|Why is aspirin so toxic? How do cats become poisoned? Symptoms Diagnosis Treatment Avoidance|
At a glance:
Aspirin (acetylsalicylic acid) is a popular non-steroidal anti-inflammatory drug (NSAID) that has a number of therapeutic uses including treating fever (antipyretic), reducing inflammation, blood thinning (by preventing the formation of blood clots) and relieving pain (analgesic).
Unfortunately, aspirin is also one of the most common causes of poisoning in cats. Unlike humans, cats metabolise aspirin very slowly and it is extremely easy to give a cat a fatal overdose in as little as a single tablet. Aspirin poisoning can occur in cats of all ages, however, kittens and senior cats are at greater risk.
UDP-glucuronosyltransferase 1-6 (UGT1A6) is an enzyme produced by the liver and encoded by a gene of the same name. UGT1A6 evolved to help detoxify plant-based toxins (phytoalexins) by a process called glucuronidation.
As cats evolved, mutations occurred on this gene to the point where it only produces a minute amount of UGT1A6 compared to other animals and people. As a cat’s diet is primarily carnivorous, it had no need for UGT1A6 to help it detoxify plant-based toxins. Mutations of this gene had no impact on survival of the cat species. One study lead by Michael Court in 1997 found 18 species of cat, from cheetas to lions, which suggests that the mutations occurred the common ancestor.
Cats also lack N-acetyltransferase 2, an enzyme in humans encoded in the NA2 gene, N-acetyltransferase 2 detoxifies certain drugs and metabolites.
This means that it takes considerably longer for the body to metabolise aspirin than it would in a human or dog. The biological half-life of aspirin in cats is approximately 40 hours compared to 7.5 hours in dogs.
- When a pet owner intentionally gives the cat a high dosage of aspirin.
- When a pet owner gives a smaller amount of aspirin over a prolonged period. Because cats are not able to metabolise the drug as quickly as other animals, levels quickly build up resulting in toxicity.
- When a pet owner accidentally gives a product containing aspirin such as Pepto-Bismol.
- Accidental ingestion, such as eating an aspirin dropped on the floor. This is more likely to occur in dogs than cats who tend to be more fussy about what they eat.
- Deliberate poisoning.
Aspirin overdose causes inflammation, bleeding, ulceration and perforation of the stomach, bone marrow toxicity, metabolic acidosis and damage to the kidneys and liver.
Symptoms begin within 3-6 hours of ingestion, but may be slower to occur if lower doses are given over a period of time. The first signs of aspirin toxicity include loss of appetite and vomiting. As many body systems are affected, symptoms can be varied and include the following.
Gastrointestinal disturbances are usually the first symptoms to appear, aspirin irritates the gastric mucosa. Symptoms include:
- Vomiting (which may contain blood)
- Black and tarry feces (melena)
- Loss of appetite
- Abdominal pain
- Stomach ulceration and perforation
Respiratory centre stimulation:
Aspirin stimulates the respiratory centre (located in the brain):
- Rapid breathing (tachypnea)
- Hyperthermia (elevated body temperature)
- Respiratory alkalosis (increased respiration elevates the blood pH)
Acute kidney failure:
- Loss of appetite
- Increased thirst
- Jaundice (yellowing of the gums and mucous membranes)
- Nervous system disorders including hyperexcitability, loss of balance (ataxia), depression, seizures.
- Aspirin inhibits the formation of platelets, which are required to help your body form blood clots. This can lead to spontaneous bleeding. You may notice red spots under the skin, known as petechiae.
- Bone marrow toxicity can result in anemia, symptoms may include pale gums, lethargy. Heinz body formation, hemolysis (blood cell destruction) and thrombocytopenia (low platelet count) may also be present.
- Increased respiration and urination due to metabolic acidosis and acute kidney failure can result in dehydration and electrolyte imbalances.
Diagnosis is based on a history of exposure to the drug, however, in the case of deliberate poisoning, the pet owner may not be aware that the cat has ingested aspirin.
Tests your veterinarian may perform include:
- Complete blood count – To check white and red blood cell count. Anemia, Heinz bodies, and thrombocytopenia may be noted.
- Biochemical profile – To monitor kidney and liver function and electrolytes.
- Urinalysis – To determine how concentrated the urine is which can help to evaluate kidney function.
- Blood gas test – To monitor respiratory gas alkalosis or anion-gap metabolic acidosis as toxicity progresses.
Prompt diagnosis and treatment are vital if your cat is to recover from aspirin poisoning. There is no antidote, the goal of treatment is to remove the drug from the body and offer supportive care.
- Ventilate the cat if it is experiencing breathing difficulty
- Provide supplementary oxygen where necessary
- Administer medications to control seizures
- Re-establish hydration and electrolyte balance with the administration of intravenous fluids. This also helps the kidneys to flush the toxin out of the body via the urine (known as diuresis). Furosemide may be administered to enhance this.
- Maintain normal body temperature if the cat has become hyperthermic via external cooling such as fans.
- Induce vomiting if the exposure was recent (within the past 2-4 hours) or gastric lavage (stomach pumping), followed by administration of activated charcoal to prevent further absorption.
- Hemodialysis is a process of purifying the blood of a cat whose kidneys are not working properly.
- Sodium bicarbonate will be administered intravenously to correct metabolic acidosis and alkalinize the urine, treatment with sodium bicarbonate can lead to hypokalemia (potassium deficiency) and therefore should be administered carefully with frequent monitoring of urine or blood pH.
- Medications such as sucralfate and misoprostol to protect the gastrointestinal tract.
The prognosis is favourable if treatment commences prior to the onset of symptoms, otherwise, it is poor.
- Never administer medications to your cat unless prescribed by a veterinarian.
- Always follow your veterinarian’s directions to the letter.
- Have one member of the household responsible for administering medication so that your cat isn’t accidentally medicated twice.
- Keep all medications away from cats and children.
- Don’t store human and pet medications together.
- If you suspect your cat has had aspirin, take him to the veterinarian immediately.
Aspirin can be used in very small doses to prevent blood clots, reduce fever and relieve pain but only under strict veterinary supervision. It is absolutely vital that cat owners do not self-medicate their cat, particularly aspirin because this will most likely have lethal consequences for the cat.
The safe dose for cats is 10mg/kg every 48 hours. Use with caution in cats with liver or kidney disease, gastric ulcers, coagulation disorders, asthma, pregnancy, in kittens or senior cats.
Stop blood thinners (including aspirin) 1-2 weeks prior to surgery.