Zika, the mosquito-borne virus that triggered public health alarm bells last summer, has receded from the spotlight. But, experts say, expect the virus to pose a renewed threat this year.

How great of a threat? That’s where it gets tricky.

No locally acquired cases of the virus have been reported in the United States this year. But as public health agencies gear up for mosquito season, uncertainty remains around what resources states may need and whether they will receive adequate federal support. In addition, researchers still have questions about how the virus works and its long-term effects. These forces could complicate efforts to track outbreaks and provide accurate information about prevention and disease management.

“We still have much to learn. And much remains to be done,” Lyle Petersen, director of the Centers for Disease Control and Prevention’s Division of Vector-Borne Diseases, said last month at a House Energy and Commerce subcommittee hearing.

Zika, which can cause birth defects if contracted by pregnant women, is primarily transmitted by a mosquito typically found in southern areas of the country such as the Gulf Coast. It hit the Americas in 2015 and reached the United States via infected travelers.

Last year, 5,102 cases were reported in the United States — most people contracted the virus while traveling in South and Central America. Sixty-four babies in the U.S. have been reported to be affected since the government began reporting outcomes last June. In rare cases, people who had not traveled abroad contracted Zika through a local mosquito or sexual contact. Puerto Rico, a U.S. territory, had 34,963 confirmed cases in 2016.

Again this year, determining if areas are experiencing Zika transmission will require active surveillance and testing. Most people who contract the virus don’t show visible symptoms.

Implementing that testing is challenging, experts said.

“Should you begin universal screenings in communities that are at risk?” asked Jeff Engel, executive director of the Council of State and Territorial Epidemiologists, which represents the specialists working in public health departments. “That is a tough resource question and probably is not feasible.”

Last year, Congress appropriated $1.1 billion toward fighting Zika. That was split among research, overseas response and state public health efforts. In April, the CDC warned states those federal dollars were running out. They are projected to last through this fiscal year.

Meanwhile, the White House budget released Tuesday proposes an emergency fund that would finance responses to outbreaks like Zika. But it also would cut $1.3 billion from the CDC and slash $838 million from the National Institute of Allergy and Infectious Diseases, which orchestrates vaccine development. So far, Congress has said little about approving more funding.

“Funding from Congress has been critical for our response to Zika. However, additional support will be needed,” said Rick Bright, director of the federal Biomedical Advanced Research and Development Authority, a subsidiary of the U.S. Department of Health and Human Services, at the hearing.

Some advocates also warn an ongoing hiring freeze at HHS could limit its capacity to combat Zika.

“Obviously we live in tough times when it comes to budget constraints,” said Tom Skinner, a CDC spokesman. If those constraints thwart surveillance this year, experts warned, it will make it harder to know precisely when Zika hits. That puts women of childbearing age — for whom the virus’ consequences are most dramatic — at greater risk.

“I’m worried we missed [many cases of] Zika last year and we’ll miss Zika this year,” said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, in Houston. “If you’re a woman of reproductive age, living on the Gulf Coast of the United States, or Arizona or Southern California, and you’re pregnant or might be pregnant — you don’t really know if Zika’s in your area.”

But long-term follow-up will probably require more funding, noted Chris Van Deusen, a spokesman for Texas’ Department of State Health Services.

And leveraging prenatal care to push Zika testing has issues, too. Many women most at risk are low-income, Van Deusen noted, and therefore less likely to interact with the health care system. Those people won’t be tested or accounted for in the state’s Zika tracking efforts.

That, Hotez said, compromises authorities’ ability to discover Zika when it hits and then get the word out. “The only way we identify Zika outbreaks like [last year’s] in South Florida is through serendipity,” he said.

Testing is another concern. Current tests are unreliable in identifying Zika among patients previously infected with related viruses, such as dengue.

Limited resources could also strain states. Last year, the volume of patient samples sent for testing overwhelmed laboratories in some affected areas, noted Kelly Wroblewski, director of infectious disease at the Association of Public Health Laboratories.

Many states also lack the infrastructure to adequately address an outbreak. Surveillance of mosquito-borne diseases is virtually nonexistent in many states, according to a 2014 report from the Council of State and Territorial Epidemiologists. In many areas, mosquito eradication has shrunk along with dwindling public health budgets.

“We need better mosquito control methods, we need better surveillance,” Petersen said at the May hearing. “We need to develop more of a nationally standard approach to vector control and laboratory testing. … This will require a sustained effort to rebuild infrastructure.”

The scarcity of resources is compounded by scientists’ limited knowledge of the virus.

Experts are “discovering new things every day,” particularly regarding how the virus plays out after birth, Engel said. Beyond an obvious condition like microcephaly — which results in an abnormally small head and brain impairment — it’s tough to assess Zika’s impact. Preliminary work suggests children without microcephaly but whose mothers were infected may have developmental issues later in life. But their severity — and likelihood of appearing — is unclear.

“It’s the problem with these rapidly emerging diseases; you can’t get the research up quickly enough,” said Scott Weaver, director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch, in Galveston.

Shefali Luthra and Carmen Heredia Rodriguez, Kaiser Health News

Kaiser Health News is an editorially independent part of the Kaiser Family Foundation.