Disease in birds - a masterclass
Several diseases circulate in wild bird populations. Like diseases that affect humans, many of these are mild. However, there can be implications of even mild avian disease for an individual’s survival and/or reproductive success, and more serious diseases can cause population decline or even extinction, as well as posing a risk to humans if they can cross the species barrier and infect us too.
There are many types of disease-causing processes, which can be categorised into non-infectious and infectious. Non-infectious diseases are those that cannot be transmitted between affected animals, such as most types of cancer, whereas infectious diseases – often caused by bacteria, viruses or parasites – are transmissible. The modes of disease transmission vary depending on the agent and the way the disease manifests itself. We can then further categorise infectious diseases by whether they are endemic (already in the population), or emerging (they have only recently been detected or are increasing in their geographic range, or the number of species or animals they are affecting).
Salmonellosis is one of the most common bacterial diseases diagnosed in garden birds, particularly during the winter months and in seed-eating birds such as House Sparrows and finches. Salmonella bacteria are transmitted by the ingestion of food that has been contaminated with infected bird droppings, which is known as the faeco-oral route.
Meanwhile, infection with Suttonella ornithocola, a recently discovered bacterium that is known to affect birds of the tit family, can result in a pneumonia-like respiratory condition. It is thought that the bacterium is spread by aerosol transmission between individual birds. Although the external signs of the disease are non-specific, they may include lethargy and fluffed-up plumage, and sometimes breathing difficulties.
Avian pox is one of the most noticeable, and therefore commonly reported, viral diseases of garden birds. It is thought to be transmitted between birds in three main ways: by biting insects (in this situation, known as a ‘vector’ for transmission), by direct contact between birds, and indirectly by contact with contaminated surfaces (such as perches and bird tables). Research on viral DNA from affected tits in Britain has revealed that the strain causing disease here is the same as that which had previously been documented in tit species elsewhere in Europe. This information, together with patterns in disease emergence and spread in Britain, indicates that the outbreak began in Southeast England, and suggests that the virus most likely arrived from mainland Europe.
Another viral disease with various possible transmission routes is Psittacine Beak and Feather Disease (PBFD). PBFD affects parrots around the word, including free-living Ring-necked Parakeets in Britain. The PBFD virus can be spread by direct or indirect contact with infectious particles (such as feather dander), faeco-orally, and vertically (from a female to her offspring). As the name suggests, the disease can lead to severe beak and feather abnormalities, but may also cause immunosuppression and lead to overwhelming secondary infections.
We lost an estimated 67% of the British Greenfinch population between 2007 and 2017.
Trichomonosis is probably the best known parasitic disease of wild British birds, which is spread via fresh saliva. This can occur when adult pigeons and doves feed their young on crop milk, when adult birds feed one another with regurgitated food during the breeding season, or through sharing food or drinking from water sources contaminated by an infected bird. However, there are multiple other types of parasites (such as tapeworms, mites, lice), each with their own lifecycles and transmission routes.
For example, Syngamus trachea, most commonly referred to as gapeworm, is a parasitic roundworm that affects some species of insectivorous garden birds including Blackbirds, Starlings and corvids. Its lifecycle involves an intermediate arthropod host (e.g. an earthworm, snail or slug) where the larvae develop, and infection occurs when the intermediate host is ingested by a bird.
Skin infection with Cnemidocoptic mites can lead to a condition in garden birds commonly known as ‘scaly foot’. These mites are found deep within the skin layers where they cause proliferative lesions that typically develop around the digits and foot, but sometimes extend up the entire leg. Chaffinches are most frequently affected, although other species have occasionally been observed with the condition. The lesions tend to develop slowly, over weeks to months, and affected birds usually remain bright and active. However, severe cases do occur, resulting in lameness and even digit loss, alongside an increased risk of predation or entanglement. The parasite has a wide distribution across Great Britain and can be transmitted between birds either through direct or indirect contact. In addition to infection with Cnemidocoptic mites, affected birds can also be found to be infected with Chaffinch papillomavirus, causing symptoms similar to ‘scaly foot’.
There is a growing need for continued surveillance to enable rapid detection of new and emerging disease threats to safeguard Britain’s wild bird populations
Effects of disease
Population declines and species extinctions have both been recorded as a result of disease, particularly on islands where endemic species are vulnerable to introduced pathogens. Closer to home, wild birds in Great Britain are also affected. One such example is the epidemic mortality and population decline that has been observed in British Greenfinches since 2006. Research has shown that as a result of an ongoing outbreak of the disease finch trichomonosis, caused by the protozoan parasite Trichomonas gallinae. We lost an estimated 67% of the British Greenfinch population between 2007 and 2017.
Animal welfare concerns
The welfare of animals affected by disease can also be compromised, sometimes substantially. Avian pox, for example, is a viral disease that has been reported in birds for decades, mostly affecting species such as Dunnock, Woodpigeon and Starling. More recently however, avian pox has started affecting birds belonging to the tit family, predominantly Great Tits. Unlike in other species, in which the skin growths that the virus can cause tend to be mild, Great Tits commonly develop large, tumour-like growths. Whilst affected birds can recover, the growths may interfere with the bird’s vision or ability to feed or fly properly,vand thus make them vulnerable to trauma or predation. So while investigations have revealed that avian pox is unlikely to be having a major impact on the Great Tit
population as a whole, the disease can compromise the welfare of affected birds.
Public, pet and livestock health
Although the risks to public health from wildlife disease in the UK are considered to be very low, some pathogens carried by wild birds can be transmitted, directly or indirectly, to people. One example of this is salmonellosis. In garden birds, the disease is caused by infection with specific strains of Salmonella Typhimurium bacteria, which, although research has shown them to be highly host-adapted, also have the ability to affect humans on rare occasions. It is important to ensure that sensible hygiene precautions, such as wearing gloves when handling feeders, bird baths or tables, and always washing hands thoroughly afterwards, are adopted as a routine.
There is also potential for disease transmission between wild and domesticated species. For example, domestic cats have been documented with gastroenteritis-like symptoms due to infection with Salmonella bacteria after the predation of a sick garden bird. Disease transmission between wild birds and livestock, such as poultry, can also occur.
Given what is known about the disease conditions affecting British birds, and their associated transmission routes, the risk of disease spread is likely to increase wherever birds congregate in large numbers. Although gardens are becoming an increasingly important
habitat for wild birds, garden feeding stations concentrate feeding opportunities within a relatively small area and encourage close contact between species that would otherwise not occur. This is likely to play an important role in disease transmission, particularly if hygiene levels are poor, and highlights the need for best-practice garden management in order to reduce the risks.
Disease prevention in your garden
- Offer a variety of food types from accredited sources.
- Feed in moderation, so that feeders are typically emptied every 1-2 days.
- Regularly rotate feeder sites to avoid accumulation of waste food or droppings.
- Ensure optimal hygiene at feeders and baths with frequent cleaning and disinfection.
- If you see a sick or dead garden bird, report it to the team of wildlife veterinarians at GWH via www.gardenwildlifehealth.org
On a global scale, disease is one of many threats facing wild birds, and the numerous examples cited in this article illustrate its potential to significantly impact their populations and welfare, as well as human and domestic animal health. In a changing world, with concurrent threats such as climate change and invasive species, there is a growing need for continued surveillance to enable rapid detection of new and emerging disease threats in order to safeguard Britain’s wild bird populations.
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