Veterinary Science
Division, Avian Health Unit, Ayr
Outbreaks of mortality
in wild birds in gardens in the U.K. were first reported in the
mid 1960s, 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 and occurred in
greenfinches (Carduelis chloris) and house sparrows (Passer
domesticus).
Mortality incidents have
continued, and since 1994 post mortem examinations have been
carried out by the Veterinary Science Division of S.A.C. on over
200 finches or sparrows found dead in Scotland. The
results from these investigations have shown that, in addition to
Salmonella typhimurium, a strain of the bacterium Escherichia
coli referred to as E. coli O86
is responsible for many
of the deaths in wild birds.
During these seven years
a seasonal pattern has emerged. Salmonella
typhimurium (often a particular type referred to S.
typhimurium DT40) typically causes mortality in the months
December to March, mostly in greenfinches and to a lesser extent
in house sparrows. E. coli O86 usually causes
mortality in the months March to June, especially in greenfinches
and siskins (Carduelis spinus). Both bacteria
have also caused deaths in goldfinches (Carduelis carduelis)
and chaffinches (Fringilla coelebs), although in smaller
numbers, and occasionally Salmonella spills over into
other species such as the great tit (Parus major). Dead
birds or sick birds are usually found in the vicinity of the bird
feeders. If seen alive the birds appear fluffed up,
reluctant to fly, appear to be breathing heavily and may look as
if they are having difficulty in swallowing.
Fig 3, Fig 4, Fig 5.
Deaths from E coli O86 have occurred in (left to right)
goldfinches, siskins and chaffinches.
The post mortem
examination of birds dying from salmonellosis often reveals
substantial yellow/orange areas of damage to internal organs such
as the gullet, liver, spleen (an organ that 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 bird’s
stomach even if it continues to eat. The post mortem findings in
birds dying from E. coli O86 are different. This
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 special laboratory media for the
culture and identification of the bacteria from the carcases.
Fig 6, Fig 7, Fig 8.
Post mortem lesions of salmonellosis – (left to right)
ulceration and necrosis (cell death) of oesophagus (gullet);
necrotic spots in liver; enlarged spleen with necrotic areas.
Fig 9 and Fig 10. Post
mortem appearance of two siskins with E coli O86 infection –
no necrotic areas but typically oesophagus (and sometimes gizzard)
packed with food material such as peanuts, sunflower seed etc.
Deaths in garden birds
in the U.K. have become a regular occurrence each winter, and the
winter of 2000-2001 looks like being the same. On
January 12th 2001 The Royal Society for the Protection
of Birds reported unprecedented numbers of telephone calls from
the public reporting sick and dead birds in gardens. Some
dead birds have been examined by the Zoological Society of London,
some by the Veterinary Science Division of SAC in Scotland, and
the results confirm that Salmonella is again the major
cause of the mortality. So far in 2001, salmonellosis has been
confirmed by SAC in the greenfinch, chaffinch, goldfinch, house
sparrow, and also the tree sparrow (Passer montanus),
another bird whose population in the U.K. is in dramatic decline.
Deaths in siskins in the second week of February 2001 were the
result not of salmonellosis but of E coli O86 infection,
which in past years has usually occurred later in the year, from
March onwards.
Fig 11 and Fig 12. In
January 2001 deaths from salmonellosis were diagnosed in tree
sparrows, another species of bird whose population is in decline
in the UK. As before, substantial damage to the oesophagus can be
seen.
Deaths from
salmonellosis have not been confined to sparrows and finches at
bird tables in the U.K. Since 1988, many finches have
been found dead around garden feeders in the United States and
Canada, mostly pine siskins (Carduelis pinus) and common
redpolls (Carduelis flammea), also evening grosbeaks (Coccothraustes
vespertinus), house sparrows, and American goldfinches (Carduelis
tristis). As in the U.K. incidents, the type of Salmonella
involved was Salmonella typhimurium phage type 40.
Cases of salmonellosis were also reported in domestic cats that
had preyed on sick birds around bird feeders.
A different strain of Salmonella
typhimurium described as S. typhimurium DT160 caused
outbreaks of mortality in house sparrows in New Zealand in 1999/2000.
At the same time there was evidence of disease in humans (including
one death), and in young farmed ducks and quail, dogs and cats,
deer and horses. In one incident more than 400 dead
birds were found at one location on one day. This
strain of Salmonella typhimurium also caused a small
outbreak of mortality in house sparrows in Central Newfoundland,
Canada, in February/March 1999.
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 Salmonella
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 occurs, contaminating the food and
water, the feeders and drinkers, and the surrounding environment.
Under these conditions, the bacteria then have the chance to
overwhelm the birds and cause their deaths.
Clearly prevention is
very important, and is based on preventing a build up of these
potentially lethal bacteria. Regular cleaning and
disinfection of bird tables, feeders and drinkers will help, as
will a 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.
This greater awareness
of the causes of mortality in finches comes at a time when the
populations of some of the UK’s wild birds are in decline,
especially woodland species (down by 20% since the mid 1970s) and
farmland species (down by 40% in the same period). In
the report The State of the UK’s Birds 1999,
published by The Royal Society for the Protection of Birds and
The British Trust for Ornithology, attention is drawn to the
continued and alarming decline of once-common species such as the
redpoll (down by 92% since the 1970s), the house sparrow (down by
58%) and the tree sparrow (down by 87%), three bird species known
to be susceptible to salmonellosis. Indeed, the
Report suggests that, such is the severity of the decline in
house sparrow and redpoll numbers, they should be considered for
inclusion on the Birds of Conservation Concern (BoCC) red list of
endangered species.
In addition to causing
disease in finches, E. coli O86 and Salmonella
typhimurium phage types 40 and 160 can also occasionally
cause disease in humans, especially young babies. 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.
Much remains unknown
about the occurrence of E. coli O86 and Salmonella
typhimurium in healthy and sick wild birds of different
species, and about the factors that allow these organisms to
build up and cause disease. Two Trusts with an
interest in wild bird conservation, The Dulverton Trust
and The Game Conservancy Trust, are therefore funding a
three-year study to look at the significance of these organisms
in wild birds.
To further investigate
the role of healthy carrier wild birds feeding at bird tables,
composite samples of droppings from apparently healthy wild birds
are being collected on a regular basis from two sites in
southwest Scotland and tested for Salmonella, E. coli O86,
and other important bacteria.
One of the sites (Site A)
has a history of deaths in wild birds from Salmonella and E.
coli O86 in previous years, the other site (Site B) has no
history of mortality in garden birds. Thirty-five
samples have been collected at Site A from September 2000 to the
end of January 2001, and 30 samples from Site B between October
2000 and the end of January 2001. On Site A, the site
with the history of deaths in garden birds in previous years, 13
of the 35 samples were positive for Salmonella typhimurium
and two for E. coli O86. One sample was also
positive Yersinia enterocolitica, another bacterium that
can cause illness in humans but doesn’t cause problems in
birds.
In contrast, on
Site B where there has been no such history of illness in birds,
only one of 30 samples was positive for Salmonella (a
different phage type of Salmonella typhimurium) and none
for E. coli O86. However six of the samples
from Site B were positive for Yersinia enterocolitica. Most
of the isolations of Salmonella, E. coli O86 and Yersinia
enterocolitica occurred in December and January. Despite
the frequent isolation of Salmonella from the faeces at
Site A, few ill birds have been seen and salmonellosis has only
been confirmed in one bird, a chaffinch, from this site to the
end of January.
Although this project is
at a very early stage some interesting (and perhaps surprising)
trends are beginning to emerge. The examination of
wild bird faeces at bird tables has shown that a remarkably high
proportion of samples of pooled faeces from some bird tables may
be positive for Salmonella without major mortality being
observed. It is possible that the majority of birds
at Site A remained healthy despite the presence of Salmonella,
or alternatively wild birds were indeed dying but their carcases
were not found. It is therefore possible that the
same thing is happening at other bird tables, and that the extent
of the problem of salmonellosis in garden birds is far greater
than is currently realised. However the study also
highlights the differences between populations of wild birds at
different sites. This difference in bacterial
populations in birds at different sites may help to explain why
deaths in garden birds are observed in some areas but not others,
and possible reasons for the differences will be explored.
This study will continue
in 2001 and 2002, but the results so far support the view that
potentially dangerous bacteria build up around some bird tables
in the late autumn, posing a threat to the birds feeding there.
The results also reinforce the fact that wild bird droppings at
bird tables frequently contain bacteria that can cause illness in
humans, and proper personal hygiene must be observed after
cleaning bird tables or handling sick or dead birds.
Anybody finding dead
garden birds can contact Tom Pennycott, Senior Veterinary
Investigation Officer of the Avian Health Unit, S.A.C. Veterinary
Science Division, Ayr at 01292 520318 for further information and
advice. If a post mortem examination of small birds such as
garden finches is to be carried out and the bodies are to be sent
by Royal Mail, they must be securely packaged as follows:
a)
Wrap the body in sufficient absorbent material (e.g. kitchen roll)
to absorb all possible leakage.
b)
Place the wrapped body in a leak-proof plastic bag (e.g. a
freezer bag) and seal with a metal or plastic tie or something
similar.
c)
Place the sealed bag in a strong clip-down container (e.g. a
margarine container) or a strong cardboard or polystyrene box,
filling any empty space with absorbent material and sealing the
container with self-adhesive tape.
d)
Details such as name, address and telephone number of sender, and
any other additional relevant information such as number and
species of birds found dead, should be enclosed in a separate
sealed plastic bag and attached to the outside of the container.
e)
All the above should be secured in a padded bag or covered by
strong brown paper.
f)
The outer cover should be labelled in bold capitals “PATHOLOGICAL
SPECIMENS – FRAGILE – HANDLE WITH CARE” and
addressed to Tom Pennycott, Avian Health Unit, S.A.C. Veterinary
Science Division, Auchincruive, Ayr, KA6 5AE. The
sender’s name and address should also be included on the
outer cover.
Mr.
Tom W. Pennycott
Senior
Veterinary Investigation Officer, Avian Health Unit
S.A.C.
Veterinary Science Division
Auchincruive
Ayr KA6 5AE
February 12th
2001