SÃO PAULO—Brazil’s southernmost state halted the use of a mosquito larvicide that an Argentine doctors’ group warns could be behind the recent surge of babies born with microcephaly.
The ban was imposed despite assertions by the federal government and U.S. health authorities that there is no scientific basis linking use of the chemical to the birth defect.
Health officials in Rio Grande do Sul over the weekend suspended the use of the larvicide Pyriproxyfen to destroy mosquito eggs and larvae in the state’s drinking water supplies, in what they called a “preventive measure.”
The decision came just days after the Argentine group Red Universitária de Ambiente y Salud (University Network of Environment and Health), released a report that links the pesticide to the huge increase in Brazil in recent months of suspected cases of microcephaly, in which infants are born with shrunken heads and underdeveloped brains.
Medardo Ávila Vazquez, a pediatrician and neonatal development specialist at the Universidad de Córdoba who belongs to the group, acknowledged that the group hasn’t done any lab studies or epidemiological research to support its assertions, but it argues that using larvicides may cause human deformities.
The Argentine group’s assertions were quickly rejected by Brazilian and U.S. health authorities, and run contrary to the beliefs of many health authorities in Brazil and internationally that the more likely cause of the rise in suspected microcephaly cases is the mosquito-borne Zika virus that is rapidly spreading across the Americas.
Mr. Ávila Vazquez said the group is calling on Brazil and other governments in the region to be extremely cautious about drawing fast conclusions about the relationship between Zika and microcephaly, and it warns that fumigating, especially by airplanes, is dangerous and won’t solve the mosquito problem. The group comprises about two dozen people in Argentina, mostly doctors, who are concerned about the use of insecticides and other agrochemicals and their impact on human health.
“We think it is likely that Pyriproxyfen is the problem,” Mr. Ávila Vazquez said.
The Argentine doctors’ report is the latest twist in the mounting global battle to decipher the Zika virus and its potential health effects and halt its spread. The World Health Organization earlier this month declared the Zika virus a global public-health emergency because of its possible links to microcephaly and Guillian-Barré, a rare disorder in which the body’s immune system attacks nerve cells. The agency called for more research to determine whether there is a causal link between microcephaly and Zika.
That uncertainty has allowed room for a slew of new theories on the cause of the outbreak, put forward with wildly varying degrees of evidence, to proliferate on social media.
Brazil’s Health Ministry issued a statement that there is no evidence linking larvicide to microcephaly. “Unlike the relationship between the Zika virus and microcephaly, which has had its confirmation attested in tests that indicated the presence of the virus in samples of blood, tissue and amniotic fluid, The association between the use of Pyriproxyfen and microcephaly has no scientific basis.” the ministry said.
Dr. Francis Collins, director of the U.S. National Institutes of Health, called the Argentine assertions “sketchy.”
“Let me advise caution in offering credence for this larvicide theory…unless and until there is more data to support it,” he said. “The situation in Brazil and elsewhere will not be assisted by attaching unwarranted credibility to this interesting but speculative theory.”
Dr. Anthony S. Fauci, director of the NIH’s National Institute of Allergy and Infectious Diseases, said that while no theory should be rejected out of hand at this stage, and a causal link between Zika and microcephaly hasn’t been proven, the discovery of Zika in the brains of miscarried fetuses and babies who died soon after birth “strongly suggests direct involvement of the virus.”
Another group, the Brazilian Association of Collective Health, which advocates against the widespread use of pesticides, and is cited in the Argentine physicians group’s report, also denounced the assertion of any link between microcephaly and pesticide use. It cautioned against “spreading untruths and content without any (or enough) scientific basis.”
Tokyo-based Sumitomo Chemical Co.
Ltd., which manufactures Pyriproxyfen, issued a statement rejecting any connection between microcephaly and the use of the larvicide. The company said the product has been used in other countries including France, Denmark, Spain, Turkey, the Dominican Republic and Colombia. The company that Pyriproxyfen has been approved by the WHO to combat mosquitoes and that extensive testing has shown that it isn’t carcinogenic and doesn’t cause nervous system damage or affect reproductive ability.
Nationwide, Brazil has 41 confirmed cases of Zika-related microcephaly, plus an additional 421 confirmed cases of microcephaly whose cause hasn’t yet been determined, according to the most recent figures from the health ministry. Another 3,852 suspected cases still are under investigation.
Rio Grande do Sul, along the Argentine border, is among the Brazilian states least affected by the microcephaly outbreak, with just one confirmed case since the beginning of 2015, according to Brazil’s Health Ministry.
The state’s health secretary, João Gabbardo Dos Reis, said that Pyriproxyfen isn’t necessary because the state’s water supplies are clean.
“Although we have no indication that the larvicide has a link with microcephaly cases, it is also true that we do not have any strong evidence that it has no links,” he said.
But in one of the hardest hit regions, the northeastern state of Bahia, officials said they would continue to use larvicide.
“If we did not use [Pyriproxyfen] we would have more cases yet,” said Bahia Health Secretary Fábio Vilas-Boas.
—Tom Burton and Taos Turner contributed to this article.