Hives (urticaria) are raised, red, puffy patches that appear on the skin, often suddenly, and they may itch. They are part of an allergic type of reaction: cells in the skin called mast cells release histamine and other chemicals, and these make small blood vessels leak fluid into the surrounding tissue [1].
The skin is only one of the places where this reaction can show. When it spreads through the body it is called anaphylaxis, which can also affect the lungs and the gut and can be fatal [1].
If your cat has hives with swelling of the face, trouble breathing, vomiting, weakness or collapse, see a vet at once.
Signs
What you may notice on the skin:
- Raised patches: red, swollen bumps or welts that can come up within a short time.
- Swelling of the face: puffy lips, eyelids or muzzle. In a review of cats with a severe allergic reaction after a vaccine, facial swelling was one of the skin signs reported [2].
- Itch: some cats scratch or groom the area, others do not seem bothered.
In cats, a severe reaction often shows up away from the skin. In the same review of 57 cats that reacted within an hour of a vaccine, signs most often involved breathing (in about seven in ten), circulation such as collapse (about six in ten) and the gut, mostly vomiting (about half). Skin signs were seen in only about one cat in ten [2]. So a cat can be in serious trouble even without visible hives.
Causes
Reactions of this type can be set off by insect and snake venom, a range of medicines, vaccines and food [1]. For bites, see fly and mosquito bites on dogs and cats.
Food can affect a cat’s skin in many different ways, so a food reaction is one possible cause of skin signs in cats [3]. The exact trigger is not always clear, and your vet will help work it out.
How vets diagnose hives
Your vet will look at the skin, check your cat’s breathing and circulation, and ask what happened in the hours before the hives appeared. A severe allergic reaction is recognised from the pattern of signs rather than from a single test [1]. If the skin problem keeps coming back, your vet may suggest further tests to look for the cause.
If food is suspected, the usual way to find out is a diet trial run by your vet, because blood, saliva and hair tests for food allergy have not proved reliable [3]. See also food allergy skin signs in dogs and cats.
Treatment
Your vet will decide whether your cat needs treatment and which kind. A severe allergic reaction is an emergency that needs medicine given by a vet straight away [1]. Avoiding the trigger, once it is known, helps stop the problem coming back.
At home, as directed by a vet:
- tell your vet about any insect sting, new medicine, vaccine, food or product before the hives began;
- give only the medicines your vet prescribes;
- if your cat’s face swells or its breathing changes, see a vet at once.
Outlook
How a cat does depends on how severe the reaction is [1]. Mild skin signs often settle, but a severe reaction can be life threatening. In the Japanese review, most of the cats reported with anaphylaxis after a vaccine died, which is why the authors urge vets to be ready for it [2]. If your cat has reacted to a vaccine before, tell your vet before the next one.
These are statements about groups of animals. They cannot predict how one cat will do. Your vet can explain what this means for your cat.
When to see your vet
See a vet at once if your cat has:
- swelling of the face or head [2];
- fast or difficult breathing, vomiting, weakness, pale gums or collapse [2];
- any of these signs soon after a sting, injection, vaccine or new medicine [1].
See your vet soon if your cat has:
- red, swollen patches on the skin;
- hives together with other signs of being unwell;
- hives that keep coming back.
FAQ
Do dogs get hives too?
Yes. See hives and facial swelling in dogs.
Can I give my cat a human allergy medicine?
No. Give only the medicines your vet prescribes for your cat.
The bottom line
Hives are raised, red, swollen patches of skin caused by an allergic type of reaction. Stings, medicines, vaccines and food are known triggers. In cats a severe reaction often shows as breathing trouble, vomiting or collapse rather than skin changes, and it needs a vet at once. Only a vet can find the cause, and your vet will advise on treatment.
This article is for general information only and does not replace advice from your veterinarian. If your cat has swelling or trouble breathing, see a vet at once.
Sources
- Shmuel DL, Cortes Y. Anaphylaxis in dogs and cats. J Vet Emerg Crit Care (San Antonio). 2013;23(4):377-94. PMID 23855441. https://pubmed.ncbi.nlm.nih.gov/23855441/
- Yoshida M, Mizukami K, Hisasue M, et al. Anaphylaxis after vaccination for cats in Japan. J Vet Med Sci. 2022;84(1):149-152. https://pmc.ncbi.nlm.nih.gov/articles/PMC8810323/
- Mueller RS, Unterer S. Adverse food reactions: pathogenesis, clinical signs, diagnosis and alternatives to elimination diets. Vet J. 2018;236:89-95. PMID 29871756. https://pubmed.ncbi.nlm.nih.gov/29871756/