Detection of anatoxin-producing Phormidium in a New Zealand farm pond and an associated dog death

Over the last decade, proliferations of benthic mat-forming Phormidium have been reported with increasing frequency in New Zealand’s cobble-bed rivers. The dominant species within these mats (Phormidium autumnale) commonly produces a range of neurotoxins, collectively known as anatoxins. Ingestion of toxic Phormidium-dominated mats has resulted in the death of an estimated 100 dogs at New Zealand’s rivers since 2005. In September 2014, a 12-week-old border terrier died rapidly after contact with floating ‘algal mats’ at a small farm pond near Kaikoura (South Island, New Zealand). The aim of this study was to establish whether ingestion of the algal mats was the cause of the death and if the dominant species was similar to that found in rivers. Environmental samples from the pond and the dog’s stomach contents contained moderate levels of anatoxin-a (25 µg L−1 and 36 µg kg−1 wet weight, respectively) and high levels of dihydroanatoxin-a (2,118 µg L−1 and 4,437 µg kg−1, respectively). The dominant organism in the environmental sample was cultured and a polyphasic approach used for characterisation. Morphological analysis of environmental samples and six isolates indicated the dominant species was most similar to Ph. autumnale. Further confirmation was provided by 16S ribosomal RNA gene sequences which showed a high percentage identity (≥ 99%) to strains isolated from Phormidium-dominated mats in New Zealand rivers. Identification of anatoxin-a and dihydroanatoxin-a in the stomach contents, environmental samples and isolates, in concert with the rapid dog death provide strong evidence that anatoxin poisoning was the cause of death. To our knowledge this is the first report in New Zealand of a dog death related to ingestion of toxic Phormidium mats from a farm pond. This study highlights the wide diversity of habitats that toxin-producing Phormidium can inhabit, and the need for greater awareness of the risks Phormidium-dominated mats pose to human and animal health.

近十年来，新西兰砾石底河流中底栖席状生长的席藻属（Phormidium）种群增殖现象的报道频率日益升高。这些席体中的优势物种为秋生席藻（Phormidium autumnale），其通常可产生一类统称为鱼腥藻毒素（anatoxins）的神经毒素。自2005年以来，新西兰河流中因误食以有毒席藻属物种为主的席体而死亡的犬只数量已达约100只。2014年9月，新西兰南岛凯库拉附近一处小型农场池塘中，一只12周大的边境㹴在接触漂浮的“藻席”后迅速死亡。本研究旨在明确该犬只的死亡是否因误食藻席所致，以及其优势藻种是否与河流中发现的席藻属物种一致。该池塘的环境样本以及该犬只的胃内容物中均检出中等浓度的鱼腥藻毒素a（anatoxin-a）（分别为25 μg·L⁻¹和36 μg·kg⁻¹湿重），以及高浓度的双氢鱼腥藻毒素a（dihydroanatoxin-a）（分别为2118 μg·L⁻¹和4437 μg·kg⁻¹湿重）。研究人员对环境样本中的优势生物体进行了分离培养，并采用多相分类法（polyphasic approach）开展分类鉴定。对环境样本及6株分离株的形态学分析结果显示，其优势物种与秋生席藻（Phormidium autumnale）高度相似。16S核糖体RNA基因（16S ribosomal RNA gene）序列分析进一步验证了这一结果：该序列与新西兰河流中从席藻属主导席体分离得到的菌株的同源性高达99%以上。胃内容物、环境样本及分离株中均检出鱼腥藻毒素a与双氢鱼腥藻毒素a，结合该犬只的快速死亡现象，可为“鱼腥藻毒素中毒是其死亡原因”提供强有力的证据。据我们所知，这是新西兰首例报道犬只因误食农场池塘中的有毒席藻属藻席而死亡的案例。本研究凸显了产毒席藻属物种可栖息的生境类型极为多样，同时也提醒人们需进一步提升对席藻属主导藻体所带来的人类与动物健康风险的认知。