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Here’s another reason it will be hard to get rid of Zika: Mosquitoes can pass the virus to their offspring in their eggs.

It’s not a surprising finding. Mosquitoes infect their larvae with other viruses, too, including Zika’s close relative the dengue virus.

But it’s another obstacle for people trying to get rid of Zika and the mosquitoes that spread it.

In this May 23, 2016, file photo, an Aedes aegypti mosquito sits inside a glass tube at the Fiocruz institute where they have been screening for mosquitos naturally infected with the Zika virus in Rio de Janeiro, Brazil. AP

“It makes control harder,” said Dr. Robert Tesh of the University of Texas Medical Branch in Galveston. “Spraying affects adults, but it does not usually kill the immature forms — the eggs and larvae. Spraying will reduce transmission, but it may not eliminate the virus.”

Usually, it takes people plus mosquitoes to spread a virus. The mosquitoes bite actively infected people, incubate the virus for a while, and then bite other people to spread it. If no people in an area are infected, no virus spreads. Sometimes an animal can act as a reservoir — birds can keep West Nile Virus spreading, for instance.

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So-called vertical transmission allows the virus to spread even if all the adult mosquitoes in an area die out.

Tesh and colleagues infected Aedes aegypti and Aedes albopictus mosquitoes with Zika and then tested the eggs they laid.

The Aedes aegypti mosquitoes — the main carrier of Zika — did occasionally pass the virus to their eggs, they reported in the American Journal of Tropical Medicine and Hygiene. But it wasn’t most or even much of the time — about one in every 290 times.

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That means it’s probably not a major way Zika sticks around, Tesh said.

“I think it just another survival mechanism for the virus to make it through the season,” Tesh told NBC News.

“I think it just another survival mechanism for the virus to make it through the season."

Zika has spread explosively across Latin America and the Caribbean over the past year. It’s caused two small outbreaks in Florida and is expected to lead to more.

Once thought to be a benign virus, it’s now been shown to cause horrendous birth defects if a pregnant woman gets it, and can cause a paralyzing condition called Guillain-Barré syndrome.

There’s no drug to treat it and no vaccine to prevent it. The World Health Organization and the Centers for Disease Control and Prevention say the best way to fight Zika is to fight the Aedes mosquitoes that carry it.

But that’s not easy. Aedes are container breeders — they can lay their eggs in small containers and need just a tiny bit of water to hatch.

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They live in and around houses and like crowded urban areas where spraying is difficult.

Spraying insecticides to kill adult mosquitoes cannot wipe out Aedes, Tesh and other experts said. They lay their eggs right above the water line in a small container, a discarded tire or some trash, Tesh explained.

“So when that container is filled and the water covers the eggs, the eggs hatch,” he said.

“You have to scrub the inside of the container. That is the way to get rid of the eggs."

The eggs can survive being dried out, and they stick really well. They’re impervious to insecticides or other chemicals, he added. So simply emptying containers regularly does not necessarily get rid of the mosquitoes.

“You have to scrub the inside of the container. That is the way to get rid of the eggs,” he said.

Dr. Peter Hotez, who heads the tropical medicine program at Baylor College of Medicine in Houston, said the trade in used tires probably helped spread Aedes mosquitoes back into countries such as Brazil, which had eradicated them decades ago with heavy insecticide use.

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There was some good news in the study: Aedes albopictus, the so-called Asian tiger mosquito that has a much broader ranges than Aedes aegypti, did not transmit Zika to its eggs. Researchers still are not clear whether tiger mosquitoes can infect people with Zika, although they can carry the virus.

The Florida Department of Health reported one more locally transmitted case of Zika Monday in an outbreak affecting Miami beach.