Pharmacokinetics and therapeutic potential for repeated oral doses of trimethoprim/sulphachlorpyridazine in horses.

This study examines how a powder formulation of two drugs, trimethoprim and sulphachlorpyridazine, behaves in the body of horses when given repeatedly, and how effective these doses are in treating certain bacterial infections.

Research Methodology

• The research was conducted on eight healthy horses, where a powdered formulation of two drugs, trimethoprim and sulphachlorpyridazine, was administered.
• Each horse received these drugs 12-hourly mixed with their feed for a duration of five days. The dosage given was 5 mg/kg trimethoprim and 25 mg/kg sulphachlorpyridazine.
• At first, the intake of the medicated feed was very variable, primarily because the horses needed to adjust to the taste. However, after a few days, all the horses started to consume the medicine-filled feed regularly.
• Before the start of this medication schedule, and at the end, faecal samples were collected from the horses and analyzed for the presence of salmonella bacteria. All samples were found to be negative for this pathogen.

Observations and Findings

• When the results from this experimental setting were compared with a previous single-dose administration of these drugs, it was noticed that the repeated doses led to higher plasma concentrations of the drugs. Furthermore, these drugs stayed longer in the system, as suggested by the higher area under the curve.
• The drug concentrations achieved in the blood were found to be above the minimum levels required to inhibit the growth of certain bacteria that can lead to respiratory infections in horses. These include Streptococcus zooepidemicus, Streptococcus equi, Actinobacillus equuli, and Rhodococcus equi.
• A synergistic relationship was also seen between the two drugs, meaning their combined effect was greater than their individual effects when administered separately. However, the degree and nature of this synergy varied depending on the species of the bacteria.

Conclusion

• This study suggests that the combined use of trimethoprim and sulphachlorpyridazine has therapeutic potential in treating bacterial infections in horses, particularly respiratory ones. The fact that repeated doses resulted in sufficiently high and enduring drug concentrations in the body, and were also effective against various microbial pathogens, further support this finding.
• However, it is important to note that horses might initially refuse the medicated feed due to the change in taste. Therefore, care should be taken to ensure proper intake of the medicaments during the initial days of this treatment regimen.