Botulism vs. colic
It may be tough to differentiate botulism from colic, because horses with botulism are often lying down and not eating, and
as a result, they may be designated as having colic.
"There have been several occasions, even at a place such as New Bolton Center, where a botulism case was sent to surgery [for
colic]," Johnson says. "During surgery, no surgical lesions were found in the gastrointestinal tract. The practitioner then
realized, after recovery, that it was botulism."
One clue to distinguish botulism from colic is that horses with botulism usually look worse standing up than lying down, Johnson
explains. Additionally, they are often more agitated, have a higher heart rate when standing and have more muscle tremors.
When they are lying down, horses with botulism tend to relax. They might not go into lateral recumbency, but even sternal,
their muscle tremors stop and their heart rate decreases.
Also, unlike horses with colic, those with botulism don't do "flank watching" and frequently don't go back and forth between
lateral and sternal recumbency as frequently as a horse with abdominal pain tends to do.
Another factor that distinguishes botulism from colic is the horse's response to an analgesic, says Johnson. "Giving a horse
with abdominal pain an analgesic, such as flunixin, xylazine or detomidine, will usually lead to clinical improvement for
the duration of action of the analgesic," Johnson states. "But in horses with botulism, you give them an analgesic and they
don't necessarily look better. The tremors continue, and they may still lie down, as it is not affecting the muscle weakness,
though it would affect the pain of the colicky horse. This exemplifies another finding to prompt putting botulism on your
Other distinguishing signs of botulism
Johnson also notes that once veterinarians have seen a few botulism cases, they're much better at diagnosing it again. Those
familiar with the disease will notice additional cranial nerve or subtle deficits, such as weak eyelid tone, dilated pupils,
slow pupillary eye reflexes, and weak tail and anal tone. It's also common to notice muscle fasciculations and tremors over
the triceps, pectorals or even the hind leg muscles. And although the horse's gait is initially very normal, eventually weakness
and a tendency to excessive recumbency will become apparent, says Johnson.
Although the PCR test, which was developed by Johnson and her colleagues at New Bolton Center, shortens the time to get a
definitive diagnosis, she doesn't recommend waiting for the results before treating a symptomatic horse, as it will still
be several days for the PCR results to come back. "If you have a botulism suspect case, it should be treated within that interval,
rather than waiting," advises Johnson.
It also often helps to confirm the source—and the type—of the toxin. This could be important if someone has recently acquired
a large quantity of hay that's possibly contaminated with carrion. "If someone is trying to decide whether to get rid of all
that feed, knowing the source of the toxin can help with the decision," Johnson explains.
If a veterinarian knows that a horse has been eating a "risky" food—from a large round bale of hay or one that has been improperly
stored, or from a source with known carcass or carrion contamination—it should heighten the suspicion of botulism. Feeding
haylage or silage is also considered a risk factor. "Though not a common occurrence, it does happen every once in a while,"
There is also a tendency to blame the water as a source of the botulism toxin. "When you have outbreaks with multiple horses
involved, it's almost never the water," Johnson explains.