What is an allergy?
In dogs, adverse reactions to food such as loose stools, diarrhoea or even vomiting can be due to feeding a new diet that is substantially different from whatever the dog was used to being fed. If symptoms appear rapidly, these symptoms might be caused by ingested toxins.This sort of reaction is very different from a food allergy, which not only causes symptoms that occur several hours if not days after the offending food has been eaten, but symptoms can also affect the skin and airways in addition to more obvious digestive complaints.
Because of this wide range of different symptoms, and the fact that they don't necessarily appear immediately, it can be difficult to recognise when your dog has developed an allergy. In addition, many dog owners are still unaware that dogs can actually develop an allergy.
But food allergies are a very common cause of itching, and account for about 10 to 15 percent of all allergic skin diseases in dogs.
In a nutshell
- An allergy is an acquired sensitivity to a substance.
- This substance is then called an allergen.
Food allergy or sensitivity?Generally speaking, an allergy always involves the immune system. The immediate allergy, with life-threatening symptoms is very rare (think of peanut allergies in children). The most common form is the food allergy with delayed onset of symptoms as described above. This is also often referred to as a food sensitivity or hyper-sensitivity.
The science bitWhen food is digested, it is broken down into its smallest constituents, or down to molecular level. The process of digestion is completed when food particles are either moved into the bloodstream to be transported to body cells for further use, or excreted if they are not needed by the body. In the healthy dog, the barrier between intestines and bloodstream allows only the smallest food particles to pass through, and the dog's immune system recognises food protein as such and lets it travel into the bloodstream.
In an allergy, protein molecules that previously were allowed to pass are now sought out and attacked by the immune system. This happens because the immune system mistakes the harmless food proteins for foreign invaders (such as viruses or parasites) which would present a danger to the body and therefore not be allowed to enter the bloodstream. It is important to note that a food allergy can only develop to a protein that the dog has ingested at least once before - otherwise the immune system would not "recognise" and attack it. Food allergens have often been part of a dog's diet for months or years before the immune system responds in this misguided way.1
Ingestion of the offending food stimulates this immune response which results in the release of chemicals, including histamine, which in turn cause the main symptoms of an allergic reaction. The main consequences are inflammation of the intestinal lining, the key site of the immune response, and inflammation of the skin and other sites in the body.
Why does a dog develop a food allergy?
In contrast to dogs with seasonal inhalant allergies (e.g. pollen), dogs with food allergies tend to itch and have other symptoms all year round. Therefore, it is essential to find and remove the type of food that triggers the allergic response.
PrognosisThe prognosis for a dog with food allergy is good if the allergens can be identified and eliminated from the dog's diet. Some dogs may develop new food allergies with time, and will need to have their diet adjusted as necessary.
Find out how exactly an exclusion diet works.
1 John K. Dunn, Textbook of small animal medicine, 1999, p428-429
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