Allergies

Introduction

Allergies and hypersensitivities are a disorder of the immune system. The immune system in pets is designed to defend the body against harmful pathogens. However, in our allergic pets, after exposure to certain stimuli or substances, the immune system overreacts, producing an undesirable immune response to certain allergens that would be considered harmless in a non-allergic animal. This allergic response, or hypersensitivity, can manifest as itchy, red and inflamed skin.

Our pets can be diagnosed with four common allergic skin diseases. These are:

• Flea Allergy Dermatitis
• Atopic Dermatitis
• Allergic Contact Dermatitis
• Cutaneous Adverse Food Reactions

Clinical signs for each of these diseases can be very similar, and sometimes there can be an overlap of diseases, where your pet may have two concurrent allergies, for example, Flea Allergic Dermatitis and Atopic Dermatitis. The following sections will elaborate on each of these four common allergies.

Whilst allergies are a common cause of skin disease, many other diseases can also cause itchy, red and inflamed skin. Along with taking a history and performing a clinical exam, your veterinarian may also perform skin tests to rule out these other causes.

Exposure to these allergens is via inhalation or absorption through the skin. Once the animal has had exposure to the allergen, it will mount an exaggerated immune response and develop clinical signs such as itching and inflammation of the skin. Animals with Atopic Dermatitis will scratch, chew, lick or rub areas of the body such as their paws, face, ears, axillae (armpits) and abdomen. The location of the itch can vary between animals, and the severity of the itch can vary also. Some animals may only display itchy ears, while others may suffer from severe itching in many different body locations.

A diagnosis is made by ruling out other causes of itching, such as parasites, food allergy, contact allergy, and bacterial and fungal infections. The history of the pet’s itching and the sites of itching are assessed by your veterinarian to ensure that they fit with a diagnosis of Atopic Dermatitis. 

Once a diagnosis has been made, an allergy test can be performed. The allergy testing aims to identify the environmental allergens the animal is allergic to, not diagnose the presence of Atopic Dermatitis. The allergy tests most useful for Atopic Dermatitis are intradermal skin tests or blood tests. Once the allergens have been identified via testing, desensitising vaccines can be prepared for that animal.

Desensitising vaccines are the only treatment choice that alters the animal’s immune response to benign environmental stimuli. These vaccines aim to desensitise your pet to specific allergens by giving controlled amounts of the vaccine via injection or orally under the tongue. Over time, after receiving regular doses of the vaccine, your pet’s immune system will become desensitised to the allergen and will not mount an exaggerated immune response when exposed to the allergen in normal conditions.

We can use many other medications and treatment modalities in pets with Atopic Dermatitis, and sometimes a combination of these is required to control and manage this complex disease.

A diagnosis of FAD is usually based on the location of skin lesions and the itch resolving after a flea control trial. While evidence of fleas or flea “dirt” (faeces) can help with the diagnosis, they are not always easy to find, especially in cats who groom themselves well.

Flea Allergy Dermatitis is best managed by ensuring the pet is on regular flea prevention and that the pet’s environment has been treated for fleas. All other in-contact pets, or pets in the same household, should also be put on a regular flea prevention program as this will help to minimise exposure.

If you suspect your pet has fleas or is suffering from Flea Allergy Dermatitis, contact your veterinarian to discuss the most appropriate flea control plan.

The most common allergens are plants such as “Wandering Trad” and “Moses in the Cradle”, however, other substances such as cement, floor polishes and carpet deodorisers have been shown to affect pets. Because this type of allergy requires direct contact between the skin and the offending substance, only sparsely-haired areas in contact with the ground are usually affected. We are less suspicious of contact dermatitis when very hairy areas are affected by dermatitis.

Common symptoms include red, inflamed bumps (macules and papules), which can be very itchy. The affected areas can include the abdomen and underside of the chest, underneath the tail, front of the neck and chin and the inside of the ear pinnae (ear flaps). Over time, these lesions may spread to involve larger areas as hair loss progresses in nearby areas. Depending on the offending substance, clinical signs may be seasonal or non-seasonal.

A diagnosis of contact dermatitis is made by combining information from the pet’s history with the observation of the location of the skin lesions and demonstrating that the clinical signs resolve when the pet is isolated from possible allergens. To maximise the effect of the isolation trial, we usually recommend that the pet is first bathed with a gentle, hypoallergenic shampoo to remove possible allergens. A “patch test” can also be performed by your veterinarian, which involves applying the suspected allergens to the skin and seeing if a localised reaction occurs. This can help determine which plants or materials are likely to cause contact dermatitis.

Once diagnosed, management of these cases relies on avoidance of the allergens. However, this may not be possible in every case, and topical anti-inflammatory medications can be useful in management. In more severe cases, systemic anti-inflammatory and anti-itch medications may be required.

Clinically, Cutaneous Adverse Food Reactions are difficult to distinguish from other types of allergies, such as Atopic Dermatitis. CAFR is a potential cause of non-seasonal itching and occasional gastrointestinal signs in dogs and cats. The clinical signs you might see with CAFR are similar to Atopic Dermatitis. The distribution of lesions and areas affected may range from just ear inflammation to whole-body itching. Some pets may have concurrent gastrointestinal signs such as vomiting, diarrhoea, flatulence or even increased toileting habits. A common presentation in cats is itching around the neck and face; however, it can also be generalised.

These patients are prone to secondary bacterial and yeast infections similar to Atopic Dermatitis. The presence of these secondary infections can complicate reaching a definitive diagnosis and can make management more difficult.

Food allergies cannot reliably be tested for with blood tests. An elimination diet trial must be performed to determine whether a food allergy is present. This consists of feeding your pet either a hydrolysed diet from your veterinarian or a home-cooked diet that contains ingredients your pet has not been exposed to (a novel diet). The trial should be continued for 8–12 weeks or as your veterinarian advises. When doing a food trial, you may need to discuss alternative non-flavoured parasite prevention with your veterinarian to avoid possible contamination with a food type you are trying to avoid.

At the end of the 8-12 week period, if the itching stops, your veterinarian will guide you through the reintroduction of your pet’s previous diet to identify the cause of your pet’s food allergy. A diet that avoids the specific allergen can then be chosen for long-term use. It is important that the long-term diet chosen for your pet is complete and balanced.

The prognosis for CAFR is good once the food allergen has been identified and can be avoided. Occasionally, animals may experience further dietary intolerances and may require repeat elimination trials, as they can become intolerant to other foods over time.