Experts say there's no evidence that an increase in birth defects in Brazil, which has coincided with an outbreak of Zika virus, is due to pesticides used to kill mosquito larvae.

An Argentine environmentalist site last week posted a claim, under the heading Report from Physicians in the Crop-Sprayed Town, that a surge birth defects in Brazil is due to a chemical called pyriproxyfen, used to fight mosquitoes, rather than the Zika virus itself. Zika is spread by a species of mosquito called Aedes aegypti, and possibly Aedes albopictus.

The Argentine environmentalists' claims have stoked conspiracy theories online and in social media, causing some to proclaim the Zika virus a hoax. On its web site, the group claims that spraying mosquitoes using planes is "criminal, useless and a political maneuver" to make it appear that governments are taking action. The root of Zika, the group claims, "lies in inequality and poverty."

The group also opposes the use of genetically engineered mosquitoes, which are being tested as a way to reduce mosquito populations. Use of these altered mosquitoes has been "a total failure, except for the company supplying mosquitoes," the group said.

Brazil's Ministry of Health has rejected the environmentalists' claims, noting that the World Health Organization has approved pyriproxyfen as safe.

While the WHO has said there's not yet definitive evidence that Zika virus can cause birth defects, scientists say the links between the mosquito-borne virus and birth defects are growing stronger. Although the Zika virus was first diagnosed in monkeys in Uganda in 1947, it was not suspected of causing birth defects until last fall, when Brazil reported a huge spike in microcephaly — in which babies are born with abnormally small heads. The increase in birth defects occurred roughly six months after the first cases of Zika were diagnosed in Brazil, suggesting that mothers were infected in their first trimester of pregnancy, a critical time for fetal brain development.

Research on Zika and microcephaly "overwhelmingly point to a virus as the cause of microcephaly," said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. "The focus needs to be on mosquito control to prevent new Zika cases as the epidemic advances."

Genetic material from the Zika virus also has been found in the brains, placenta and amniotic fluid from several infants with microcephaly, including ones which were miscarried or aborted by women infected with Zika while pregnant.

Prenatal infections are known to cause extensive brain damage in fetuses and newborns. These infections include toxoplasmosis, caused by a parasite found in undercooked meat; rubella, also known as German measles; herpes; syphilis; and cytomegalovirus, a type of herpes virus that causes mild symptoms in most people, but can cause severe harm in newborns or people with weakened immune system.

Although toxins have been known to cause microcephaly, scientists contacted by USA TODAY said the notion that pyriproxyfen could cause these birth defects doesn't make any sense.

"It seems very odd that this would be the cause" of microcephaly, said biologist Laura Harrington, professor and chairwoman of the entomology department at Cornell University in New York.

The active ingredient in pyripyroxyfen imitates a hormone in mosquitoes that allow the normal growth of the immature, or larval, stages of insects, and prevent insects from developing to the adult stage, Harrington said. Humans don't have this hormone.

On its web site, the environmental group claims that microcephaly has only occurred in places in Brazil where larvicide has been used. But the Brazilian Ministry of Health notes that microcephaly cases have increased even in places that haven't used the pesticide.

Also, a recent analysis of microcephaly cases in French Polynesia found that rates of the condition increased there after a Zika outbreak in 2013 to 2014. No one noticed the spike in microcephaly cases until researchers went back to their records to look for it.

"There's a lot of alarmism going around right now, and we would all benefit if it would get tamped down a bit," said Grayson Brown, director of the public health entomology laboratory at the University of Kentucky in Louisville.

"The reason that the pesticide is found in areas with microcephaly is because it's being used to control Aedes aegypti and Zika," Brown said. "The larvicide would not be used in areas lacking the mosquito vector. The whole notion is misplaced cause and effect."

The Argentine environmentalists are "are using this tragedy to promote their own agenda" of opposing pesticide and genetic engineering," Brown said.

Fighting the mosquitoes that spread Zika will require a "close collaboration" between government and individuals, Brown said. Unfounded claims related to Zika "undermine the confidence that citizens need to have in their government at this time and, as such, makes control more difficult. It is really quite irresponsible and counterproductive."