Of the various types of acute respiratory diseases, perhaps none is more puzzling and variable than influenza. In mild forms, this disease may be difficult to distinguish from the common cold. Yet influenza may also be a major contributing cause of death during epidemics, as in the case of the current epidemic – now threateningly coming close to becoming a pandemic (initially called, but now appears to have been misidentified as, swine influenza) – that started in Mexico and is believed to have so far killed 149 people in that country.
To date, the 1918-1919 pandemic is still considered the most disastrous influenza outbreak, with more than 20 million people succumbing to it. Two other influenza pandemics occurred in 1957-1958 and in 1959-1960. Both these influenza pandemics were less severe than the 1918-1919 pandemic; still, they caused large numbers of deaths. Also, these last two influenza pandemics were caused by the influenza A, or what is known as the Asian strain virus.
A virus belonging to this same group A influenza virus is believed to be the cause of swine influenza. A nonpathogenic bacterium, called Haemophilus suis, enhances the severity of this acute communicable disease of the respiratory tract. The disease is noted for its two basic characteristics: rapid spread through a herd of swine (hence, the name given to this type of influenza) and sudden onset in individuals. Swine influenza is rarely fatal; secondary infections or complications are the usual causes of death in actual cases.
Animals infected with swine influenza are weak and depressed. The virus probably finds its way into the body by means of the food and water infected with discharges from the respiratory tract of diseased animals. Larvae of a parasitic nematode, specifically a lungworm, harbor the virus and may act as vectors.
Swine influenza first appeared at the time of the 1918 human influenza pandemic, during which millions of swine were affected. Apparently at the time, swine influenza occurred as a new disease. Since the late 1918, the disease in swine has taken place annually in the fall and early winter in the United States. There were indications that the virus might have been the main cause of the 1918 human influenza pandemic. Apart from swine-to-swine transmission, it is obvious that infection of humans by swine virus is possible.
Even in this time of modern medicine and technology, there are still no effective prophylactic measures against swine influenza. Its control depends on sanitation and good swine housing. Serum-neutralization and complement fixation tests may confirm clinical diagnosis. Besides its suspected reemergence in Mexico, swine influenza is of special interest to epidemiologists and microbiologists as it is one of the rare cases known in which the severity of disease is enhanced by the synergistic activity of two dissimilar microorganisms.
1. “The Influenza Pandemic of 1918” by Molly Billings, June, 1977 modified RDS February, 2005 – http://virus.stanford.edu/uda/
2. “Influenza Pandemics of the 20th Century” by Edwin D. Kilbourne, New York Medical College, Valhalla, New York, USA, on the Emerging Infectious Diseases Journal (online), National Center for Infectious Diseases, Centers for Disease Control and Prevention – http://www.cdc.gov/Ncidod/EID/vol12no01/05-1254.htm
3. “Swine Influenza” – http://www.spc.int/rahs/Manual/Porcine/SIE.HTM
4. “Swine Influenza Virus (SI), Flu”, on ThePigSite Quick Disease Guide – http://www.thepigsite.com/diseaseinfo/118/swine-influenza-virus-si-flu