With today's trend of resorting to dietary modification and the addition of herbs and supplements into our meal-plan as a natural way of combating and preventing common human ailments, one should be very careful in how far the companion cat can be included in these recommendations. Many natural substances which aid human ailments may have an equally beneficial effect on the cat, but equally many can also have detrimental effects on the cat and her health.

The use of garlic and other members of the Lily family such as onions, shallots, and chives in the cat's diet is not advised. Ingestion of these plants in a raw, cooked, or dried form can lead to damage of the red blood cells, which are rejected by the body from the bloodstream, and a continued use of garlic or onions can eventually result in hemolytic anemia. If the anemia is not controlled by discontinuing the use of the plants, it can potentially lead to death. Less significant but important is that raw garlic and onions irritate the mouth, esophagus, and stomach and can cause or exacerbate ulcers.

"An alkaloid, N-propyl disulphide, present in both cultivated and wild onions, chives, and garlic, affects the enzyme, glucose-6-phosphate dehydrogenase, in red blood cells that interferes with the hexose monophosphate pathway. Oxidation of hemoglobin results because there is either insufficient phosphate dehydrogenase or glutathione to protect the red blood cells from oxidative injury. The resulting formation of Heinz bodies within erythrocytes is characteristic of onion poisoning. (...)"
Marielle Gomez-Kaifer, PhD, Department of Chemistry, University of Miami

"Onion poisoning has been described in the cat by Kobayaschi (1981) who reported a hemolytic anemia and increase in Heinz bodies in cats that had consumed onion soup. (...)"

Excerpt from Nutrient Requirements of Cats, Revised edition by the National Research Council.
Ref.: Kobayaschi, K. 1981. Onion poisoning in the cat. Feline Pract. 11:22.

"Some people may have difficulty with eating raw garlic as it can be irritating to the intestinal linings."
Benjamin Lau, M.D., Ph.D. is a leading authority on garlic and health, professor of microbiology and immunology at the School of Medicine at Loma Linda University

"Allicin, which gives garlic its odor, is a strong oxidant, that is, a chemical that creates free radicals, which in excess, can be dangerous. Allicin can cause stomach irritation and, in rare cases, hemolytic anemia, destruction of red blood cells. If placed directly on the skin, allicin can cause blistering."
"Cats or Dogs should never be given raw, un-aged garlic because it is very irritating to the mouth, esophagus, and stomach and can cause or exacerbate ulcers."
The Garlic Information Center hotline, 1-800-330-5922 at Cornell University Medical College.

What are Heinz bodies?
"Heinz bodies are pieces of oxidized hemoglobin; on a Romanowsky-stained blood smear, they appear as pale, circular structures that often protrude from the side of the RBC."
The Merck Veterinary Manual 8th edition, page 1208

What is Hemolythic anemia?
"The immune response can become directed against the body's own blood cells, including red blood cells, neutrophils, and platelets. In some instances, the cause of this is unknown, and the body is truly reacting against itself (an autoimmune disease). However, this is fairly uncommon, and usually the reaction against the body's own cells is initiated by exposure of the cat to a foreign antigen (...)"
The Cornell Book of Cats 2nd edition, page 276