Blue Green Algae Poisoning in Cattle
|Algae strewn water troughs such as the ones in this collage should be avoided on dairy farms
Blue green algae, also known as cyanobacteria, are microorganisms with characteristics that fall between those of bacteria and plants. They grow in water bodies in tropical, subtropical and temperate regions all around the world. In Michigan, blooms of blue green algae occur any time from late spring to early fall (June through September or October) but more especially in July and August.
For blooming to occur, the right combination of environmental conditions must exist and these include warm sunny weather with temperatures ranging from 50 to 86 degrees Fahrenheit, phosphorus concentrations in excess of 30 micrograms/liter, and high water nitrogen content. Some of these environmental conditions are consistent with water pollution, generally referred to as eutrophication.
Around the world blue green algal blooms are found both in fresh and brackish water bodies. In Michigan, blue green algal blooms are associated mainly with fresh water bodies. For the past 3 years there have been more than usual inquiries or reports of blue green algal poisoning in cattle in Michigan. In summer of 2008 there was an unconfirmed case on a dairy farm in Michigan in which nine out of 80 cows died suddenly.
Toxic Species and Compounds
There are hundreds of blue green algal species but only a handful are associated with poisoning in animals. Common poisonous species include Microcystins, Anabaena, Planktothrix, Nostoc, Oscillatoria, and Anabaenopsis. These species of blue green algae produce different kinds of toxins of which two classes -- microcystins and anatoxins -- are responsible for most animal deaths. Microcystins poison the liver, whereas anatoxins target the nervous system.
The majority of blue green algal poisoning is caused by microcystins. Naturally, these toxins are found inside algal cells, but stressful conditions such as treatment of water with algaecides or natural death of cells results in release of these potent toxins.
Cattle can be affected by drinking water containing toxins or intact blue green algal cells. In small lakes or large ponds of water, wind effect tends to concentrate the blooms on one side of the water body. Cattle are usually poisoned when they drink from the windward side of these stagnant water bodies where the blue green algae have accumulated.
However, toxic blue green algae also will grow in stagnant small paddles of water and in water collection vessels on farm if left uncleaned for a long time. The microcystins will poison the liver rather acutely. In some cases, affected cattle die within a few hours of exposure. In subacute cases, death may come in a day or so. Microcystin LR, the predominant prototype microcystin, causes massive centrilobubar hepatic necrosis.
Shock is a result of hemorrhage within the liver caused by massive liver injury. In a natural setting of an algal bloom several microcystins (with different potencies) are produced. In some cases anatoxins are present at the same time.
Anatoxins are the second class of blue green algal toxins most responsible for livestock poisoning in Michigan. The anatoxins are structurally different from the microcystins and are also structurally different among themselves.
As mentioned earlier, these target the nervous system. They are produced by Anabaena, Planktothrix, Oscillaria, and Microcystin species. Some of these same species also produce microcystins. It is possible to have both microcystins and anatoxins produced in an algal bloom.
Two common anatoxins are produced (i.e., Anatoxin a and Anatoxin a(s). Anatoxin a is a nicotinic agonist at the cholinergic receptors. Intoxication results in a very rapid (minutes to a few hours) onset of rigidity and muscle tremors, paralysis and death by respiratory paralysis.
On the other hand, Anatoxin a(s) is an irreversible acetylcholinesterase inhibitor. In this regard, its toxic mechanism is similar to that of organophosphorous or carbamate insecticides with the exception that Anatoxin a(s) does not cross the blood-brain barrier while the insecticides do.
Clinical signs are all related to the effects of this toxin to the peripheral nervous system and include salivation, lacrimation, urination, diarrhea (SLUD), tremors, ataxia and convulsions. Affected animals die of respiratory paralysis. There are no gross or histological lesions in animals that have died of anatoxin intoxication.
Death comes quickly, usually in a matter of minutes to hours and may occur in the vicinity of the contaminated water body. There is no antidote for Anatoxin a intoxication. Treatment is supportive and includes respiratory support and anti-seizures medications. Atropine is an antidote for Anatoxin a(s) intoxication. 2-PAM is not effective and should not be given. The rest of the treatment is supportive and involves respiratory support and anti-seizure medications.
Diagnosis of microcystin intoxication involves water analysis for microcystins, microscopic water and gastrointestinal content examination for the cyanobacteria, and tissue analysis for bound microcystins. For morphological identification, a water sample in a glass jar in the ratio of 50:50 water: neutral buffered formalin solution can be analyzed at the diagnostic lab. There is no antidote to microcystins and there is no proven effective therapy. In this regard, prevention is the cure.
Diagnosis for Anatoxin a(s) intoxication includes whole blood cholinesterase assay, analytical confirmation of the toxin in water, stomach contents, and in the tissues. The brain cholinesterase test is not useful as these toxins do not cross the blood-brain barrier. Morphological identification of toxin-generating algae in water is also beneficial.
Diagnosis of Anatoxin a intoxication commonly relies on morphological identification of toxin generating cyanobacteria in water or stomach contents as well as analytical confirmation for the toxin(s) in water, stomach contents and tissues. As is true for microcystins, methods for analytical confirmation for Anatoxins are not commonly available in veterinary diagnostic laboratories. Thus, blue green algal intoxication is probably under diagnosed.
In summary, blue green algal poisoning should be considered in cattle presented with acute episodes of liver and/or neurological disease.
These clinical signs are caused by microcystin and/or anatoxin toxin poisoning. Although these two classes of toxins are responsible for most cyanobacterial intoxications, there are other cyanobacterial toxins produced in fresh waters that potentially could cause toxicity with different clinical signs.
More information on health effects of blue green algae poisoning are online at http://www.cdc.gov/hab/cyanobacteria/facts.htm
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