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The Premier Online Magazine devoted
to Persian & Exotic Shorthair Cats

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Anaphylaxis In Cats

What Is Anaphylaxis?

Anaphylaxis is a sudden onset, total body hypersensitivity reaction in response to exposure to an allergen.

What Allergens Cause Reactions?

Allergens implicated in anaphylactic reactions include:

  • Antibiotics
  • Vaccines
  • Insect stings, bites and other venoms
  • Foods
  • Other particles which may be inhaled

An Allergic Reaction Can Be Fatal

Anaphylactic reactions can lead to death of a cat in one of two ways – either by a sudden severe decrease in blood pressure (vasculogenic shock) or by asphyxiation (suffocation) due to spasm and fluid-filling of the airways.


In order for an anaphylactic reaction to occur, sensitization to the agent must first happen. This sensitization may occur over a long period of time or may be from only one prior exposure. During sensitization, the antigens are “presented” to the cells of the immune system which then produce antibodies. These antibodies are now ready and waiting for the next dose of allergen. The degree of sensitization can be influenced by the nature and dose of the antigen, the site of exposure, and the cat’s inherent immune response to that allergen. These different factors all play a role and help to explain why one cat may react to a vaccine whereas another will be perfectly fine.

What Is An Anaphylactoid Reaction?

AnaphylactOID reactions do not require previous exposure to an allergen and may occur on the first exposure to an agent. They follow a different cellular mechanism in the body, but the end result is the same.

Regardless if your cat is having an anaphylacTIC or anaphylacTOID reaction, you must act quickly.

How Common Is Allergic Reactions In Cats?

The incidence of anaphylactic reactions to medications is a number that cannot readily be calculated – accurate numbers are not available because not all clients report mild reactions to their veterinarians, and not all veterinarians report reactions to the proper authorities. However, in researching this article, it would appear that practitioners from all parts of North America have seen anaphylactic reactions in cats to both vaccines and to other injectable and topical medications.

Anaphylaxis vs Allergic Response: Many of the vaccine reactions that owners report to me do not meet the time factor or symptomatology of true anaphylaxis. These reactions may be at least partially psychological in nature if they usually occur in fearful or high-strung pets. Fear causes the release of many physiological mediator chemicals that make drug reactions more likely. In mild cases. In this case the only signs are low fevers, sluggishness and loss of appetite. Pets that develop a mild first case often go on to develop more severe cases during subsequent exposure to the allergen.

Cats Are Not Small Dogs

There are well recognized differences between dogs and cats when it comes to anaphylaxis. In cats, 66% of reported clinical signs following vaccine administration involved the gastrointestinal tract (vomiting/diarrhea), 22% involved the respiratory tract (difficulty breathing), and 12% involved the skin. In dogs, over 50% of the clinical signs were skin-related (hives & swelling of face/ears) and only 6% respiratory.

Dr. Annette Litster, Research Manager for the Centre for Companion Animal Health at the University of Queensland, has found that in the cat, serotonin released from platelets is the major chemical that drives the anaphylactic reaction. This is quite different from dogs, in which histamine and mast cell tryptase are the major chemicals. Much work is still being done to fully evaluate the implications of these findings, but it may mean that antihistamines such as diphenhydramine (Benadryl) may be less effective in prevention and treatment of reactions in cats and other anti-serotonin drugs such as cyproheptadine (Periactin) might be the way to go.

Symptoms of an Allergic Reaction

The anaphylactic reaction may take only minutes to develop, and the route of exposure likely influences how fast the signs are seen. For example, the reaction to a vaccine administered SC would in theory be slower onset than if the allergen was inhaled or administered intravenously. It is therefore a very good idea to keep a close eye on your kitties when you are giving them any kind of injections, whether it is vaccines or antibiotics, for at least 15 minutes.

The first signs of anaphylaxis in cats include:

This is the time you should be simultaneously injecting the cat with epinephrine (instructions to follow) and calling your closest veterinary clinic. In mild instances, one or two injections of epinephrine may be all that is required, but more serious cases need intravenous fluids, steroids, antihistamines, oxygen and close monitoring.

Neomycin Alert

Three different veterinarians reported acute systemic anaphylactic reactions in cats after administration of topical eye medication containing bacitracin, neomycin, and polymyxin. Within minutes of administering the medication all three cats developed vomiting and then respiratory signs, cardiovascular collapse, and death.

One other veterinarian reported a similar reaction to a different triple antibiotic/hydrocortisone ointment. In the human literature, topical application of bacitracin has been associated with severe anaphylaxis; in the cats the exact cause has not been identified, but most clinicians feel it was the neomycin in the product that caused the reaction, as it is a known sensitizing agent.

Necropsy Findings

Often after a sudden otherwise unexplained death of a cat, either the owner or the veterinarian will suggest a necropsy (post-mortem examination, in humans it is an autopsy) to try to find out why the cat has died. Sometimes we get lucky and a reason may be found, such as a ruptured blood vessel, a serious but previously undiagnosed heart defect, or a bowel obstruction.

Since death can occur rapidly due to acute shock, the absence of abnormal findings at necropsy does not exclude a diagnosis of anaphylaxis. Necropsy findings after fatal anaphylaxis are surprisingly variable, not only between species because of species differences, but also within species. In the cat, reported findings include pulmonary edema, emphysema, hemorrhage and congestion, and edema of the lower half of the small intestine, but these organs may be macroscopically normal.

A necropsy can help to rule out other causes of sudden death, but doesn’t always give us the answer we so desperately want: “Why did my kitty die?”

For more information read the article titled, Necropsy.


Anaphylaxis and anaphylactoid reactions are medical emergencies that require immediate treatment to prevent a catastrophic outcome. Treatment is the same for both conditions and early intervention is critical to successful management.

If the reaction has occurred at home, then epinephrine should be administered as soon as possible, and injections may be repeated every 10-15 minutes as required. Once at the veterinary clinic, intravenous fluid therapy, steroids, antihistamines and oxygen may be required to stabilize the cat. Most cats respond quickly and favorably to aggressive therapy, and usually once they are back on their feet and acting normally a relapse will not occur (providing of course there isn’t repeated exposure to the agent that caused the reaction in the first place).


Epinephrine (epi) can be injected either intramuscularly or SC at home. Epinephrine (also called adrenalin in other parts of the world) is available for injection in different concentrations so you must be very careful when reading the label.

Epi most commonly is found in vials of 1mg/ml – labeled 1:1000 – or 0.1mg/ml – labeled 1:10,000. For the cat the usual dose is 0.1ml of 1:1000, or 0.1mg. If you have the 1:10,000 then you need to give 1 ml to get the same amount.

Overdosing epi can cause extremely fast heart rates (tachycardia), as well as vomiting/diarrhea, but fortunately it is a very short acting drug so the effects wear off after 15 minutes or so.

A convenient method of administration is with the product EpiPenJr. ©, which contains 0.15mg of epinephrine. There is also the regular adult EpiPen© which contains 0.3mg, so make sure you get the right one. Your veterinarian should be able to write a prescription for this product.

Read the instructions carefully BEFORE you need to use it – during an emergency is not a good time to learn something new. To administer to the cat, grab the rear leg with your hand so that the thigh bone is between your thumb and fingers, hold the pen as directed by the instructions and very firmly press the “firing end” against the skin in the bunch of muscle you’ve grabbed with your other hand. Hold it there for 10 seconds. The needle is pretty short so you don’t have to worry about it going right through the leg, but you also need to make sure it’s going all the way through the fur (especially in our long-haired kitties!).

There is a nice website at that has a good animation – it’s designed for people, but it’s a good video.


As previously mentioned, serotonin appears to play an important role in anaphylaxis in cats. Cyproheptadine (Periactin) is an effective anti-serotonin drug which is usually readily available from the pharmacy with a prescription or may even be carried by your veterinary clinic.

Cyproheptadine has traditionally been used in cats as an appetite stimulant at a dose of 2-4 mg per cat, and has even shown promise in helping feline asthma cases, at a dose of 1 mg. Research is still underway to see how effective this drug may be for anaphylactic reactions in cats. Unfortunately there is only an oral form of the drug available which limits its use in the emergency situation.

Prevention of Reactions

Sadly, there is no way to predict which cats are going to have an anaphylactic reaction and to what allergens. It goes without saying that if a cat has had a reaction to a particular drug then you should think long and hard and fully evaluate the risk:benefit ratio before giving that drug again. It should also be noted that cross-reactions can occur, in which one drug, such as penicillin, can sensitize the system to a similar drug such as a cephalosporin. If your cat has had a reaction to Duplocillin (penicillin) in the past, make sure you always discuss this with your vet before ANY OTHER antibiotic injections are given.

I would also suggest that if your cat has been given any injectable medications (including vaccines), topical or eye medications, you wait in the reception area for at least 15 minutes afterwards. In most clinics, 15 minutes is enough time for you to be already on your way home. Hang out, chat, read a poster or a pamphlet – you probably don’t need to but that one time you do, you’ll be awfully glad you did.

If for some reason you absolutely must re-administer a drug that your cat has reacted to in the past (usually this will be a Rabies vaccine) then pre-treatment with diphenhydramine (Benadryl) at 2mg/kg by mouth a couple hours in advance may help to lessen the reaction. There currently is no research that proves pre-treatment with cyproheptadine is beneficial.

In Conclusion

For breeders giving their own vaccinations and antibiotics, it is a wise precaution to include epinephrine in their medicine chest. It is important to recognize the symptoms of an allergic reaction and to act promptly.


  Dr. Leah Montgomery, DVM

Dr. Montgomery attended the University of British Columbia, graduating with a Bachelor of Science in Agriculture, then earned her Doctorate from the  Western College of Veterinary Medicine. She owns Shaughnessy Veterinary Hospital in beautiful Port Coquitlam, BC, Canada. The multi-veterinarian practice boasts an all female staff of vets, technicians, assistants and support staff. The practice is a full-service veterinary hospital with a complete radiology suite, full in-clinic blood testing capabilities, a dedicated operating room with a surgical laser, and acupuncture/chiropractic treatments.

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