Cats occasionally develop allergies to medications that are prescribed to them, such as Erythromycin, an antibiotic ointment used for skin, sinus, and eye infections. Contact allergies of all sorts are caused by the overly aggressive response of specialized immune cells, called mast cells, to a protein that the body sees as an invader. In cats, this response by the immune system can cause an uncomfortable and unsightly skin condition, and in rare cases can lead to an anaphylactic reaction. Allergies to Erythromycin are relatively common compared to other medications.

When cats develop allergies to topical medications like Erythromycin, an antibiotic ointment used to treat skin, sinus, and eye infections; they may develop itchy and uncomfortable skin conditions that are prone to infection.

Symptoms of allergies in cats are typically exhibited as skin problems, regardless of the type of allergy. These can include: 

If your pet experiences a rare anaphylactic response, the reaction will be very rapid and will include swelling of the face, particularly around the eye, intense itching, and difficulty breathing. If your pet exhibits these symptoms, immediate medical attention is required.

The specialized cells of the immune system that are tasked with the role of protecting the body from dangerous pathogens are known as mast cells. When the cat’s immune system is stimulated by a particular protein, it may trigger the mast cells to release a naturally occurring compound known as histamine. Histamine has an inflammatory effect on all of the tissues it comes into contact with and in rare cases, allergies that are linked to medications such as Erythromycin may induce an anaphylactic reaction, in which the cat experiences massive swelling, respiratory distress due to swelling of the lung tissues, and a dramatic drop in blood pressure. This type of response occurs rapidly and is frequently fatal if not treated quickly.

If your cat is showing signs of anaphylactic shock shortly after having the medication administered, the timing of the reaction will help your veterinarian make a preliminary assumption that the allergy is likely to be related to the antibiotic, and this medication will be avoided in the future. Signs of milder, skin-related reactions may be delayed by several hours, which may cloud the origin of the symptoms. The signs and symptoms that your cat exhibits are typically indicative of an allergy or infestation and will generally prompt your cat’s doctor to collect a sample from an affected area using a technique known as skin scraping, as well as collecting blood samples. The skin samples that are collected will then be evaluated by an examination of the cells using a microscope, known as cutaneous cytology, and standard blood tests are likely to show an increase in a type of white blood cell known as eosinophils, which can also indicate the presence of an allergic reaction.

The results of these tests may help the examiner to make an initial diagnosis, at which time a patch test, also known as an intradermal skin test, may be recommended to narrow down the origin of the symptoms. In a patch test, minuscule amounts of the antigens that the veterinarian suspects, as well as common allergens in the area, are injected under the skin in order to induce a localized reaction. Many veterinary professionals may find it simpler and less invasive to diagnose the trigger by eliminating the suspected allergens from the feline’s environment first, in this case, by changing to an alternate antibiotic medication.

A complete change in the type of antibiotic will need to be made for the symptoms to cease, and in many cases, symptoms will continue for a time after the medication has stopped being administered. Several varieties of relatively effective antihistamines are available for feline use, both formulated specifically for cats and some are formulated for humans. Although there are some antihistamines that are formulated for humans that are valuable for use with cats as well, it is essential that you never give your pet human medications without the advice and supervision of a veterinarian as even those that are safe to use may require adjustments to the dosage that can be species-specific. Hydrocortisone is used in many shampoos and salves that can help to relieve the skin discomfort that is associated with allergies.

It is important to note that your pet should be prevented from licking these preparations off their skin as it can make them ill if consumed in large amounts. If neither antihistamines or hydrocortisone preparations are successful in reducing the allergy-related symptoms, then corticosteroid injections or oral tablets may be recommended. Although cats can develop side effects from the corticosteroids, it is much less common than it is for dogs.

As allergic reactions tend to become more aggressive over time, your veterinarian may recommend that you get a prescription for a dose of epinephrine if your cat has experienced a moderate to severe allergic reaction involving hives or any interference with breathing. EpiPens and EpiPenJrs are both too powerful for most cats as they are typically under 20 pounds, and you will usually be given a syringe with the right sized dose for your pet. Use of epinephrine to stop an allergy attack should be followed by a trip to the veterinary emergency room, even if your cat appears to be fully recovered. Epinephrine is a short-acting drug, and the allergic reactions are known to resume without proper medical treatment.