Avian Influenza (‘Bird Flu’)

Avian Influenza (‘Bird Flu’) and Pigeons
Gordon A Chalmers, DVM
Lethbridge, Alberta, Canada
Email: gacdvm@telus.net

The current worldwide concern about the spread of Avian Influenza among humans and poultry in southeast Asia also has many pigeon fanciers interested in the role of pigeons, especially racing pigeons, in this serious disease. On the basis of these concerns, it seems appropriate and timely to take a look at this disease, for the benefit of fanciers.

Influenza has been known since 1878, and is caused by a Type A influenza virus. There are three types of influenza viruses, namely A, B & C.
Type A viruses have been recovered from humans, swine and horses, and occasionally, from birds and other mammals. Types B and C are usually isolated only from humans.
The virus currently infecting in birds in southeast Asia is a Type A influenza virus.

Thousands of influenza viruses belonging to many subtypes have been recovered from domestic and avian species over the world. Infections in domestic or confined birds have been associated with several forms of the disease, ranging from inapparent to mild upper respiratory disease, to loss of egg production, through to acute fatal disease. At present, Avian Influenza is recognized in two forms: 1) Highly pathogenic avian influenza, also called Fowl Plague, and 2) Low pathogenic avian influenza. (‘Pathogenic’ refers to the ability of an infecting agent to produce disease – hence, a virus that is highly pathogenic is capable of producing severe disease and often, acute death.) The highly pathogenic form spreads rapidly among flocks of poultry and is often highly fatal. It has been noted that one gram (about 1/30th of an ounce) of contaminated droppings from infected chickens can contain enough highly pathogenic virus to infect 1,000,000 birds. Conversely, the low pathogenicity form generally causes only mild disease.

In general, free-flying wild birds do not develop significant disease following infection with influenza viruses, but infections are widespread in many of these birds. It is particularly important to note that influenza viruses are readily recovered from migratory waterfowl, especially ducks, over the world. There is a great deal of speculation about the importance of this very large reservoir of influenza viruses in wild birds. This reservoir may serve as a source of viruses for other species, including
humans, lower mammals, and birds. The high rate of infection allows for the maintenance and emergence of new and potentially highly dangerous strains, by means of mutation and/or genetic reassortment.
Species of birds that become infected with the virus of Avian Influenza, shed it from the respiratory tract, from the eyes, and in droppings. Spread of the virus can occur by means of droplets of liquid sneezed by infected birds, or in their droppings. Vehicles, equipment, cages, clothing, and insects in contact with these infective droplets, or with droppings from infected birds, are ready means of spread.

Now, what is the role of pigeons in Avian Influenza? To answer that question more fully, we can look at some past examples of outbreaks of the disease in poultry in North America, plus the current one in Asia, and the tests that were conducted on pigeons collected from areas in which the disease occurred.
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In late 1992, antibodies to Avian Influenza were found in blood samples from a commercial flock of turkeys in the northeastern USA. (Note that antibodies are protective substances that are produced by the defensive network of the body in response to an infection). Investigation showed that there was a possible association between this flock and live bird markets. An influenza virus designated H5N2 was isolated from birds in one location. State jurisdictions were concerned by this situation and took both control and eradication measures when appropriate. Public poultry markets, shows and exhibitions were quarantined and premises on which the virus were isolated and depopulated.

Because of the very great concern by state and federal officials for the health and economics of the poultry industry in several jurisdictions in the USA, racing pigeons were included under the umbrella of domestic poultry, and accordingly, racing was banned in a number of concerned states. To date, however, the body of scientific evidence indicates strongly that pigeons are not involved in the transmission of Avian Influenza to domestic poultry.
During an outbreak of Avian Influenza (H5N2) nine years earlier (1983-84), again in the northeastern USA, scientists conducted a survey of wildlife to determine the potential of wild birds to spread disease locally among farms, or to carry the virus to more distant locations. Included in this survey were 1) wild and free-flying domestic ducks and geese, 2) wild or free-flying domestic birds closely associated with poultry farms, poultry manure, or poultry carcasses, 3) mice and rats found inside and around houses containing infected poultry, and 4) wild birds of any species reported sick or dead within the quarantine zone.

Tracheal (windpipe) and vent swabs from birds, and lungs from mice and rats were examined for virus. As well, in some instances, toes from birds and rodents were also collected for the same purpose. When feasible, blood samples were also obtained from birds and small rodents. Attempts to isolate virus were conducted on 4,132 birds and rodents collected within the quarantine zone. Included in this number were 473 pigeons (92.6% of these pigeons were obtained from known infected farms), 81 pigeon feet (all of them from influenza-affected premises), and seven mourning doves. None of the 4,132 samples was positive for influenza virus. Blood samples from 2,147 non-aquatic birds, including 383 pigeons, were negative for antibodies to Avian Influenza – an indication that infection by this virus had not occurred in these birds. An additional 313 birds, including 50 pigeons, collected from the quarantine zone, were also negative for influenza virus. It is important to note that experimental attempts to infect pigeons with this strain of Avian Influenza did not result in either multiplication of the virus in these pigeons, or evidence of antibodies in the blood. The results of all of these studies indicated that pigeons were not infected with Avian Influenza and did not spread it.

In the 1993 outbreak in the USA, in the period from February to May, blood samples were collected from 17 flocks of meat varieties of pigeons, mainly White Kings located within the quarantine area, for evidence of antibodies to Avian Influenza. Flock sizes varied from 2000 - 3000 birds, and represented a total of about 34,000 - 51,000 birds. Approximately 10 birds per flock were sampled, for a total of 160 birds. In every instance, all pigeons tested were negative for antibodies to Avian Influenza.

Another study published in 1996 on the susceptibility of pigeons to Avian Influenza, found that groups of pigeons inoculated with two strains of highly pathogenic influenza virus or two strains of nonpathogenic virus remained healthy during the 21-day trial period, did not shed virus, and did not develop antibodies to this disease – further evidence that pigeons are not a factor in the spread of this disease,

More recent evidence from experimental work in 2001/02 has shown that pigeons infected experimentally with the highly pathogenic form of the virus (designated H5N1, and of Hong Kong origin) did not develop signs of this disease and did not have detectable changes to this disease in their tissues. As well, virus was not found in their tissues and neither was it re-isolated from swabs of tissues. These findings indicated once again that pigeons (along with starlings, rats and rabbits used in these studies), are largely resistant to infection with this virus.

Despite these reassuring findings, fanciers should be aware of the very slight possibility that if a returning race bird, or any wild bird for that matter, drops into a poultry farm on which the chickens are infected with Avian Influenza, it could pick up the influenza virus on its feet as it walks through droppings from these infected chickens. If this bird were to fly to another poultry farm, in theory it could be a mechanical means of spreading the virus on its feet to chickens on the second farm. The importance of this fact is that Type A influenza viruses can remain viable for long periods at moderate environmental temperatures, for four days in water at 22oC (72oF), and for over 30 days at 0oC (32oF). However, as noted in earlier studies, the feet of pigeons collected from affected poultry farms were examined for influenza virus and all were found to be negative.

Given this information from the scientific literature, it is important to note the non-role of pigeons in the spread of Avian Influenza, and the fact that pigeons themselves are not infected by this virus.The reasons for the understandable caution and concern by regulatory agencies when they are faced with outbreaks of Avian Influenza include the fact that it can be a very costly disease. For example, the US government spent over $60 million in 1983-84 to eradicate a highly pathogenic H5N2 virus in poultry flocks (both chickens and turkeys).

I hope that this brief look at Avian Influenza and the non-role of pigeons in the spread of this disease to poultry will be of some assurance to concerned fanciers. Further information on this disease can be obtained from federal or provincial/state governmental agencies.