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Allergic Reactions in Pets

Updated: Jan 13, 2023

Imagine you're walking your young, healthy dog through the neighborhood after supper. He's sniffing in the grass and weeds, having fun it seems. It's a completely normal walk, until you get home and notice his snout now looks a little bit puffy and he keeps rubbing his face on the rug. But he doesn't have any pain, and he drinks and eats and seems otherwise fine, so you almost forget about it. Then, about 15 minutes later he comes in the living room and you can't even recognize him. He threw up, his face is now blown up like a balloon with bright red ears, he has itchy red welts all over his trunk and neck, and he seems miserable. Better break out the antihistamines and call the vet, because your dog is having an ALLERGIC REACTION!

Allergic reactions (hypersensitivity reactions) are a response by the immune system to a foreign substance. Recognizing a foreign invader, the immune system goes into overdrive, releasing massive amounts of histamine in an inflammatory cascade to repel the invader. This is a natural process, but the problem occurs when the effects of the immune response are more damaging to the body than the foreign invader itself!

The two most common causes of an allergic reaction in our pets are vaccines and insect stings, followed by plants, food, and medications. Allergic reactions can be immediate (within seconds to minutes of exposure) or delayed (30 minutes to 8 hours after exposure), and may be classified as mild, moderate, or severe. Mild cases may only have mild itchiness, moderate cases may have some swelling and hives, and severe cases may result in life threatening anaphylactic shock. Fortunately, our pets don't suffer from airway obstruction problems due to allergic reactions as often as humans do, and airway obstruction is not common even with severe facial swelling (angioedema). Exceptions would be breeds or individuals with pre-existing airway conditions such as English bulldogs.

How do we treat allergic reactions? First try to decontaminate. If your pet has something on its coat or skin that it is allergic to, washing it off will help. A drop of Dawn soap and a lukewarm rinse will help remove any plant oils or dislodge any fire ants or wasp stingers that may be present.

Then we medicate. For mild reactions, benadryl (diphenhydramine) is sometimes all that's needed. This common anti-histamine helps to neutralize the massive amounts of histamine in the circulation (histamine causes redness, hives, swelling, and itching). A benadryl injection will work faster, but if you've got the 25mg tablets at home you can give your dog roughly one tablet per 30 pounds of body weight. The effects of benadryl are usually first seen within 30 minutes, and last for 6 to 8 hours. The main side effect of benadryl is sedation.

For moderate reactions, a quick trip to the vet is definitely in order. Your pet will probably get a shot of benadryl in addition to an injection of a potent steroid like dexamethasone. Steroids calm down the immune system and suppress the inflammatory cascade, so it is like treating the reaction at the source. It can take up to 4 hours for the dexamethasone to work, but the effects do last for 24 hours. Side effects may include increased thirst, appetite, and urination.

For moderate to severe cases i.e. true anaphylaxis, your vet may choose to give all of the above plus an epinephrine injection. This is the ingredient in Epi-Pens, and it can be life saving in severe reactions. Side effects of epinephrine include rapid heart rate and high blood pressure. Your pet will also likely need IV fluids and intensive care to treat an anaphylactic reaction.

If your pet has a history of allergic reactions, make sure to let your vet know prior to any future vaccine appointments. They may wish to pre-treat with Benadryl. It is usually a good idea to wait at your vet for 30 minutes or so after any vaccines, just in case there is a reaction. And avoid the use of vaccine clinics, where they may not have the oxygen, IV fluids and medications needed to treat a severe reaction. If your pet has a severe or anaphylactic reaction to vaccines, they should not be vaccinated anymore.

No matter how careful we are however, allergic reactions can still happen. The most important thing is to be prepared and educated for the possibility. Have your vet's phone number handy, and the local animal ER as a backup. Know your pet's dose of Benadryl and have some on hand. If you see signs of a reaction, decontaminate, medicate, call your vet, and get in the car if you think you need to. Taking these steps will ensure that your pet gets through an allergic reaction safely!

Durbin Creek Animal Hospital (904) 770-7615

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