MATAMOROS, Mexico — Maria Sam had lost count of her 9-year-old son’s seizures in the nearly three months since they applied for asylum in the United States but were told to wait on the Mexican side of the Rio Grande.
But his most recent medical episode was enough to prompt Ms. Sam, who is from Guatemala, to pack up their few belongings and return to the border post to test the Trump administration’s “Remain in Mexico” asylum policy and its stated exemption for medical emergencies.
“Yesterday was the worst because he turned purple,” Ms. Sam said last Wednesday, as her son David clutched her jacket and they prepared to walk to the international bridge that connects this dangerous area of Mexico to Brownsville, Texas. “They had to call an ambulance and bring him to the hospital.”
Also at the port of entry on that cold, rainy afternoon was a Honduran mother with a child who has autism. Nearby, a 7-year-old whose rare brain disorder, lissencephaly, gave her a life expectancy of only three more years, danced to the holiday jingle “Mi Burrito Sabanero” playing on the cellphone of a volunteer doctor. That doctor had just asked American immigration officials to allow the families in on a medical exemption after evaluating them.
A little more than a year ago, these families, fleeing violence and upheaval in Central America, probably would have requested asylum, been detained for a short period in the United States and released to await an adjudication of their cases — the “catch and release” process that infuriates President Trump. While in custody, the migrants were entitled to treatment at public hospitals and clinics of the United States, or by the federal government at a detention center.
But in January 2019, the administration introduced the Migrant Protection Protocols, or M.P.P., which empowered officers to return migrants south of the border to wait for the duration of their cases. Senior officials with the Department of Homeland Security were quick to note the exemptions for migrants who could establish a sufficient fear of torture or persecution or had known physical or mental health issues.
But as the policy returned around 60,000 migrants to Mexico, those exemptions were largely ignored, according to immigration lawyers, American doctors in Matamoros and the migrants themselves.
At a makeshift camp in Matamoros, tents crowd a muddy levee, housing around 2,500 migrants. Families cook on homemade stoves built out of old washing machines. From a trailer, Dr. Maura Sammon, the medical director for Global Response Management, leads a team of doctors, some of whom are migrants themselves.
Dr. Sammon said the team treated at least 40 patients a day. She listed the more serious medical issues: sickle cell anemia, hypoxia, third-degree burns and sepsis. Other patients included a 70-year-old with chest pains, children with epilepsy or development disorders, a migrant with H.I.V. as well as one with ovarian cancer.
“This is 100 percent a creation of M.P.P.,” Dr. Sammon said. “It is not a virtual wall — it is a wall. You see how close that river is. You see people looking at that river every day and saying, ‘The United States is right there.’”
Her team sends patients in need of emergency care to a nearby hospital, but the care can be inadequate. A boy who went to the hospital with appendicitis was discharged, then his appendix ruptured, Dr. Sammon said. Some migrants refuse to go to the hospital for fear of being kidnapped by cartel organizations, the same threat that prompted the State Department to advise Americans not to travel to Matamoros. On Thursday, a drive-by shooting near the camp forced the evacuation of Dr. Sammon’s medical team.
Homeland security officials say the new asylum policy, more commonly known as Remain in Mexico, quelled a surge of migration last year and eased overcrowded detention facilities in the United States. Forcing migrants to wait in Mexico has also discouraged those unlikely to qualify for asylum from participating in the process, officials say.
A medical issue by itself usually has not been enough to gain entry into the United States, and was rarely grounds for a claim of asylum, typically granted to those fleeing political oppression and violence. Foreigners with health conditions typically have obtained visitor visas and must prove they can sufficiently pay for medical treatment in the United States.
But before Remain in Mexico went into force, immigration officials were required to provide care for sick asylum seekers in their custody. That changed when the administration began returning migrants, many of them already ill, to areas with little access to treatment.
“Their understanding is it’s Mexico’s responsibility to care for the migrants, but are they checking on that?” asked Theresa Cardinal Brown, the director of immigration and cross border policy at the Bipartisan Policy Center.
Of the more than 61,097 migrants subjected to Migrant Protection Protocols, just 263 have been granted some form of relief and entered the United States, according to data from the Transactional Records Access Clearinghouse, or TRAC.
Beyond health care, immigration groups say M.P.P. has also denied migrants access to lawyers. More than 25,600 migrants have pending cases while other cases have been closed.
In raw numbers, the policy appears to be working. Mr. Trump has celebrated eight straight months of declining arrests at the border. In January, Customs and Border Protection recorded more than 36,600 arrests at the border, down about 75 percent from the more than 144,100 arrests in May.
“While I think it’s fair to say the administration is having an impact on reducing the number of people who enter, the human cost is appalling,” said Cecilia Muñoz, who was the director of the White House Domestic Policy Council under President Barack Obama. “The Trump administration is banking on the notion that if the problem is on the other side of the border, it’s not our problem, even though it is of our making.”
Mr. Trump’s anti-immigration messaging was initially used by smugglers who bought radio ads in Central America and warned migrants that the United States would soon be shutting the border, helping fuel a surge in illegal crossings last spring.
But as word gets out about Remain in Mexico, and Mexico has increased enforcement at its southern border with Guatemala, deterring such crossings may be succeeding. The number of migrants returned under the policy has declined for five straight months. More than 12,400 migrants were returned to Mexico in August. In January, that number was below 2,000, according to TRAC.
That decline has come about not only because asylum seekers must remain in Mexico. The administration is also turning to other programs for fast-tracking deportations along the border, and Guatemala has given the Trump administration permission to deport asylum seekers to Guatemala, even if they are from northern Mexico.
Most of the thousands of migrants waiting in Matamoros arrived before those other programs, and for some, the wait has been devastating.
Yodalys, 52, a migrant from Cuba who asked to be identified only by her first name for fear of retaliation, said in an interview that she did not hope for entry into the United States just to see her children. She also hoped it would save her vision.
She was returned to the Matamoros camp last summer after traveling to the border from Cuba and asking for protection in the United States. She already had high blood pressure and epilepsy. Weeks after arriving at the camp, she noticed that she was losing her vision. She went to a doctor in October and learned that a parasite derived from the disease toxoplasmosis had infected her eyes.
She was unable to get the medical care she needed in Matamoros, so an immigration lawyer, Charlene D’Cruz, tried bringing Yodalys to the Brownsville port of entry to be admitted to the United States under the Migrant Protection Protocols’ medical exemption.
She was denied four times, Ms. D’Cruz said. Then this month, for unclear reasons, officers told Ms. D’Cruz they would allow Yodalys to be released into the United States.
By then, “I could only see shadows,” Yodalys said in a phone interview from her daughter’s home in Florida.
She has completely lost vision in one eye, and doctors say her sight could have been saved if she had received treatment in the United States sooner. But she expressed no ill will.
“I’m very grateful to the United States for finally letting me in,” Yodalys said.
Officers with Customs and Border Protection make decisions on medical exemptions for the policy on a case-by-case basis, according to Matthew Dyman, a spokesman for the agency. Officers are also offered the option of bringing migrants to American hospitals before returning them to Mexico. Mr. Dyman referred to a medical directive by the agency in December that required all migrants who cross the border to be given a medical screening.
The agency cannot publicly standardize medical conditions that would permit entry into the United States because smugglers facilitating migration to the border would then advise migrants to tell officers that they have those conditions, Mr. Dyman said.
But some say that allows the agency to be inconsistent in its application of the exemption. Dr. Sammon said she had witnessed officers allowing parents of children with developmental disorders into the United States on some days and deny others with the same or more imminent health concerns on other occasions. “No rhyme or reason,” she said.
Ms. Sam hoped the officers at the port would use their discretion to allow her and David to enter the United States so they could reunite with their relatives. Returning to Guatemala was no longer an option; her abusive husband had threatened multiple times to kill her.
After more than two hours, port officers gave Ms. D’Cruz, the lawyer, their decision. The port officers deemed that there was sufficient care available in Mexico. The families were denied entry.
Ms. Sam said she remained hopeful. “That’s all I have for my children,” she said.
Then each of the mothers and their children walked down the bridge for another night in the tents.
Paulina Villegas contributed reporting from Mexico City.