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El Niño may increase breeding grounds for mosquitoes spreading Zika virus, WHO says

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© WHO Archive/TDR /Mark Edwards

Geneva, 22 February 2016—Above-average rainfall caused by El Niño is expected in parts of South America until May 2016, and could cause floods and increases in diseases spread by mosquitoes, including malaria, dengue, chikungunya and the Zika virus.

A recent WHO report on the health consequences of El Niño forecast a rise in vector-borne diseases, including diseases spread by mosquitoes, in Central and South America, particularly in Ecuador, Peru, Paraguay, Uruguay, Brazil and Argentina. Severe drought, flooding, heavy rains and temperature rises are all known effects of El Niño—a warming of the central to eastern tropical Pacific Ocean.

Zika is a virus spread to humans by Aedes mosquitoes- the same mosquitoes that spread dengue, chikungunya and yellow fever. The Aedes mosquito breeds in standing water.

“We could expect more mosquitoes capable of spreading the Zika virus because of expanding and favourable breeding sites due to the weather effects of El Niño,” said Dr Raman Velayudhan, Coordinator, Vector Ecology and Management, Department of Neglected Tropical Diseases.

The symptoms known to be caused by Zika infection tend to be mild. It is estimated that three out of four people infected with Zika virus do not show symptoms.

More concerning is the unproven link between Zika and an increased number of reports of babies born with unusually small heads, or microcephaly. WHO announced that the recent cluster of microcephaly and other neurological abnormalities reported in the Americas region constitutes a Public Health Emergency of International Concern.

Steps are being taken to prevent and reduce the health effects of El Niño, in particular by reducing the mosquito populations that spread Zika virus. WHO and partners are working together to provide support to ministries of health to:

  • strengthen any action that helps control mosquito populations such as source reduction measures targeting main mosquito breeding spots, distribution of larvicide (insecticide that is specifically targeted against the larval life stage of the Aedes mosquito) to treat standing water sites that cannot be treated in other ways (cleaning, emptying, covering), etc.;

  • strengthen vector surveillance (e.g. how many breeding sites in an area, percentage of sites reduced);

  • monitoring of the impact of actions to control the mosquito populations

  • build country and WHO response efforts to address El Niño related disasters and

  • share information across the region for planning a rapid response and recovery.

“Households can also help reduce mosquito populations,” said Dr Velayudhan. “Containers that can hold even small amounts of clear water such as drums, buckets, flower pots, roof gutters or used tyres should be emptied, cleaned or covered so that mosquitoes cannot use them to breed, including during severe drought.”

The current El Niño from 2015 to 2016 is expected to be the worst in recent years and comparable to the El Niño in 1997-1998 which had major health consequences worldwide. For example, in Ecuador, the 1997-1998 El Niño event affected 60% of the population and outbreaks of malaria—also spread by mosquitoes—increased by 440%. While adverse weather effects of El Niño are expected to wind down by mid-2016, the health impacts may last throughout 2016 and beyond.