Hundreds of migrants and their children seeking to enter the U.S. from Mexico are arriving with illnesses, forcing U.S. Customs and Border Protection to seek additional medical assistance and boost medical screenings, the agency disclosed Monday.

Between Dec. 22 and Sunday, the agency reported 451 cases referred to doctors or other providers, including 259 children. Among the children, half of the cases involved kids under the age of 5.

The ill migrants have been arriving with all kinds of ailments, many with flu or pneumonia that can be particularly pervasive and dangerous this time of year. Seventeen migrants have been hospitalized, including six children, according to the agency..

"The U.S. Border Patrol is doing everything in its power to handle this crisis," said Commissioner Kevin K. McAleenan in a statement. But he added, "The status quo is not acceptable" given the wave of immigrants arriving.

Secretary of Homeland Security Kirstjen Nielsen said in a statement last week that the Border Patrol has detailed 139,817 migrants on the Southwest border in the past two months. That compares to 74,946 for the same period last year. These include 68,510 family members and 13,981 unaccompanied children.

Making matters worse, McAleenan said central American refugees are now arriving directly by bus, not just by foot, and that a "robust smuggling cycle continues."

Disclosure of the numbers of sick migrants come amid heightened concern about the medical condition of children in U.S. custody. Previously, the agency disclosed an 8-year-old boy from Guatemala died in custody on Christmas Eve, the second immigrant child to pass away in detention this month.

The boy, Felipe Gomez Alonzo, had been treated and released for cold and fever symptoms during nearly a week in U.S. custody, the agency said.

The Border Patrol has been receiving assistance from the Coast Guard, which has sent medical teams to the border in the Yuma and Tucson, Arizona, and Rio Grande Valley sectors of the border. The Public Health Service also said it will deploy teams to help.

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The Border Patrol is also changing procedures to enhance medical evaluations of children. Parents are being interviewed about their children's medical condition and history and kids' blood pressure, pulse and temperature are among the vitals being checked by assessment teams, McAleenan told reporters in a conference call.

"We have always had an agent review the condition of the children," McAleenan said. "What we are providing now is a medical professional" for the evaluations.

He said he said the cases being referred involve illnesses believe to have been contracted in transit to the U.S., not while in U.S. custody. "Many were ill before they departed their homes," he said.