The main culprit in the Salton Sea’s bird die offs
is avian botulism. The bacteria Clostridium botulinum, responsible for botulism
outbreaks, is a common bacteria in wetland ecosystems. It is found in wetland
and in the tissues of most wetland inhabitants, including aquatic insects,
mollusks, crustacea, and many vertebrates, including healthy birds. It can
cause explosive outbreaks in waterfowl, shorebirds and recently, pelicans
and other fish-eating birds at the Salton Sea.
Little is known about the natural
factors that cause the bacteria to leave the spore state and begin producing
toxin. Several factors may play a role, including the bacterial host strain
and environmental characteristics, such as temperature and salinity. Once
birds are infected, a maggot cycle can begin and spread the bacteria to large
numbers and species of birds.
There are two types of botulism that have been found at the Salton Sea. Type
C botulism is typical for waterfowl and the most common botulism strain found
in wildlife. Type C botulism poses virtually no human health risks. The second
type of botulism is Type E. This strain has been found largely in fish eating
birds. It is much more rare than Type C, but poses greater risks to humans.
There have been cases of Type E botulism in humans mostly resulting from the
consumption of improperly prepared fish.
Toxin production takes place in decaying animal carcasses. Flies deposit eggs
on carcasses, which are fed upon by resulting maggots. These maggots then
concentrate the toxin, and the toxic maggots are ingested by birds. These
birds then die, leading to the proliferation of more maggots. As the cycle
accelerates, major die-offs occur. Fish eating birds, such as pelicans, are
believed to get sick when they eat fish that have concentrated the toxin in
their intestines. These dying fish become easy prey for the birds that then
ingest fatal doses of the toxin.
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Birds initially lose ability to control muscles and appear weak and limp.
They have an inability to fly, followed by the inability to walk. The infected
bird may be able to propel itself using its wings to “paddle”
over land or water. Eventually, control of neck muscles is lost and the bird
can no longer hold up its head, resulting in drowning.
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Due to the rapid spread of botulism through the maggot cycle, carcass removal
is critical to prevent and minimize the spread of the disease. Carcass removal
at the Salton Sea is as prompt and as complete as possible, using airboats
that have been purchased specifically for the disease program. Birds in the
early stages of botulism are sent to rehabilitation centers, where they are
cared for and later released. Surveillance continues after the end of an outbreak
to insure that the event does not reoccur.
Those working on an outbreak wear rubber gloves and bag all carcasses. These
are then incinerated immediately. Disinfection of equipment and clothing is
not necessary as this disease spreads mainly through a maggot driven cycle.
In some cases, water is drawn off an impoundment if it proves to be a "hot
spot". However, quick removal of all carcasses to prevent maggot growth
is the most important method to control the spread of botulism.
At the beginning of an outbreak event, several carcasses of various species
are collected for analysis to determine the presence of botulism toxin and
the particular strain. Specimens are double bagged and packed into a cooler
with blue ice and newspaper. Information detailing where specimens were found
and any symptoms seen in the field are included. For further information on
our wildlife disease program, please call 760-348-5278.
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