ONTARIO: Neurotoxic cyanobacterium (blue-green alga) toxicosis in Ontario

In May 2006, 3 dogs (2 Labrador retrievers and a weimaraner) in a group of 11 dogs died suddenly and unexpectedly within 1 h after swimming in a local pond at a dog sitter’s farm. The dogs had spent approximately 5 min at the pond during a supervised walk around the farm, and several of the dogs were observed swimming in the pond and eating the vegetation. When the dogs arrived back at the house, 2 dogs became weak and collapsed, with shallow breathing. The 3rd dog developed similar signs within minutes. A 4th dog, a Labrador retriever-cross, subsequently developed similar signs, but survived. Hyperglycemia and acidosis were documented from 2 of the dogs from which antemortem blood was collected. The only dog submitted for postmortem examination had no gross or microscopic lesions in tissues. Gastric content was negative for organophosphates, carbamates, strychnine, and the mycotoxins penetrim A and roquefortine. Brain cholinesterase activity was reduced at 0.6 μmol/g/min (reference range, 3.2 +/− 1.6 μmol/g/min). Pond water and stomach contents were positive for the neurotoxin anatoxin-a and negative for the hepatotoxin microcystins by liquid chromatography tandem mass spectrometry (LC/MS/MS). Blue-green alga produces 4 types of toxins: hepatotoxins, neurotoxins, lipopolysaccharide endotoxins, and cytotoxins. Hepatotoxins, particularly microcystins, appear to be the most common toxins identified in toxic blooms. Neurotoxins appear to be much rarer in toxic blooms. The most common blue-green algal neurotoxin is anatoxin-a, but all the neurotoxins interfere with the transmission of signals in neurons or across the neuromuscular junction, leading to muscular paralysis and, in severe cases, death due to respiratory failure. Animal poisoning associated with toxic blooms of cyanobacterium have been reported in all continents, except Antarctica, (1,2) and there have been a few reports of domestic animals and wildlife consuming freshwater contaminated with toxic blue-green alga blooms in Canada (3). These reports have been predominantly microcystin-containing hepatotoxic blooms occurring in Saskatchewan and Alberta. Microcystins have been identified in Ontario in the past few years as causing disease in dogs and humans. The authors could find no reports of neurotoxic cyanobactrial blooms from Ontario affecting domestic animals. This report of neurotoxic cyanobacterial (blue-green alga) toxicosis in dogs is noteworthy in that it may be the first report of this in Ontario and in that the early seasonal occurrence, in May rather than in late summer, was unexpected.

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