Researchers from the University of Warmia and Mazury in Olsztyn, Poland, were curious as to why horses in their region had small strongyles that were showing resistance to ivermectin. The scientists tested the ivermectin absorption rate at different times of year.
The first phase of the study took place in May; nine warmbloods were given a standard dose of ivermectin and had their blood drawn at regular intervals to determine medication levels. Fecal samples to test efficacy were also taken before and at regular intervals after deworming.
The same protocol was repeated with the same horses in November.
The research team found that ivermectin absorption varied by season. The drug was found in the horse's blood in significantly higher concentrations in the spring, four hours after administration, than it was in the fall. However, the effect was temporary; maximum ivermectin levels were found within 36 hours after administration in both spring and fall.
The scientists suspect that this variability in absorption is related to diet, rather than time of year. In the winter, the study horses consumed hay instead of grass. Hay has more crude fiber that can absorb the ivermectin until it is broken down further in the horse's digestive tract. They noted that the slower absorption does not affect the drug's efficacy and there was complete elimination of parasite eggs in both groups within four days.
The researchers note that diet may affect the absorption of other drugs and medications administered to horses.
Read more at EQUUS magazine.
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