American health officials on Sunday declared a public health emergency over increasing cases of swine flu, saying that they had confirmed 20 cases of the disease in the United States and expected to see more as investigators fan out to track down the path of the outbreak. Officials said they had confirmed eight cases in New York, seven in California, two in Kansas, two in Texas and one in Ohio, and that the cases looked to be similar to the deadly strain of swine flu that has killed more than 80 people in Mexico and infected 1,300 more.