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The boy, who was nearly 2 years old, arrived in the border city of Brownsville with "underlying health issues" on April 4 and developed flu symptoms four days later, the Texas Department of State Health Services said. He was taken to a Brownsville hospital April 13 and transferred to the following day to a hospital in Houston, where he died Monday night.
Texas Health Services Commissioner Dr. David Lakey said it is "highly likely" the child contracted the illness in Mexico, though that hasn't been confirmed. The boy is one of 16 confirmed swine flu cases in Texas, where Gov. Rick Perry has issued a disaster declaration and schools have shut down across the state out of fear of the virus.
The cause of the boy's death was pneumonia caused by the flu virus, Cameron County Judge Carlos Cascos said. Texas Children's Hospital, where the boy died, said in a statement he was suffering from "acute respiratory illness."
Officials did not specify what underlying health issues the boy had before arriving in the U.S.
State health officials declined to identify the boy or his family, citing privacy concerns, medical confidentiality and "the absence of an obvious health threat from the boy to the public at large."
State health officials said the boy would not have been infectious when he flew from Mexico City to Matamoros, across the border from Brownsville. None of his close contacts have developed symptoms.
President Barack Obama said he wanted to extend his "thoughts and prayers" to the child's family.
Health officials in Brownsville are trying to trace his family's trip to find out how long they were in the area, who they visited and how many people were in the group, Cascos said.
The boy's family members "are healthy and well," Houston's health director, Dr. David Persse, said at a Wednesday news conference.
The toddler was about 2 years old. Houston officials said he was 23 months old, but state officials said he was 22 months old and could not immediately explain the discrepancy.
The Centers For Disease Control and Prevention confirmed the death earlier Wednesday.
The news came as two young brothers in Massachusetts became the first to test positive in the state for swine flu, health officials confirmed Wednesday.
The boys, ages 8 and 10, who became sick after a vacation to Mexico, are from Lowell -- about 35 miles northwest of Boston. Neither child attended school after returning home from the trip.
Nevada confirmed its first case Wednesday -- a 2-year-old girl from the Reno area who was not hospitalized and is recovering.
Children, especially those younger than age 5, are particularly vulnerable to flu and its complications, and every year children die from seasonal flu.
According to the CDC, more than 20,000 children younger than age 5 are hospitalized every year because of seasonal flu. In the 2007-08 flu season, the CDC received reports that 86 children nationwide died from flu complications.
Authorities have confirmed at least 93 swine flu cases in the United States. In addition to the 16 in Texas, they've identified 51 cases in New York, 14 in California, three in Maine; two in Kansas, two in Massachusetts, and one each in Indiana, Ohio, Arizona and Nevada. The CDC also said Michigan had two, but state officials maintained only one was confirmed.
The virus that has sickened more than two dozen students at a Roman Catholic high school in New York City forced another two city schools to close for the rest of the week. Chicago school officials shut down an elementary school Wednesday after one child contracted a probable case of swine flu.
The Mexican Civic Society of Illinois on Wednesday canceled a Cinco de Mayo event at Chicago's Navy Pier this weekend because of concerns about the flu, said spokeswoman Evelia Rodriguez.
She said hundreds of performers and vendors had planned to travel from Mexico, but some had trouble getting flights. Cinco de Mayo celebrates Mexico's defeat of a French army on May 5, 1862.
Swine flu is suspected of killing more than 150 people and sickening more than 2,400 in Mexico.