Zinc phosphide is a rodenticide that has been used since the 1930s, although only registered for use in Australia since 1997. Zinc phosphide treated grain was used extensively in recent mice plagues to control mice damage in crops through perimeter baiting of crops. A recent case of zinc phosphide toxicity in ewes was primarily of interest due to a similar post-mortem appearance to anthrax. Anthrax is an important cause of death in the western Riverina in summer, generally presenting as rapid death, with generalised haemorrhagic lesions, proceeding a brief period of significant pyrexia.
Crossbred ewes of mixed ages which had been in a wheat stubble paddock for 3 weeks and had just commenced lambing were found dead or dying, most within 50-100 metres of the dam, in early March 2015.
Clinical signs observed included rapid respiration and elevated body temperatures (to 42.1C). Of the affected ewes that were still alive, most were in lateral and sternal recumbency with tetanic spasms and spastic paralysis of their limbs. Those still standing appeared unaware of their surroundings and were ataxic when pushed.
Freshly dead ewes had either blood-tinged stable foam or non-clotting dark blood discharges from the nose.
Two freshly dead ewes were initially tested for anthrax using ICT test kits on blood taken from the nasal discharge. Both tests were negative.
Post-mortem lesions were generally haemorrhagic in nature and included generalised ecchymotic and petechial haemorrhages observed in the subcutis, liver, lungs, endocardium and epicardium. The duodenum also had large splash haemorrhages over the serosal surface. The spleen was swollen with pulpy contents and splash haemorrhages over the surface.
A small quantity of wheat grain was found in the rumen, leading the owner to check a bulker bag of poisoned wheat in the corner of the paddock and to a diagnosis of zinc phosphide toxicity.
Dam water tested negative for blue-green algae.
Samples were submitted for TSE exclusion and were negative. The main histopathology findings were generally non-specific, including degenerative changes in the liver and kidney, and mild to moderate vacuolation of the deep grey matter layer of the cerebral cortex, either due to oedema or an artifact. Histopathology of the heart confirmed the endocardial haemorrhage, extending into subendocardial myocardium (Erika Bunker, veterinary pathologist, EMAI). There were no histopathological lesions observed in the spleen.
No available treatment was known for this condition at the time. The immediate treatment of the mob with oral sodium bicarbonate or magnesium hydroxide to reduce acid production may have reduced phosphine gas production. In this case, deaths were limited to those already dead or showing clinical signs at the time of my visit, possibly indicative of only those sheep having found the toxic grain. A total of 25 affected sheep died within 24 hours of the onset of clinical signs.
Zinc phosphide is the active ingredient in a variety of pellets and tracking powders used as rodenticides. A common product used during mouse plagues is Mouse-off® (Australian Control Technologies). Related products include aluminium and magnesium phosphide. Their toxicity is largely due to the release of phosphine gas, favoured by acidic and moist conditions. The phosphine gas then acts on organs most sensitive to oxidative injury such as lungs, heart, kidney, liver and brain. (Schnitker, 2010). This phosphine gas release will have a fishy odour and is also highly toxic for humans. Care should be taken during autopsy procedures if zinc phosphide, or other phosphide, toxicity is suspected.
A minimal amount of grain appeared to be present in the rumen of the autopsied ewes. The oral LD50 for sheep of zinc phosphide is between 20-40 mg/kg LW (McGirr, 1953) to 60-70 mg/kg LW (Gupta, 2007).
The two main differentials in this case were anthrax and blue-green algae toxicity. Anthrax could be ruled out on the basis of negative ICT tests, and blue-green algae toxicity was eliminated following water testing. If the poisoned wheat had not been found, a possible test for zinc phosphide is to test rumen fluid with silver nitrate impregnated filter paper (not something generally carried). (NB. This test is not available on EMAI’s list of available tests, and Erika Bunker was unaware of it being used there (E Bunker pers. comm.). Phosphide oxidises rapidly, so in the case of availability of lab tests, it is important that rumen samples are put into airtight containers frozen and dispatched as soon as possible.