An outbreak of the mumps virus in the U.S. government’s crowded immigration detention facilities is adding a new strain to a system that the secretary of homeland security warned months ago had reached its “breaking point.”

Officials from Immigration and Customs Enforcement, the Homeland Security agency responsible for the long-term detention and deportation of people who are in the country illegally, said Friday that they have quarantined 5,200 migrants at 39 detention facilities across the country, most after exposure to mumps.

The agency said it has confirmed 334 mumps cases since September. Mumps is considered a highly contagious but not life-threatening disease.

The quarantines, which were first reported by CNN, come as the Trump administration has struggled to manage an ongoing influx of migrants across the U.S.-Mexico border, where immigration authorities have apprehended nearly 600,000 people since October. ICE agents have apprehended another 34,500 people in the interior of the United States, according to official government statistics. The agency says it is currently holding about 52,560 people in detention facilities across the country, 5,000 more than it forecast in its 2019 budget.

Nathalie Asher, the executive associate director for ICE Enforcement and Removal Operations, said that the need for quarantines to prevent the spread of infection will result in lengthier detention times, and compound backlogs across the system.

Asher blamed the influx of migrants — most of whom are families and children — at the southwest border as a “significant” factor in the mumps outbreak.

“With 75% of our current detention population coming to us directly from the border, the fact that we’re seeing mumps in nearly 40 facilities across the country and recent outbreaks of mumps in Central America, the preponderance of evidence points to the major influx at our Southwest border being, at minimum, a significant contributing factor of these occurrences,” Asher said in a statement.

Another ICE official said news reports of mumps outbreaks in Honduras starting in late 2018 and the occurrence of mumps in dozens of ICE facilities also supported that assessment.

There were no confirmed reports of mumps among ICE detainees before 2018, the official said.

Mumps outbreaks in the United States are rare. The virus is largely preventable through the same widely used vaccine that protects against measles and rubella. However, health officials have warned in recent months that declining vaccination rates in the United States have spawned measles outbreaks in cities across the country, including New York City and Portland, Ore.

The Centers for Disease Control and Prevention also reported a spike in mumps outbreaks, infecting thousands from January 2016 to June 2017. The largest outbreak sickened nearly 3,000 people in a close-knit community in Arkansas.

So far this year, the CDC has documented more than 1,000 mumps cases, including hundreds in Texas and Pennsylvania alone.

The government has not provided an assessment of vaccination rates among detained migrants, most of whom come from the Central American countries of Guatemala, Honduras and El Salvador.

Those three countries have high vaccination rates that are generally on a par with the United States, according to figures published by the CDC. However, access to medical care is extremely limited in rural areas.

In the United States, the most at-risk members of the population, as with other communicable diseases, are the unvaccinated and those living in close or unhygienic quarters, such as prisons, health experts say.

ICE says that all detainees in its custody receive a health screening within the first 12 hours and that the agency offers the MMR — for measles, mumps and rubella — vaccine to those who do not present symptoms. Officials say they work quickly to quarantine those who do exhibit symptoms.

But auditors and human rights advocacy groups have criticized the Department of Homeland Security for poor hygiene and limited access to medical care in its facilities, which range from government-run detention centers to remote private jails, and several detainees have died of communicable diseases in recent months.

An internal report released last week by the department’s inspector general titled “Concerns about ICE Detainee Treatment and Care at Four Detention Facilities” documented a litany of ICE standards violations, including poor hygiene, expired food, moldy bathrooms, inadequate medical care and nooses found in detainee cells — among other problems — at ICE facilities it investigated in California, New Jersey, Louisiana and Colorado.

Immigrant advocates have also criticized the Trump administration for its aggressive push to detain thousands of migrants, including those who do not appear to pose a threat to public safety.

ICE said earlier this week that more than two-thirds of the 140 people it arrested in a recent five-day operation across the Midwest had no prior criminal history.

On Thursday, sheriff’s deputies in Arizona found the body of a 7-year-old girl in a remote stretch of desert borderland. Officials said the girl was part of a group of five Indian nationals whom smugglers had led across the border into the desert wilderness where the temperature recently reached 108 degrees, U.S. Customs and Border Protection said in a statement.

Border Patrol agents apprehended the other members of the group separately: two adult women, and later, a mother and her 8-year-old child who were suffering from dehydration.