Pyriproxyfen: 5 Fast Facts You Need to Know

Pyriproxyfen microcephaly

Pyriproxyfen targets mosquito larva and may be the real cause of microcephaly, not Zika, according to Argentinian doctors. (Getty)

Pyriproxyfen, an insecticide, is rumored to be the real cause of increased cases of microcephaly in Brazil, not the Zika virus. (Read more about microcephaly here.) A group of scientists in Argentina claim that their studies show Pyriproxyfen is the real culprit. But other scientists are dismissing these claims. What is Pyriproxyfen? Is it used in the United States and could it pose a danger here? What products use Pyriproxyfen in the United States?

Here’s what you need to know.

1. Brazil Added Pyriproxyfen, an Insecticide That Targets Hormone Pathways, to Its Water Supply

microcephaly Pyriproxyfen

Did Pyriproxyfen cause the increase in microcephaly cases in Brazil, not Zika? (Getty)

Pyriproxyfen is a broad-spectrum insect growth regulator that’s used on houseflies, mosquitoes, and cockroaches, among other insects, according to a WHO study published in 2007. The insecticide is also used to kill fleas and ticks on pets and is manufactured by Sumitomo Chemical. It’s sold across the world, including in the United States.

In Brazil, Pyriproxyfen was added to the water supply in 2014 to combat the larva of mosquitos that carried the Zika virus. Some scientists now believe that Pyriproxyfen was the real cause of the increased microcephaly cases, not the Zika virus. But other experts disagree.

2. Argentinian Scientists Claim Pyriproxyfen Is the Real Microcephaly Culprit, but Other Scientists Disagree

Pyriproxyfen larvicide, microcephaly

Is Pyriproxyfen the cause of birth defects and microcephaly in Brazil or is Zika? (Getty)

Argentina-based doctors with the group Physicians in Crop-Sprayed Towns (PCST) published a report questioning if Pyriproxyfen might be linked to increased cases of microcephaly in Brazil, Fox News Latino reported.  The group said that microcephaly was being reported in areas of Brazil where Pyriproxyfen had been added to the drinking water, Tech Times reported. In the state of Pernambuco, where the Aedes aegypti mosquito lives in high numbers, Pyriproxyfen was added to the water reservoirs. Pernambuco was the first state to notice a microcephaly problem and accounts for 35 percent of the cases in Brazil.

PCST added that Zika epidemics in locations where Pyriproxyfen wasn’t added to the drinking water had not reported increased cases of microcephaly. They cited Colombia as an example, where 3,177 pregnant women with Zika gave birth but there was not a single case of microcephaly.

Other scientists disagree with the group’s findings and still believe that microcephaly is tied to pregnant women catching the Zika virus from infected mosquitos. Tirumalai Kamala, an immunologist, has pointed to two reports that show evidence of Zika in the placenta and brains of fetuses lost in miscarriages, Inquisitr reported. WHO has not said that Zika definitely causes microcephaly, but has said the circumstantial evidence for Zika causing microcephaly is suggestive and worrisome.

Sumitomo Chemical issued a statement saying there was “no scientific basis” for the doctors’ claim. It added that WHO has approved Pyriproxyfen since 2004 and the EPA has approved it since 2001.

3. One Brazilian State Suspended Pyriproxyfen’s Use in the Water Supply

Pyriproxyfen microcephaly

Mosquitoes carrying the Zika virus are thought to put pregnant women at risk of microcephaly. But could Pyriproxyfen be the cause? (Getty)

Rio Grande do Sul, a state in Brazil, suspended the use of the larvicide Pyriproxyfen, Fox News Latino reported. The suspension was done in case Argentina-based doctors were right about the insecticide possibly being the source of microcephaly.

“We cannot run the risk,” said Joao Gabbardo, Health Secretary for Rio Grande do Sul. Health Minister Marcelo Castro said the rumor lacked any logic or sense.

4. Pyriproxyfen Is Used in the United States, but Not in Drinking Water

Pyriproxyfen zika virus

Health workers fumigate in an attempt to eradicate the mosquito which transmits the Zika virus on January 28, 2016 in Recife, Pernambuco state, Brazil. (Getty)

Pyriproxyfen was first introduced to the U.S. in 1996 to protect cotton crops against whitefly. It’s also used to protect other types of crops. In the U.S., it’s often marketed under the name Nylar. The pesticide is not used in the drinking water, like it is in Brazil.

According to a 2012 EPA study posted in the Federal Register, Pyriproxyfen is registered in the U.S. for flea and tick control in the home and on pets, as well as indoor and outdoor ant and roach control. Formulas include carpet powders, foggers, aerosols, shampoos, bait, and pet collars.

Many flea and tick over-the-counter medicines for pets contain Pyriproxyfen. According to Mercola, Bayer says the insecticide targets the hormone pathways of insects but doesn’t affect mammals. However, back in 2011, Dr. Karen Becker, a veterinarian, wrote on Mercola’s website that Pyriproxyfen was a newer chemical and an EPA test showed it decreased body weight and toxicity in the offspring of animals exposed to the chemical. She recommended not using it on pets, but not all vets agree with her assertion.

5. The EPA Determined That Pyriproxyfen Has a Low Toxicological Profile, but a WHO Report Noted Effects on Animal Fetuses

Pyriproxyfen brazil

Microcephaly is a devastating birth defect that’s increasing in Brazil. (Getty)

According to, the EPA did not find high toxicity with Pyriproxyfen: “Pyriproxyfen is of low acute toxicity by oral, dermal, inhalation, and ocular routes of exposure.” The EPA went on to say that Pyriproxyfen is not a neurotoxic chemical and there’s no evidence of prenatal or postnatal sensitivity in rats or rabbits. There are also no adverse effects on the immune system.

According to WHO, Pyriproxyfen mainly affects the liver in mice, rats, and dogs, and can change cholesterol concentrations. It may also lead to anemia at high doses. The WHO paper did mention that in rats’ development, there was some effects on growth at very high levels but no signs of neurotoxicity and no recorded effects in a more conventional study. However, there was “an increased frequency of skeletal variations … in fetuses” when administered at a high dose of 300 mg/kg of body weight per day. At higher doses in rabbits, there were some signs of abortion and premature delivery.

A bulletin published by WHO in February 2016 read, in part: “Currently, the association with the ZIKV infection is the most explored possibility. Evidence of perinatal transmission … together with its strong neurotropism and its documentation in amniotic fluid of foetuses with microcephaly are factors that favour this hypothesis. However, if the ZIKV were indeed introduced in Brazil at the World Cup in mid 2014, the outbreak of microcephaly would have preceded it.”

The bulletin added:

ZIKV has been identified in Africa over 50 years ago, and neither there nor in the outbreaks outside Africa, such an association with microcephaly has been reported. Many other potential factors need to be considered as the cause of the outbreak.”

Among those factors are associated infections, viral infections, and teratogens exposure such as to vaccines or drugs used in early pregnancy. WHO added that malnutrition should also be considered.

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