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Outbreaks of mortality in wild birds in gardens in the UK were first reported in the mid 1960’s, when members of the general public began to put out bags of peanuts to feed the wild birds. In these first outbreaks most deaths were due to infection with the bacterium Salmonella Typhimurium (abbreviated to S. Typhimurium) and occurred in Greenfinches (Carduelis chloris) and House sparrows (Passer domesticus).Mortality incidents have continued, and since 1995 many post mortem examinations have been carried out at the Avian Health Unit (AHU) of the Veterinary Services Group of SAC (Scottish Agriculture College). The results from these investigations have shown that in Scotland two strains of S. Typhimurium, DT40 and DT 56 variant, cause most of the deaths during outbreaks of mortality, but a strain of the bacterium Escherichia coli referred to as E. coli O86 is also responsible for some of the deaths in wild birds.
Outbreaks of mortality are most often seen in Greenfinches, Chaffinches (Fringilla coelebs) House Sparrows and Siskins (Carduelis spinus), but other birds such as Goldfinches (Carduelis carduelis), Tree Sparrows (Passer montanus|) and Great Tits (Parus major) have also been affected in smaller numbers. During outbreaks of disease, dead birds or sick birds are usually found in the vicinity of the bird feeders. If seen alive the birds are fluffed up, reluctant to fly, and may look as if they are breathing heavily or have difficulty swallowing.
The post mortem examination of birds dying from salmonellosis often reveals substantial Yellow/Orange areas of damage to internal organs such as the oesophagus (gullet), crop, liver, spleen (an organ which tries to fight off diseases) and sometimes the lungs and lower part of the digestive tract. The damage to the gullet can be so severe that it causes a partial blockage, preventing food getting to the birds stomach even if it continues to eat. The post mortem findings on birds dying from E. coli O86 are different. The bacterium doesn’t cause such obvious damage to the internal organs but has the ability to produce toxins (poisons) that prevent the digestive tract from working properly. Birds dying from E . coli O86 typically have much food in the gullet but little further down the digestive tract.
Confirmation of the cause of death requires specialist laboratory facilities for the culture and identification of the bacteria.
Wild bird mortality survey 1995-2002
To find out about salmonellosis in wild birds, post mortem examinations were carried out by the AHU of SAC Veterinary Services on the carcases of 685 wild birds between 1995 and 2002. Carcases were submitted directly to the AHU, or in the case of injured or diseased birds were first taken to a wildlife rehabilitation centre. Salmonella was isolated from191 of the 685 wild birds examined. Most of the isolates were the strains of S. Typhimurium referred to earlier, namely DT 40 and DT 56 (variant), and were most commonly isolated from Greenfinches, Chaffinches and House Sparrows. S. Typhimurium DT 40 was the commonest strain of salmonella, found in 118 Greenfinches, 10 House Sparrows and 5 Chaffinches. S. Typhimurium DT 56 (variant) was less common, found in 9 Greenfinches, 15 House Sparrows and 7 Chaffinches. A marked seasonal pattern was noted, with 86% of isolates of DT 40 occurring in the three months January to March. Isolates of DT 56 were less concentrated, with 81% of isolates occurring in the five months November to March. The seasonal patterns for Greenfinches and House Sparrows are shown in Figs 9 and 10.
E. coli O86
The other bacterium commonly causing mortality in garden birds is E. coli O86. Of 64 birds dying from E. coli with the O86 profile between 1997 and 2002, 91% occurred in the months February to May. Thirty-nine of the birds were Siskins, 23 were Greenfinches, one was a Chaffinch and one was a Goldfinch.
Deaths in wild birds in North America, New Zealand and Scandinavia.
Deaths form salmonellosis have not been confined to Sparrows and Finches at bird tables in the U.K. Since 1998, many finches have been found dead around garden feeders in the United States and Canada, mostly Pine Siskins (Carduelis pinus) and Common Redpolls (C. flammea), also Evening Grosbeaks (Coccothraustes vespertinus), House Sparrows and American Goldfinches (C. tristis). As in most of the U.K. incidents, the type of salmonella involved was S. Typhimurium DT 40. Cases of salmonellosis were also reported in domestic cats that had preyed on sick birds around feeders.
A different strain of salmonellosis, described as S. Typhimurium DT 160, caused outbreaks of mortality in House Sparrows in New Zealand in 2000, where there was evidence of disease in humans, (including one death) and in young farmed Ducks and Quail, dogs, cats deer and horses. In one incident in New Zealand more than 400 dead birds were found at one location on one day. Small numbers of greenfinches, Goldfinches and Blackbirds (Turdus merula) were also affected. Mortality reached a peak in the New Zealand winter months of July to August, decreasing to smaller numbers in the following spring and summer. This strain of S. Typhimurium was also found in 15% of wild Sparrows trapped in 1979 in Guelph, Canada, and caused a small outbreak of mortality in House Sparrows in Central Newfoundland, Canada in February and March 1999.
Mortality from salmonellosis in finches and House Sparrows at garden feeding sites has been a regular occurrence in Norway. Of 441 garden birds with salmonellosis in Norway between, 1969 and 2000, over half were Bullfinches (Pyrrhula pyrrhula), although Greenfinches, Siskins, Redpolls and House Sparrows were also involved. As in Britain there was a marked seasonal incidence, with most cases occurring in January to April. Most of the isolates from Norway were S. Typhimurium DT 40 or another strain referred to as S. Typhimurium U277. Deaths have also occurred in wild finches in Finland and Sweden.
Why are the deaths occurring?
Although the mortality incidents in the U.K. usually occur at sites providing supplementary feeding for wild birds, the food is not believed to be the initial source of the bacteria, but rather the cause of the congregation of large flocks of birds in a small area. Some birds probably carry small numbers of S. Typhimurium and E. coli O86 in their intestines, and when the birds congregate at the bird tables and feeding stations a build up of these bacteria may occur, contaminating the feeders and drinkers and the surrounding environment. Under these conditions, the bacteria may then have the chance to overwhelm the birds and cause their deaths.
Clearly prevention is very important, and is based on avoiding a build up of these potentially lethal bacteria. Regular cleaning and disinfecting of bird tables, feeders and drinkers may help, as will periodic change of feeding sites. If the birds can be spread out by using several different feeding sites, so much the better. The areas beneath the feeders can also quickly become contaminated and should be kept as clean as possible, with any uneaten food removed.
Illness in humans and cats.
Human illness from E. coli O86 and from wild bird strains of salmonella is currently uncommon in the U.K., but there is the potential for spread to humans. Several years ago in England, S. Typhimurium DT 160 caused gastro-enteritis in humans who had eaten food prepared in a kitchen in which Sparrows were contaminating the food preparation areas, and this strain also caused disease in humans in New Zealand in 2000. An outbreak of gastro-enteritis in humans was seen in Norway and Finland in 1987, caused by a strain of S. Typhimurium U277 identical to that found in Bullfinches. A link with chocolate produced in one factory was established, and it was suspected that contamination from wild birds may have occurred. Investigations in Norway have also revealed that some “sporadic” cases of sickness and diarrhoea in humans are caused by wild bird strains of S. Typhimurium, especially in young children, and can be associated with direct contact with wild birds or their droppings or playing in areas where wild bird droppings have been observed. Rubber gloves should therefore be worn when cleaning bird tables or if the carcases of dead birds have to be handled, and hands must be thoroughly washed, especially before preparing food. Wild birds should be excluded from food preparation areas.
Pet cats may be at risk from salmonellosis from wild birds. An outbreak of disease in cats in Sweden was seen in February to April 1999, caused by wild bird strains of salmonella. Affected cats lost their appetite and many developed vomiting or diarrhoea. At the same time people were finding sick or dead wild finches, mostly Siskins and Redpolls, in gardens and the same strain of S. Typhimurium was isolated from some of the birds. It was concluded that the sick finches were easy prey for the cats which then became ill themselves. Spread to humans was also reported, either directly from wild birds or from cats infected from wild birds.
The decline in House Sparrows in Great Britain.
At one time considered to be a serious pest of cereal crops and crop stores, the breeding population of House Sparrows in G.B. has fallen from about 12-15 million pairs in the 1970’s to a current figure of around 6 million pairs. Declines were first seen in farmland but the greatest have been in urban and suburban gardens, and it is likely that different factors are responsible for the decline in the different habitats. Farmland is no longer as attractive to Sparrows as it once was, due to a multitude of factors, such as improvements in grain harvesting and storage, fewer weed seeds and invertebrates, changes in grassland management and fewer farmyards in which the birds can forage. However the greatest impact on total population size in Britain has been the decline in the urban and suburban populations. A report by the British Trust for Ornithology (BTO) to the Department for Environment, Food and Rural Affairs (DEFRA) in August 2002 suggested a number of possible reasons for the fall in the urban and suburban sparrows, including predation by cats and Sparrowhawks, a lack of nest sites due to house renovations, the development of Brown Field sites, and air pollution. The BTO reported also highlighted the possible importance of infectious disease, spread at bird tables and other forms of feeders in gardens.
Studies by SAC Veterinary Services have shown that infection with S. Typhimurium is a common cause of death in House Sparrows found dead in urban or suburban habitat by members of the public in the winter and spring. Between 1998 and 2002, salmonellosis was diagnosed in 22 of 24 House Sparrows (from nine different sites) submitted to the Avian Health Unit during the months of November to March. Monitoring wild bird faeces at garden feeding station in South-West Scotland has also shown that a strain of S. Typhimurium known to cause deaths in finches and House Sparrows can become established in flocks of wild birds feeding at bird tables and persist for over 30 months. It is therefore possible that infection with S. Typhimurium is a contributory factor to the decline in House Sparrows in some locations.
Acknowledgements.
Much of the work carried to investigate deaths in wild birds was funded by The Dulverton Trust and The Game Conservancy Trust. SAC also receives financial support from the Scottish Executive Environment and Rural Affairs Department. An additional generous donation from CJ Wildbird Foods Ltd, Upton Magna, Shrewsbury, will enable further monitoring to be carried out in 2004. The help of all organisation, companies and individuals involved is gratefully acknowledged.
Mr Tom W. Pennycott
Senior Veterinary Investigation Officer, Avian Health Unit
SAC Veterinary Services
Auchincruvie
Ayr. KA6 5AE.
January 2004.
Outbreaks of mortality are most often seen in Greenfinches, Chaffinches (Fringilla coelebs) House Sparrows and Siskins (Carduelis spinus), but other birds such as Goldfinches (Carduelis carduelis), Tree Sparrows (Passer montanus|) and Great Tits (Parus major) have also been affected in smaller numbers. During outbreaks of disease, dead birds or sick birds are usually found in the vicinity of the bird feeders. If seen alive the birds are fluffed up, reluctant to fly, and may look as if they are breathing heavily or have difficulty swallowing.
The post mortem examination of birds dying from salmonellosis often reveals substantial Yellow/Orange areas of damage to internal organs such as the oesophagus (gullet), crop, liver, spleen (an organ which tries to fight off diseases) and sometimes the lungs and lower part of the digestive tract. The damage to the gullet can be so severe that it causes a partial blockage, preventing food getting to the birds stomach even if it continues to eat. The post mortem findings on birds dying from E. coli O86 are different. The bacterium doesn’t cause such obvious damage to the internal organs but has the ability to produce toxins (poisons) that prevent the digestive tract from working properly. Birds dying from E . coli O86 typically have much food in the gullet but little further down the digestive tract.
Confirmation of the cause of death requires specialist laboratory facilities for the culture and identification of the bacteria.
Wild bird mortality survey 1995-2002
To find out about salmonellosis in wild birds, post mortem examinations were carried out by the AHU of SAC Veterinary Services on the carcases of 685 wild birds between 1995 and 2002. Carcases were submitted directly to the AHU, or in the case of injured or diseased birds were first taken to a wildlife rehabilitation centre. Salmonella was isolated from191 of the 685 wild birds examined. Most of the isolates were the strains of S. Typhimurium referred to earlier, namely DT 40 and DT 56 (variant), and were most commonly isolated from Greenfinches, Chaffinches and House Sparrows. S. Typhimurium DT 40 was the commonest strain of salmonella, found in 118 Greenfinches, 10 House Sparrows and 5 Chaffinches. S. Typhimurium DT 56 (variant) was less common, found in 9 Greenfinches, 15 House Sparrows and 7 Chaffinches. A marked seasonal pattern was noted, with 86% of isolates of DT 40 occurring in the three months January to March. Isolates of DT 56 were less concentrated, with 81% of isolates occurring in the five months November to March. The seasonal patterns for Greenfinches and House Sparrows are shown in Figs 9 and 10.
E. coli O86
The other bacterium commonly causing mortality in garden birds is E. coli O86. Of 64 birds dying from E. coli with the O86 profile between 1997 and 2002, 91% occurred in the months February to May. Thirty-nine of the birds were Siskins, 23 were Greenfinches, one was a Chaffinch and one was a Goldfinch.
Deaths in wild birds in North America, New Zealand and Scandinavia.
Deaths form salmonellosis have not been confined to Sparrows and Finches at bird tables in the U.K. Since 1998, many finches have been found dead around garden feeders in the United States and Canada, mostly Pine Siskins (Carduelis pinus) and Common Redpolls (C. flammea), also Evening Grosbeaks (Coccothraustes vespertinus), House Sparrows and American Goldfinches (C. tristis). As in most of the U.K. incidents, the type of salmonella involved was S. Typhimurium DT 40. Cases of salmonellosis were also reported in domestic cats that had preyed on sick birds around feeders.
A different strain of salmonellosis, described as S. Typhimurium DT 160, caused outbreaks of mortality in House Sparrows in New Zealand in 2000, where there was evidence of disease in humans, (including one death) and in young farmed Ducks and Quail, dogs, cats deer and horses. In one incident in New Zealand more than 400 dead birds were found at one location on one day. Small numbers of greenfinches, Goldfinches and Blackbirds (Turdus merula) were also affected. Mortality reached a peak in the New Zealand winter months of July to August, decreasing to smaller numbers in the following spring and summer. This strain of S. Typhimurium was also found in 15% of wild Sparrows trapped in 1979 in Guelph, Canada, and caused a small outbreak of mortality in House Sparrows in Central Newfoundland, Canada in February and March 1999.
Mortality from salmonellosis in finches and House Sparrows at garden feeding sites has been a regular occurrence in Norway. Of 441 garden birds with salmonellosis in Norway between, 1969 and 2000, over half were Bullfinches (Pyrrhula pyrrhula), although Greenfinches, Siskins, Redpolls and House Sparrows were also involved. As in Britain there was a marked seasonal incidence, with most cases occurring in January to April. Most of the isolates from Norway were S. Typhimurium DT 40 or another strain referred to as S. Typhimurium U277. Deaths have also occurred in wild finches in Finland and Sweden.
Why are the deaths occurring?
Although the mortality incidents in the U.K. usually occur at sites providing supplementary feeding for wild birds, the food is not believed to be the initial source of the bacteria, but rather the cause of the congregation of large flocks of birds in a small area. Some birds probably carry small numbers of S. Typhimurium and E. coli O86 in their intestines, and when the birds congregate at the bird tables and feeding stations a build up of these bacteria may occur, contaminating the feeders and drinkers and the surrounding environment. Under these conditions, the bacteria may then have the chance to overwhelm the birds and cause their deaths.
Clearly prevention is very important, and is based on avoiding a build up of these potentially lethal bacteria. Regular cleaning and disinfecting of bird tables, feeders and drinkers may help, as will periodic change of feeding sites. If the birds can be spread out by using several different feeding sites, so much the better. The areas beneath the feeders can also quickly become contaminated and should be kept as clean as possible, with any uneaten food removed.
Illness in humans and cats.
Human illness from E. coli O86 and from wild bird strains of salmonella is currently uncommon in the U.K., but there is the potential for spread to humans. Several years ago in England, S. Typhimurium DT 160 caused gastro-enteritis in humans who had eaten food prepared in a kitchen in which Sparrows were contaminating the food preparation areas, and this strain also caused disease in humans in New Zealand in 2000. An outbreak of gastro-enteritis in humans was seen in Norway and Finland in 1987, caused by a strain of S. Typhimurium U277 identical to that found in Bullfinches. A link with chocolate produced in one factory was established, and it was suspected that contamination from wild birds may have occurred. Investigations in Norway have also revealed that some “sporadic” cases of sickness and diarrhoea in humans are caused by wild bird strains of S. Typhimurium, especially in young children, and can be associated with direct contact with wild birds or their droppings or playing in areas where wild bird droppings have been observed. Rubber gloves should therefore be worn when cleaning bird tables or if the carcases of dead birds have to be handled, and hands must be thoroughly washed, especially before preparing food. Wild birds should be excluded from food preparation areas.
Pet cats may be at risk from salmonellosis from wild birds. An outbreak of disease in cats in Sweden was seen in February to April 1999, caused by wild bird strains of salmonella. Affected cats lost their appetite and many developed vomiting or diarrhoea. At the same time people were finding sick or dead wild finches, mostly Siskins and Redpolls, in gardens and the same strain of S. Typhimurium was isolated from some of the birds. It was concluded that the sick finches were easy prey for the cats which then became ill themselves. Spread to humans was also reported, either directly from wild birds or from cats infected from wild birds.
The decline in House Sparrows in Great Britain.
At one time considered to be a serious pest of cereal crops and crop stores, the breeding population of House Sparrows in G.B. has fallen from about 12-15 million pairs in the 1970’s to a current figure of around 6 million pairs. Declines were first seen in farmland but the greatest have been in urban and suburban gardens, and it is likely that different factors are responsible for the decline in the different habitats. Farmland is no longer as attractive to Sparrows as it once was, due to a multitude of factors, such as improvements in grain harvesting and storage, fewer weed seeds and invertebrates, changes in grassland management and fewer farmyards in which the birds can forage. However the greatest impact on total population size in Britain has been the decline in the urban and suburban populations. A report by the British Trust for Ornithology (BTO) to the Department for Environment, Food and Rural Affairs (DEFRA) in August 2002 suggested a number of possible reasons for the fall in the urban and suburban sparrows, including predation by cats and Sparrowhawks, a lack of nest sites due to house renovations, the development of Brown Field sites, and air pollution. The BTO reported also highlighted the possible importance of infectious disease, spread at bird tables and other forms of feeders in gardens.
Studies by SAC Veterinary Services have shown that infection with S. Typhimurium is a common cause of death in House Sparrows found dead in urban or suburban habitat by members of the public in the winter and spring. Between 1998 and 2002, salmonellosis was diagnosed in 22 of 24 House Sparrows (from nine different sites) submitted to the Avian Health Unit during the months of November to March. Monitoring wild bird faeces at garden feeding station in South-West Scotland has also shown that a strain of S. Typhimurium known to cause deaths in finches and House Sparrows can become established in flocks of wild birds feeding at bird tables and persist for over 30 months. It is therefore possible that infection with S. Typhimurium is a contributory factor to the decline in House Sparrows in some locations.
Acknowledgements.
Much of the work carried to investigate deaths in wild birds was funded by The Dulverton Trust and The Game Conservancy Trust. SAC also receives financial support from the Scottish Executive Environment and Rural Affairs Department. An additional generous donation from CJ Wildbird Foods Ltd, Upton Magna, Shrewsbury, will enable further monitoring to be carried out in 2004. The help of all organisation, companies and individuals involved is gratefully acknowledged.
Mr Tom W. Pennycott
Senior Veterinary Investigation Officer, Avian Health Unit
SAC Veterinary Services
Auchincruvie
Ayr. KA6 5AE.
January 2004.