Australian Shepherd Health & Genetics Institute

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An Unpleasant Itch

Canine Allergies

by C.A. Sharp

First published in the Fall 2001 issue of Double Helix Network News  Rev. May 2013


Scratch.  Scratch.  Scratch.  Where humans sneeze and wheeze, dogs itch.  Dogs can have allergies, just as we do.  Like us, dogs can have respiratory or digestive problems caused by allergies, but most likely they will itch when exposed to something they are allergic to.

Allergic reactions are rarely fatal for dogs.  Allergies are a persistent nuisance and, for some especially sensitive dogs, a source of ongoing misery.  A severely allergic dog may itch constantly, damage its skin and coat with constant scratching, biting and rubbing, and suffer from secondary problems like bacterial and yeast infections that take a foothold in its damaged skin.  An allergic dog can also have chronic and occasionally severe respiratory or digestive problems or, in the worst-case scenario, succumb to anaphylactic shock.  However, with proper diagnosis and treatment, most dogs can live in relative comfort.

The problem of canine allergies seems to be growing worse, with more and more dogs affected.   This is in part a matter of perception.  We are much more aware of allergies today because science has learned a great deal about them in recent years.  This knowledge is often discussed in the media, heightening our awareness and sometimes influencing us to call something an “allergy” when it really is not.  Diagnosis of canine allergies should be made by a veterinarian; not through the owner’s assumptions.  Increased scientific knowledge of immune system function and dysfunction has also lead to better diagnosis.

Allergies that would have been unrecognized in times past are today identified for what they are.  In addition, we and our dogs are exposed to potentially irritating substances—ranging from food preservatives to cleaning solvents to garden chemicals—which our grandparents, not to mention our dogs’ great, great, grandparents, never encountered.    Sensitive dogs and people have many more things to react to.

One of the reasons people and dogs may be more sensitive is our efforts to clean things up in order to prevent various infectious diseases.  An unfortunate downside of that very beneficial effort has been decreased opportunity for young immune systems to learn what they need to react to.  When they don’t do so, they are more prone to cause allergic reactions to substances that aren’t a threat.

Allergies are the physical expression of the immune system’s over-reaction to one or more substances, called “allergens,” that are not normally irritants and which will not bother a normal individual.  Allergens can range from pollens and molds to common food items.  Flea-bite dermatitis is the most common canine allergy; the allergen involved is the saliva of fleas.

Like other immune-mediated diseases, severe allergies are genetically predisposed.  The specific allergies a dog suffers from result from interactions between its genetic make-up, its immune system, and the environment in which it lives.  Because severe allergies are integral with a dog’s genetics, they cannot be cured.  However, the problem can be minimized with proper veterinary treatment and reduced exposure to the offending allergens.  Treatment is for life.

Environmental factors include exposure to allergens, parasite load and the administration of vaccines.  Even though allergies generally don’t develop until a dog is at least six months old, allergen exposure usually takes place before the dog is four months of age.  An allergy does not develop unless there has been prior exposure, which allowed the immune system to recognize the allergen and “decide” that it needed to be attacked if encountered again.  This attack upon subsequent exposure is what causes the allergic reaction.  Exposure can occur through breathing or ingesting the allergen or from getting it on the skin.

If a dog has parasites, the immune system will react to their presence.  The greater the parasitic “load,” the greater the stress on the dog’s immune system.  This can lead to a variety of health problems, including severe allergic reactions if the dog is also exposed to additional allergens.   Fleas are the most problematic parasites where allergies are concerned, since their saliva so frequently causes a reaction, but heartworm and intestinal parasites can also set up the dog for allergy attacks.

Both killed and modified live vaccines are potentially allergenic, though for very different reasons.  Killed vaccines contain chemicals called adjuvants that enhance the efficacy of the vaccine without exposing the dog to the pathogen.  The adjuvants can cause an allergic reaction.  In the modified live vaccines, the toxins produced by the pathogen are what cause the reaction.  The benefits of vaccination far outweigh the risk.  The “core ”diseases for which the animal is vaccinated can be fatal.  Dog owners should not avoid vaccinating, but should work with their veterinarians to implement a vaccination protocol that gives the dog sufficient protection from infectious diseases.  Use only those vaccines for diseases to which your dog is a risk and allow an adequate interval between vaccinations so that the dog’s immune system is not overwhelmed.

Atopic dermatitis, a hypersensitivity reaction of the skin, is the second most common form of allergic reaction in dogs.  When an individual is exposed to an allergen, usually by inhaling it, the immune system begins producing Immunoglobulin E (IgE), a special type of cell designed to target the allergen.  The IgE activates mast cells that release several different substances including histamine, a chemical that causes itching, inflammation and swelling.  Most mast cells are found around the feet, ears and anus so allergic reactions of the skin appear more commonly in these areas.  If the skin within the ear is affected, the dog may also develop secondary ear infections.  Dogs can also experience allergic respiratory problems, digestive problems and eye irritation, but these are much less frequently seen than the skin reactions

Respiratory reactions include an asthma-like chronic bronchitis.  Affected dogs have a dry, hacking cough that can be triggered by exertion or by pressure on the trachea.  Other dogs may have PIE, pulmonary infiltration with eosinophilia, an allergic reaction in the lungs.  Eosinophils are a type of white blood cell.  White blood cells are the foot soldiers in the immune system’s army.  When faced with an infection or allergen, the body produces white cells to fight it.  In PIE, the body produces too many of these cells in the lungs, causing respiratory distress.

Food allergies can manifest as digestive problems or skin reactions.  In humans, food allergy is over-diagnosed.  This is probably also the case in dogs.  A number of foods contain substances that can cause mast cells to release histamine, leading to an allergy-like reaction even in a normal individual.  Any food can cause reactions in an allergy-prone dog, but some are more likely culprits than others. The portion of an allergen to which the immune system reacts is called an epitope.  The proteins found in wheat have over 50 epitopes, so it is not surprising that allergic dogs often react to wheat-based feeds.  Affected dogs tend to vomit within a couple hours of eating and may sometimes have loose stools.   Skin reactions are not unusual.  Affected dogs may have difficulty maintaining weight, despite a good appetite.  Severely allergic individuals have chronic diarrhea, significant weight loss and poor coat quality.  Food allergies often arise after the dog has suffered a case of infectious enteritis.

The most severe—and potentially fatal—form of allergic reaction is anaphylactic shock.  It can occur after eating something containing an allergen, an injection of drugs or vaccine, or the bite of an insect.    Affected dogs will have difficulty breathing.  Their gums will be pale due to a drop in blood pressure.  Immediate veterinary treatment is necessary.

Allergies may commence as early as six months of age and have been reported to begin as late as seven years, though most affected dogs will have shown signs by the time they are two or three years old.  Depending on the allergens that the dog reacts to, its problems may initially be seasonal, but in those with the genetic potential for serious allergies they will advance into a year-round condition.

Any individual may develop minor to moderate allergies sometime during life, but for those with the wrong set of genes, the disease can have a significant quality of life impact.   A dog will not have severe allergies unless it has the genes that cause allergies.  This means breeders need to take allergies seriously.  Severely allergic dogs should not be bred.  Their healthy relatives should be bred to mates without family history of severe allergies. If allergies are a frequent problem in a family of dogs, the breeder would be wise to hold off breeding those dogs until they are at least three years of age to have some degree of assurance that they are not allergic.

The higher the level of inbreeding, the more likely a dog is to develop allergies.  The more inbred a dog is, the more likely it is to have reduced diversity in its Major Histocompatibility Complex (MHC), the group of genes which govern the functioning of the immune system.  Lack of MHC diversity reduces the efficiency of the immune system and may increase the risk of immune-mediated diseases, including allergies.

Severely allergic females often have fertility problems.  It is uncertain whether these are secondary to the allergies or the level of inbreeding.  Breeders should be aware of their dogs’ coefficients of inbreeding (COI).  To detect inbreeding that is not apparent in the common three to five generation written pedigrees, the COI should be calculated over several more generations.  How many generations depends on the genetic history of the breed, but for most ten will be adequate.  If the COI is high (12.5% or more), mates should be selected which will give a COI in the puppies that is lower than that of the parent with the family history of allergies.

As with any inherited problem, breeders would do well to record as much information as possible on the allergy status of numerous relatives of the dogs they intend to use for breeding.  This includes “his sisters and his cousins and his aunts”—those dogs not directly on the pedigree.  The more affected family members a dog has, the more likely it is to develop allergies or produce young who will.  For Australian Shepherds, ASHGI’s IDASH Pedigree Analysis Service can help with this.

Understanding what allergies are and what causes them, coupled with knowledge of the genetics involved, can help breeders reduce the numbers of dogs scratch, scratch, scratching away.