After three weeks of laboratory testing, Australian officials have determined that botulism or a scrub tick infestation likely caused the deaths of 22 horses at a Kooralbyn, Queensland, property, according to a Nov. 2 press release from Biosecurity Queensland.

"Botulism is a bacteria that lives in soil and produces a toxin that affects the nervous system if ingested," said Queensland Chief Biosecurity Officer Jim Thompson. "The other likely scenario is scrub tick poisoning. Witnesses have reported that the horses first noticed to be in distress on Oct. 6 had been carrying large numbers of ticks. Fatal infestations of scrub tick on adult horses have been recorded in the past."

According to a fact sheet from the New South Wales, Australia, Department of Primary Industries, scrub ticks–also known as paralysis ticks–are native to Australia and are most frequently found on the nation’s East Coast. The arachnids "inject a toxin causing paralysis that can be fatal in domestic animals, both pets and livestock," and "more than 80,000 cases of tick toxicosis, mainly in domestic pets are treated each year in eastern Australia."

Thompson said both botulism and scrub tick infestations are known to cause "progressive muscular paralysis, as was seen with these horses." He added that it is often difficult to differentiate between the two conditions