Sonny Bono Salton Sea National Wildlife Refuge Complex
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Wildlife Disease

Avian Botulism

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 sediGull (upper right), American White Pelican (lower left).  (USFWS)ment 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.


Mode of transmission

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.
Disease Control.  (USFWS)
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Disease Control

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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Last updated: May 23, 2011