Several species of blue-green algae are capable of producing toxins. Cyanotoxins commonly associated with freshwater algal blooms include microcystins/nodularins (hepatotoxic), cylindrospermopsin (hepatotoxic), anatoxin-a/anatoxin-a(s) (neurotoxic), and saxitoxins (ie, paralytic shellfish poison; neurotoxic). Microcystins and anatoxin-a/anatoxin-a(s) are the most common cyanotoxins in North America and occur across the country. Cyanotoxins can affect any species. The following discussion focuses on the effects of microcystins and anatoxin-a/anatoxin-a(s) on dogs.
Microcystins are cyclic heptapeptides that inhibit serine threonine protein phosphatases and lead to reorganization of hepatocyte infrastructure and eventual hepatocyte apoptosis.3,4 Microcystins are transported into cells by organic anion transport proteins, which are highly expressed in hepatocytes in relation to other cell types.5,6 Increased production of necessary transport proteins in hepatocytes can explain the propensity for microcystins to target the liver following ingestion; this could also be because the liver is the first organ exposed to ingested toxins absorbed from the GI tract. There are reports of microcystin intoxication causing lesions in the kidney.7,8
Anatoxin-a is a bicyclic amine alkaloid that is a potent acetylcholine-receptor agonist and exhibits greater affinity for nicotinic receptors than for muscarinic receptors.2,9 Anatoxin-a(s) is a guanidine methyl phosphate ester that is a naturally occurring organophosphate and potent inhibitor of acetylcholinesterase.10,11
Although predicting the occurrence of algal blooms can be challenging, common environmental conditions exist, including warm (>70°F [21°C]), stagnant waters with some mechanism of nutrient accumulation. Nitrogen and phosphorus are the main nutrients implicated in eutrophication (ie, abnormal accumulation of nutrients, commonly due to surface runoff), which leads to harmful algal blooms that often appear as a thick scum, similar to paint, on the surface of a body of water.12 Nitrogen and phosphorus can enter bodies of water from many sources (eg, fossil fuel combustion; stormwater discharge; groundwater pollution; agricultural and residential fertilizers; urban, industrial, and agricultural waste).13 Cyanotoxin poisonings typically occur in late summer or early fall, as water temperatures rise and rainfall tapers off.