A few words on the feeding of garlic
Time and again dog owners write to me to educate me about the danger of feeding garlic to dogs. A few scientific studies have determined that plants of the Allium family, especially onions and garlic, can be toxic to dogs. These reports of Allium species being potentially toxic to dogs have been adopted and disseminated by many pet websites without any critical reflection or analysis of the above-mentioned studies. The result has been quite a bit of hysteria about feeding garlic to dogs.
When reading the full text of the relevant studies (Lee et al., 2000; Hu et al., 2002; Yamato et al., 2003; Cope, 2005), however, the situation is not quite as black & white as some pet websites might lead you to believe.
Some toxic substances act as oxidants upon entering the body and can overwhelm the antioxidant abilities of red blood cells. Garlic, onion, ramsoms (wild or bear’s garlic) and other Allium plants contain Sulphur compounds which can reduce the enzyme glucose-6-phosphate dehydrogenase (G6PD), which protects the cell walls of red blood cells. The oxidants then oxidize the hemoglobin, damaging the red blood cells and triggering the formation of Heinz bodies. Heinz bodies are basically clumps of damaged hemoglobin resulting from exposure to high oxidant levels. The loss of red blood cells eventually leads to anemia, in this case Heinz body anemia, as the loss of red blood cells is due to the formation of Heinz bodies. If this process continues unimpeded, the reduction of red blood cells could over time lead to severe anemia and the animal could die.
Heinz body anemia is classified as a regenerative anemia, which simply means that it will resolve itself in a matter of days when the causative toxins (in this case onions or garlic) are removed from the diet. Besides onions, garlic or other Allium plants, causes of Heinz body anemia in dogs can be through exposure to vitamin K3 (menadione), naphthalene, propylene glycol, benzocaine, methylene blue, phenyl-hydrazine, acetaminophen and excessive copper or zinc.
In cats, Heinz body anemia can be provoked by the same toxins listed above as well as phenacetin, phenazopyridine, propofol, salmon-based diets and foods containing onion or garlic powder. Cats with hyperthyroidism, diabetes, renal failure and lymphoma are also more susceptible to forming Heinz bodies. Healthy cats tend to have up to 10% Heinz bodies in their blood at any time due to the inefficiency of their spleens in removing Heinz bodies and the increased susceptibility of their hemoglobin to oxidative damage.
Unlike cat blood, canine blood does not normally contain any Heinz bodies at all.
To see what all this means in a practical sense, let’s look at the studies.
In studies using onions, hemolytic changes developed after ingestion of 15-30 g onions/kg body weight/day and a toxic effect was seen only after administration of over 50 g onions/kg body weight/day (over at least 2 days) (Cope, 2005). In this study, the dogs (5 Pekinese) developed hemolytic anemia after receiving a toxic dose of onions for several days. Another study (Yamato et al. 1998) found that some oriental breeds are particularly susceptible to developing Heinz body anemia. The anemia provoked in these studies were all reversible.
In a study on garlic (Lee et al. 2000), there was “what looked like” changes in red blood cells only after administration of more than 5 g garlic/kg body weight/day, but none of the dogs in this study developed hemolytic anemia. Also, the observed changes in the red blood cells were reversible within a very short period of time. In another study with various garlic extracts, there were changes in red blood cells only after an amount of 1.5 ml garlic extract/ kg body weight/day was administered. Apparently, the form of garlic – extract, fresh, dried – plays an important role.
To put things in perspective, in my diet plan I recommend one fresh clove of garlic for a 30 kg dog up to 3 times per week. A large clove of fresh garlic weighs approximately 3 g.
This is 0.0001% of body weight of a 30 kg dog, or 0.1 g garlic/kg body weight/3 x week.
Or based on a week: 0.3 g garlic/kg body weight/week.
Compare the toxic dose of 0.5% of body weight, or 5 g garlic/kg body weight/day.
Or based on a week: 35 g garlic/kg body weight/week.
According to my feeding plan, a 30 kg dog would receive 3 cloves of fresh garlic per week or a maximum 9 g of garlic per week.
Toxic, however, would be at least 116 times that amount. For a 30 kg dog that would mean ingesting 350 cloves of garlic or 1050 g of garlic per week!
It is therefore not really possible to cause hemolytic anemia inducing damage to the red blood cells at the dosage I recommend, or even 10 times that dosage (which of course I do NOT recommend). Apart from that, it is extremely unlikely that a dog would eat this amount of garlic or, if he actually ate that much, not regurgitate it immediately.
The health promoting properties of the administration of small amounts of fresh garlic vastly outweigh the absolute minimal risk of any toxicity in this case. That said, garlic is a valuable medicinal plant and should be used accordingly. It is unnecessary to feed garlic for no reason and it really doesn’t do much by way of ectoparasite control (fleas, ticks). Onions, on the other hand, should not be fed to pets. Also, when feeding garlic always use fresh cloves. Garlic extracts and powders are much more concentrated and can lead to problems much more easily than whole, fresh garlic.
P. S. By the way, people can get Heinz body anemia by the consumption of large quantities of onion plants (and horses, and other mammals …).
- Hematologic changes associated with the appearance of eccentrocytes after intragastric administration of garlic extract to dogs. Am J Vet Res. 2000 Nov;61(11):1446-50.; Lee KW, Yamato O, Tajima M, Kuraoka M, Omae S, Maede Y.
- An experimental study of hemolysis induced by onion (Allium cepa) poisoning in dogs. J Vet Pharmacol Ther. 2008;31(2):143–149. Tang X, Xia Z, Yu J.
- Isolation and Identification of Organosulfur Compounds Oxidizing Canine Erythrocytes from Garlic (Allium Sativum). Journal of agricultural and food chemistry. 50. 1059-62. 10.1021/jf011182z. Hu, Qiuhui & Yang, Qing & Yamato, Osamu & Yamasaki, Masahiro & Maede, Yoshimitsu & Yoshihara, Teruhiko. (2002).
- Induction of onion-induced haemolytic anaemia in dogs with sodium n-propylthiosulphate. Vet Rec. 1998;142(9):216–219. Yamato O, Hayashi M, Yamasaki M, Maede Y.
- Allium species poisoning in dogs and cats. Vet Med. 2005:562–566. Cope RB.
- Small Animal Clinical Diagnosis by Laboratory Methods, 5th Edition; Michael D. Willard, Harold Tvedten. Publisher: Elsevier