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Botulism in Birds
Avian botulism is a paralytic disease caused by ingestion of a
toxin produced by the bacteria, Clostridium botulinum. This
bacteria is widespread in soil and requires warm temperatures, a
protein source and an anaerobic (no oxygen) environment in order
to become active and produce toxin. Decomposing vegetation and
invertebrates combined with warm temperatures can provide ideal
conditions for the botulism bacteria to activate and produce
toxin. There are several types of toxin produced by strains of
this bacteria with birds being most commonly affected by type C
and to a lesser extent type E.
Birds either ingest the toxin directly or may eat invertebrates
(e.g. chironomids, fly larvae) containing the toxin.
Invertebrates are not affected by the toxin and store it in their
body. A cycle develops in a botulism outbreak when fly larvae
(maggots), feed on animal carcasses and ingest toxin. Ducks that
consume toxin-laden maggots can develop botulism after eating as
few as 3 or 4 maggots.
Clinical Signs/Field Signs
Healthy birds, affected birds, and dead birds in various stages
of decay are commonly found in the same area. The toxin affects
the nervous system by preventing impulse transmission to muscles
which results in flaccid paralysis. Consequently, birds are
unable to use their wings and legs normally or control the third
eyelid, neck muscles, and other muscles. Birds with paralyzed
neck muscles cannot hold their heads up and often drown. Death
can also result from water deprivation, electrolyte imbalance,
respiratory failure, or predation.
Lesions
There are no specific lesions associated with this disease.
Diagnosis of botulism is based on demonstration of the toxin in
serum from sick birds, or tissue samples from dead birds such as
clotted heart blood, stomach contents, or liver.
Wildlife Management Significance
Outbreaks occur from coast to coast in the United States and
Canada, generally from July through September. Thousands of birds
may die during a single outbreak.
Prompt removal and proper disposal of carcasses by burial or
burning (in accordance with applicable ordinances) is highly
effective in removing toxin and maggot sources from the
environment. If possible avoid altering water depth by flooding
or drawing down water levels during hot weather. This may
increase invertebrate and fish die-offs, a protein source for the
bacteria.
Providing mildly affected birds with fresh water, shade and
protection from predators may help them recover from the
intoxication. Botulism antitoxin is available but requires
special handling and must be given early in the intoxication.
Birds that survive a botulism outbreak are NOT immune to botulism
toxin.
Public Health Significance
Botulism in people is usually the result of eating improperly
home-canned foods, which contain types A or B toxin. Type E toxin
has been associated with improperly smoked fish. People, dogs,
and cats are generally thought to be resistant to type C toxin,
but a few cases have been reported in people and dogs. Thorough
cooking destroys botulism toxin in food.
Cause
Bacteria -- Clostridium botulinum, Types C and E toxins
Host
Type C toxin: waterfowl, shorebirds, colonial waterbirds, and
others
Type E toxin: gulls, loons, and others
