Merck Manual

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Pet Owner Version

Algal Poisoning


Wayne W. Carmichael

, PhD, Wright State University

Reviewed/Revised Oct 2020 | Modified Nov 2022

Algal poisoning is a severe and deadly condition caused by heavy growths of toxic blue-green algae in water, including drinking water and water used for agriculture, recreation, and aquaculture. Deaths and severe illness of livestock, pets, wildlife, birds, and fish occur worldwide. Poisoning usually occurs during warm seasons when the algal water blooms are larger and last longer. Most poisonings are seen among animals drinking algae-infested fresh water. Animal size and species sensitivity influence the degree of poisoning. Depending on bloom densities and toxin content, animals may need to ingest only a few ounces or up to several gallons of water to be poisoned.

More than 30 species of blue-green algae, producing a number of different toxins, have been associated with toxic water blooms. Some species of algae produce toxins that damage nerve tissue. Others produce toxins that damage the liver, and others damage both nerve tissue and the liver. Death occurs within a few hours when the nervous system is affected because animals stop breathing. Liver failure causes death within a few days.

An affected animal may have muscle tremors, watery or bloody diarrhea, difficulty breathing, and go into a coma before death. A greenish algal stain may be seen on the mouth, nose, legs, and feet. When nerve tissue is affected, signs progress from muscle spasms to decreased movement, abdominal breathing, a bluish tinge to the skin and mucous membranes, convulsions, and death. Signs in birds are similar and also include spasm of the back muscles, which causes the head and legs to bend backward and the trunk to arch forward. In smaller animals, leaping movements often occur before death. In horses that survive sudden poisoning, the nose, ears, and back become sensitive to light, followed by hair loss and skin sloughing.

Affected animals should be moved to a protected area out of direct sunlight, away from the contaminated water supply. Ample quantities of water and good quality feed should be made available. Surviving animals have a good chance for recovery. Activated charcoal slurry may be beneficial if given shortly after toxin ingestion.

Keeping animals away from the affected water supply is essential. If no other water supply is available, animals should be allowed to drink only from shore areas kept free (by prevailing winds) of dense surface scums of algae. Cyanobacteria can be controlled by adding copper sulfate or other algicidal treatments to the water. Copper sulfate is best used to prevent bloom formation, and care should be taken to avoid water that contains dead algae cells, either from treatment with algaecide or natural aging of the bloom. This is because toxin is freed into the water after breakdown of the intact algae cell walls. Algaecide use should comply with local environmental and chemical registration regulations.

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