Equine Disease Quarterly | EQUINE
is a true medical emergency, with survival linked to
prompt intervention through aggressive antibiotic
treatment and wound debridement.
The Clostridium genus consists of over 150 known
species of Gram-positive, anaerobic, spore-forming
bacteria. The spore-forming ability of these bacteria
allows survival for long periods of time in the
environment. When spores encounter a location
without oxygen, such as damaged muscle, they
are triggered to proliferate and produce exotoxins,
which cause extensive tissue and vascular damage.
The clostridial species that commonly cause myositis
include C. perfringens, C. septicum and C. chauvoei.
Clostridial myositis has been reported following
intramuscular inoculations of vaccines, ivermectin,
antihistamines,
phenylbutazone,
vitamins,
prostaglandins, and most commonly, flunixin
meglumine. Infrequently, cases occur following
inadequate perivascular administration of compounds,
foaling trauma, or puncture wounds. In a study by
Peek et al in 2003, stallions and Quarter horses were
overrepresented, and the authors hypothesized this
might be due to the heavy muscling of these groups.
The mechanism by which the bacterial spores arrive
in the muscle of the horse is unknown. It is possible
spores are introduced at the time of injection. Another
theory is that bacteria are translocated from their
normal environment in the intestine, in times of
inflammation or colic and arrive in the muscle via
the bloodstream. No association linking whether or
not the injection site is cleaned prior to injection and
the development of myositis has been established. A
higher incidence of myositis with irritating substances
such as nonsteroidal drugs and vitamins is reported,
potentially due to increased tissue damage and
creation of an oxygen-free environment.
Diagnosis involves aspirating a small amount of fluid
for anaerobic culture and Gram-staining to look for the
presence of gram-positive rods. To treat the infection,
large incisions are made into the muscle and fascia
to expose the bacteria to oxygen and debride dead
tissue. General supportive care is critical because
these bacteria produce toxins that have secondary
effects on the horse, including the potential to
reduce the contractility of the heart. Clostridial toxins
may also result in anemia, thrombocytopenia, and
leukopenia. Horses are commonly treated with high
doses of intravenous penicillin, intravenous fluids,
cardiovascular support, and wound care. Hyperbaric
therapy, where available, is suggested as an adjunct to
routine treatment.
Survival has been reported to range from 31% to
73% and appears to be better for infections with
C. perfringens compared with C. septicum or C.
chauvoei. Horses that survive the initial toxemic stages
of disease have an improved prognosis. The wounds
created by a combination of infection and treatment
are usually large, and may take weeks to months to
heal entirely. Horses which do not survive show signs
of intravascular coagulation and multi-organ failure.
There is no definitive prevention for clostridial myositis.
When giving intramuscular injections, use large and
well vascularized muscle groups, and when possible
avoid giving irritating substances in the muscle if there
is an alternative route such as oral or intravenous
administration.
CONTACT:
Rebecca Ruby, BVSc (dist), MSc, DACVP
[email protected]
(859) 257-8283
UK Veterinary Diagnostic Laboratory
University of Kentucky
Lexington, KY
• Volume 21 Issue 3 | September 2019 •
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