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The Mosquito That Started Today's Zika Outbreak in Brazil Was Killed Off 50 Years Ago

Brazil’s government has declared “war” on the Aedes aegypti mosquito but conditions have changed since the country eliminated the insect in 1958, when it was spreading yellow fever.
Photo by Percio Campos/EPA

When President Dilma Rousseff put the Brazilian army at the forefront of her government's campaign against the Zika virus, and its link to birth abnormalities, she said it amounted to "war against the mosquito."

It is a war that Brazil already fought half a century ago and won, temporarily, when it eliminated the Aedes aegypti mosquito from the country and brought yellow fever under control.

But today, experts say, urban growth and greater civil liberties mean a repeat victory over the same mosquito, which is now spreading Zika, requires new tactics and weapons that might not be successful anyway.


"Currently there is a greater complexity of the urban environment that makes it difficult to eradicate the Aedes aegypti mosquito again," said Rafael Freitas, entomologist at the Oswaldo Cruz Institute, Brazil's leading research institute based in Rio de Janeiro.

Related: Brazil Steps Up Its War Against Zika

Freitas highlighted the fact that today's cities are much bigger and that state forces cannot gain access to houses as easily as they could before. In the original campaign, he said, homes would simply be sealed while sulphur was sprayed to kill off the insects.

"The resident was forced to leave home, with no care for where he went, whether it was a relative's house or even a public square," he said. "Nowadays it is not possible to perform such procedures because of private property rights, for example."

Aedes aegypti is a particularly difficult species of mosquito to control. It breeds easily in small amounts of standing water, which means it can easily flourish in urban areas, particularly the many poor Brazilian barrios that do not have running water. It also bites during the day, which means mosquito nets provide scant protection.

Brazil was declared free of the insect in 1958, but it remained present in neighboring countries, such as Venezuela and Suriname. By the end of the 1960s controls had been relaxed and defenses lowered, and the mosquito was back.

Since then, Aedes aegypti has remained present in every Brazilian state, carrying Dengue, Chikungunya, and now the Zika virus.


Zika was first detected in Brazil in the Spring of 2015. By the end of the year, doctors were linking the mild fevers and discomfort it typically causes with a rise in the number of babies born with neurological problems characterized by abnormally small heads, known as microcephaly.

The language of war in the government's campaign against Zika since then, however, seems more like an effort to convince the public that it is serious, than any real indication of full-scale repression of the insect.

Public servants have been given powers to forcibly enter abandoned properties or places where there is a suspected breeding site and the owner is absent. But the more than 200,000 troops deployed to confront the Zika epidemic have primarily been employed in tasks designed to raise awareness of the risks, and to encourage the public to keep their homes mosquito free.

Meanwhile, researchers have been highlighting more sophisticated methods for trying to control the mosquito and the diseases it spreads.

Among the projects is the Eliminate Dengue programme in which a naturally occurring bacteria, Wolbachia, is used in mosquitos to reduce the transmission of Dengue.

Related: Genetically Modified Mosquitoes Will Guard the World Cup Against Dengue

The bacteria is found in 60 per cent of insects and when inserted into Aedes aegypti, it has proven capable of stopping dengue. Two batches of mosquitoes carrying the Wolbachia bacteria have been released in Rio de Janeiro so far.


There have also been trials of genetically-modified mosquitoes, whose offspring do not survive. British company Oxitec have been producing these self-limiting mosquitoes at a laboratory in São Paulo. They have released males with modified genes to breed with females — only females bite — in trial areas. Last month, the company announced it would be extending its trials.

Some of the measures to control the mosquito in the past, however, have come under fierce criticism.

A report from Argentinian doctors' organisation, Physicians in the Crop Sprayed Town, suggested pyriproxyfen, a larvicide used in drinking water supplies to control mosquitoes, was behind the cases of microcephaly.

The group claims that this would explain why other places affected by Zika, that has now spread to dozens of countries in the Americas, have not reported similar increases in the rates of babies born with microcephaly.

"Malformations detected in thousands of children from pregnant women living in areas where the Brazilian state added pyriproxyfen to drinking water is not a coincidence," the report says. "The Ministry of Health is placing the blame on the Zika virus for this damage, while trying to ignore its responsibility, and ruling out the hypothesis of direct and cumulative chemical damage caused by years of endocrine and immunological disruption of the affected population."

The doctors argued that the larvicide, which caused malformations in mosquitoes, may also therefore be responsible for malformations in babies.


The Brazilian health ministry immediately refuted this and said there had been incidents of microcephaly in areas that had not used pyriproxyfen in water supplies.

A government statement said that, unlike with Zika, there is no scientific evidence suggesting a link with larvicide in drinking water.

The World Health Organization's coordinator of water, sanitation, hygiene and health, has also ruled out a link between pyriproxyfen and microcephaly.

"There's absolutely no concern for reproductive effects that have been raised for this chemical," Bruce Gordon told NPR last week. "Pyriproxyfen actually mimics a hormone found in invertebrates, so it basically interferes with their development, but mammals don't have that development process."

Meanwhile, researchers from the Oswaldo Cruz Institute have published the first full genetic sequencing of Zika virus as found in a baby with microcephaly.

Related: Are Mosquitoes Behind an Increase in Brazilian Babies Born with Abnormally Small Heads?

The research adds to the mounting evidence of a link between the virus and microcephaly, which has not yet been firmly established. It also revealed strong similarities between the strain of the virus in Brazil and those found in a 2013 outbreak in French Polynesia that was not associated with any reported rise in cases of abnormal births.

"Although the Zika virus has been the only agent found in amniotic fluid, further studies are needed to confirm that this virus is the only cause of microcephaly in these cases, even if this is so far the strongest evidence," said Ana Bispo, head of the virus laboratory at the Oswaldo Cruz Institute. "We need urgent studies to understand the biological mechanisms involved in the infection and its possible link with birth defects."


Meanwhile, Brazil has teamed up with specialists from the US-based Center for Disease Control to study pregnant women and babies with microcephaly in the north eastern state of Paraiba to try to fully understand the link between the two.

Beyond Brazil, the country that has launched the most obviously vigorous response to Zika has probably been Cuba, where the state has greater powers to require the population to cooperate.

President Raúl Castro certainly sounded more convinced of the warlike rhetoric he used in his announcement that he was mobilizing 9,000 troops in a major operation to fumigate the island.

"Every single Cuban must take up this battle as a personal matter," Castro wrote in a national message on Monday, despite the fact that the Cuban health authorities say they have still yet to confirm a single case of Zika.

"Our people will once again demonstrate our organizational capacity to maintain the health standards reached by the Revolution," he continued. "We are required to be more disciplined and demanding than ever before in similar tasks."

Related: Here's What You Should Know About the Zika Virus

Alan Hernández contributed to this report.

Follow Donna Bowater and Alan Hernández on Twitter: @DonnaBow and @alanpasten