This report outlines a case in which approximately 100 of 1200 young merino ewes died suddenly, following treatment of a dam using copper sulphate. The cause of death is suspected to be a cyanobacterial neurotoxin.
A producer in the southern Central West Local Land Services region contacted one of us, (AS), after finding approximately 100 of 1200 young merino ewes dead around one of the two dams in a paddock of irrigated sorghum in late March 2018.
The property owner had recently noticed an algal bloom on the surface of this dam. After seeking local advice, he had treated the algae using copper sulphate. The previous afternoon, a 'couple of handfuls' of copper sulphate granules were sprinkled by hand directly over the bloom, where it had blown to one side of the dam. Most carcases were located within 50 metres of this dam.
The dead animals were in lateral recumbency, and many displayed evidence of a struggle, with paddle marks around the head and feet.
Post-mortem examination was conducted on several carcases. On the basis of decomposition, death was estimated to have occurred at least 12 hours earlier, and autolysis was too advanced to identify any detailed gross pathology. Brief survey of the livers of most animals appeared to be within normal limits, although the liver of one sheep was slightly enlarged, with tan discolouration and red mottling of the surface extending into the liver parenchyma. Jaundice was not present in any of the carcases, eliminating copper poisoning as a cause of the deaths.
Dam water analysis found a sparse population of Microcystis algae present. While autolysis affected histopathological interpretation of the fixed liver sample submitted, the pathologist did not observe hepatic necrosis, the expected finding in cases of blue green algae poisoning where hepatotoxins are involved.
Blue green algae are also known as cyanobacteria. There are many different varieties, and some of these species produce toxins. Hepatotoxins and neurotoxins may be considered the most significant in terms of livestock disease and mortality.
The clinical findings in cases of blue green algae poisoning can vary from sudden death due to massive hepatic necrosis or the effects of neurotoxins, to photosensitisation and sub-acute mortality, depending on the type and quantity of toxin consumed. Post-mortem findings may include gross liver pathology in cases where hepatotoxins are involved, or no specific lesions, where neurotoxicosis is suspected.
Toxins produced by blue green algae are stored within the algal cell, and are only released into the water when the algae die, although 'leakage' of toxin from the cell can also occur when algae are exposed to an acidic stomach environment. Intermittent, small-scale algae death or cell turnover, may mean that a water source has only low levels of toxins present. However, death of large numbers of algae, such as when treating an algal bloom, may liberate large quantities of toxin into the water supply rendering it highly poisonous.
Historically, copper sulphate has been widely used to treat algae in farm dams. Other commercial products are now available, and approved specifically for the control of blue green algae. Use of any algicide increases the risk of poisoning due to simultaneous algae death and toxin release. Copper sulphate is no longer recommended, both because it also kills other forms of aquatic life, and because it can be toxic to ruminants, particularly those which have pre-existing liver damage due to pyrrolizidine alkaloid poisoning.
Dam water analysis in this case only yielded Microcystis algae. Microcystis aeruginosa produces microcystin LR, a known hepatotoxin. Other toxic blue green algae that are known to produce neurotoxins such as Anabaena and Aphanizomenon were not identified, however could have been present prior to copper sulphate treatment.
While not able to be confirmed through laboratory testing, the history of treatment with copper sulphate, presentation of sudden death around the dam, and lack of gross pathology at post-mortem in this case are consistent with a diagnosis of neurotoxic blue green algae poisoning. This should provide a reminder of the risks of using algicides in farm dams where blue green algae are present.