Atropine is a key treatment for a common eye condition in horses, but its use has been associated with an increased risk of colic.
Researchers, in a just-published study, investigated the effects of atropine use in the eyes of horses, recommending a dose rate which minimised its effects on gut action.
Lena Ström and her colleagues noted the central role of atropine sulfate use in the treatment of equine uveitis, which is a leading cause of blindness in horses.
However, its frequent administration may reduce intestinal action, increasing the risk of colic.
They noted there was limited knowledge of the drug’s passage and action through the horse’s system.
“This knowledge gap could impede the use of atropine because of the presumed risk of unwanted effects,” they noted in the journal BMC Veterinary Research. “Additional information could therefore increase safety in atropine treatment.”
Atropine sulfate (1mg) was given to horses in two experiments: In the first part, it was given intravenously and topically (manually as eye drops and under the eyelids through a lavage system) to six horses to document atropine disposition with each method of administration.
Blood samples were collected regularly and plasma was analyzed for atropine.
The study team found that atropine plasma concentration fell below the lower limit of quantification (0.05 micrograms per litre) within five hours, after both topical and intravenous administration.
They found that the bioavailability after administration of an ophthalmic preparation as an eye drop or topical infusion were 69% and 68%, respectively. The terminal half-life was short, at just 48 minutes.
After a single dose of 1mg atropine sulfate given as eye drops in the conjunctival sac, the plasma atropine concentration was of the same order of magnitude as after the intravenous administration, they noted.
In the second experiment, topical ophthalmic atropine sulfate and a control treatment were given to four horses in two dosing regimens to assess the effects on gastro-intestinal action.
The frequency of gut noise was monitored and used to estimate the degree of gut action.
The researchers noted a statistically significant decrease in intestinal action after administration of 1mg of topical ophthalmic atropine sulfate every three hours compared to the control horses, but not after administration of the drug every six hours.
Clinical signs of colic were not observed under any of the treatment protocols.
The authors said the data and simulations indicate that eye drops given at one and three-hour intervals will lead to atropine accumulation in plasma over 24 hours, but that a six-hour interval allows total washout of atropine between two topical administrations.
If constant corneal and conjunctival atropine exposure is required, a topical constant rate infusion at 5 micrograms per kilogram every 24 hours offers a safe alternative, they said.
They noted that while no signs of colic were observed in any of the evaluated treatment protocols, clinical dosage regimen recommendations with no or negligible risk for colic remain to be firmly determined.
However, taking the plasma exposure after topical administration into consideration, data and simulations indicated that eye drops given at a one-hour interval will lead to a sustained atropine accumulation in plasma over 24 hours, whereas a six-hour interval allows a total washout of atropine between doses.”
The study was centred at the Swedish University of Agricultural Sciences.
Ström, L., Dalin, F., Domberg, M. et al. Topical ophthalmic atropine in horses, pharmacokinetics and effect on intestinal motility. BMC Vet Res 17, 149 (2021). https://doi.org/10.1186/s12917-021-02847-4