When Patricia Pena hosted a Zika awareness class near the Texas border with Mexico on Tuesday, only four people showed up.

“Even though there’s been a lot of announcements on TV about it and how to protect yourself, families are still very naive when it comes to the information on Zika,” said Pena, who works with La Frontera Ministries, a community nonprofit.

While Zika cases in south Florida drew most of the headlines last year, the mosquito-borne virus also struck in the Rio Grande valley. As mosquito season ramps up again, activists and health workers fear that the region is at risk of an outbreak.

More than 1.3 million people live in the Valley, many in deprived neighbourhoods known as colonias, where conditions are ripe for mosquitoes to breed: sprawling settlements limit the effectiveness of spraying, standing water is common, and many houses lack window screens or air-conditioning.

“You have a lot of these families who don’t even have money to get rid of their garbage,” Pena said, “and their houses are infested with all kinds of creatures, including mosquitoes.” She added that many people don’t have equipment to cut their grass, which could hide breeding pools, and that the streets lack proper drainage.

“It’s going to hit the poorest people,” said Joseph McCormick, regional dean of the University of Texas School of Public Health. “People that live in areas where mosquitoes are going to breed, areas where they have poor housing, just like it is in South America.”

Michael Seifert, a community organiser in Brownsville, suggested that until people see local cases of babies with microcephaly – a birth defect that can be caused by Zika – the virus will be viewed as a sort of “urban legend”, distant and unlikely.

It is real enough, if not widespread: according to state health department statistics, 10 Zika cases have been documented in Texas this year and 320 in 2016 and 2015. About 250 women and children have shown evidence of infection reported to the federal Zika Pregnancy Registry.

Last November the state’s first reported case of local mosquito-borne Zika infection was in Brownsville, leading the federal Centers for Disease Control and Prevention (CDC) to designate the city, of about 185,000 people, a Zika “cautionary area” which pregnant women should consider avoiding.

There were six cases of local transmission via mosquitoes reported in Brownsville in the final two months of 2016. This month the state health department issued a recommendation that all pregnant women in six border counties be tested in their first and second trimesters, as well as others exhibiting possible Zika symptoms.

Efforts to fight Zika and accurately assess the risk and spread are complicated by the lack of access to healthcare: there is no public hospital in the region and the rate of people without health insurance is among the highest in the nation.

Tracking Zika is complicated further by the large volume of cross-border traffic and significant numbers of undocumented immigrants, especially since the Donald Trump’s harsh deportation efforts have caused many people to fear any interaction with authorities.

Seifert said he recently heard from a county official that it’s unclear where Zika cases originated, in part becase “people are too afraid to tell us if they were traveling to Mexico and back, because of the whole Trump mess”.

“If you show up at my door and you’re a health person, and I’ve been shown to have Zika and they say, ‘were you traveling?’ – I’m not so sure I’ll tell you that.”

A 25-mile drive from South Padre Island, one of the country’s most popular spring break destinations, Brownsville stands across from the Mexican city of Matamoros, separated by the Rio Grande.

“When they say we have travel cases of Zika, it’s not like somebody flew in from Brazil,” said Lisa Mitchell-Bennett, a project manager at the School of Public Health. “They went to visit their aunt or their boyfriend 100 metres across the river on the weekend, or they went to their family Sunday dinner.”

McCormick, a member of the CDC’s global advisory group, said the numbers probably do not tell the whole story: 80% of Zika cases do not show symptoms, including cases that cause birth defects, and officials have struggled to develop an improved tracking system.

“We have to have better surveillance so that we know if the virus is expanding, where is it going,” McCormick said. “We need human surveillance and some mosquito surveillance to know where the mosquitoes are.

“This is the tip of the iceberg,” he added. “Without some kind of surveillance of actually testing people we have no idea, frankly, what’s going on.”

With many important questions still to be answered, including for how long a man infected with Zika could sexually transmit it, McCormick said that severe budget cuts to medical research proposed by the Trump administration are worrying.

“More resources are going to have to come but it’s not clear it’s going to come from this administration,” he said. “That part of it’s bleak.”

“There are a lot of unknowns here that are distressing everybody in the public health world.”

Mosquitoes can be a year-round problem in parts of the southern US and in Texas, where an unusually warm winter and rainy spring have created conditions for mass breeding. West Nile virus has already been detected this year in some mosquito pools in the Dallas area.

Dengue fever cases have previously occurred in the valley, and some experts are concerned that other insect-borne tropical diseases transmitted are more prevalent in Texas than previously though. Some fear Chagas disease, typically spread to humans by a “kissing bug” bite around the mouth or eyes, followed by defecation while feeding on blood that transmits a parasite. Chagas can cause fatal heart conditions that may go undetected for decades.

With Zika, Mitchell-Bennett said it can be hard for authorities to explain their fears: they promote early testing, stress that the disease can spread through sex, and warn people to use repellent, stay indoors and drain standing water.

“I think it’s pretty clear that we’re going to get more cases,” Mitchell-Bennett said. “When, how quickly, I don’t think anybody knows.”