There’s still no confirmed cause of death for 19 horses that died within a short period of time on a Kooralbyn, Queensland (Australia) farm. Sudden death and Queensland invariably conjures up thoughts of Hendra virus, considering the large number of Hendra cases that have occurred in the area this year, but that particular disease has already been ruled out.

Large numbers of horses dying on a farm around the same time obviously raises a lot of concern. Often, diagnosis is difficult and takes time. One problem is even deciding where to start. For an outbreak like this there are various considerations:

  • The usual (infectious) suspects: Common things occur commonly, and strange things are often caused by atypical presentations of common diseases. Hendra’s been ruled out but other infectious causes can’t be ignored.
  • Toxin-associated bacterial diseases: This mainly refers to botulism, which causes adult horses to get sick after eating feed contaminated with botulinum toxin. Poorly fermented feed or feed that has (accidentally) incorporated the remains of an animal that died of botulism can be the source. Botulism outbreaks are usually fairly characteristic clinically since animals with botulism have pretty consistent and remarkable clinical signs of illness. Presumably, it’s low on the list here as I haven’t seen it mentioned in any reports.
  • Toxins: A toxic cause is pretty high on the list, and can be hard to trace. Potential culprits include toxic plants or inadvertent access to toxic chemicals (I remember an outbreak of dead horses I was involved with a few years ago in which the horses died of pesticide exposure… on an organic beef farm!). Malicious poisoning always has to be considered as well.
  • New diseases: New diseases don’t come along very often, but they can. Similarly, diseases that have been rare can sometimes peak out of nowhere, but something like this is much less likely than the categories above.
  • Miscellaneous: For example, if all the horses died inside, electric shock would have to be considered. I assume that’s not the case here.

Diagnosis of the cause in an outbreak like this requires a lot of effort, including careful consideration of any signs of illness that were identified before death, good post mortem examinations (necropsies) of dead animals, thorough investigation of the farm looking for toxic plants and other toxic substances, investigation of other farms in the area to see if there are any unexplained horse deaths, investigation of any unexplained deaths of other animals in the area (e.g. looking for abnormal numbers of dead birds on the farm or in the area), testing of samples from horses for various toxins and testing tissues for possible infectious causes. Not easy or cheap, but hopefully effective, and hopefully will ultimately help save other horses from the same fate.