Infectious Diseases of Garden Birds - Minimising the Risks

Infected Great TitSecond Edition. Copyright September 1998. UFAW

Infectious Diseases of Garden Birds - Minimising the Risks
James K Kirkwood BVSc PhD CBiol FIBiol MRCVS¹
& Shaheed K Macgregor HTEC MSc²

1 UFAW, The Old School, Brewhouse Hill, Wheathampstead, Hertfordshire AL4 8AN
2 Institute of Zoology, Regent's Park, London NW1 4RY


Contents

Foreword
Warning - disease risks to humans

  1. Introduction
    The impact of infectious diseases
    Possible consequences at the population level
    Possible consequences to welfare
  2. Some infectious diseases of garden birds
    Salmonellosis
    Colibacillosis
    Yersiniosis
    Pasteurellosis
    Chlamydiosis
    Trichomoniasis
    Warts (viral papillomas)
    Avian pox
    Aspergillosis
  3. Hygiene - minimising the risks of infectious diseases at garden bird feeders
  4. What to do if a disease outbreak occurs
    Determining the cause
    Treatment
    Euthanasia of moribund birds
  5. Obtaining advice and arranging for investigations
    Advice
    Collection and postage of carcases for examination

Foreword

These notes have been produced in response to the growing number of requests that we receive for information about the causes of disease and mortality incidents in garden birds and what can be done to prevent them. Few studies of the diseases of garden birds have been undertaken and data on the subject are scarce. This document is concerned with infectious diseases only and provides brief notes on a selection of those judged (from our experience and from the literature) to be among the most common.

It is hoped that as further information becomes available it will be possible to produce a more comprehensive publication providing a firmer basis for the development of preventative measures.

We are grateful to the many persons that have provided us with details of disease incidents over the past few years and to the many colleagues with whom I have discussed various aspects of these diseases.

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Warning - disease risks to humans

Some infections of garden birds can cause severe and potentially fatal disease in humans and some can be acquired from birds which show no signs of disease themselves. Always observe scrupulous personal hygiene when handling or cleaning bird tables or feeders (use rubber gloves, do not bring these or the feeders into your house, and always wash your hands after handling feeders). Dead birds are likely to present a particular risk. Avoid handling them at all if possible. If they must be handled - then always use disposable rubber gloves (and dispose of these hygienically after use). Carcases should be disposed of hygienically (ideally by incineration on a bonfire). Do not collect carcases for post mortem examination unless arrangements have been made for the examination at a veterinary laboratory and follow the instructions provided below in Sections 4 and 5, or others you may be given by the laboratory at which the examination is to be undertaken, very carefully. In no circumstances can UFAW accept any liability for loss, damage, disease, injury or death, associated with provisioning wild birds or dealing with disease or mortality outbreaks among them.

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1. Introduction

Although mortality rates of garden birds, especially of birds in their first year, are quite high, most people rarely see ill or dead birds with the exception of those killed by cars, domestic cats or through flying into glass windows. Wild birds are, however, susceptible to a wide range of infectious and non-infectious diseases but rather little is known about the prevalence of these diseases or the factors which may precipitate outbreaks. The aim of this leaflet is to describe some common infectious diseases of garden birds, to provide information about how they may be diagnosed, and to make general recommendations about their control and prevention.

The impact of infectious diseases

People judge the importance of disease outbreaks in different ways. One important factor which influences attitudes and responses to disease outbreaks is the degree to which humans are responsible for the outbreak. Diseases are a feature of life and most people accept that we should be very cautious about interfering with natural processes even if, like naturally-occurring outbreaks of disease in wild animals, they have unpleasant effects. On the other hand, if it is found that a disease outbreak in wildlife is due, in some way, to human activities then many people feel that it is right that we should take steps to control the problem and prevent it happening again. Another important point to note is that whilst some take the view that disease outbreaks are unimportant unless they threaten the viability of a species or have a large impact on its population size (at say the national or global level), others judge seriousness in terms of the suffering caused by the disease to individual wild birds. These are conservation and welfare concerns. They are distinct but equally valid ethical issues.

Possible consequences at the population level.

At the population level, the impact of an infectious agent can vary from none to catastrophic. The possible outcomes include:

  1. the infection becomes established in the population but has no significant effect on numbers (the proportion dying of the disease would otherwise have died of some other cause);
  2. the infection causes a decline in numbers leading to loss of the population;
  3. the infection causes a decline in numbers to a low level at which loss of the population due to other chance events becomes more likely;
  4. the infection causes a temporary decline in numbers followed by recovery to the original level (eg in cases in which there are too few susceptible birds, following the deaths of some and the growing immunity of others, to sustain the infection);
  5. the infection becomes established in the population and causes a decline in numbers to a new and lower plateau level;
  6. the infection becomes established and causes cyclic or chaotic fluctuations in population size.

The effects, at the population level, of the common infectious diseases outlined in Section 2 below have not been studied. There is no evidence that any pose a threat to population viability other, perhaps, than at the local level. Some (for example salmonellosis and trichomoniasis) can certainly cause temporary reductions in local population sizes during epidemics but whether the presence of these infections results in population sizes being held at below the carrying capacity of the environment is unknown.

Possible consequences to welfare.

The impact of infectious agents on welfare can also vary greatly depending on the species or type of infectious agent, the species of the host, and the circumstances (including for example, the age, sex, and immune status of the host and the intensity of exposure to the agent). The possible outcomes of infection on welfare vary from no effect to serious tissue damage and dysfunction causing severe and prolonged pain and distress (that level of stress in which significant resources are being diverted into the response to the detriment of the bird and in which it is conscious of and suffering as a result of the effort required for this).

Diseases which may cause severe effects at the population level may have little impact on welfare (eg chronic chlorinated hydrocarbon poisoning led to catastrophic population declines in raptors due to eggshell thinning but may not have caused significant pain or stress). Conversely, some diseases which cause severe pain and distress may have little or no impact at the population level (eg salmonellosis almost certainly cause severe pain and distress to large numbers of greenfinches each year but there is no evidence that the disease threatens the national population size).

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2. Some infectious diseases of garden birds

Many viral, bacterial, fungal, protozoal and parasitic diseases can occur in wild birds in the UK. Some are much more common than others and information about those which may be among the most frequent is provided below. Control and prevention of outbreaks of these diseases (if appropriate - see above) depends upon knowledge of the cause but with few exceptions there are no easy ways of diagnosing this (eg from the behaviour or appearance of the bird). Therefore when disease outbreaks occur it is usually necessary to arrange for post mortem examinations. Advice about this is presented in Sections 4 and 5 below.

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Salmonellosis

Agent: The disease is caused by species of the bacterium Salmonella and, most commonly, Salmonella typhimurium. A variety of strains have been isolated from diseased garden birds but that most frequently associated with disease in greenfinches appears to be 'phage type 40 (others isolated from greenfinches include 129,160 and U165).

Epidemiology: No surveys of the occurrence of outbreaks of this disease have been undertaken in the UK but it appears to be a common cause of epidemics in garden birds especially during the colder months from December through to April. Surveys indicate that rates of carriage of strains of S. typhimurium by garden birds is low. Whether outbreaks are typically due to spread of infection from symptomless carriers or to exposure to strains from other sources (eg rodents, food or other) is not known, but evidence suggests that the common strain in greenfinch mortality incidents - S.typhimurium phage type 40 - is primarily associated with wild birds. It has also been isolated from gamebirds, horses, calves and other domestic animals and can also cause disease in humans (see below). This strain has been diagnosed as the cause of mortality incidents in wild birds in Europe and also in the USA and Canada. The main route of spread is likely to be via faecal contamination of food at feeding sites. Salmonella species can survive and, in suitable conditions (perhaps met for example in moist uneaten food on a bird table on a warm spring day), multiply in the environment. The seasonal pattern of incidence is probably attributable to the greater risk of spread of infection in the winter months when large numbers of birds gather at feeding sites but it is also possible that cold weather may present a stress which reduces the birds' immunity. In some incidents there appears to be a sex bias, with more deaths in males than females.

Species susceptible: A wide range of species of garden birds is susceptible to salmonellosis but seed- eating passerines, especially greenfinches appear to be particularly at risk (and, males more than females, it appears). In typical incidents, most deaths occur in greenfinches with chaffinches, house sparrows and other species being affected to a lesser extent. However, patterns vary and in one case, five bullfinches died but greenfinches were unaffected.

Clinical signs: As in many diseases, affected birds appear fluffed-up, weak and lethargic. As also occurs with some other diseases, birds with salmonellosis are often observed to stay close to feeders or water baths and to continue to try to feed until just before death. Birds are often reported as being visibly ill for several days before they die although some birds die more rapidly and whilst still in moderately good body condition.

Pathology: At post mortem examination affected birds are typically found to be very thin. In finches, the infection often causes ulceration and abscess formation in the wall of the crop and in some cases this appears to cause crop stasis. Although the dead birds are usually very thin, their crops may be full. The infection may be disseminated through the body causing enlargement of the liver and spleen and focal lesions in the spleen, liver, lungs and at other sites including joints and eyes.

Risks to human or domestic animal health: A wide range of species including humans is susceptible to salmonellosis. Cases of disease caused by phage type 40 are recognised by the Public Health Laboratory Service every year and it is thought that these are associated with spread of infection from bird droppings. S. typhimurium can cause serious disease in humans especially in the young, old or others with diseases that may compromise the immune system. Domestic dogs and cats could acquire the disease through eating carcases of affected birds.

Diagnosis: The pattern of the epidemic, clinical signs and findings on gross post mortem examination may be suggestive and isolation of S. typhimurium from affected organs helps to confirm the diagnosis. However, it must be remembered that isolation of Salmonella does not necessarily indicate that salmonellosis was the cause of death as some birds are symptomless carriers.

Impact on population: During salmonellosis outbreaks quite large proportions of flocks may die (it can be over 50% but is often less) and noticeable declines in the numbers of the affected species are often reported by garden bird watchers. However there is no evidence that the disease is a threat to populations other than on a local level.

Impact on welfare: Salmonellosis can affect many tissues (see Pathology above). Typically the time course of the disease is chronic (days) and affected birds become very thin before death. The disease causes high levels of stress (distress) and almost certainly severe pain associated with tissue damage and also, in many cases, crop impaction.

Treatment: Salmonellosis is difficult to treat in small captive birds and it is unlikely that facilities, resources or expertise to treat wild birds affected with this disease will be available. Treatment under field conditions is not possible since therapy depends upon regular administration of antibiotics (among other things) and administration of antibiotics to free-living populations via food or water would not be adequate for treatment of ill birds and would merely encourage the development of drug-resistant strains in the others.

Control and prevention: Control and prevention depend upon minimising the risk of transmission to susceptible birds through hygienic measures and by taking steps to minimise densities at feeding stations. See Section 3 below.

References and further reading

Brittingham, M.C. & Temple, S.A. (1988) Avian disease and winter bird feeding. The Passenger Pigeon 50, 195- 203.

  • Brittingham, M.C., Temple, S.A. & Duncan, R.M. (1988) A survey of the prevalence of selected bacteria in wild birds. Journal of Wildlife Diseases 24, 299-307.
  • Cizek, A., Literak, I., Hejlicek, K., Treml, F. & Smola, J. (1994) Salmonella contamination of the environment and its incidence in wild birds. Journal of Veterinary Medicine B 41, 320-327.
  • Cornelius, L.W. (1969) Field notes on salmonella infection in greenfinches and house sparrows. Bulletin of the Wildlife Disease Association 5, 142-143.
  • Gerlach, H, (1994) Bacteria. In Avian Medicine. B.W. Ritchie, Harrison, G.J. & Harrison, L.R. (eds). Wingers Publishing Inc, Lake Worth, Florida. Pp 947-983.
  • Kapperud, G. & Rosef, O. (1983) Avian wildlife reservoir of Campylobacter fetus subsp jejuni, Yersinia spp and Salmonella spp in Norway. Applied and Environmental Microbiology 45, 375-380.
  • Keymer, I.F.K. (1993) Bacterial infections of wild birds. In Proceedings of the BOU Symposium on Diseases and Parasites of Birds. Madingley Hall, Cambridge, 8th-10th September 1993. British Ornithological Union, Oxford.

Kirkwood, J.K., Holmes, J.P. & Macgregor, S.K. (1995) Garden bird mortalities. Veterinary Record 136, 372

  • Kirkwood, J.K. & Macgregor, S.K. (1998) Salmonellosis in provisioned free-living greenfinches (Carduelis chloris) and other garden birds. Proceedings of the European Association of Zoo- and Wildlife Veterinarians Second Scientific meeting 21-24 May, 1998, Chester, UK. Pp 229-233.

Macdonald, J.W. & Cornelius, L.W. (1969) Salmonellosis in wild birds. British Birds 62, 28-30.

  • Pennycott, T.W., Ross, H.M., McLaren, I.M., Park, A., Hopkins, G.F. & Foster, G. (1998) Causes of death of wild birds of the family Fringillidae in Britain. Veterinary Record 143, 155-158.
  • Prescott, J.F., Poppe, C., Goltz, J. & Campbell, G.D. (1998) Salmonella typhimurium phage type 40 in feeder birds. Veterinary Record 142, 732.
  • Stroud, R.K. & Friend, M (1987) Salmonellosis. In Field guide to wildlife diseases. M. Friend (ed). US Dept of Interior Fish and Wildlife Service Publication 167. Pp 101-106.

Wilson, J.E. & Macdonald, J.W. (1967) Salmonella infection in wild birds. British Veterinary Journal 123, 212- 219.

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Colibacillosis

Agent: The disease is caused by species of the bacterium Escherichia coli. A variety of strains have been isolated from birds but the strain that has been associated most frequently with mortality incidents in garden birds appears to be a non-haemolytic strain: E Coli O86.

Epidemiology: E coli O86 has only recently been discovered as a cause of mortality in garden birds and no surveys of the occurrence of outbreaks of the disease have been undertaken. However, it appears, like Salmonella typhimurium, to be a common cause of epidemics in garden birds especially during the colder months from December through to April. In a study undertaken in Scotland, it was found to be the major problem diagnosed in finches in the Highland Region in 1994 and 1995. As with S typhimurium, the main route of spread is likely to be via faecal contamination of food. In some incidents there appears to be a sex bias, with more deaths in males than females.

Species susceptible: E coli O86 has been diagnosed as the cause of death in incidents involving greenfinches, siskins, chaffinches and blue tits. It may affect a wider range of species however, it has been reported that the organism was not cultured in a survey of 101 non-finch birds dying of other causes.

Clinical signs: As in many diseases, affected birds appear fluffed-up, weak and lethargic. Birds are thin at death indicating that the course of the disease is typically a few days.

Pathology: At post mortem examination affected birds are typically found to be very thin and to have empty or almost empty crops.

Risks to human or domestic animal health: E coli O86 can cause outbreaks of diarrhoea in infants.

Diagnosis: The pattern of the epidemic, clinical signs and findings on gross post mortem examination may be suggestive and isolation of E coli O86 helps to confirm the diagnosis, but it is possible that the organism may be carried by some birds without causing disease.

Impact on population: Outbreaks may affect quite large proportions of garden bird flocks (over 20 sick or dead birds have been observed at one time) and apparent declines in garden populations have been noted during outbreaks. However there is no evidence that the disease is a threat to populations other than at a local level but little is known about it at this stage.

Impact on welfare: The time course of the disease appears to be days and affected birds become very thin before death.

Treatment: It is unlikely that facilities, resources or expertise to treat wild birds affected with this disease will be available.Treatment of colibacillosis in wild birds under field conditions has not been reported and is unlikely to be possible since therapy would depends upon regular administration of antibiotics (among other things) and administration of antibiotics to free-living populations via food or water would not be adequate for treatment of ill birds and would merely encourage the development of drug-resistant strains in the others.

Control and prevention: Control and prevention depend upon minimising the risk of transmission to susceptible birds through hygienic measures and by taking steps to minimise densities at feeding stations. See Section 3 below.

References and further reading

Foster, G., Ross, H.M., Pennycott, T.W., Hopkins, G.M. & McLaren, I.M. (1998) Isolation of Escherichia coli O86:K61 producing cyto-lethal distending toxin from wild birds of the finch family. Letters in Applied Microbiology 26, 395-398.

Pennycott, T.W., Ross, H.M., McLaren, I.M., Park, A., Hopkins, G.F. & Foster, G. (1998) Causes of death of wild birds of the family Fringillidae in Britain. Veterinary Record 143, 155-158.

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Yersiniosis

Agent: Yersiniosis is the name given to the disease caused by the bacteria Yersinia pseudotuberculosis and Y. enterocolotica.

Epidemiology: Y. pseudotuberculosis is spread by faecal contamination of food and can survive in the environment for at least several days. The organism has been cultured from the droppings of apparently healthy birds and a small proportion of individuals of some species may therefore be carriers. Wild rodents can also carry and excrete the organism and contamination of food by rodents may be a significant factor in some cases. Cases appear to occur more commonly in the winter months.

Species susceptible: Yersinia infections can affect a range of species of garden birds including tits, finches, dunnock, blackbird, and songthrush. Patterns of incidence in captive bird collections suggest that there is species variation in susceptibility.

Clinical signs: Affected birds may be seen fluffed-up and lethargic before death. The disease can be acute or chronic and death may occur whilst birds are still in quite good body condition or not until they have become emaciated.

Pathology: These infections may cause enteritis but can be disseminated through the body, causing focal areas of tissue death and granuloma formation (a type of tissue reaction) in lungs, liver and spleen.

Risks to human and domestic species: Y. pseudotuberculosis can cause disease in humans and some domesticated species.

Diagnosis: The disease cannot be distinguished from other systemic illnesses on the basis of the behaviour and appearance of affected birds. Finding multiple pale areas in liver and spleen at gross post mortem is suggestive but culture of the organisms from these sites is necessary to confirm the diagnosis.

Impact on populations: It appears that the disease is sporadic and can cause localised garden bird mortality incidents involving several species.

Impact on welfare: Yersiniosis can cause acute disease leading rapidly to death but the course of the disease is probably days in many cases. The pathology must be associated with high levels of stress (distress) and the lesions (tissue injuries) almost certainly cause severe pain and the clinical signs and behaviour of affected birds support this.

Treatment: If diagnosed early enough, captive birds with yersiniosis can be treated using appropriate antibiotic therapy. Treatment of cases in free-living birds is very unlikely to be feasible for the reasons outlined in the section on salmonellosis.

Control and prevention: As in salmonellosis, control and prevention rest upon measures to minimise the risk of transmission to unaffected birds - hygiene and avoidance of high densities at feeding sites.

References and further reading

Brittingham, M.C., Temple, S.A. & Duncan, R.M. (1988) A survey of the prevalence of selected bacteria in wild birds. Journal of Wildlife Diseases 24, 299-307.

Kapperud, G. & Rosef, O. (1983) Avian wildlife reservoir of Campylobacter fetus subsp jejuni, Yersinia spp and Salmonella spp in Norway. Applied and Environmental Microbiology 45, 375-380.

Gerlach, H, (1994) Bacteria. In Avian Medicine. B.W. Ritchie, Harrison, G.J. & Harrison, L.R. (eds). Wingers Publishing Inc, Lake Worth, Florida. Pp 947-983.

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Pasteurellosis

Agent: The bacterium Pasteurella multocida or other Pasteurella species.

Epidemiology: P. multocida can be isolated from the respiratory tracts of a proportion of healthy wild birds and is also carried by rodents and, frequently, in the mouths of dogs and cats. It has caused large- scale mortality incidents in waterfowl on occasions in the USA. In these outbreaks transmission is probably direct from bird to bird or via contamination of environments where birds (eg waterfowl) gather in high densities. Such epidemics have not been observed in wild birds in the UK. In garden birds, cases tend to occur sporadically and are frequently secondary to bite or scratch wounds caused by dogs or cats.

Species susceptible: A wide range.

Clinical signs: The disease can be acute or chronic. There are no specific signs and the disease cannot be diagnosed from the appearance or behaviour of affected birds. In the late stages, affected birds are likely, as with other severe illnesses, to be lethargic, weak, fluffed-up and to have their eyes closed or part-closed at times.

Pathology: The infection may spread throughout the body and affect many organs. Areas of necrosis and inflammation may be seen in the liver and spleen. In chronic cases, infection may be localised at bite or scratch sites with formation of pus.

Risks to human and domestic species: The organism can affect a wide range of species but does not present a significant risk to fit humans or other mammals.

Diagnosis: This depends on isolation of the organism from affected tissues. Pasteurellosis can be suspected in birds that become ill after wounding by a dog or cat.

Impact on populations: Cases are usually sporadic and the disease is therefore very unlikely to have a significant impact.

Impact on welfare: Birds affected with Pasteurella infections almost certainly suffer pain and severe stress (distress) prior to death.

Treatment: The disease may respond to antibiotics if treated early and, if brought into temporary captivity for treatment, the chances of survival of birds injured by dogs or cats is likely to be increased significantly by administration of antibiotics as soon as possible after the injuries have been caused.

Control and prevention: There is no approach to the prevention of sporadic, spontaneous cases. Prevention of cases associated with cat bites depends on exclusion of cats or measures to minimise the risk of cat attacks at feeding sites and elsewhere.

References and further reading

Gerlach, H, (1994) Bacteria. In Avian Medicine. B.W. Ritchie, Harrison, G.J. & Harrison, L.R. (eds). Wingers Publishing Inc, Lake Worth, Florida. Pp 947-983.

Macdonald, J.W., Owen, D., Spencer, K.G. & Curtis, P.E. (1981) Pasteurellosis in wild birds. Veterinary Record 109, 58.

Fiennes, R.N.T.-W. (1982) Diseases of bacterial origin. In Diseases of cage and aviary birds. M.L. Petrak (ed). Lea & Febiger, Philadelphia. Pp 535-598

Friend, M. (1987) Avian cholera. In Field guide to wildlife diseases. M. Friend (ed). United States Department of the Interior Fish and Wildlife Service. Resource Publication 167, 69-82.

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Chlamydiosis

Agent: Chlamydiosis, which is also known as ornithosis or psittacosis, is caused by the intracellular organism Chlamydia psittaci. There are a variety of strains and these differ in their virulence.

Epidemiology: Chlamydiosis is quite a common disease of birds, especially psittacines (parrots) in captivity. There have been very few studies of infection rates in free-living wild birds but there is evidence that the organism is quite frequently carried by pigeons and some raptors. The carriage rates in garden passerine birds are not known. There are very few reports of disease and mortality in wild birds in Britain due to chlamydiosis but this is probably because few investigations have been undertaken. Birds can carry the infection for long periods (probably years) without showing signs of the disease and, in captivity, it is recognised that development of the disease, and/or onset of shedding of the organism, is often precipitated by stress. In some cases in wild collared doves the disease appeared to be secondary to stress due to Trichomonas infections (see below). Transmission between birds is via faeces and, under conditions of close contact, also via aerosols produced on sneezing. The agent can survive outside the body for at least several days and may also be spread in feather dust and dried faeces.

Species susceptible: The disease has been reported in wild robins and collared doves in Britain but it is likely (and apparent from knowledge of the disease in captive birds) that a wide range of species are potentially susceptible. In one incident in which the disease was confirmed in robins, deaths were also reported in dunnocks, and also a great tit and a coal tit.

Clinical signs: The clinical signs can vary considerably and the disease cannot be diagnosed on the basis of these. The infection may be very acute and cause sudden death or the disease may progress more slowly over days or even weeks. Signs may include: fluffed-up plumage, lethargy, nasal and ocular discharges, tremors, and watery droppings. Recovery from the clinically apparent disease is rare. Outbreaks in wild birds may be associated with few signs other than mortality.

Pathology: Carcases may show signs of conjunctivitis, nasal discharge and diarrhoea - faecal staining of plumage around the vent. The disease affects a range of tissues and at post mortem examination enlargement of the liver and spleen is often apparent and there is fibrinous peritonitis, pericarditis and air sacculitis. Enteritis and bronchopneumonia may also be apparent.

Risks to human or domestic animal health: C. psittaci can cause severe disease (psittacosis or ornithosis) in humans: an atypical pneumonia characterised by 'flu'-like symptoms: high fever, headache, debility and shortness of breath. The disease can be fatal if not treated. In Britain the disease is an occupational hazard for parrot keepers and for those in the domestic waterfowl industry. Wild birds have very rarely been shown to be a source of infection but this is very probably because the association is hard to prove. Birds with known or suspect chlamydiosis must be handled only with the utmost care and by trained persons.

Diagnosis: Diagnostic methods include histopathology and cytology (examination of smears) using special stains or immunofluorescent methods, or detection of antigens using ELISA (immunological) techniques or by genetic methods. Since healthy birds can carry the organism, its detection in the absence of typical pathology, does not confirm that it was the cause of death.

Impact on populations: The prevalence and impact of the disease in wild bird populations is unknown. It is known to cause sporadic and local mortality incidents.

Impact on welfare: The disease causes widespread tissue damage and affected birds show signs of being in severe pain and distress.

Treatment: In captivity, chlamydiosis in birds can be treated with antibiotics (including macrolides and fluoroquinolones). It is very unlikely that treatment of affected free-living birds would be feasible.

Control and prevention: If mortality incidents are found to be due to chlamydiosis, measures to improve hygiene at bird feeders and to minimise densities, and thus risks of cross-infection, at bird feeders are likely to help in reducing the likelihood of further cases.

References and further reading

Bracewell, C.D. & Bevan, B.J. (1986) Chlamydiosis in birds in Great Britain. 1. Serological reactions to chlamydia in birds sampled between 1974 and 1983. Journal of Hygiene, Cambridge 96, 447-451.

Brand, C.J. (1989) Chlamydia infections in free-living birds. Journal of the American Veterinary Medical Association 195, 1531-1535.

De Gruchy, P.H. (1983) Chlamydiosis in collared doves. Veterinary Record 113, 327.

Gerlach, H, (1994) Chlamydia. In Avian Medicine. B.W. Ritchie, Harrison, G.J. & Harrison, L.R. (eds). Wingers Publishing Inc, Lake Worth, Florida. Pp 947-983.

Gough, R.E. & Bevan, B.J. (1983) Isolation and identification of Chlamydia psittaci from collared doves (Streptopelia decaocto) Veterinary Record 112, 552.

Keymer, I.F. (1974) Ornithosis in free-living and captive birds. Proceedings of the Royal Society of Medicine 67, 733-735.

Simpson, V.R. & Bevan, R (1989) Chlamydia psittaci infection in robins. Veterinary Record 125, 537.

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Trichomoniasis

Agent: Trichomoniasis, also known as 'canker' in pigeons and 'frounce' in raptors, is caused by the motile, single cell parasite Trichomonas gallinae.

Epidemiology: Cases can occur at any time of year but young birds seem particularly susceptible. Large- scale epidemics can occur in columbiforms (see below). The infection is probably mainly spread via food or drinking water freshly contaminated with saliva or, possibly, droppings from an affected bird. Unlike Salmonella, Trichomonas cannot survive for more than a few minutes outside its host.

Species susceptible: A wide range of species of birds is susceptible to trichomoniasis including garden birds, game birds and raptors. However, pigeons and doves may be the main carriers. These species, and especially the collared dove, also appear to be particularly susceptible and large-scale epidemics can occur.

Clinical signs: In addition to showing signs of general illness, affected birds may dribble, regurgitate food, and appear to have difficulty swallowing. In some cases there is laboured breathing. Sometimes swelling of the crop is visible or palpable. Mildly affected birds may recover but the disease, which progresses over several days or even weeks, is often fatal. Affected birds become emaciated.

Pathology: This organism typically causes yellow plaques on, and necrosis of, the walls of the mouth, nasal cavities, oesophagus or crop. However infection can become more widespread through the body and can involve the liver and other organs.

Risks to human and domestic animal health: The species of Trichomonas which affect wild birds do not affect humans or other mammals but can cause disease in domestic poultry.

Diagnosis: The epidemiology and clinical signs may suggest trichomoniasis. The lesions seen at post mortem examination are fairly characteristic but those of other diseases (avian pox, Capillaria infestations) can be similar. Diagnosis can be confirmed by detecting the organism in direct smears from live or very freshly dead birds, or by histopathology.

Impact on population: Reports suggest that during outbreaks quite high proportions of local populations of collared doves can die (in some cases over 50%) and in columbiforms it may well be a factor that sets a constraint to population densities (outbreaks appear to occur where densities are high). In species other than pigeons and doves the disease appears to be sporadic.

Impact on welfare: The tissue damage caused by this disease and its consequences are almost certainly severely painful. Affected birds often show signs of hunger (they move towards food) but appear unable or unwilling to eat. The course of the disease is prolonged and affected birds suffer distress.

Treatment: Trichomoniasis, except in the late stages, can be effectively treated in captive birds with metronidazole. However, effective and targeted dosing of free-living birds under field conditions would be difficult.

Control and prevention: Control and prevention rest on minimising risk of transmission through good bird table hygiene and measures to limit high densities of birds at feeding sites. See Section 3 below.

References and further reading

Grainer, EC. & Baxter, WL (1974) A localised epizootic of trichomoniasis in mourning doves. Journal of Wildlife Diseases 10, 104-106.

Grainer, EC. & Ritchie, B.W. (1994) Parasites. In Avian Medicine. B.W. Ritchie, Harrison, G.J. & Harrison, L.R. (eds). Wingers Publishing Inc, Lake Worth, Florida. Pp 947-983.

Keymer, IF (1982) Parasitic diseases. In Diseases of cage and aviary birds. M.L. Petrak (ed). Lea & Febiger, Philadelphia. Pp 535-598.

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Warts (viral papillomas)

Agent: The Fringilla papillomavirus (FPV).

Epidemiology: The epidemiology of the disease has not been studied.

Species susceptible: Chaffinches and, to a lesser extent, bramblings. In a large survey of birds captured for ringing in the Netherlands, papillomas were found on 330 (1.3%) of some 25,000 chaffinches examined and both sexes were affected. However, cases usually occur in clusters and quite high proportions of local populations may be affected in outbreaks.

Clinical signs: The disease causes warty outgrowths on the foot or tarsometatarsus (the bare part of the leg). Usually only one limb is affected. The growths vary from small nodules to large irregular shaped and deeply-fissured masses which almost engulf the entire lower leg and foot and which can distort the toes. Affected birds usually seem in otherwise good health but some may show signs of lameness and hop mainly on the unaffected foot and digits may be lost. The warts grow slowly and may progress over many months.

Pathology: The growths have a similar structure to warts in mammals and are due to excessive growth of the keratinised layers of the skin.

Risks to human and domestic species: None known.

Diagnosis: The clinical appearance is strongly suggestive but other diseases can cause swellings on the legs and feet: infestation with Cnemidocoptes mites (there is some evidence for an association between mange due to Cnemidocoptes infestations and the occurrence of papillomas), bacterial infections (bumblefoot), poxvirus infections. Diagnosis can be confirmed by histology or detection of papillomavirus particles.

Impact on populations: It seems unlikely that this disease has an impact on population densities.

Impact on welfare: Even birds with large papillomas often appear to behave normally so, in some cases, the growths may be little more than an inconvenience and relatively minor irritation. However, lameness is sometimes observed and this clearly indicates pain.

Treatment: None. The outcome of the disease is unclear. Birds may die through being incapacitated or through developing secondary infections but it is possible that in some cases the lesions may regress spontaneously.

Control and prevention: The fact that cases occur in clusters suggests that the presence of affected birds presents a risk to others that are susceptible. The mode of transmission is not known but it seems likely that the virus may be spread via surfaces the birds stand or perch upon. If so, hygiene measures and steps to minimise crowding at perching or feeding sites may reduce the risk.

References and further reading:

Blackmore, D.K. & Keymer, I.F. (1969) Cutaneous diseases of wild birds in Britain. British Birds 62, 316-331.

Lina, P.H.C., van Noord, M.J. & de Groot, F.G. (1973) Detection of virus in squamous papillomas of the wild bird species Fringilla coelobs. Journal of the National Cancer Institute 50, 567-571.

Macdonald, J.W. (1965) Mortality in wild birds. Bird Study 12, 181-195.

Macdonald, J.W. & Gush, G.H. (1975) Knemidocoptic mange in Chaffinches. British Birds 68, 103-107.

Osterhaus, A.D.M.E. & Moreno-Lopez, J. (1993) Papovaviruses. In J.B. McFerran and M.S. McNulty (Eds) Virus infections of birds. Elsevier Science Publishers B.V., Amsterdam. Pp 147-151.

Washington, D. (1964) Unusual growths on the feet of chaffinches. British Birds 57, 184.

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Avian Pox

Agent: There are a variety of Avipoxvirus species and these affect different species of birds.

Epidemiology: Very little is known about the prevalence of pox virus infections and the diseases caused by these in wild birds in Britain. One survey indicated that the disease was prevalent in woodpigeons and that it appeared to be relatively common in house sparrows and possibly other small passerines, but very few data are available and very few investigations have been carried out. Pox viruses enter through breaks in the skin or mucous membranes (eg the lining of the mouth). They are very resistant and can survive for a long time in the environment in dried pox scabs, so birds may acquire infection from contact with environment which has been contaminated by infected birds. The disease is also spread between birds by biting parasites such as mosquitoes and mites or by simple mechanical spread by flies. Direct transmission between birds perhaps facilitated by territorial aggression. It is thought that some birds can carry the virus without showing clinical signs. In woodpigeons immature and adult birds of both sexes are affected and outbreaks appear to occur mainly in the winter months.

Species susceptible: Pox virus diseases have been recognised in a wide range of bird species including raptors, game birds, domestic fowl, pigeons and a variety of passerines. Most species of wild birds are probably susceptible to at least one of the avipoxviruses. Some of the pox viruses are very host-specific - that is they do not infect or cause disease in other species of birds others may cause mild disease in some species but severe disease in others. In Britain cases among garden birds have been recorded in house sparrows, woodpigeons, blackbird, dunnock and starling. In Norway, among wild birds, the disease was noted to affect the titmice (Paridae) most commonly.

Clinical signs: The disease occurs in three main forms: cutaneous and diphtheritic (in which pox lesions occur mainly on the skin or mucous membranes) and septicaemic (in which there is a rapidly fatal systemic spread of infection). In the cutaneous form lesions occur mainly on the unfeathered parts - face, legs and feet - starting as small blisters and developing into wart-like nodules which, in birds that survive, become raised, brown-coloured scabs which, in due course, drop off. In tits pox lesions were noted especially at the base of the beak and the rim of the beak and eyes. Affected birds may show few signs of systemic illness and may recover fully. In the diphtheritic form, pox lesions may occur in the mouth, oesophagus, upper respiratory tract, conjunctiva and eyelids. The clinical signs depend upon the sites affected and the severity but may include conjunctivitis (with eyes partially closed), laboured breathing, difficulty in eating or drinking. Some of the avian pox viruses cause high mortality in susceptible species.

Pathology: See Clinical Signs above.

Risks to human and domestic species: Pox viruses of wild birds are not known to cause disease in humans or other mammals but some can affect species of domestic poultry.

Diagnosis: The clinical signs in birds that can be closely examined may be suggestive but pox lesions in the mouth could be confused with other diseases (eg trichomoniasis). The diagnosis can be confirmed by histopathology, by demonstration of virus particles, in samples of the scabs, with electron microscopy, or by virus isolation.

Impact on population: There is no evidence that poxviruses affect dynamics or viability of populations of wild birds in Britain but the subject has not been studied.

Impact on welfare: Mild cases may be associated with little pain or stress but severe cases are likely to be associated with severe discomfort and distress.

Treatment: There is no specific treatment for poxvirus disease (in captivity, supportive therapy may improve the chances of survival and reduce discomfort).

Control and prevention: It seems likely that poxvirus diseases occur sporadically in wild birds in Britain. It is possible that crowding at feeding stations might facilitate spread and thus, if cases occur, consideration should be given to minimising this.

References and further reading

Blackmore, D.K. & Keymer, I.F. (1969) Cutaneous diseases of wild birds in Britain. British Birds 62, 316-331.

Gerlach, H. (1994) Viruses. In Avian Medicine. B.W. Ritchie, Harrison, G.J. & Harrison, L.R. (eds). Wingers Publishing Inc, Lake Worth, Florida. Pp 862-874.

Hill, J.R. & Bogue, G. (1977) Epornitic of pox in a wild bird population. Journal of the American Veterinary Medical Association 171, 993-994.

Holt, G. & Krogsrud, J. (1973) Pox in wild birds. Acta vet. Scand. 14, 201-203.

McGaughey, C.A. & Burnet, F.M. (1945) Avian pox in wild sparrows. Journal of Comparative Pathology and Therapeutics 55, 201.

Tripathy, D.N. (1993) Avipox viruses. In J.B. McFerran and M.S. McNulty (Eds) Virus infections of birds. Elsevier Science Publishers B.V., Amsterdam. Pp 5-15.

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Aspergillosis

Agent: The fungus Aspergillus fumigatus is the main cause but other Aspergillus species are sometimes involved.

Epidemiology: Aspergillosis is typically a disease of birds that are in poor condition or in sub-optimal health for some other reason (eg food shortage). Exposure occurs through the inhalation of fungal spores but these are ubiquitous and in healthy birds exposure does not usually lead to disease. Large scale outbreaks have been reported in the USA where birds have been exposed to very high densities of spores at dumps of mouldy grain but in the UK it occurs sporadically and mainly in debilitated individuals. There is no evidence that the infection can spread from one bird to another.

Species susceptible: Probably all species are susceptible. (The disease is especially common in sea birds when brought into captivity in a terrestrial environment).

Clinical signs: The disease is usually chronic in duration. Affected birds lose body condition and show signs of general malaise: reduced appetite, weakness, lethargy, lack of exercise tolerance and fluffed-up plumage. In the late stages evidence of respiratory difficulties may be observed including laboured, noisy, open-mouthed breathing. The tail may move up and down with each breath. Affected garden birds are likely to take refuge in undergrowth and die unseen.

Pathology: Aspergillus fumigatus grows, as does a mould on bread, on the linings of the air sacs and in the lungs. At post mortem these white to green plaques of Aspergillus can be seen. Typically carcases are emaciated but death can occur quickly at an earlier stage if the trachea becomes blocked by a detached plaque.

Risks to human and domestic species: A wide range of animals, including humans, can get aspergillosis but, as in wild birds, it is rare in fit individuals. There is no evidence that infection can spread from affected birds to humans.

Diagnosis: The disease cannot be diagnosed from the behaviour or appearance of affected birds. The appearance of the plaques of fungal growth on the air sacs or in the lungs at post mortem are fairly characteristic and the diagnosis can be confirmed by demonstration or culture of the organism from these.

Impact on population: The disease is sporadic and is very unlikely to have a significant effect on population densities.

Impact on welfare: The disease causes debility and malaise and, in some cases, respiratory difficulties.

Treatment: Aspergillosis is difficult to treat unless diagnosed at an early stage and, in wild birds, it is very unlikely to be diagnosed until too late a stage for successful treatment.

Control and prevention: Mouldy food (especially on a large scale, eg dumps of mouldy grain or straw) can be a source of high densities of spores which are likely to increase the risk of the disease in wild birds. Moulds can thrive in the bases of peanut feeders especially when these have become wet from rain. Peanut feeders should be of a design that allows complete cleaning.

References

Bauck, L. (1994) Mycoses. In Avian Medicine. B.W. Ritchie, Harrison, G.J. & Harrison, L.R. (eds). Wingers Publishing Inc, Lake Worth, Florida. Pp 947-983.

Keymer, IF (1982) Mycoses. In Diseases of cage and aviary birds. M.L. Petrak (ed). Lea & Febiger, Philadelphia. Pp 535-598.

Locke, L.N. (1987) Aspergillosis. In Field guide to wildlife diseases. M. Friend (ed). United States Department of the Interior Fish and Wildlife Service. Resource Publication 167, 145-150.

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3. Hygiene - minimising the risks of infectious diseases at garden bird feeders

Knowledge of the epidemiology and causes of diseases in garden birds is incomplete and research is needed in this area (for example, to elucidate the source of Salmonella typhimurium 'phage type 40 and the effects of variation in bird densities at feeding sites on its transmission). However, it is possible to suggest a number of measures that may assist in minimising the risks of disease outbreaks.

The spread of some of the infections mentioned above and others not covered is very likely to be facilitated by crowding, particularly at feeding sites where foods can become heavily contaminated with droppings or, in the case of trichomoniasis, drops of saliva. Where large populations of birds regularly feed at the same place there is a potential for infected or carrier birds to transmit infection to susceptible individuals (Kirkwood, 1998; Pennycott, 1998; Hartup et al, 1998).

  1. If large numbers of birds visit your garden for feeding, provide food at several sites and change these regularly so that the feeding sites do not become heavily contaminated. Avoid feeding large numbers of birds at the same place (eg one small patch of the lawn or, unless it is regularly cleaned, on one table) over long periods.
  2. Suspended feeders should not have sills on which food particles and droppings can collect. Keep the ground beneath these feeders, where birds often pick up morsels that have dropped, clean (regularly swept) or move the feeders regularly. Sweepings and brushings should be disposed of hygienically and ideally by incineration.
  3. Keep surfaces on which birds feed clean (brush them daily to avoid heavy faecal contamination and dispose of brushings hygienically). If food is put out on the ground, do not put it in the same place every day.
  4. Feeders and bird tables should be thoroughly cleaned on a regular basis (depending on how dirty they become), and daily if a disease outbreak occurs. After cleaning, they can be disinfected by washing or soaking for a few minutes in a 5% sodium hypochlorite solution or using a safe disinfectant such as (Tamodine-E, Vetark, PO Box 60, Winchester SO23 7LS). Rinse all surfaces very thoroughly after disinfection.
  5. If water is provided in drinkers or bird baths, it should be clean (changed daily) and drinkers/bird baths should be kept clean and regularly disinfected as described above (rinse thoroughly afterwards).
  6. Mouldy foods may contain toxins which can cause disease in birds and can also provide a substrate in which Salmonella can multiply. Use fresh and good quality foods and dispose hygienically of any left uneaten.
  7. Observe careful hygiene when cleaning bird tables, feeders and water baths. Use rubber gloves and do not take these or feeders or water baths into your house (they can be cleaned using a bucket in the garden). As mentioned above, a number of infectious diseases of birds can also affect humans and/or a variety of domestic animals.
  8. Store foods where they cannot be contaminated by rodents.
  9. Watch for signs of illness in birds at feeding sites and seek advice promptly if disease outbreaks occur so that the cause can be diagnosed (see Sections 4 and 5 below). Appropriate control and prevention measures depend on knowledge of the cause.

References

Kirkwood, J.K. (1998) Population density and infectious disease at bird tables. Veterinary Record 142, 468.

Pennycott, T.W. (1998) Population density and infectious disease at bird tables. Veterinary Record 142, 523.

Hartup, B.K., Mohammed, H.O., Kollias, G.V. & Dhondt, A.A. (1998) Risk factors associated with mycoplasmal conjunctivitis in house finches. Journal of Wildlife Diseases 34, 281-288.

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4. What to do if a disease outbreak occurs

Determining the cause: Measures to treat, prevent or control diseases, and assessment as to whether such measures are appropriate, depend on knowledge of the cause. It is therefore important that the cause of outbreaks is diagnosed if possible. This is likely to require post mortem examination of one or more carcases (or perhaps detailed clinical examination of affected birds before death) by a veterinarian although, in some cases (eg chaffinch leg warts), a good guess can be made as to the cause based on the circumstances, species affected, and signs.

Treatment: It is appropriate to direct efforts at reducing the likelihood of outbreaks and preventing spread of infectious diseases at feeding sites rather than at treating cases once they have occurred. For the reasons mentioned above, in most cases, treatment of birds suffering from the diseases listed here is unlikely to be applicable (however UFAW would be happy to advise about this if cases occur).

Euthanasia of moribund birds: Wild birds are protected under the Wildlife and Countryside Act 1981 under which it is an offence to intentionally kill, injure or take any wild animal. However, the Act allows that: '....a person shall not be guilty of an offence by reason of...the killing of any such animal if he shows that the animal had been so seriously disabled...that there was no reasonable chance of its recovering.' Thus, the law allows, for humane reasons, the euthanasia by a humane method of wild birds that are moribund. Euthanasia can be done (by a veterinary surgeon) by administering an overdose of anaesthetic.

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5. Obtaining advice and arranging for investigations

Advice: The Institute of Zoology and UFAW will endeavour to provide advice on handling disease incidents and, resources permitting, will try to arrange for investigations of the cause or to provide advice about this if you telephone the Institute of Zoology:

0207-449-6685

(Mortality incidents thought to have been caused by pesticides should be reported to the MAFF Wildlife Incident Unit, Central Science Laboratory, York. Tel 0800-321600).

Collection and postage of carcases for examination:

Carcases must not be sent without prior arrangement by telephoning 0207-449-6685

If it is possible to arrange for post mortem examinations to try to diagnose the cause of mortality incidents, details will be provided then as to how carcases should be collected, packaged and despatched for analysis.

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Acknowledgements

We are most grateful to Dr Ian K Keymer, Mr Vic R Simpson and Mr J Dick Best for helpful comments on a previous draft.

Universities Federation for Animal Welfare

(Founded 1926)

UFAW is an internationally recognised scientific and educational charity which adopts an objective and realistic approach to animal welfare by looking at the behaviour and needs of animals, making rational judgements and giving expert advice on how particular species should be cared for and managed.

The Federation holds symposia and workshops; carries out and sponsors research into many aspets of the biology and welfare of farmed, companion, wild, zoo and laboratory animals; produces standard texts on animal care and management and publishes technical reports, as well as the quarterly scientific journal Animal Welfare.

UFAW is an independent registered charity supported by donations, legacies and membership subscriptions.

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