Garlic has a long history of possible health benefits but its safety has been questioned because other members of the Allium family (i.e. wild onion) are known to potentially cause hemolytic anemia.
In 2005, Pearson et al published a study showing doses above 200 mg/kg (3.2 oz for a 450 kg/990 lb horse) for 71 days could cause “hematologic and biochemical indications of Heinz body anemia, as characterized by increases in Heinz body score (HBS), mean corpuscular volume (MCV), mean corpuscular hemoglobin, platelet count, and serum unconjugated and total bilirubin concentrations and decreases in RBC count, blood hemoglobin concentration, mean corpuscular hemoglobin concentration, and serum haptoglobin concentration.”
Heinz bodies are small bubble-like structures on red blood cells. They form after oxidative damage to hemoglobin. Garlic interferes with an enzyme system that normally protects hemoglobin from oxidative stress.
That dosage had been voluntarily consumed but is larger than would typically be fed as a supplement. I had also had personal experience with hemolytic anemia developing in a barn of horses being fed 1 to 2 kg/day of a commercial feed containing garlic in an undisclosed amount. Anemia resolved upon stopping the feed.
Most recently, Saastamoinen et al reported that 83 days of feeding as little as 32 mg/kg (1/2 ounce daily for a 450 kg/990 lb horse) also caused slight decline in hemoglobin, hematocrit and red blood cell counts compared to matched control horses not fed garlic. Unfortunately, they did not check for Heinz body formation, bilirubin or haptoglobin so it is difficult to draw direct comparisons. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6356413/ .
While the changes found by the 3 month study of Saastamoinen et all were relative small, so was the dosage used. Horses will vary in their sensitivity to garlic’s effects but it is clear that doses of garlic much smaller than previously thought may cause red cell changes. If you are feeding garlic long term it is wise to monitor for Heinz body formulation and changes in red cell count and parameters.
Eleanor Kellon, VMD