CHICAGO (Reuters) - Health experts are bracing for Zika virus to spread to the United States by April or May, borne by a mosquito that craves human blood, feeds during the day and lives under beds and inside closets.

A health agent shows a chemical compound to kill mosquito larvae during an inspection for the Aedes aegypti mosquito inside a house at Vila Canoas slum, as part of efforts to prevent the spread of the Zika virus, in Rio de Janeiro, Brazil, February 2, 2016. REUTERS/Pilar Olivares

Until now, the best weapon against disease-carrying mosquitoes in the United States has been outdoor pesticide fog sprayed by truck and airplane. But health experts fear the typical approach will do little to eradicate the Aedes aegypti mosquito that carries Zika.

Controlling that mosquito requires pesticide sprayed under beds, on the walls and in closets, said Gonzalo Vazquez-Prokopec, who studies disease transmission patterns of mosquitoes at Emory’s School of Public Health’s Department of Environmental Sciences.

“We know fogging is not effective,” Vazquez-Prokopec said.

Though there could be localized U.S. outbreaks, most likely along the Gulf Coast, federal officials said they hope the wide use of air conditioning, window screens and regular garbage collection will mitigate the risk.

The World Health Organization declared the Zika outbreak an international health emergency this week after evidence linking the virus to microcephaly, a devastating birth defect that can cause unusually small heads and permanent brain damage. Brazil has reported 3,700 suspected cases of microcephaly. The outbreak is now affecting at least 25 countries and territories, most of them in Latin American and the Caribbean, and could infect up to 4 million people in the Americas, according to the WHO.

More than 30 people in the United States have been confirmed to have Zika after traveling to an affected country. There has been one report of transmission within the United States, but experts believe that will increase as the weather warms up, the local mosquito population multiplies and many more travelers return to the country.

“All it takes is one of those individuals who arrives back in the United States at the stage where they have virus in their blood,” said Scott Weaver, an expert in mosquito-borne viral diseases at the University Texas Medical Branch’s Galveston National Laboratory. At that point, he said, a single mosquito biting the affected person could spread the disease to others.

White House spokesman Josh Earnest said Monday the risk of transmission now is “quite low,” but as temperatures rise, “we want to make sure that we have got a strategy to try to limit the spread of this disease when that happens.”

The U.S. Centers for Disease Control and Prevention is working on a control program for Zika, which will likely involve public education about eliminating breeding sites and spraying to kill mosquito larvae and adult mosquitoes, especially in areas experiencing outbreaks, said spokesman Tom Skinner.

Until then, the CDC is circulating guidelines developed for combating chikungunya, a close cousin to Zika carried by the same types of mosquitoes. Local health departments are also sorting out their approach to fight Zika..

“If it’s going to happen, I think it will happen in the warmer months, likely in April and May,” said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston.

Zika thrives in impoverished areas, spreading widely in garbage-filled neighborhoods and in homes and apartments with no screens on the windows, conditions that are present in many Gulf Coast communities in the United States, Hotez said.

THE VECTORS

The Aedes aegypti mosquito that carries Zika also transmits dengue fever and chikungunya. Aedes aegypti is mostly found in southern parts of the United States, such as the coastal regions of Texas, Louisiana, Mississippi, Alabama and Florida.

Experts believe it arrived on slave trade ships from Africa, spreading yellow fever in port cities, including a 1793 outbreak in Philadelphia that wiped out 10 percent of the city’s population of 50,000.

Unlike Aedes aegypti, most mosquitoes common to North America feed at night and live in wooded areas.

Recent research suggests the pest may be adapting to colder temperatures. David Severson at the University of Notre Dame discovered a population of Aedes aegypti mosquitoes that has spent the past four winters underground in Washington, D.C.’s Capitol Hill neighborhood

Aggressive abatement involving indoor and outdoor fogging and breeding ground eradication between 1947 and 1970 nearly wiped out Aedes aegypti. At the time, the mosquitoes were the source of yellow fever in across the Americas. But budget cuts and the development of an effective yellow fever vaccine ended eradication efforts, and Aedes aegypti populations rebounded.

Scientists believe Aedes albopictus, or the Asian tiger mosquito, also is capable of spreading Zika. This aggressive biter arrived in the United States in 1985 and has replaced Aedes aegypti in some places. Its range includes at least 32 U.S. states as far north as Illinois and Pennsylvania and in pockets as far west as California.

Aedes albopictus breeds in small containers of water, bites during the daytime and lives near population centers. A less picky eater, it also feasts on pets and wild animals.

Researchers in Brazil are studying whether the Culex species, a carrier of the West Nile virus commonly found in many southeastern U.S. states, might carry Zika, which could explain the rapid spread in Brazil. These mosquitoes rest in the daytime and bite at dusk or after dark.

PUBLIC HEALTH PROBLEM

All of this poses a challenge for U.S. health departments, which have faced pressure to reduce mosquito abatement activities amid budget cuts and increasing concerns over exposure to pesticides.

“The current methods we have some shortcomings,” said the CDC’s Dr. Anne Schuchat. “We’re going to need to work in future on identifying better options.”

Brazil’s government has mounted a door-to-door campaign and has authorized public health officials to enter properties by force if necessary. Health workers search for potential breeding spots and in some areas use indoor foggers, applying pesticides that stick to walls.

“That is not going to fly in the United States,” said Joseph Conlon, technical advisor for the American Mosquito Control Association, which represents researchers, public health officials and pesticide makers.

There are no pesticides registered by the Environmental Protection Agency for indoor application, Conlon said. Instead, abatement will likely focus on typical breeding sites, from birdbaths to potted plants, dog bowls, tin cans, tires and other places likely to become inundated with water.

“Our best bet is to remove the breeding habitats,” he said. “It’s a lot harder to do than you would think. People don’t want to change their habits,” he said.