Hives in a cat usually fade within 24 hours, and the 2 hours after the first welt matter more than the next 2 days. Most reactions peak in 20 to 40 minutes, so a cat that is still swelling at the 2-hour mark, or whose voice or breathing has changed, needs to be seen rather than watched. Three trigger classes cover most cases, and naming the class sets both the watch window and the recheck.
The 2-hour line
The first wheal appears 5 to 30 minutes after contact with the trigger, peaks at 20 to 60 minutes, and flattens over 6 to 24 hours. If the trigger is still on the skin, such as shampoo, plant sap, or a flea product, new wheals can keep arriving for 2 to 6 hours, which is why a single look at minute 15 proves nothing. Measure the largest wheal every 15 minutes and write the number down. Growth that continues at 120 minutes, a wheal that crosses 3 cm, or any new site after the 2-hour mark is the point to call. Once 2 hours have passed with no growth and no breathing change, the risk of laryngeal swelling has largely passed, though muzzle swelling can still creep for up to 6 hours.
What a wheal actually looks like
A wheal is raised 1 to 5 mm, measures 2 to 10 mm across, and has a pale center with a red rim; the hair over it stands up. Individual lesions come and go within 6 to 24 hours and leave no mark, which separates hives from most other skin disease. Stroking the skin firmly produces a raised line within 60 seconds, and that line is a useful confirmatory sign at home. Typical sites are the face, the pinnae, the paws, and the belly; a cat with lesions only on the trunk is more likely to have something else.
Angioedema is the same reaction, deeper
When the swelling sits in the deeper layer, the skin looks normal but the muzzle, eyelids, or pinnae thicken by 2 to 4 mm and feel firm rather than doughy. Eyelids can close within 20 minutes. The dangerous version is sublingual and laryngeal: a voice that changes pitch, an inspiratory noise heard across the room, drooling, or a cat that stretches its neck forward to breathe. Those 4 signs move the cat from the watch list to the emergency list regardless of how the skin looks.
3 trigger classes
Class matters more than the individual cause, because the class sets the window and the tests worth doing.
| Class | Typical examples | First wheal (minutes) | Where it starts | Watch (hours) |
|---|---|---|---|---|
| 1a. Sting or bite | Bee, wasp, ant, spider | 5-15 | Muzzle, paw | 4 |
| 1b. Contact | Plant sap, plastic bowl, shampoo, flea product | 30-120 | Chin, muzzle, belly | 2 |
| 2. Food or oral | New food, treat, milk, fish, supplement | 20-90 | Face, ears, belly | 2 |
| 3. Infection or immune | Vaccine, transfusion, bacterial or viral infection | 10-60 | Face, then generalised | 6 |
Class 1a is the one that escalates fastest and the one where a sting on the face of a kitten under 4 months deserves a visit even if the swelling looks mild. Class 2 is the one that repeats: 2 or more episodes in 8 weeks is the pattern that justifies an elimination trial. Class 3 is the one to take seriously after any injection, because a reaction that starts at minute 10 can be at the larynx by minute 40.
What to do in the first 2 hours
Photograph the lesion every 15 minutes with a coin or a ruler beside it, so size is a number and not an impression. Note the time of the last meal and any new product used in the previous 24 to 48 hours. Apply a cool compress for 10 minutes, not ice directly on the skin. Take off a plastic collar and swap the plastic bowl for ceramic, since both are common contact triggers. Keep the cat in a quiet room at 20 to 24 degrees Celsius and do not start a food trial in the middle of a reaction, because a new food muddies the record.
The 8-week food trial
If there have been 2 or more episodes in 8 weeks and no sting or contact trigger is obvious, an 8-week elimination trial with 1 protein and 1 carbohydrate is the standard next step. Feed nothing else for those 8 weeks, including treats and toothpaste, then reintroduce 1 food at a time over 7 days each. Keep a 4-column diary: date, food, minutes to first wheal, and largest size in mm. Hives that stop during the trial and return within 7 days of the challenge are as close to a diagnosis as this problem gets.
When to call the vet today
Any breathing change, voice change, drooling, or neck stretching is an emergency now, not in 2 hours. So is swelling that is still growing at the 2-hour mark, hives with vomiting or weakness, a sting inside the mouth, or a first episode in a kitten under 4 months. Lesions that have not faded after 24 hours, or that leave crusts and pigment, are probably not hives, and the next step is a skin exam rather than another wait.
Five steps for this week
- Photograph the next episode at minute 0, 15, 30, 60, and 120 with something for scale.
- Write down every food, treat, shampoo, and flea product used in the 48 hours before it started.
- Switch to ceramic or steel bowls and note the date, because plastic contact is a common and fixable cause.
- Time the swelling: if it is still growing at 120 minutes, the plan is a visit, not a photo.
- Read the cost estimate before an elimination diet and skin workup, and check vestibular disease if the cat also tilts its head, since facial swelling and a head tilt together point away from hives.
Terms mentioned in this article
Frequently asked
How long do hives last on a cat?
Each wheal fades in 6 to 24 hours without leaving a mark, though new ones can appear for 2 to 6 hours while the trigger is still on the skin. A lesion still present at 24 hours, or one that leaves a crust, is probably not urticaria and needs a skin exam.
Why is the 2-hour mark so important?
Most reactions peak in 20 to 40 minutes and laryngeal swelling develops early. A cat that is stable at 120 minutes with no growth and no breathing change has passed the highest-risk window, though muzzle swelling can still progress for up to 6 hours.
What are the 3 trigger classes?
Contact and environment, which includes stings, plant sap, plastic bowls, and shampoos; food or oral exposure; and infection or immune events such as vaccines, transfusions, and bacterial or viral infection. The class sets the watch window and the tests worth running.