Despite relatively intensive efforts, no cause has been identified for the deaths of 22 horses ona Kooralbyn farm in Queensland, Australia. Readily identifiable causes have been ruled out, leaving two main theories:
As mentioned in an earlier post, one possibility is botulism It can be very hard to diagnose botulism in horses, and the disease is often presumptively diagnosed by ruling out all other reasonable options.
The other possibility is tick paralysis, which unfortunately is equally difficult to diagnose definitively. Certain types of ticks can cause progressive and severe paralysis which may look similar to botulism in some ways. Scrub ticks (the name typically used for Ixodes holocyclus) were noted on one of the first affected horses, but there’s no information about whether all of the other horses had ticks too, and if so whether they were ticks that can cause paralysis. Kooralbyn is in a region where these paralysis ticks can be found and spring (e.g. now in Australia) tends to be the peak time for paralysis cases, so it’s a reasonable consideration.
It’s frustrating but far from unusual not to be able to make a solid diagnosis in outbreaks. Beyond the frustration factor, it’s also a concern because it makes it hard to take precautions to prevent it from happening again.