More work is needed to identify safe garlic doses for horses, according to researchers, who found both positive and negative health effects from its use.
Garlic (Allium sativum) has been used in the diets of humans and animals for centuries because of its supposed positive health effects.
In horse nutrition and care, garlic is typically used to treat respiratory diseases and infections in the lungs, and to provide relief from the symptoms of coughs.
The wider onion family (Allium species) is rich in an active component of organosulfur compounds that are associated with the above-mentioned beneficial properties, but these plants have also been linked with toxicosis in mammals.
One of those toxins is N-propyl-disulfide, which alters the enzyme glucose-6-phosphate dehydrogenase in red blood cells. This interferes with the cells’ ability to prevent oxidative damage to hemoglobin.
Indeed, ingestion of onions can cause hemolytic anemia in horses, and there are scientific papers that point out adverse effects of garlic in humans and various animal species, including horses, cattle, birds, rats and dogs.
Garlic is also claimed to have many other positive effects and is, consequently, often included as a supplemental feed for horses.
However, studies on its effects in horse nutrition are scarce, and the dosage for any beneficial effects is not known. There is also little information on possible adverse health effects to determine the safe use of garlic for horses.
Researchers in Finland set out to test the hypotheses that garlic supplementation may help to clear mucus in the airways, but also causes declining blood-related values when fed for prolonged periods.
Researchers Markku Saastamoinen, Susanna Särkijärvi and Seppo Hyyppä used 12 Finnhorse mares in their study, described this week in the journal Animals.
The horses underwent three endoscopic examinations of their airways during the study. After the first one, at the start of the study, they were paired based on their matched health status and upper respiratory tract characteristics.
One horse in each pairing was then fed 20 grams of garlic flakes each day, equivalent to 32mg of garlic for every kilogram of body weight. The other horse in each pair served as the control.
Otherwise, the horses’ feed and housing conditions were the same.
The garlic supplementation continued for 83 days, with the second endoscopic examination occurring after 41 days and the last one at the end of the experiment.
Blood samples were taken for analysis at the same intervals as the endoscopic examinations.
The garlic supplementation seemed to reduce the tracheal exudate score based on the endoscopy examination, the study team reported. Indeed, clinical signs disappeared in three of the six horses fed the garlic supplements. One horse remained without any signs during the study period, while clinical signs remained in two horses from the garlic-supplemented group.
Among the control horses, clinical signs remained in three horses, fluctuated in two, and disappeared in one.
There were no statistically significant differences in the blood parameters between the groups. However, the garlic-supplemented horses showed a slight declining trend in hemoglobin, hematocrit (the ratio of the volume of red blood cells to the total volume of blood) and red blood cells.
Levels in the control horses remain higher.
The average final hemoglobin value of the garlic-supplemented horses was at the lowest limit of the normal range for Finnhorses, or even below it, the researchers reported.
These findings may indicate slight anemia in the garlic supplemented horses, they said.
The authors said the decrease in hemogloblin and the other blood values would be more critical to oxidatively stressed hard-working horses than those in light work.
“The safe limit of garlic supplementation potentially depends on factors such as differences between individual horses, and the health status and exercise level of the horse.”
Low intake levels (15 mg/kg BW dried garlic) were unlikely to result in a risk of adverse effects in healthy, non-exercising, non-oxidatively stressed adult horses, they said.
However, they noted that other researchers had pointed out that the form of garlic supplementation (dry, fresh, garlic oil, extract) may contain different substances with different biological effects.
“To our knowledge, this is the first organized study in controlled conditions to show possible positive and negative health effects of garlic supplementation in horses,” Saastamoinen and his colleagues reported.
“Although this study shows that dried garlic may help to remove tracheal mucus, it also points out that there may be a risk of adverse effects on hemoglobin levels and red blood cell amount if fed with garlic for long periods of time.
“The supplementation level (32 mg/kg BW) of dried garlic fed to horses seemed to reduce the tracheal symptoms and accumulation of tracheal exudates, but may also cause decreased hematologic values when fed continuously for a nearly three-month period.”
The trio stressed that the experiment was on a small scale and the results should be considered preliminary.
“Consequently, further research is needed to identify safe garlic doses and supplementation duration for horses, as well as to examine the positive and preventive health effects.”
The researchers pointed to previous work on the use of herbs and herbal extracts in sport horses, in which the authors pointed out that the traditional use of herbs is not always properly based on dosages and safety is not guaranteed. Supplements considered safe in humans and other species are not always safe in horses, it was noted.
“The authors of a recent study suggested that the usage of garlic as a feed additive should be monitored carefully because of the detrimental effects of overdosing.”
Another suggested said it was possible that even low supplementation levels may be detrimental when the period of supplementation is long in duration, suggesting that herbal supplements in horses should be evaluated to verify possible negative side effects, followed by standardization of the dosage.
Saastamoinen and Särkijärvi are with the Natural Resources Institute in Finland, and Hyyppä is with Ypäjä Equine College.
Garlic (Allium Sativum) Supplementation Improves Respiratory Health but Has Increased Risk of Lower Hematologic Values in Horses
Markku Saastamoinen, Susanna Särkijärvi and Seppo Hyyppä
Animals 2019, 9(1), 13; doi:10.3390/ani9010013