An influenza isolate submitted to CDC from Nevada for reference analysis has been identified as a swine influenza-like virus, antigenically similar to A/New Jersey/76.* This continues a pattern of occasional human infections with swine influenza viruses currently circulating in pigs.

The virus was isolated from a 4-year-old girl who was hospitalized in Las Vegas on February 6 with a temperature of 104 F (40 C), cough, left lower lobe pneumonia, and pancytopenia. The child had been diagnosed in December 1981 as having acute lymphocytic leukemia, for which chemotherapy had been administered continuously since that time. When she visited Los Angeles for intravenous chemotherapy in an outpatient department on January 29, 1982, her leukemia was considered to be in remission, and she had a clear chest on physical examination and no symptoms of respiratory infection. Between January 30, the date of her return to Las Vegas, and February 6, the child visited only 2 homes, occupied by close family members and their friends, in residential subdivisions in Las Vegas. She was in contact with 14 persons including 9 children or young adults. Thirteen of the contacts were interviewed recently, and none reported either respiratory illness at the time of the child's influenza illness or reported contact with pigs. None of these contacts had serum hemagglutination-inhibition titers of 10 to swine influenza-like viruses that could not be explained by the individual's age or probable receipt of swine influenza vaccine during military service in 1976-1977. There were no community morbidity indices indicating influenza outbreaks in Las Vegas from January through April, but 2 influenza A(H1N1) (closely related to A/England/333/80) and 18 influenza B viruses were isolated in 2 hospitals performing virus diagnosis.

After the child was hospitalized on February 6, her pneumonia progressed despite therapy with broad-spectrum antibiotics. On February 11 she was transferred to a Los Angeles hospital where she died on February 14. Laboratory tests in Nevada and Los Angeles were negative for any infectious agent except influenza virus. Interviews at the hospitals in Las Vegas and Los Angeles have not identified any unusual respiratory illnesses among patients or staff in contact with the child. Antibody prevalence to swine influenza-like virus in young children in Las Vegas and Los Angeles, as well as contacts of the patients in the Los Angeles hospital, will be studied. No other human isolations of swine influenza-like viruses have been reported to CDC this winter. Reported by O Ravenholt, MD, Clark County Health District, J McCusker, MD, RR Belliveau, MD, Southern Nevada Memorial Hospital, P Reichelderfer, PhD, Sunrise Hospital, BA Neyland, MD, Las Vegas, JH Carr, MD, State Epidemiologist, Nevada State Dept of Human Resources; J Cherry, MD, UCLA, B Agee, MD, Los Angeles County Health Dept, R Roberto, MD, J Chin, MD, State Epidemiologist, California Dept of Health Svcs; Field Svcs Div, Epidemiology Program Office, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: This incident is consistent with the infection in 1974 by swine influenza-like virus of an immunocompromised child, who subsequently died (2). Sporadic isolations of swine influenza-like virus from children or young adults were reported in 1976-1977 (3) and also in 1978-1979 and 1979-1980 (4). The sporadic isolations after 1974 were not associated with unusual clinical symptoms, and the source of these infections, as well as the case in 1974, could be attributed to exposure to swine. Serologic evidence of swine influenza infection involving some members of a farming family exposed to pigs has also been reported (5). The single proven outbreak of swine influenza among humans in 1976, at Fort Dix, New Jersey, is believed to have involved person-to-person transmission over a few weeks in the atypical environment of the training camp (6). Low-level person-to-person transmission may also have been responsible for 3 reported serologically diagnosed cases of swine influenza in 1975-1977, when, as in the case of the Las Vegas child, no exposure to swine could be identified (3,7).

The lack of other swine influenza-like virus isolations from humans in Nevada or California this winter, despite the isolation of numerous influenza B and human strains of influenza A(H1N1) viruses in these states this winter, and the lack of respiratory illness among any of the child's family contacts (many of them apparently susceptible as judged by their antibody status and age) support the conclusion that swine influenza-like viruses that currently circulate in pigs have not demonstrated the propensity to cause human epidemics. Nevertheless, transmissions from swine to human, and subsequently from person to person at generally low levels will probably continue to be detected occasionally by virus surveillance activities.

References

1. CDC. Influenza nomenclature. MMWR 1980;29:514-5.

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

3. CDC. Influenza Surveillance Report No. 92, January 1981, pp 36-7.

4. CDC. Influenza Surveillance Report No. 93, in press.

5. O'Brien RJ, Noble GR, Easterday BC, Kendal AP, Shasby DM, Nelson DB, Hattwick MAW, Dowdle WR. Swine-like influenza virus infection in a Wisconsin farm family. J Infect Dis 1977;136 (Suppl.):S390-6.

6. Top FH Jr, Russell PK. Swine Influenza A at Fort Dix, New Jersey (January- February 1976). IV. Summary and speculation. J Infect Dis 1977;136 (Suppl.):S376-80.

7. Thompson RL, Sande MA, Wenzel RP, Hoke CH, Gwaltney JM. Swine-influenza infection in civilians. N Engl J Med 1976;295:714-5. *Redesignated as H1N1 subtype following a revision of influenza nomenclature (1).

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