Swine influenza virus infections in humans have been reported in the United States, Canada, Europe, and Asia. There are no unique clinical features that distinguish swine influenza in humans from typical influenza. Although a number of the case patients had predisposing immunocompromising conditions, healthy persons are also clearly at risk for illness and death from swine influenza. The high proportion of fatalities in this case series likely reflects a strong case ascertainment bias. The majority of case patients reported contact with pigs, consistent with seroepidemiological studies that have demonstrated increased rates of swine influenza virus infection in people with occupational swine exposure [36
Bias resulting from heightened awareness and increased surveillance soon after the 1976 Fort Dix episode may have resulted in a transient increase in identification of cases that would otherwise have been attributed to human influenza. Of the 6 North American cases after 1982, 3 were identified because the cases were fatal in healthy young subjects and because a cause was actively pursued. Two cases occurred in laboratory workers with known exposure to swine influenza, so the index of suspicion was high. Therefore, it is likely that nonfatal cases of swine influenza continue to occur but are not identified. Furthermore, fatal cases may be missed when they occur in persons with predisposing conditions in whom death due to influenza would not be unusual.
Persons who work with swine may play an important role in the mixing of influenza virus strains, leading to reassortment and development of novel progeny strains with pandemic potential. People with exposure to pigs may be among the first to be infected in the event of a novel virus becoming epizootic in swine herds, and those who work with swine may serve as a bridge for transmission of the virus to their communities [39
]. A policy of vaccinating swine workers annually with human influenza vaccine would decrease the risk of reassortment events.
Sporadic cases of swine influenza in humans, combined with seroepidemiological studies demonstrating increased risk of swine influenza in occupationally exposed workers, highlight the crucial role that this group may play in the development of new strains of influenza virus. Persons who work with swine should be considered for sentinel influenza surveillance and may be an important group to include in pandemic planning.