Continuing growth in cases of the H1N1 Flu Outbreak Virus (just renamed by the U.S. Department of Agriculture) has raised many questions about the severity and control of the virus and the future implications for the swine industry, according to Iowa State University (ISU) veterinary diagnostician Greg Stevenson, who spoke at a World Pork Expo seminar sponsored by Fort Dodge Animal Health.

Interestingly, the proportion of swine respiratory cases in which influenza virus is detected at the ISU Veterinary Diagnostic Laboratory (VDL) has been declining since 2003 (see Figure 1). “That steady decrease has likely been due to changes in management as well as increased vaccine pressure,” Stevenson says.

Changing Flu Viruses

It's important to remember that H1N1 flu viruses are not new to swine, he points out. Historically, the original or “classic” swine flu was type H1N1. Only in recent years (1998) have both H3N2 and H1N2 viruses (called triple reassortants because they contain gene segments of avian, human and swine viruses) evolved from a mixing of that original classic swine influenza virus (SIV) with human and avian flu viruses.

Figure 2 depicts the proportion of cases of all three types of SIV identified at the Iowa State VDL since 1999.

Most times when reassortment of flu viruses occurs, the impact is minimal because the new virus does not survive and spread, he says.

“But once in a while one of the new reassortant viruses becomes established in a population, and that is what is happening with this new H1N1 Flu Outbreak Virus infecting people,” Stevenson explains.

The federal Centers for Disease Control and Prevention became concerned because of the apparent establishment of the H1N1 virus in the human population and its transmission among people, unusual events for H1N1 viruses.

To be clear, the H1N1 flu viruses that are currently most common in pigs are different in their makeup from the H1N1 strain recently discovered in humans — but they are also different from the classic H1N1 flu virus that exclusively circulated in swine prior to 1990, Stevenson explains.

“We do still find this classic strain in our swine population, but not very commonly. The viruses that we have today in our pigs are mostly triple reassortant H1N1 strains, comprised of genes from the classic swine H1N1 virus and also genes from human and avian viruses,” he states.

Key Questions

Stevenson provided answers to several key questions:

• Where did this novel H1N1 flu virus infecting humans come from?

  The virus is a quadruple reassortant and contains mostly genes from the triple reassortant H1N1 viruses found in swine in North America as well as two genes from flu viruses found only in swine in Europe and Asia. It was originally assumed that this novel virus must have originated from pigs in Mexico, but pig herds in close proximity to humans that were first infected have been negative, he asserts.

• Is the novel virus present in Iowa swine?

  Stevenson says ISU diagnostic lab virologist K-J Yoon, DVM, compared 125 H1N1 swine isolates to sequences obtained from the new H1N1 Flu Outbreak Virus. “Absolutely none of this flu outbreak virus was detected in our testing of samples,” Stevenson reports.

• Will the Iowa State standard diagnostic test for influenza viruses in swine detect the H1N1 Flu Outbreak Virus?

  The standard influenza tests will identify all flu viruses, but it will take further testing to identify the novel H1N1 virus; Iowa State has developed a rapid test with that capability, he says. It is expected that other state diagnostic labs will also have access to a rapid diagnostic test, which will only test positive for the H1N1 Flu Outbreak Virus and negative for all known North American swine influenza viruses.

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