Pigeons and avian influenza (bird flu)
Avian influenza (bird flu) in feral pigeons - what are the risks?
Also commonly known as:
Bird flu, fowl plague, bird influenza, Asian bird flu, HPAI, LPAI, H5N1, H7N2, H7, H5N2
Influenza has been known about since 1878 and is caused by a type ‘A’ influenza virus. It has historically been known as ‘fowl plague’. There are three types of influenza virus: type A, type B and type C. Most forms of influenza are solely associated with humans, but the type A influenza virus has been found in pigs, horses and occasionally in birds and other mammals. Types B and C are human-specific and are not found in animals, mammals or birds. The type associated with recent outbreaks of avian influenza (bird flu) in south-east Asia is the type A influenza virus.
Thousands of influenza viruses, belonging to many sub-types, have been found in both domesticated and wild birds all over the world. Currently, avian influenza is recognised in two forms:
- Highly pathogenic avian influenza (HPAI)
- Low pathogenic avian influenza (LPAI)
The highly pathogenic form ('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) is the most virulent form of the disease and can spread rapidly, particularly when found in intensively farmed domestic poultry. The mortality rate for birds infected with the highly pathogenic form can be up to 100% and the disease can develop so fast that in some cases birds will die without ever having showed any signs that they had contracted the disease. The highly pathogenic form is so virulent that one gram of infected chicken excrement can contain enough highly pathogenic virus to infect 100,000 birds. Conversely, the low pathogenic form results in a milder, less significant form of the disease with infected birds rarely becoming ill or demonstrating symptoms, but they still have the potential to pass the disease on to other birds or animals. Certain low pathogenic forms can, however, mutate into highly pathogenic strains.
Avian influenza (bird flu) is a notifiable disease in the UK and is listed in section 88 of the Animal Health Act 1981. Section 15 (1) of the Act says:
“Any person having in their possession or under their charge an animal affected or suspected of having one of these diseases must, with all practicable speed, notify that fact to a police constable.”
Loosely translated this means that if you suspect or are aware of the presence of a notifiable disease there is a legal obligation to notify a DEFRA Divisional Veterinary Manager immediately. DEFRA is the Department of the Environment, Food and Rural Affairs and is responsible for overseeing animal health in the UK.
Avian influenza in birds is spread via secretions from the eyes, respiratory tract and from faecal matter, with droplets of liquid sneezed by infected birds spreading the disease extremely rapidly in environments where large numbers of birds are housed. In wild birds the situation is different with many migratory birds (including waterfowl, sea birds and shore birds) carrying the virus for long distances and being implicated in the international spread of the disease. Migratory waterfowl - most notably wild ducks - are the natural reservoir of bird flu viruses and these birds are also the most resistant to infection. They can carry the virus over great distances and excrete it in their droppings, yet develop only mild and short-lived illness themselves. There is a great deal of speculation about the importance of this very large reservoir of influenza viruses in wild birds as it is 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.
Some strains of avian influenza can be transmitted to humans and other animals but this is normally only the case following high levels of exposure to infected birds and/or their faecal matter. People most at risk would be those involved in intensive farming, in particular the poultry industry. These strains will normally only cause mild symptoms in humans but a current south-east Asian strain has caused a number of deaths. Although humans can be infected from birds the current highly pathogenic H5N1 strain does not readily infect people and there is very little chance, if any, for human-to-human spread of the disease.
The main significance for human health is that birds could be the source of new strains of influenza virus. Existing bird strains could mutate to form a new strain, which could, in turn, readily infect humans. Likewise, if mammals are infected with both human and avian strains of the disease at the same time the mixing of genetic material from the two viruses might produce new strains. These strains would have the potential to spread readily between humans. If a new strain of avian flu was to mutate, humans would have little or no immunity to it and a serious worldwide epidemic could occur.
Although avian influenza has hit the headlines on numerous occasions over the last few years the disease is yet to have a major impact in the UK, with only minor outbreaks which have been confined to poultry production units. The most recent outbreaks of avian influenza have been the low pathogenic strain with an outbreak of H7N2 in Conwy, North Wales in May 2007 and more recently an outbreak of the H7 strain in St Helens, Merseyside in June 2007. In both cases restrictions that were imposed on the sites concerned were removed promptly, in the case of Conwy within 5/6 weeks of investigation and in the case of St Helens within a day of investigation. The most recent outbreak of highly pathogenic avian flu was the strain H5N1 identified on a poultry unit in Holton, Suffolk in February 2007. Amongst restrictions imposed on the facility were a 3 km Protection Zone and a 10 km Surveillance Zone along with a much wider Restriction Zone. All restrictions were removed from the farm just over one month later.
Although avian influenza has been restricted, in the main, to the mass production of poultry, the feral pigeon has inevitably been identified as one species that has the potential to carry and pass the disease onto humans based on the birds’ close association with man. The main focus of attention has been the racing pigeon industry due to the fact that it involves the transportation of pigeons across international borders. Clearly, if pigeons are released in EU countries where avian influenza is active, when they return to lofts in the UK there is clear potential for those birds to carry and transmit the disease to other domesticated birds, wild birds and animals and indeed humans. As it is migratory birds that are considered to be the main carriers of avian influenza, racing pigeons must also fall into this category as they are frequently required to cross international borders.
Much research has been carried out to better understand the threat posed by the pigeon (feral and domesticated) and research is ongoing in many parts of the world, particularly in those countries worst affected. A thorough scientific research programme was undertaken following the outbreak of the highly pathogenic strain H5N2 in the north-eastern United States (in 1983/4) to assess the potential for wild birds to spread disease amongst local farms. The following species were included in this survey:
- Wild and free-flying domestic ducks and geese
- Wild or free-flying domestic birds (including pigeons)
- Dead or sick birds within the quarantine area
Attempts to isolate the virus were conducted on a sample of 4,132 birds, of which 473 were pigeons, and of this number 92.6% were collected from infected farms. A further 81 feet, taken from dead pigeons, were also assessed for the purposes of the research - this is because pigeons commonly feed on agricultural sites and by walking in infected faecal matter the birds could potentially pass on the disease. In order to assess the sample, tracheal (throat) and vent (anus) swabs were taken from each bird. None of the 4,132 birds collected tested positive for the H5N2 strain. Blood samples taken from 383 pigeons were also negative for antibodies (antibodies are protective substances that are produced by the defensive network of the body in response to an infection) to avian influenza, an indication that infection by this virus had not occurred in these birds. An additional 50 pigeons, collected from within the quarantine zone, were also negative for the influenza virus. Experimental attempts made to infect pigeons with the highly pathogenic H5N2 strain of avian influenza did not result in either multiplication of the virus or any 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 another outbreak of avian influenza in the USA in 1993 (in the period February to May) blood samples were collected from 17 flocks of pigeons located within the quarantine area for evidence of antibodies to avian influenza. Flock sizes varied from 2000 - 3000 birds and represented a total of between 34,000 and 51,000 birds. Approximately 10 birds per flock were sampled (a total of 160 birds) and in every instance all pigeons tested were negative for antibodies to this 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 non-pathogenic virus, remained healthy during the 21-day trial period. The sample did not shed virus and did not develop antibodies to this disease - further evidence that pigeons are not a factor in the spread of avian influenza. More recent scientific evidence, from experimental work in 2001/2002, has shown that pigeons infected with the highly pathogenic form of the virus (designated H5N1 of Hong Kong origin) did not develop signs of this disease and did not have detectable changes to the disease in their tissues. Neither was the virus found in their tissues and nor 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 highly pathogenic strain of the virus.
It is quite clear from all the information available that avian influenza continues to be a threat to both humans and birds, but the likelihood of its transmission to humans as a result of contact with the feral pigeon or its faeces is virtually nil. The feral pigeon is reputed to be the ultimate disease-carrier, harbouring the capability to spread a huge variety of diseases to both humans and other birds and animals, but in reality this is a myth. As can be seen from the findings of several research programmes, the feral pigeon is at the bottom of the list of those species that have the potential to spread avian influenza and it is likely that this is the case with most of the other diseases that are commonly associated with the pigeon.