An influenza virus isolated from a 32-year-old Wisconsin woman who had been exposed to pigs has been identified as a swine influenza virus. On September 13, 1988, when the woman was in her 36th week of pregnancy, she was admitted to a community hospital with a 1-week history of fever to 102 F (38.9 C), headache, myalgias, 3 days of nonproductive cough, and 1 day of progressive shortness of breath. The patient had radiologic evidence of pneumonitis with bilateral lower lobe consolidation and was treated with broad-spectrum antibiotics. On September 14, she was transferred to a tertiary-care hospital for ventilatory assistance and management of late-stage pregnancy. Specimens for bacterial, viral, and fungal cultures and serologic testing were obtained, and antibiotic therapy was continued. Because of mechanical difficulty in ventilating the patient, labor was induced, and on September 17 she gave birth to a healthy 2990-g infant. Despite intensive support, her respiratory failure persisted, and she died September 21.

The patient's clinical course and findings of postmortem examinations were consistent with primary viral pneumonia. All cultures for bacteria and fungi were negative. Influenza virus was isolated postmortem from a sputum specimen obtained on September 14 and was identified by monoclonal antibody as type A virus. When further tested by hemagglutination inhibition (HI) tests at CDC, the virus was unreactive with antisera to strains currently prevalent in the world but reacted with antisera to influenza A(H1N1) viruses related to swine influenza. A paired serum specimen showed a fourfold or greater rise in HI antibody against swine influenza virus. Results of preliminary HI tests performed at the University of Wisconsin with monoclonal antibodies to the hemagglutinin of swine influenza virus indicate that the isolate resembles viruses isolated during the summer from pigs in Wisconsin. Further laboratory studies of the recent Wisconsin isolate are being undertaken to confirm its similarity to enzootic strains.

On September 3, 4 days before the onset of illness, the woman had attended a county fair and had visited the display area of a pig barn. Veterinarians working at the fair stated that influenza-like illness had occurred among pigs at the fair.

Preliminary investigations have detected no outbreaks of influenza-like illness in the surrounding communities. Further investigation is under way to determine whether the swine influenza virus may have infected others with known exposure to pigs at the fair or from the ill woman to persons with whom she had close contact.

Reported by: J Kaufman, MD, M Hassan, MD, M Rytel, MD, R Chayer, MD, P Volkert, MD, P McKinney, MD, J Michaels, C Farmer, PhD, Milwaukee County Medical Complex; G Sedmak, Milwaukee Bureau of Laboratories, Milwaukee; BC Easterday, DVM, VS Hinshaw, PhD, Dept of Veterinary Medicine, Univ of Wisconsin, Madison; J Berg, D Hopfensperger, M Proctor, PhD, JP Davis, MD, State Epidemiologist, Wisconsin Dept of Health and Social Svcs. TF Smith, PhD, Mayo Clinic, Rochester, Minnesota. WHO Collaborating Center for Influenza, Influenza Br, and Epidemiology Office, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Influenza A viruses circulate naturally in many nonhuman hosts including swine, horses, and numerous avian species (1). Such viruses are theoretically reservoirs from which strains capable of infecting humans may evolve; however, the genetic composition of most animal influenza viruses appears to biologically restrict the range of hosts and prevent the viruses from crossing the species barrier. In some cases, however, this barrier appears to have been circumvented. For example, "Hong Kong flu" emerged in 1968 by probable genetic reassortment between the previously circulating human strains of "Asian flu" and an animal virus that donated the gene for the new virus hemagglutinin (2).

Unlike the 1968 event, the case in Wisconsin probably represents direct transmission of influenza virus from a pig to a human host. Virus isolation on other occasions has proven that viruses genetically similar to those found in swine in the United States have caused human infections. The first proven case was in 1974 and involved a child with Hodgkin's disease who had known contact with pigs (3). Since then, other laboratory-confirmed cases have occurred in persons who either resided on farms or visited locations where contact with pigs was possible (4). Infections also have occurred without demonstrated exposure to swine, including the 1976 cluster of cases in Fort Dix, New Jersey, which showed limited person-to-person transmission (5), and possibly a case in a child in Las Vegas (6). In these situations, initial undocumented transmission of influenza virus from swine to humans is likely to have occurred, and the infected person probably initiated limited chains of person-to-person transmission (possibly with subclinical infections), which eventually led to the laboratory- diagnosed cases. In addition to the recent Wisconsin case, other human infections with swine influenza viruses have been fatal (4). The fatalities have occurred in human hosts who were immunosuppressed or otherwise compromised by stressful occupational or medical conditions. However, continuous transmission of swine influenza virus in humans has not occurred.

References

1. Kilbourne ED. Influenza. New York: Plenum Medical Book, 1987.

2. Palese P, Kingsbury DW, eds. Genetics of influenza viruses. New York: Springer-Verlag/Wien, 1983.

3. Smith TF, Burgert EO Jr, Dowdle WR, Noble GR, Campbell RJ, Van Scoy RE. Isolation of swine influenza virus from autopsy lung tissue of man. N Engl J Med 1976;294:708-10.

4. Dowdle WR, Hattwick MAW. Swine influenza virus infections in humans. J Infect Dis 1977;136(suppl);S386-9.

5. Hodder RA, Gaydos JC, Allen RG, Top FH Jr, Nowosiwsky T, Russell PK. Swine Influenza A at Fort Dix, New Jersey (January-February 1976). III. Extent and spread and duration of the outbreak. J Infect Dis 1977;136(suppl):S369-75.

6. Patriarca PA, Kendal AP, Zakowski PC, et al. Lack of significant person-to-person spread of swine influenza-like virus following fatal infection in an immunocompromised child. Am J Epidemiol 1984;119:152-8.

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