Cause of Australia’s Mysterious Horse Deaths Found

The sudden deaths of five horses on 5 Oct. 2011 put Australian horse owners on high alert. Since June of this year, 22 horses have died due to Hendra virus, making 2011 the worst year on record. While the horses in Kooralbyn, Queensland had not shown signs of the Hendra virus, Australians had reason to be concerned.  

Within two days the number of equine deaths on the Kooralbyn property rose from five to 16. Laboratory tests ruled out Hendra virus as a possible cause and authorities began searching the property for various toxins.  Samples taken from necropsies performed on the deceased horses were also undergoing testing for infectious diseases.

As the number of surviving horses continued to decline and the death toll reached 21 on 11 Oct., investigators began looking at the possibility that the horses were intentionally poisoned. The 25 quarter horses were valuable racehorses that had been moved from Toowoomba (in Southern Queensland) to the Kooralbyn property at the end of September.  While the owner of the horses began expressing concern of foul play by an opponent of racing, veterinarians continued to investigate causes including toxic plants or poisoned water on the property, tick-borne diseases, and botulism.  

Eight days after the first deaths occurred, various media reports stated, on 13 Oct. 2011, that necropsy results confirmed the deaths were due to tick paralysis. Ixodes holocyclus, is known as the “paralysis tick,” and poses a threat to both animals (wild and domestic) as well as humans along the eastern coast of Australia. For more information on animal health and diseases in Queensland, visit Biosecurity Queensland at:

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