to exclude mosquitoes, frequently emptying them (at least once per week), or treating them with insecticide, e.g., temephos 1% sand granules, will contribute to the prevention of dengue outbreaks. However, this may not be sufficient in settlement areas where there are small, freshwater collections in other artificial containers and miscellaneous debris. Depending on feasibility, chemical larvicides may also be applied under these conditions. Portable equipment for space spraying will be needed in areas with difficult road access. To the extent possible, affected populations may be provided with mosquito repellents.
Source reduction is the mainstay of dengue prevention. This should be backed up by chemical control whenever mosquito populations expand rapidly.
Where is the risk of Japanese Encephalitis most acute and what should be done?
The risk of Japanese encephalitis (JE) outbreaks is less than malaria and dengue and restricted to Sri Lanka. When large stretches of flooded area are washed out by monsoon rains and turn into freshwater, populations of the mosquitoes Culex tritaeniorrhynchus and Culex gelidus (vectors of JE) may rapidly build up, and may start biting humans (instead of their normal hosts of - domestic animals - which will have been decimated by the disaster). In normal circumstances, pigs are the amplifying hosts of the JE virus; in this emergency situation, direct transmission from herons and similar birds to humans cannot be excluded.
- vaccination of vulnerable groups (children) in areas at risk;
- personal protection from mosquito bites through the use of insecticide treated mosquito nets
- use of insect repellents