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Swine Flu: Birds, Pigs, and People

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The rapid emergence of a new strain of influenza (Swine flu) in Mexico, the U.S., Canada, and other countries is quickly reminding the world that viruses and other microbes are not limited by differences in species or political boundaries.


While we have been watching for several years to see if the H5N1 strain of avian influenza (“bird flu”) in Asia and Africa changes to transmit easily among humans, a new variant in the western hemisphere seems to have already made that transition. This new strain, Swine Flu, appears to have been formed from components of four viruses from three different animal species: a bird virus, two pig viruses, and also a human influenza virus.


Swine flu is a respiratory disease of pigs caused by Type A Influenza. While swine flu viruses do not normally infect humans, infections have been known to occur, and cases of human-to-human spread of swine flu viruses have been documented. Diseases that can be spread from animals to people are referred to as zoonoses


What is a virus?
Viruses are a simple life form basically composed of a genetic core and an outer covering. Unlike some other viruses, the influenza viruses replicate without the internal mechanism that reduces mutation (cancer cells in our own bodies also lack that mutation-reducing mechanism). These viruses then all have a chance to mutate and thereby form new strains, though most of the new strains do not survive.


Influenza viruses are also notable in that they are “segmented,” and these eight genetic segments are in fact interchangeable, which makes the virus very adaptable. For the flu, one set of these gene segments affects how the flu virus attaches or sticks to the host cell it infects. Other segments affect the ability of the virus to replicate and to leave the infected cell in order to infect new cells. The avian influenza virus that originated in Asia and which has been killing both poultry and humans for the last few years has Type Five of the first type of segment and Type One of the second, hence the name “H5N1” for the avian flu.


In our lifetimes, we are exposed to one or another of these influenza virus segments as they circulate around the world, and as a result people develop at least partial immunity to those segments. Unfortunately, few people alive today have any immunity to the H5N1 avian influenza because in their lifetimes they have not been exposed to either the H5 or N1 segments, which is what makes that strain dangerous.




Humans have some resistance to new flu
The new human flu cases have been laboratory-confirmed as something else, H1N1. It is moving very quickly around the world with people traveling from country to country. Fortunately, many people have been exposed to the H1 segment either naturally or because it has been included in human flu vaccines for a number of years, and may provide some level of immunity to this new H1N1 strain. This has not been determined yet in the particular case at hand, but has been found to be true with other strains in people and in animals.


Notably, however, while H1N1 influenza has been circulating in pigs in the US, without causing problems for humans, the version infecting people right now has a unique combination of some human flu genes, some bird flu genes, and genes from swine flu found in both North America and Asia. This mixing occurs when avian or mammal (e.g., pigs, humans, horses and dogs) hosts are infected with several strains of virus at the same time, and those interchangeable virus segments do what they normally do: mix, and match up with new strains. Right now, the virus infecting people outside of Mexico does not appear to be very dangerous, but we do not yet know why infected people in Mexico died. Because influenza viruses continually change, it is not possible to predict how the virus will behave over the coming weeks or months.

What can we do?
For ourselves, annual flu vaccines—which are produced based on the best scientific assessment of which strains of virus will most likely be circulating in the near future—are a good idea. Some years these forecasts are better than others, but in general they typically hit the mark.


Personal hygiene is another good idea. Hand washing with soap or using a rub-on hand disinfectant such as the alcohol-based products and being careful about rubbing the eyes, nose and mouth can greatly reduce exposure. Flu viruses can survive on objects and surfaces briefly, but they are easily killed with common disinfectants or hot water. The flu virus dies within a second of being exposed to temperatures as low as 162 degrees Fahrenheit (water boils at 212F), so cooking food instantly kills the virus. For people that work with live animals that can be infected with the flu, such as pigs, poultry, horses and dogs, good personal hygiene is critical. Finally, cooking food eliminates all risk and it’s important to know that you will not get swine flu from eating pork products.




*One World, One Health?
The situation developing first in North America and spreading throughout the world is another example of the fact that infectious organisms have no respect for boundaries among species or countries. Roughly two-thirds of all known infectious diseases (more than 1400) are zoonotic diseases, those shared between people and animals, so it makes little sense to tackle the challenges of human, domestic animal and wildlife health separately based on the old “silo model” of academic disciplines.

Rapid transportation coupled with the global movement of people and goods enhances the transmission of infectious diseases quickly around the world. The number of international travelers is now approaching one billion per year and at any given moment there are approximately 35,000 ships at sea moving goods between countries. The idea that infectious diseases are restricted to a single country is naïve, since from the microbe’s point of view it’s just one small world. To address diseases effectively, therefore, we have to accept the fact that all species around the world are in this together, and that the health of people, animals, and our planet are inseparable.


Written and reviewed by William B. Karesh, DVM, Vice President, Global Health Program, Wildlife Conservation Society, President, OIE Working Group on Wildlife Disease, Cheif of Party, Global Avian  Influenza Network for Surveillance of Wild Birds, Co-chair, IUCN Species Survival Commission.


For more information on Swine Influenza (Flu), visit the Centers for Diseace Control and Prevention 


*One World, One Health is a registered trademark of the Wildlife Conservation Society

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