(CNN) -- U.S. medical experts investigating a novel swine flu outbreak in California and Texas will examine samples from Mexico, where a respiratory illness has killed at least 20 people.
As a precaution to avoid further contamination, schools and universities in Mexico City and the state of Mexico were closed Friday, said National Health Secretary Jose Angel Cordova. He indicated Friday the schools may remain closed for a while.
In the United States, seven cases of a previously undetected strain of swine flu have been confirmed in humans, the Centers for Disease Control and Prevention said. All of the patients have recovered, officials said. None of the patients had direct contact with pigs.
Five of the cases were found in California, and two in Texas, near San Antonio, said Dr. Anne Schuchat, the CDC's interim deputy director for science and public health program.
The Mexican samples will be tested at the centers based in Atlanta, Georgia, spokesman David Daigle told CNN by e-mail. The samples were taken from an affected area just north of Mexico City. Canada also is testing samples from Mexico "and has placed a travel alert for travel to Mexico," Daigle said.
The Public Health Agency of Canada issued a respiratory alert for Mexico on Wednesday, recommending that health providers "actively look for cases" in Canada, particularly in people who've returned from Mexico within the last two weeks.
An alert issued Friday by the International SOS medical and consulting company said more than 130 cases of a severe respiratory illness have been detected in south and central Mexico, some of which are due to influenza.
"Public health officials in Mexico began actively looking for cases of respiratory illness upon noticing that the seasonal peak of influenza extended into April, when cases usually decline in number," the medical alert said. "They found two outbreaks of illness -- one centered around Distrito Federal (Mexico City), involving about 120 cases with 13 deaths. The other is in San Luis Potosi, with 14 cases and four deaths."
Authorities also detected one death in Oaxaca, in the south, and two in Baja California Norte, near San Diego, California.
The majority of cases are occurring in adults between 25 and 44 years of age.
The CDC reported Tuesday that two California children in the San Diego area were infected with a virus called swine influenza A H1N1, whose combination of genes had not been seen before in flu viruses in humans or pigs.
The seven patients range from age 9 to 54, the CDC's Schuchat said. "The good news is that all seven of these patients have recovered," she said.
The first two cases were picked up through an influenza monitoring program, with stations in San Diego and El Paso, Texas. The program monitors strains and tries to detect new ones before they spread, the CDC said. Other cases emerged through routine and expanded surveillance. The human influenza vaccine's ability to protect against the new swine flu strain is unknown, and studies are ongoing, Schuchat said. There is no danger of contracting the virus from eating pork products, she said.
The new virus has genes from North American swine and avian influenza; human influenza; and swine influenza normally found in Asia and Europe, said Nancy Cox, chief of the CDC's Influenza Division.
Swine flu is caused by type A influenza, according to the CDC. The virus does not normally infect humans, but cases have occurred among people, especially those with exposure to pigs. There also have been cases of one person spreading swine flu to other people, the CDC said. In 1988, in an apparent swine flu infection among pigs in Wisconsin, there also was evidence of a patient transmitting the virus to health workers, the CDC said.
Experts think coughing, sneezing and contaminated surfaces spread the infection among people. From December 2005 to February 2009, 12 human cases of swine flu were documented. Symptoms include fever, lethargy, lack of appetite, coughing, runny nose, sore throat, nausea, vomiting and diarrhea, the CDC said.
The new strain of swine flu has resisted some antiviral drugs.
The CDC is working with health officials in California and Texas and expects to find more cases, Schuchat said.
There's no need for alarm, but people at risk -- those who live in or have visited areas where patients live or have had contact with pigs -- should get tested if they notice symptoms, said Dr. William Short at the division of infectious diseases at Thomas Jefferson University Hospital in Philadelphia, Pennsylvania.
A pandemic is defined as: a new virus to which everybody is susceptible; the ability to readily spread from person to person; and wide geographic spread, said Dr. Jay Steinberg, an infectious disease specialist at Emory University Hospital Midtown in Atlanta. The new strain of swine flu meets only one of the criteria: novelty. [Lynnaea asks: really? Seems able to spread readily between people to me, and it's attaining a wide geographic spread pretty darn fast...]
History indicates that flu pandemics tend to occur once every 20 years or so, so we're due for one, Steinberg said. However, it is not likely to be swine flu, he said.
"I can say with 100 percent confidence that a pandemic of a new flu strain will spread in humans," he said. "What I can't say is when it will occur."