Editor’s note: An earlier version of this story did not include information about the CDC’s decision to abandon aerial spraying in San Juan. The story has been rewritten with this information.

Federal health officials have decided not to proceed with a proposal to begin aerial spraying on Puerto Rico in order to prevent the spread of the Zika virus. The move came two days after the island’s governor opposed the idea and the city of San Juan filed a lawsuit to prevent federal health officials from pursuing spraying.

The developments came amid heated debate over the extent to which spraying an insecticide called Naled will have a negative effect on human health and wildlife. In its lawsuit, which was filed on July 21 in federal court in San Juan, the city argued the spraying will “pose a significant risk to the well-being of several species of fish, wildlife, and plants.” The suit also cited “a serious risk to the general health” of San Juan residents.

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Earlier this month, Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, which has pushed for aerial spraying, argued this approach was Puerto Rico’s best defense to fight a virus that can cause microcephaly, a rare defect in which babies are born with abnormally small heads and brain damage. By last Friday, the CDC relented after Puerto Rican Gov. Alejandro Garcia Padilla, who had veto power, opposed the use of aerial spraying.

“CDC respects the decision of the Governor of Puerto Rico to not pursue aerial spraying with the insecticide Naled,” the agency told STAT. “We moved too quickly in our urgency to do all that we could to be responsive and prepared in the event officials in Puerto Rico decided to use Naled. Under no circumstances would CDC undertake application of Naled unless the government of Puerto Rico decided to do so, authorized it, and requested CDC to do so.”

Instead, the agency plans to distribute another insecticide called bti, or bacillus thuringiensis israelensis, which is not known to effect other organisms.

As reported previously, as many as 50 pregnant women in Puerto Rico are becoming infected with the mosquito-borne virus every day. But following a June 16 meeting to discuss the CDC proposal, Puerto Rico government officials had debated whether to pursue spraying. Since then, legislators held public hearings amid a growing number of protests.

The city filed its lawsuit last Thursday after learning that the CDC had transferred a “significant amount of Naled” to Puerto Rico, suggesting there is an “imminent possibility” that the agency will begin aerial spraying without consent of the local government. The decision to spray the island was, however, ultimately up to Padilla.

The city contended the CDC would violate the National Environmental Protection Act if an environmental impact statement is not obtained prior to spraying. The lawsuit also cited concerns about toxicity. Naled can be toxic if ingested, inhaled, or absorbed through the skin. Vapors or fumes can harm the mucous membranes lining the mouth, throat, and lungs, according to the Extension Toxicology Network.

However, Gina McCarthy, an administrator for the Environmental Protection Agency, had recently told the Associated Press that Puerto Rico needs to seriously consider aerial spraying. “It can be done safely and effectively and is perhaps the most important tool we can use right now to change the trajectory,” she told the news service.

The CDC suggested aerial spraying because of the rate of infection. A total of 339 pregnant women on the island have been diagnosed with Zika, and health officials believe women will eventually give birth to children with microcephaly.

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Overall, Puerto Rico has reported about 2,400 Zika cases, 44 hospitalizations, and one death.

In addition, 16 people have been diagnosed with a temporary paralysis condition known as Guillain-Barre that has been linked to Zika infections. The CDC estimates that more than 20 percent of Puerto Rico’s 3.5 million people could be infected with Zika in an outbreak expected to peak by this summer. As Associated Press wrote recently, though, local health officials have dismissed that number as being exaggerated.

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