Cholinesterase in cats and dogs: the test for insecticide poisoning

What it shows

There are two related enzymes. Acetylcholinesterase works at nerve endings and also sits on red cell membranes. Butyrylcholinesterase (BuChE, formerly called pseudocholinesterase) is made mainly by the liver and is plentiful in plasma. Depending on the lab, “cholinesterase” or “ChE” on the form refers to activity in whole blood, red cells, serum or plasma; after death the lab tests brain tissue.

When an insecticide of these groups blocks the enzyme, acetylcholine builds up and overstimulates nerves and muscles. Typical signs are drooling, small pupils, frequent urination, vomiting and diarrhoea, then muscle tremors and weakness, nervousness, lack of coordination and seizures. Signs usually begin within minutes to hours of exposure but may be delayed for more than 2 days.

Higher than reference — what it may be a sign of

The test is used to look for a decrease. Insecticide poisoning does not raise cholinesterase, so a result above the interval does not point to it; the vet reads such a result together with the rest of the panel.

Lower than reference

• Exposure to an organophosphate or carbamate — activity that is substantially inhibited, by more than half, is considered confirmatory. Clear signs of overstimulation are usually seen when activity in the brain is inhibited by more than 70%.
• The number is not a severity score — the degree of decrease in blood does not necessarily match how severe the poisoning is. In one experiment in cattle, blood activity fell to 20% of normal without signs of poisoning.

A normal result does not rule out carbamates: their bond with the enzyme is reversible and much less durable, so the inhibition often is not evident by the time the sample reaches the lab. The vet may also request testing of stomach contents for the insecticide itself, since organophosphates and their breakdown products do not stay in the body for more than about 24 hours.

Look at together with

• Acetylcholine receptor antibodies — a different test of the same nerve–muscle junction, used when weakness suggests myasthenia gravis rather than poisoning.
• Anticoagulant rodenticide, lead, ethylene glycol — other toxicology tests the vet may choose depending on what the animal could have reached.

Units and conversion

Cholinesterase is reported as enzyme activity in U/L. What matters is the comparison with the lab’s interval for the species — or, when a baseline exists, the percentage of the animal’s normal activity that remains. Methods and sample types differ between labs, so numbers from different labs are not directly comparable.

Reference range

The reference range is on your lab’s form. It applies to that lab’s method, sample type and species. A result is read as “how much of the expected activity is left”, together with the history and signs; an exposure story and a clearly reduced value support each other.

Cats and dogs

Sources

1. Merck Veterinary Manual, professional version. Organophosphate Toxicosis in Animals.
2. Merck Veterinary Manual, professional version. Carbamate Toxicosis in Animals.
3. Merck Veterinary Manual, pet owner version. Insecticide Poisoning.
4. Merck Veterinary Manual, professional version. Collection and Submission of Laboratory Samples from Animals.
5. Washington Animal Disease Diagnostic Laboratory (WADDL), Washington State University. Acetylcholinesterase (Cholinesterase) — test details.
6. Žnidaršič N, Štrbenc M, Grgurevič N, Snoj T. Potential revival of cholinesterase inhibitors as drugs in veterinary medicine. Frontiers in Veterinary Science, 2023.
7. Comparison of serum paraoxonase 1, butyrylcholinesterase, and carboxylesterase activities among dogs, cats, micro-mini pigs, and goats. Journal of Veterinary Medical Science, 2026.