Onions

Symptoms reported in cats

Most serious first.

1. 1
   Tachypnea, Dyspnea, and Cyanosis Several days after ingestion · This is a compensatory response to systemic tissue hypoxia and methemoglobinemia, with cyanosis indicating a life-threatening lack of oxygen in the blood.
2. 2
   Collapse and Death Within 24 hours (with massive doses) or several days after ingestion · Death occurs when oxidative damage to the blood, severe hemolytic anemia, and resulting tissue hypoxia are extensive and overwhelm the animal's bodily systems.
3. 3
   Lethargy, Weakness, and Depression After a lag of several days (typically 3–5 days) · These symptoms emerge as a secondary response to systemic hypoxia caused by the profound loss of oxygen-carrying red blood cells.
4. 4
   Pale or Icteric Mucous Membranes (Jaundice) After a lag of several days (typically 3–5 days) · Pale gums indicate a lack of oxygenated blood (anemia), while jaundice (yellowing of the skin/mucosa) indicates severe red blood cell destruction leading to hyperbilirubinemia.
5. 5
   Tachycardia and Heart Murmur Several days after ingestion · Tachycardia develops as the heart pumps faster to compensate for impaired oxygen transportation45. In one documented feline case, a grade IV heart murmur was also observed upon physical examination.
6. 6
   Hemoglobinuria (Dark Urine) Several days after ingestion (typically 3–5 days) · This is a hallmark sign of severe poisoning caused by damaged red blood cells rupturing and releasing hemoglobin, which is then excreted through the kidneys3. This process can lead to secondary acute kidney injury known as hemoglobinuric nephrosis.
7. 7
   Hypothermia and Cold Sensitivity Several days after ingestion · Affected cats may present with an abnormally low body temperature, measured as low as 36.6 °C in one clinical case.
8. 8
   Vomiting, Diarrhea, Abdominal Pain, and Loss of Appetite Within 1 day (or delayed up to 2–4 days) · These are usually the first clinical signs of toxicosis to appear, frequently resulting in dehydration.

Important notes

Onions (Allium cepa) are severely toxic and extremely dangerous for cats. Like other plants in the Allium family, such as garlic and leeks, onions contain complex sulfur compounds that directly attack a cat's red blood cells. When a cat eats any part of an onion, these compounds cause oxidative stress in the blood, leading the red blood cells to break apart and die. This condition, known as hemolytic anemia, makes it very difficult for the cat's body to carry oxygen. Symptoms like weakness, pale gums, fast breathing, and reddish-brown urine might not show up immediately, often appearing three to five days after the onion was eaten. All forms of onions are equally deadly: raw, cooked, dehydrated, or powdered. The toxic compounds are not destroyed by heat, and onion powder is especially dangerous because it is highly concentrated. Onion powder is frequently hidden in baby food, broths, gravies, and prepared meals. Never share onion-containing foods with your cat. If you suspect your cat has eaten any amount of onion, contact a veterinarian immediately.

• Toxic compounds: N-propyl disulfide and organosulfur compounds (alk(en)yl cysteine sulfoxides); all forms toxic
• Mechanism: oxidative damage to haemoglobin creates Heinz bodies; the spleen destroys affected red blood cells, causing haemolytic anaemia
• Symptoms: lethargy, weakness, pale or yellowish gums, rapid breathing, loss of appetite, reddish-brown urine; onset often delayed 3–5 days
• Onion powder in baby food, broths, gravies, or prepared meals is a common hidden source of toxicity
• Never share any onion-containing food with cats; contact your vet immediately if ingestion is suspected