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What Is the Origin of Swine Flu?

The influenza virus is subject to mutations from year to year. Sometimes those mutations are major and worldwide epidemics occur as few people in the world are immune to the mutated strain. Surveillance of flu activity in the U.S. includes testing samples collected from cases of flu-like illness. Two samples collected close to the U.S.--Mexico border in April 2009 tested positive for an unknown strain of influenza that would come to be known as H1N1.

  1. Influenza Virus

    • The influenza virus is an RNA virus that causes infections of the upper respiratory system. It is transmitted by contaminated droplets. The viral RNA is subject to mutations as influenza reproduces inside the cells that line the respiratory system of the host. These mutations are what lead to influenza outbreaks each year. If the mutation is significant enough worldwide, epidemics, sometimes called pandemics, ensue. There is a pandemic every few decades.

    Influenza Surveillance

    • Public health agencies in the United States and around the world conduct epidemiological surveillance for influenza year-round. This surveillance consists of knowing how the virus is spreading in the population. For this purpose, the agencies use reports from physicians and laboratories. Physicians report how many people are seeing them for flu-like illness while laboratories report on the number of tests and results, including virus strains.

    H1N1 Influenza

    • In April 2009, two children visited their health care provider in California for flu-like illness. The physician collected nasal swab samples for influenza testing and submitted them to the California department of health. At the State Laboratory, virologists were unable to identify the strain of type A influenza from those children. The samples were forwarded to the Centers for Disease Control and Prevention in Atlanta, Georgia, for more specialized testing. On April 29, 2009, the CDC announced that the samples from those children had a strain of influenza that had not been seen before.

    Flu in Mexico

    • While the testing was underway in Atlanta, the Mexican Health Secretariat had instituted control measures against a heavier-than-normal flu season in Mexico. Thousands were cramming hospitals and clinics with flu-like symptoms. Samples from those patients were collected and delivered to a specialized laboratory in Canada for influenza testing. By the time of the CDC announcement, the flu epidemic in Mexico was in its fourth week.

    Uncertainty

    • Results from CDC testing determined that the H1N1 2009 strain had in it RNA from a swine flu strain, an avian flu strain and a human flu strain. Further analysis showed that the RNA was similar to the flu circulating in Europe and Asia. The initial focus of investigating the cases in the United States looked at exposure to swine. Once the tests of the Mexican samples were known, the focus shifted to travel to Mexico. However, because the first few cases in the United States occurred close to the U.S.--Mexico border, there is uncertainty in the public health community as to whether H1N1 2009 originated in the U.S. and then was taken into Mexico while that country experienced an epidemic of seasonal flu, or whether the flu season in Mexico was heavy due to the H1N1 2009 virus only and the U.S. cases contracted it there.

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