This post has been amended to reflect changes in the story.

[DIGEST: Washington Post, WSJ, GM Watch, BBC, NPR, ABC, New Scientist, CDC, Human Rights Watch]

The Zika virus has been blamed for thousands of cases of the birth defect microcephaly, a condition in which babies are born with severely shrunken heads and brain damage. But Brazilian and Argentinian medical organizations have recently challenged that connection, claiming that the chemical larvicide Pyriproxyfen may be to blame instead.

The suggestion is one of a number of alternate theories regarding the cause of the crisis; however, health officials from the United States, Brazil and other regions have quickly rebutted any such theory.

The Argentine organization University Network of Environment and Health (REDUAS) released a report in February observing that most affected children live in areas in which the larvicide Pyriproxyfen was added in 2014 to local drinking water in an attempt to control mosquito populations. Pyriproxyfen is used to create malformations in mosquito larvae, in order to impair their development and reproductive abilities.

Credit: Source.

Their report cautions that “Many policy-makers, even PAHO and OMS, epidemiologists, public health experts, chemists and politicians in general easily forget that human beings, every one of us, have deployed embryonic development processes in which we go through very different stages. The evolution from zygote to embryo, from embryo to foetus and from foetus to newborn, is not far from the development process of the mosquito affected by pyriproxyfen. They also very easily try to ignore that in humans, 60% of our active genes are identical to those of insects such as the Aedes mosquito."

Meanwhile, the Brazilian government has refuted the doctors’ claim, and says that it uses only World Health Organization-approved pesticides. In addition, the Brazilian Association for Collective Health – cited in the REDUAS paper as having questioned the link to the birth defects to Zika, and noting the possibility of other factors, including a chemical model for mosquito control – has also discredited any link between microcephaly and pesticide use. It cautioned against “spreading untruths and content without any (or enough) scientific basis.” The REDUAS doctors acknowledge that the group has not performed any lab studies or epidemiological research to support their claims.

The Zika virus has been found in only five cases of women who gave birth to babies with microcephaly, out of a study of 3,893 cases of the malformation confirmed before January 20, 2016. A survey of more than 3,000 currently pregnant women diagnosed with Zika found no evidence of microencephaly and birth defects have not been associated with previous outbreaks of Zika, which first affected humans in the 1960s. The doctors also note that birth defects have not been found in other countries affected by Zika. Colombia has counted more than 31,000 cases of the virus, with more than 5,000 pregnant women included in the current outbreak. Colombia has not seen a single Zika-linked case of microencephaly.

Microcephaly is associated with severe intellectual impairment and motor skills problems. Geoff Woods, a clinical geneticist at the University of Cambridge who is studying affected babies, says damage impacts the brain stem and the cerebellum, which control many involuntary functions, such as swallowing, controlling body temperature and blood pressure. People born with microcephaly have typically shorter life spans. They will need specialized care for the rest of their lives.

On February 1, the World Health Organization (WHO) declared the Zika virus a global health emergency requiring international response, in the same category as the Ebola virus. WHO general director Margaret Chan called Zika an “extraordinary event” and said

the agency’s top concerns were protecting pregnant women from infection and controlling the mosquitoes that transmit the virus. The U.S. National Institutes of Health called the Zika virus a pandemic, and says the virus will ultimately reach every country in the world. The virus is thought to be transmitted mainly by the Aedes mosquito, but a handful of cases have shown that the virus can also be sexually transmitted.

Countries with past and present evidence of Zika virus transmission. Credit: Source.

The Zika virus is not usually severe or life-threatening, and in fact, only one in five people infected with Zika will ever experience symptoms, says the CDC. Its symptoms are similar to the flu, including fever, rash, conjunctivitis and joint pain. In rare cases, the virus has been associated with Guillian-Barré syndrome.

Wrong-Headed Response?

Governments around the world have issued guidelines and restrictions related to Zika, in an attempt to stem the number of babies born with birth defects.

Some Zika-affected regions have issued recommendations that women avoid getting pregnant in the next two years. However, access to birth control is limited for many women, particularly those in developing regions. Human Rights Watch notes that high rates of sexual violence, combined with restrictive laws governing women’s access to contraception, makes delaying pregnancy impossible for many women in affected countries.

The CDC has issued travel warnings for Central and South America, Samoa and Cape Verde. The International Olympic Committee (IOC) has sent an advisory to its competitors, who are readying for the 2016 Summer Olympics, which are to be held in Rio de Janeiro. The IOC says that efforts will be made to reduce breeding conditions for the mosquito, which are less present during the dry and cooler period of the year.

While many Zika-affected regions are doubling down on their use of insecticides, including Pyriproxyfen, which is being sprayed using planes and added directly to drinking water tanks and reservoirs, Rio Grande do Sul, the southernmost state in Brazil, has suspended use of the chemical. Health Secretary Joao Gabbardo said that even the "suspicion" of a correlation is enough to eliminate the potential for more harm. "We cannot run that risk," Gabbardo said.