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Epidemiolology and control of swine influenza
1 commentsConsidering the current situation we are reproducing an article by Tom Alexander previously published by 3tres3.com on 30/dec/2005.
A key factor in the epidemiology of influenza is the ability of the virus to mutate or, when cells are infected by two different strains, to recombine to produce new viruses. Either of these genetic changes results in the repeated appearance of new strains with different immunogenic structures and/or virulence, including their ability to infect different hosts.
Swine influenza has been a common acute self-limiting disease of pigs in most pig producing countries for many years. It is well covered in the literature and needs little repetition here. The common influenza virus serotypes in pigs are H1N1, H1N2 and H3N2. Different strains within these serotypes vary in pathogenicity.
Recent alarmist press reports of a new strain of the serotype H5N1, which is highly pathogenic in poultry and has caused fatal illness in people (fortunately relatively few) who have been associated with infected poultry, has raised concern that it might become more infective to people and result in a human pandemic similar to that of 1918. It has also raised concern among some discerning pig farmers and about a possible role of pigs in its future spread and what effect this might have on pig farm staff and sales of pig products.
I use the term alarmist deliberately. Certainly pigs have played an important intermediate role in the spread of influenza viruses from birds to people and may do so with variants of the new avian strain but the situation today in Europe and the Americas is very different to that of 1918. Then, the standard of living was much lower, nobody knew what a virus was, vaccines were not as advanced as they are today and there were no antibiotics. Today, it is likely that human mortality would be considerably lower. Nevertheless, it seems appropriate for veterinarians to refresh their understanding of relevant aspects of the epidemiology of influenza viruses in swine.
Influenza virus in birds is spread in their droppings and not by aerosol as it is in mammals. Wildfowl, unlike domestic poultry and other wild birds, are usually subclinically infected. They can migrate long distances spreading such viruses, for example, down through the Americas from the arctic where the virus can over-winter in ice and still be viable in cold, recently thawed, water in the spring. In spite of the depression and generalised aches in people and presumably in pigs with clinical influenza, the virus tends to remain confined in the respiratory tract and is only spread by exhalation. Where pigs have access to bird droppings, particularly those of wildfowl or domesticated ducks which have been infected by wildfowl, they may be infected by new serotypes of virus, and people in contact with infected pigs may then also become infected by aerosols. It is not surprising that many of the human pandemics of influenza were thought to have originated in China which has enormous populations of people, pigs and poultry often living in close proximity. At the time of writing, this chain of events does not seem to have occurred with the new avian strain although it has spread from South East Asia, where it has been circulating for three years, to Europe where it has been isolated recently from birds in Russia, Kazakhstan, Romania, Turkey and England. Some of this spread may have been by migrating birds although there appear to be no reports of isolation of the virus from wild birds which would be likely to make them ill or kill them.
A key factor in the epidemiology of influenza is the ability of the virus to mutate or, when cells are infected by two different strains, to recombine to produce new viruses. Either of these genetic changes results in the repeated appearance of new strains with different immunogenic structures and/or virulence, including their ability to infect different hosts. These changes could occur in the new avian virus and may then cause disease in pigs and people, but if or when this might occur, or how serious it might be, is totally unpredictable. At least one new strain has been reported, namely a drug-resistant strain. Fortunately it is less virulent than the strain of origin.
Meanwhile, it is worth reconsidering simple measures for the control of influenza viruses in pigs and people in contact with pigs. Pigs can transmit some strains of influenza virus to people, and people can transmit some strains to pigs which should be borne in mind when outbreaks of human influenza occur. Pig farmers should be aware of this and act accordingly, including the possibility of vaccinating farm staff. Pig farms should be bird-proofed so that pigs gain no access to bird droppings, for example on unprotected footpaths, loading ramps, window ledges, pig feed or bedding. Water sources should also be checked. Unfortunately, none of these are applicable to outdoor herds in which biosecurity is problematical.
Article Comments
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27-Apr-2009
Carlos H. RomeroThe author is not correct when he writes that influenza virus in birds is spread in droppings and not in aerosols. Flu virus present in the oral cavity is such that we perform surveillance for influenza viruses not only testing cloacal swabs but also oral-pharyngeal swabs. This testing is currently performed using a highly sensitive real-time RT-PCR assay.
Carlos H. RomeroThe author is not correct when he writes that influenza virus in birds is spread in droppings and not in aerosols. Flu virus present in the oral cavity is such that we perform surveillance for influenza viruses not only testing cloacal swabs but also oral-pharyngeal swabs. This testing is currently performed using a highly sensitive real-time RT-PCR assay.
