(AP) - New York City was dealing with a growing public health threat Sunday after tests confirmed that eight students at a private Catholic high school had contracted swine flu. Some of the school's students had visited Mexico on a spring break trip two weeks ago.

New York officials previously had characterized the cases as probable, but the Centers for Disease Control and Prevention confirmed that it was swine flu, Mayor Michael Bloomberg said.

About 100 students at St. Francis Preparatory School complained of flu-like symptoms; further tests will determine how many of those cases are swine flu.

Bloomberg stressed that the New York cases were mild and many are recovering, but said that parents of the students also had flu symptoms, "suggesting it is spreading person to person."

NYC officials confirmed Sunday flu strain that sickened the students is same strain as one found in Mexico.

In Mexico, health officials say swine flu has killed up to 86 people and sickened 1,400 since April 13. The World Health Organization on Saturday asked all countries to step up reporting and surveillance of the disease, as airports around the world were screening travelers from Mexico for flu symptoms.

The U.S. declared a public health emergency Sunday to deal with the emerging new swine flu, much like the government does to prepare for approaching hurricanes.

Federal health officials said Sunday that 20 swine flu cases have been reported so far in New York, Ohio, Kansas, Texas and California. Patients have ranged in age from 9 to over 50. At least two were hospitalized. All recovered or are recovering.

Swine flu is a respiratory disease of pigs caused by type A flu viruses, the CDC's Web site says. Human cases are uncommon but can occur in people who are around pigs. It also can be spread from person to person. Symptoms include a high fever, body aches, coughing, sore throat and respiratory congestion.