Influenza Type A / Swine Flu

Filed Under: Diseases, Pigs

Have you wondered why the government is so concerned about a few cases of the flu? Think there is just a lot of drama going on for ratings week? There may be more to the hype than you think. Human, avian and swine flu strains are all interrelated. The orthomyxoviruses, or influenza A viruses, of all three species can easily exchange genetic material. This genetic exchange could possibly result in a new variety of human virus, which has the potential for severe morbidity and mortality. In fact, the 1918 flu epidemic is believed to have resulted from the exchange of the genetic material of avian influenza and human influenza. The end result of the 1918 epidemic involved the death of 50 million people worldwide. The average life expectancy during this time period actually decreased by 12 years due to this one pandemic.

The 1918 pandemic killed more people than did WW1. The pandemic actually came in two waves: one mild outbreak in the late spring, then a more virulent and deadly strain following in the fall. It is now believed that the virulent strain of virus from 1918 was actually a mix of avian and human influenza A H1N1. Could it happen again? Unfortunately history has a habit of repeating itself. Current public health officials fear a genetic recombination of swine, human, and avian influenza could theoretically lead to a worldwide epidemic, possibly killing more people than that of 1918.

There is no record of swine influenza before the 1918 pandemic. It is currently believed that swine flu is a result of human viruses being transmitted to susceptible pigs and not the other way around. There are currently four types of influenza infecting pigs: H1N1, H1N2, H3N2, and the newly-characterized H2N3. The newest swine influenza strain is composed of both avian and swine influenza genes. Health officials fear that pigs could potentially be the ideal “mixing vessel” for genetic exchange and the development of a new hybrid virulent human influenza.

In 2006, two separate groups of pig production facilities were found to be infected with the H2N3 influenza virus by researchers from Iowa State University, the University of Minnesota, and St. Jude Children’s Hospital. These two production facilities both used water obtained from ponds that were visited by migrating waterfowl. The newest swine flu was found to have a similar molecular composition to the H2N3 strain that was infecting mallard ducks. Exposure to the avian flu was enough to create a whole new flu strain infecting pigs. Even mice and ferrets can be experimentally infected with the new H2N3 virus. Could avian and swine flu genes mix with human influenza genes to create a flu that the entire world’s population would be susceptible to? It could easily happen with the virus’s innate ability to change and redefine itself.

The typical incubation period for the influenza virus is from 1 to 7 days, with a shorter 1 to 3 day incubation period being more common. Viral shedding typically occurs for one week but sometimes, especially with infections in the very young, the shedding may occur for a longer period of time. Viral shedding typically peaks at 24 hours after the onset of clinical signs.

Coughing, sneezing, and contamination of common fomites such as doorknobs, countertops, etc., spread flu viruses. Rarely will the virus jump species from exposure to birds and pigs to people. The jump between species can occur, however; especially in intense production situations where humans constantly interact with their animals. Once again, it is the virus’s innate ability to change itself with the incorporation of additional genes that could make the entire human population susceptible to infection. Herein lies the concern of health organizations around the world.

Clinical signs of infection most commonly include fever, aching muscles, anorexia (no appetite), sore throat, difficulty breathing (dyspnea), coughing, and a runny nose, and in some cases it will include nausea, vomiting, and diarrhea. The development of an interstitial pneumonia is subsequently the most common cause of death by itself or in combination with secondary bacterial pneumonia.

To date two people have died as a consequence of the swine flu in the United States. In 1976 Pvt. David Lewis, stationed at Fort Dix, N.J., reported to his superior that he felt tired, achy, and run down. His commander felt at the time that it was not enough to warrant his exclusion from physical training. Twenty-four hours later Pvt. Lewis was dead and 500 of his fellow soldiers were sick with the flu. The second reported death in the U.S. was a pregnant Wisconsin woman who died 8 days following the onset of clinical signs reportedly due to pneumonia.

Due to the severity of the flu in 1976, President Gerald Ford ordered the manufacture and distribution of a swine flu vaccine. Health officials at the time were concerned about having a repeat of the 1918 epidemic. Nearly 25 million Americans were vaccinated, with a reported 25 cases of vaccinated individuals developing Guillain-Barre syndrome, which resulted in their deaths. Following these vaccine reactions, the swine flu vaccination program was discontinued. Was the prevention worth the risk? No one knows for sure but 1976 was probably not the year for the 1918 Pandemic to repeat itself. Could ‘09 be that year? Mexico reported 1,000 cases of the swine flu with over 80 deaths from March to April of ‘09. Young adults are the primary victims. Is history again poised to give us a repeat performance? We hope not. Currently Tamiflu® or oseltamivir and Relenza® or zanamivir are effective on this particular strain of the virus.

It is usually best to prevent infection. In the absence of an effective vaccine it is best to avoid exposure by avoiding those individuals suffering from clinical signs of respiratory infection or flu. Constantly wash hands and when possible and disinfect common sites of possible contamination such as doorknobs, water fountains, etc. Get plenty of rest and avoid stress whenever possible.

It is impossible to get swine flu from adequately cooked pork products. Transmission between people and pigs is possible. Those individuals at greatest risk are those individuals in agriculture who work with and process pigs.


Moellenberg, Dell Rae. “Avian Influenza Under the Microscope at CSU.” DVM News. November 2006. P. 10.

“New Swine Flu Subtype as Avian Flu Genes.” JAVMA. Vol. 232. No. 4 News. P. 497.

Radostits, Otto et al. Veterinary Medicine. 10th Ed. Saunders/Elsevier. 2007. Pp. 1331-1335.

Topics: swine flu

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