Allium species: Onion

Family

Liliaceae

Common Name

Onion (Allium cepa), chives (A. schoenoprasum), garlic (A. sativum), leeks (A. porrum), shallots, giant allium.

Plant Description

At least 700 species of Allium exist in the temperate regions of the world. These bulbous biennial or perennial plants are widely cultivated for their nutritional value and some species as colorful garden ornamentals (Fig. 36 and Fig. 37).

Growing from various sized bulbs, that have papery or fibrous outer layers, the leaves are produced basally, and can be long, linear, lanceolate, flat or grooved, terete, hollow or solid. Flowers are produced as terminal umbels, on stalks that are hollow or solid and can range in height from a few inches to 3 - 4 feet.

Flower colors vary from white to yellow, to pink or mauve. Members of the Allium genus have a characteristic onion or garlic odor when the leaves or bulbs are crushed.

Figure 36. Allium giganteum "Globemaster".

Figure 37. Allium schoenoprasum.

Toxic Principle and Mechanism of Action

Allium species contain a variety of sulfur containing compounds (alkylcysteine sulfoxides) that are converted to a variety of sulfides, disulfides, trisulfites and thiosulfonates through the action of plant enzymes once the plant tissues are damaged [1]. The typical onion odor is attributed to the disulfides and try sulfides, while the compound that causes lacrimation when peeling onions, is thiopropanol-S-oxide [2]. One compound, N-propyl disulfide, is a highly reactive oxidant that is responsible for oxidizing hemoglobin. This and other similar sulfide compounds compounds act to deplete critical enzymes such as glucose-6-phosphate and the G6P dehydrogenase that are critical in the cell membrane integrity [2,3]. Once the hemoglobin is oxidized, Heinz bodies form in the red cells, and the defective erythrocytes are removed by the spleen and reticulo-endothelial system. The resulting anemia causes generalized weakness, and can become severe enough to cause fatalities.

Poisoning from onions may occur if animals are fed whole raw onions, chopped, dehydrated, and cooked onions, or products containing onion powder. The toxicity of onions varies depending on the type of onion, growing conditions, the total amount consumed, and the animal species involved. Cats are particularly susceptible to onion poisoning, having been poisoned after being fed baby food containing as little as 0.3% onion powder [4,5]. Dogs are more tolerent of onions. An acute hemolytic anemia was reported in a dog that ingested 3 - 4 oz. of dehydrated onions [6]. Dogs fed 5.5 g per kg body weight of minced dehydrated onions, exhibited severe hematologic changes within 24 hours exposure [7]. In another study, toxic dose of raw onions in dogs has been cited as equal to or greater than 0.5% of the animal's body weight [8]. Cattle are particularly susceptible to onion poisoning, if they had diet contains greater than 20% onions [9,10]. Horses are less susceptible to onion poisoning [11], and sheep appear to be able to adapt to rations comprising 100% onions [12].

Risk Assessment

Most onion poisoning of dogs and cats results from the feeding of raw or cooked onions, or other human foods containing onion products. The risk therefore of onion poisoning in household pets is significant, especially if they are fed table scraps. Cats, in particular should not be fed any food products containing onions. Certain breeds of dog, such as the Akita, have a heritable predisposition to the hemolytic effects of onions [13]. Care should be taken to ensure that onion bulbs, whether they are of the edible or ornamental species, are stored to prevent access by household pets. Similarly, cull or spoiled onions should not be thrown on the compost pile, where dogs have access to them.

Clinical Signs

The onset of clinical signs, following the ingestion of the toxic dose of onions, may vary from one to several days, depending on the total dose of onion consumed. Infected animals become weak, the tactic anorexic and recumbent due to the developing anemia. Heart rate and respiratory rate are increased, and the mucous membranes are pale and can be jaundiced. Frequently the animal's breath smells of onion. The urine may be brown or coffee colored, indicating hemoglobinurea. Examination of the blood will reveal a decreased packed cell volume, and the presence of Heinz bodies in the red blood cells. Methemoglobin levels may also be significantly increased.

Treatment

Severely anemic animals may require a blood transfusion. Generally, however, if the animals are taken off the of the onions and are not stressed, the hemoglobinurea will resolve in 1 - 3 days and the packed cell volume will return to normal in 2 - 3 weeks. Inducing vomiting is effective if the onion has been consumed within the last 2 hours. Activated charcoal is indicated after vomiting has stopped. Pet foods containing propylene glycol should be avoided as it enhances Heinz body formation [14].