Nearly half of the horses admitted with botulism to an American veterinary hospital survived, a study of treatment records has found.
This compared to a normal survival rate of 10-30 percent in outbreaks, with the researchers noting that many such horses did not receive treatment.
Botulism is a disease caused by a neurotoxin that is considered to be one of the most potent and lethal known to mankind. The bacteria that produce it, Clostridium botulinum, are everywhere, residing in soil.
However, these bacteria produce the neurotoxin only under certain conditions, specifically moist, low-oxygen environments. The commonest route of infection for horses is spoiled ensiled feed, such as silage or balage.
Horse seem especially sensitive to the neurotoxin.
Researchers from the New Bolton Center at the University of Pennsylvania’s School of Veterinary Medicine carried out the study, investigating the veterinary records of 92 horses over six months of age with botulism admitted to the Large Animal Hospital attached to the university. The horses were admitted between 1989 and 2013.
Assistant Professor Amy Johnson and her colleagues found that 48 percent of the affected horses survived.
They found that horses admitted with a higher rectal temperature and difficulty swallowing had better odds of survival, as did those who were given antitoxin.
Horses with abnormal respiratory effort or an inability to stand had decreased odds of survival.
While the overall survival rate was 48 percent among admissions, it was significantly higher – 67 percent – for horses that arrived standing, and even higher, at 95 percent, for horses that remained able to stand throughout hospitalization.
The researchers found that complications occurred in 62 percent of cases, but they were not associated with nonsurvival.
The researchers, whose findings have been published in the Journal of Veterinary Internal Medicine, said horses that lost the ability to stand had only an 18 percent chance of survival.
“Therefore, clinicians would be justified in giving a poor prognosis (20 percent or less) to horses with botulism that lose the ability to rise, regardless of aggressive hospital treatment,” they wrote.
“Conversely, horses that retain the ability to stand throughout hospitalization have an excellent prognosis, with a 95 percent survival rate.”
Johnson and her colleagues said it was important to recognize that, although botulism was usually a rapidly progressive disease, horses might retain the ability to stand during the first few days of treatment and then become unable to stand.
“This deterioration is seen despite treatment with botulinum antitoxin because antitoxin only binds circulating toxin and does not remove toxin already bound to receptors …”
Of the 49 treated horses that lost the ability to stand, 63 percent were totally recumbent within 24 hours of hospitalization. An additional 20 percent lost the ability to stand on the second day of hospitalization, with 10% losing the ability to stand on the third day, 2 percent on the fourth day, and 4 percent on the fifth.
“Clinicians should warn owners of the small number of horses with a delayed onset of total recumbency, meaning that the most accurate prognosis can only be provided after the horse has been hospitalized for 3–5 days.”
If only the equine’s admission status was considered, the study showed that 67 percent of horses that arrived standing survived, compared to 13 percent of horses that arrived recumbent.
“These numbers suggest that early intervention is critical, and that veterinarians should aim to treat or refer cases before development of recumbency.”
The researchers found that only 57 percent of the cases were referred to the hospital as suspected botulism cases. Twenty percent were referred as colic cases, and 11 percent as neurologic/down animals.
Owners should anticipate a two-week hospitalization period, the study team said, although the range ran from two to 51 days. Nonsurviving horses were generally euthanized quickly, with a median duration of hospitalization of 1 day.
“Early identification and treatment of horses with botulism before the onset of recumbency are the most important contributing factors to survival,” they reported.
“Therefore, clinicians working in regions where botulism is endemic should maintain a high index of suspicion for the disease, particularly when examining horses for colic.
“With the current availability of reasonably priced, commercially available antitoxin, clinicians are encouraged to stock at least one unit of antitoxin so that treatment can be provided before referral for more intensive care.”
Johnson, A.L., McAdams-Gallagher, S.C. and Aceto, H. (2015), Outcome of Adult Horses with Botulism Treated at a Veterinary Hospital: 92 Cases (1989–2013). Journal of Veterinary Internal Medicine, 29: 311–319. doi: 10.1111/jvim.12502