Signs Your Pet Might Be Having A Vaccine Reaction

14 October 2016
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Vaccinating your pet is a critical responsibility of pet ownership. Without this protection against transmittable diseases, your furry friend is susceptible to a host of illnesses, some of which can become life-threatening. But every once in a while, dogs and cats can have allergic reactions to vaccines—reactions that are easy to treat, if you know what to look for and you address them right away. If your pet was just vaccinated and things seem a little "off," here are signs that they might be having an allergic reaction and what you need to do. 

Facial Swelling

Swelling around the muzzle, eyes, or neck is often one of the first signs a pet owner will notice. If it happens soon after your little one receives a vaccine, you should contact your veterinarian right away.

The vet may recommend giving an over-the-counter antihistamine and monitoring symptoms for progression. But it's possible they will want to see the animal in order to determine if more aggressive treatment is warranted. This can include antihistamine and anti-inflammatory injections, and some doctors will want to give fluids under the skin to ensure your pet stays well hydrated.

High Fever

If you suspect a fever, you can take your pet's temperature rectally with a mercury thermometer and some petroleum jelly. This gives the most accurate reading.

Normal temperatures for dogs and cats range from 100-102.5 degrees. It's okay if your pet has a low grade fever following a vaccine, but anything higher than 103 should be addressed. Your vet may not be too concerned so long as your furry friend is eating and drinking okay and a low fever is the only symptom. But most people don't take their pets' temperatures unless other symptoms are present. Be sure to note these and discuss anything else unusual when you call the vet.

Note: Do not give your pet over-the-counter medications to treat a fever as some of them can cause organ damage and other complications. Animals react differently to many human medications, so always follow the advice of your veterinarian for treating a vaccine-induced fever.

Loss of Appetite

Mild loss of appetite can be expected for a short period of time following a shot. But if your pet goes more than two days without eating, it could be attributed to a vaccine reaction. You can certainly try enticing them with a small amount of canned food or even some plain (unseasoned) chicken without the fat. If they still turn their nose up, it's time to seek medical help from your veterinarian.

This is especially true for cats because when they go too long without eating, they can develop a condition called hepatic lipidosis, and overweight felines are at a much greater risk. So always be sure that loss of appetite is addressed promptly.


Like people, animals can get hives when having an allergic reaction to most anything. Hives are a result of certain types of blood cells (mast cells) releasing histamines. This causes the red splotches to appear, and the nerve endings become aggravated, leading to itching. In fact, you may actually see your pet scratching excessively before you notice the hives.

This symptom is not a normal side effect of a vaccine, so be sure to get on the phone with the animal hospital if you observe splotches or itching.

Acute Vomiting or Diarrhea

Getting a little carsick on the way home is one thing. But if your four-legged friend is persistently vomiting or experiencing diarrhea, this can lead to dehydration. Be sure to call the vet if you observe any sort of upset stomach. The doctor may determine that Fido just has a case of bad nerves. But this could be a situation where an antihistamine is warranted. And if your pet isn't able to keep any food down, the doctor will probably want to administer the medication by injection as well as offer fluid therapy.

Trouble Breathing

It probably goes without saying, but if your pet has trouble breathing following a vaccination, you'll need to seek help immediately. The best thing to do in this situation is to call the vet and let them know you're on the way. This will allow the staff to prepare any needed medications as well as oxygen so your pet can get the treatment needed the minute you walk through the door.


Seizures are rare, but they can happen as a vaccine reaction. If you observe a seizure in your pet (collapsing, jerking movements, mouth chomping, drooling, paddling motions of the arms and legs), you need to make sure the area around them is safe. Remove any hard objects that could hurt them, or gently slide them away from a staircase. Do not try to stop the seizure by holding onto arms and legs. Also, don't put anything in their mouth as you could get bitten. Have someone time the seizure, if possible, and call the vet right away. They will likely want to see your pet and treat with medications and fluid therapy.

If your pet has been diagnosed with a vaccine reaction, ask the veterinarian about giving antihistamines before receiving vaccines in the future. Quite often, this is an effective way to prevent allergic reactions in the future.