In the popular press, the role of the mammalian immune system in protecting us from disease is often cast in simplistic terms of good and evil. Pathogens—bacteria, viruses, parasites, and fungi—are poised, ready to attack when our guard is lowered. It is a naïve model that perpetuates the notion of single, external cause of disease, with solutions to be found in pharmaceuticals and vaccines that rescue us when our own defenses are inadequate. But what happens when our immune system overreacts? When normal aspects of our environment elicit biological havoc that harms the very body the immune system is designed to protect, the stage is set for a lifetime of allergic disease.

An allergic reaction is fairly simple. It consists of two parts. In step one, the immune system mistakes a normal component of the environment, such as pollen, dander, protein, etc., for an invader—more on why this happens later. The immune system produces antibodies (specifically immunoglobulin E, or IgE) that will recognize the invader (termed the antigen) in the future. The antibodies circulate in the body, in conjunction with other types of immune cells, on the lookout for any sign of that specific antigen. 

When the antigen, perhaps spring pollen from a tree, is encountered, the primed IgE binds to the antigen, identifying it for destruction. Other immune cells, such as mast cells and basophils, attack the invader, releasing chemicals that cause tissue swelling and fluid leakage. The affected animal or person may experience inflamed, watery eyes, sneezing, coughing, red skin, ear shaking or itchy skin. If the allergen is in food, it could also cause diarrhea and vomiting. In severe allergy cases, blood pressure may drop, the tissues of the respiratory tract could swell and the ensuing anaphylactic reaction could be catastrophic.

Allergies are so common in people and in pets that we tend to dismiss them, confusing “common” with “normal”. Respiratory and skin allergies have exploded in the last thirty years, independent of better testing and identification. Most pet health insurance companies listed “allergies” as the most common reason dogs were seen by a veterinarian in 2019. This dramatic increase in allergic disease in pets mirrors the problems seen in human healthcare. What is behind the numbers?

The Hygiene Hypothesis

The “hygiene hypothesis” was put forth in the 1980s and suggests that our uber-clean modern environment fails to educate our immune system as to what is “normal” and what is “dangerous”. Frequent bathing of our pets removes normal organisms on the outside, and antibiotics and other oral medications decimate the internal biome of the gastrointestinal tract. Better hygiene undoubtedly contributed to decreases in some disease, but more of a good thing is not necessarily better. Especially in young animals, normal exposure to the outdoors, dirt, various foods and other animals is all a part of the training of the immune system. 

Additional Factors

As with most medical conditions, unraveling the knot of causation is not as simple as finding a single causative factor or theory. Excessive hygiene is partly to blame, but there is also a genetic contribution. In dogs, certain breeds are known to be more allergy-prone, and in people, having two parents with allergies results in an eighty percent chance that their children will also have allergies. The surge in pet allergies is also being linked to highly processed food ingestion, rising rates of obesity and inadequate exercise, all of which contribute to a state of whole-body inflammation that causes immune dysregulation. Over-vaccination of our pets is also a contributing factor, leading to a state of immune system hyper-alertness.

Food Allergies

Although true food allergies (as opposed to food sensitivities) are rare, they are also on the rise in dogs and cats. In people, the old advice to avoid highly allergenic foods in potentially sensitive children has been found to be completely wrong. In children with a high likelihood of peanut allergies, for example, exposure to peanuts at a very young age lessens the likelihood of developing peanut allergies. 

We have faced the same problem in our dog and cat populations. The old and unfortunately still prevalent advice, to feed only one type of food, contributes to the development of food allergies. It does so in two ways. First, repeated exposure to the same antigens (typically proteins) every day for months or years, can cause immune system sensitization. Second, feeding such a mono-diet fails to develop a healthy gut microbiome, often leading to the absence of certain crucial bacterial species. It has now been found in people that Clostridia bacteria help to prevent food allergies by causing immune cells to release a protein that makes the gut less permeable; in other words, these bacteria decrease leaky gut syndrome.

The complex allergy causation puzzle of environmental and genetic inputs is only partly understood, at best. Allergies develop in our companion animals despite healthy exposure to a diverse outdoor environment, feeding a complex and nourishing diet, and minimizing antibiotics and vaccinations. The next segment of this article will examine a range of non-pharmaceutical options for addressing allergy symptoms.

Dr. Laura Weis and her husband, Dr. Ransome Weis, own and operate Doylestown Veterinary Hospital & Holistic Pet Care, and Holiday House Pet Resort & Training Center, in Doylestown. She focuses on homeopathy and nutrition counseling for her clients within the full-service veterinary practice. Call 215-345-6000 to request an appointment. 

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