Algal Poisoning: Introduction - The Merck Veterinary Manual

Algal Poisoning: Introduction

Etiology, Epidemiology, and Pathogenesis

Cyanobacteria waterbloom

Algal poisoning is often an acute, fatal condition caused by high concentrations of toxic blue-green algae (more commonly known as cyanobacteria—literally blue-green bacteria) in the drinking water. Fatalities and severe illness of livestock, pets, wildlife, birds, and fish from heavy growths of waterblooms of blue-green algae occur in almost all countries of the world. Poisoning usually occurs during warm seasons when the waterblooms are more intense and of longer duration. Most poisonings occur among animals drinking algal-infested freshwater, but marine animals, especially maricultured fish and shrimp, are also affected. The toxins of cyanobacteria comprise 5 distinct chemical classes collectively called cyanotoxins.

Etiology, Epidemiology, and Pathogenesis:

Although toxic strains within species of Anabaena , Aphanizomenon , Cylindrospermopsis , Microcystis , Nodularia , Nostoc, and Planktothrix (Oscillatoria) are responsible for most cases of toxicity, there are >30 species of cyanobacteria that can be associated with toxic waterblooms. Neurotoxic alkaloids (called anatoxins) can be produced by Anabaena , Aphanizomenon , and Planktothrix , while saxitoxins can be produced by Anabaena , Aphanizomenon , and Lyngbya . Hepatotoxic peptides (called microcystins and nodularins) can be produced by Anabaena , Microcystis , Nodularia , Nostoc , and Planktothrix . Cylindrospermopsis can produce a potent hepatotoxic alkaloid called cylindrospermopsin. Some genera, especially Anabaena , can produce both neuro- and hepatotoxins. If a toxic waterbloom contains both types of toxins, the neurotoxin signs are usually observed first because their effects occur much sooner (minutes) than the hepatotoxins (1 to a few hours).

Poisoning usually does not occur unless there is a heavy waterbloom that forms a dense surface scum. Factors that contribute to heavy waterblooms are nutrient-rich eutrophic to hypereutrophic water and warm, sunny weather. Agriculture practices (eg, runoff of fertilizers and animal wastes) that lead to nutrient enrichment often contribute to waterbloom formation. The problem is augmented by light winds or wind conditions that lead to leeward shore concentrations of cyanobacteria in areas where livestock drink. Experiments with both toxin groups have revealed a steep dose-response curve, with up to 90% of the lethal dose being ingested without measurable effect. Animal size and species sensitivity influence the degree of intoxication. Monogastric animals are less sensitive than ruminants and birds. Depending on bloom densities and toxin content, animals may need to ingest only a few ounces or up to several gallons to experience acute or lethal toxicity.

While the species sensitivity and signs of poisoning can vary depending on the type of exposure, the gross and histopathologic lesions are quite similar among species poisoned by the hepatopeptides and neurotoxic alkaloids. Death from hepatotoxicosis induced by cyclic peptides is generally accepted as being the result of intrahepatic hemorrhage and hypovolemic shock. This conclusion is based on large increases in liver weight as well as in hepatic hemoglobin and iron content that account for blood loss sufficient to induce irreversible shock. In animals that live more than a few hours, hyperkalemia or hypoglycemia, or both, may lead to death from liver failure within a few days.

Neurotoxicosis, with death occurring in minutes to a few hours from respiratory arrest, may result from ingestion of the cyanobacteria that produce neurotoxic alkaloids. Species and strains of Anabaena , Aphanizomenon , and Planktothrix can produce a potent, postsynaptic cholinergic (nicotinic) agonist called anatoxin-a that causes a depolarizing neuromuscular blockade. Strains of Anabaena can produce an irreversible organophosphate anticholinesterase called anatoxin-a(s). Anabaena , Aphanizomenon , and Lyngbya can produce the potent, presynaptic sodium channel blockers called saxitoxins.

Clinical Findings and Lesions:

One of the earliest effects (15-30 min) of microcystin poisoning is increased serum concentrations of bile acids, alkaline phosphatase, γ-glutamyltransferase, and AST. The WBC count and clotting times increase. Death may occur within a few hours (usually within 4-24 hr), up to a few days. Death may be preceded by coma, muscle tremors, paddling, and dyspnea. Watery or bloody diarrhea may also be seen. Gross lesions include hepatomegaly due mostly to intrahepatic hemorrhage. Intact clumps of greenish algae can be found in the stomach and GI tract, and there is a greenish algal stain on the mouth, nose, legs, and feet. Hepatic necrosis begins centrilobularly and proceeds to the periportal regions. Hepatocytes are disassociated and rounded. After death, debris from disassociated hepatocytes can be found in the pulmonary vessels and kidneys. Clinical signs of neurotoxicosis progress from muscle fasciculations to decreased movement, abdominal breathing, cyanosis, convulsions, and death. Signs in birds are similar but include opisthotonos. In smaller animals, death is often preceded by leaping movements. Cattle and horses that survive acute poisoning may experience photosensitization in areas exposed to light (nose, ears, and back), followed by hair loss and sloughing of the skin.

Diagnosis:

Diagnosis is based primarily on history (recent contact with an algal bloom), signs of poisoning, and necropsy findings. Samples of the waterbloom should be taken as soon as possible for microscopic examination to confirm the presence of the toxigenic cyanobacteria and for toxin analysis. Although there are nontoxic and toxic strains of all the known toxic species, it is not possible to identify a toxic strain by visual examination. Cyanobacteria are detected by light microscopy, identified using morphologic characteristics, and counted per standard volume of water. Standard protocols for sampling and monitoring cyanobacteria as well as practical keys for the identification of toxic species are available.

Some laboratories can analyze for the toxins either by chemical or biologic assay. Animal bioassays (mouse tests) have traditionally been used for detecting the presence of the entire range of cyanotoxins based on survival times and signs of poisoning. These tests provide a definitive indication of toxicity, although they cannot be used for precise quantification of compounds in water or for determining compliance with standards for environmental levels. A number of analytic techniques are available for determining microcystins in water. Analytic techniques must provide for quantitative comparison to the guideline value in terms of toxicity equivalents. The technique most suitable in this regard is high-performance liquid chromatography, although it may still involve estimation of the concentration, and therefore only an estimate of toxicity. Commercial standards for some microcystins, nodularin, and saxitoxins are available, while those for anatoxins and cylindrospermopsin should be available shortly. Newer methods of immunoassay are also available, including commercial ELISA kits in both laboratory and field formats.

Treatment:

After removal from the contaminated water supply, affected animals should be placed in a protected area out of direct sunlight. Ample quantities of water and good quality feed should be made available. Because the toxins have a steep dose-response curve, surviving animals have a good chance for recovery. While therapies for cyanobacterial poisonings have not been investigated in detail, activated charcoal slurry is likely to be of benefit. In laboratory studies, an ion-exchange resin such as cholestyramine has proved useful to absorb the toxins from the GI tract, and certain bile acid transport blockers such as cyclosporin A, rifampin, and silymarin injected before dosing of microcystin have been effective in preventing hepatotoxicity. No therapeutic antagonist has been found effective against anatoxin-a, cylindrospermopsin, or the saxitoxins, but atropine and activated charcoal reduce the muscarinic effects of the anticholinesterase anatoxin-a(s).

Prevention:

Removal of animals 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. Some efforts have been made to erect surface barriers (logs or floating plastic booms) to keep shore areas free of surface scum, but these are not very successful. Cyanobacteria can be controlled by adding copper sulfate (CuSO₄) or other algicidal treatments to the water. The usual treatment is 0.2-0.4 ppm, equivalent to 0.65-1.3 oz/10,000 gal. of water or 1.4-2.8 lb/acre-foot of water. Livestock (especially sheep) should not be watered for at least 5 days after the last visible evidence of the algal bloom. CuSO₄ is best used to prevent bloom formation, and care should be taken to avoid water that has dead algae cells, either from treatment with algicide or natural aging of the bloom, because most toxin is freed in the water only after breakdown of the intact algae cells.

Source water management techniques for control of cyanobacterial growth include flow maintenance in regulated rivers, water mixing techniques for both the elimination of stratification and the reduction of nutrient release from sediments in reservoirs, and the use of algicides in dedicated water supply storages. Algicides will disrupt cells and liberate intracellular toxins. Algicide use should be in accordance with local environment and chemical registration regulations. In situations where multiple offtakes are available, the selective withdrawal of water from different depths can minimize the intake of high surface accumulations of cyanobacterial cells.

Water treatment techniques can be highly effective for removal of both cyanobacterial cells and microcystins with the appropriate technology. As with other cyanotoxins, a high proportion of microcystins remain intracellular unless cells are lysed or damaged, and can therefore be removed by coagulation and filtration in a conventional treatment plant. Treatment of water containing cyanobacterial cells with oxidants such as chlorine or ozone, while killing cells, will result in the release of free toxin. Therefore, the practice of prechlorination or preozonation is not recommended without a subsequent step to remove dissolved toxins.

Microcystins are readily oxidized by a range of oxidants, including ozone and chlorine. Adequate contact time and pH control are needed to achieve optimal removal of these compounds, which will be more difficult in the presence of whole cells. Microcystins, anatoxin-a, cylindrospermopsin and some saxitoxins are also adsorbed from solution by both granular activated carbon and, less efficiently, by powdered activated carbon. The effectiveness of the process should be determined by monitoring toxin in the product water.