Swine Flu -- Where Did It Come From, Where Is It Going?

The emergence of a new H1N1 influenza virus has triggered worldwide preparations for a pandemic. On April 29, 2009, the World Health Organization (WHO) announced that the problem deserved a phase 5 designation, which indicates sustained human-to-human transmission of a novel influenza strain of animal origin in two countries within one region, but with exportation to countries outside that region (see Figure 1). The first identified case, thus far, in the current pandemic occurred on March 18, 2009, in Mexico. Molecular analysis, however, suggests that the first cases may have occurred in mid-September 2008 when the most recent common ancestral virus is calculated to have emerged. Initial evidence suggested that the virus originated in swine, resulting in its being referred to as swine flu. Swine have generally been considered a frequent "mixing vessel" in which new strains of influenza virus emerge as the result of reassortment between avian and human or swine viruses (although there has been no evidence developed that the 1957 and 1968 pandemics caused by reassortments of human and avian viruses came from pigs). The resultant virus may infect humans, but usually only as the result of direct contact and with no secondary human cases. The occurrence in 1976, however, of several hundred cases of human infection and the death of Pvt. David Lewis at Fort Dix, New Jersey, led to the development and administration of a vaccine (A/NewJersey/1976/H1N1) to approximately 40 million people in the United States. Unfortunately, there were also approximately 500 cases of Guillain-Barré associated with its administration, and the program was aborted. That epidemic did not spread beyond Fort Dix. Evidence indicates that the current virus is a descendant of an ancestral triple reassortant swine virus, with elements from swine, avian, and human viruses that likely had been circulating in swine for perhaps 20 years, with limited ability to infect humans. A 17-year-old boy in Wisconsin who had butchered pigs was infected with this virus in December 2005, with no identified secondary cases. Since then, an additional 10 cases have been identified. Nine of the 11 patients had contact with pigs; all 11 survived. The triple assortant subsequently gained the ability to efficiently infect and, more importantly, spread among humans as the result of further recombination with a Eurasian swine virus, resulting in swine-origin influenza A (H1N1) virus (S-OIV). The resultant virus also has been called A/California/04/2009, since the first isolate identified was recovered from a child in San Diego. Of note, is that there is no evidence, as yet, that S-OIV is epidemic in pigs -- this is now a human, not a swine disease. In fact, the first known report of this virus in a pig population resulted from transmission from a worker who returned from vacation in Mexico to Alberta, Canada, and caused an outbreak resulting in symptomatic illness in approximately one-fourth of 2000 pigs. Thus, the only evidence of zoonotic transmission to date is from human to swine, not the other way around. Clinical symptoms of S-OIV infection are typical of influenza, except for a higher incidence of vomiting and diarrhea, each of which have occurred in approximately one-fourth of cases. The age distribution of cases is similar to that of seasonal influenza, with 60% occurring before 19 years of age. The U.S. deaths have occurred in patients with significant underlying illnesses. In California, 553 confirmed or probable cases had been identified by May 17, 2009 and 30 of these required hospitalization. 6 In contrast to other reports indicating lower sensitivity, rapid antigen tests for influenza A were positive 67% of the 24 hospitalized patients tested and in contrast to the Mexican experience, diarrhea was uncommon. Apparent pneumonia was present on chest Xray in 15 or 25 patients (60%) in whom this was performed; 10 of these had multilobar infiltrates.… [ABSTRACT FROM AUTHOR]