Objective—To determine effects on anal pressure of
horses after local injection of the external anal sphincter
with Clostridium botulinum toxin type B.
Animals—11 healthy adult horses.
Procedure—Peak and resting anal sphincter pressures
were measured with a custom-made rectal
probe that was connected to a pressure transducer.
Pressures were measured before treatment and after
injection with botulinum toxin type B (BTB) or saline
(0.9% NaCl) solution. Dose titration with 500, 1,000,
1,500, and 2,500 U of BTB was completed. Physical
changes, behavior, and anal pressure were recorded
for each horse.
Results—Injection of 1,000 U of BTB caused a significant
reduction in peak anal pressure from days 2 to
84, compared with pressure in control horses.
Maximal effect of the toxin was observed within the
first 15 days after injection, followed by a slow return
to baseline during the 168-day period. Injection of
2,500 U of BTB in the anal sphincter in 1 horse resulted
in lethargy, generalized weakness, and dysphagia
for 14 days. Adverse clinical effects were not
observed in horses after injections with 500, 1,000, or
1,500 U of BTB.
Conclusions and Clinical Relevance—The effect of
focal intramuscular injection of BTB in horses is similar
to that reported for other species. However, horses
appear to be more sensitive to BTB, compared
with other species, and clinical signs of botulism may
develop at doses exceeding 1,500 U. Injections of
BTB in the external anal sphincter of mares may be
useful to reduce incisional dehiscence after repair of
perineal lacerations. ( Am J Vet Res 2004;65:26–30)