What are they?
An allergy is an abnormal immune reaction to an otherwise harmless substance - a pollen, for example - that is termed an allergen. Like all animals, cats can (and do!) suffer from allergies, but unlike humans, most allergic diseases in cats cause skin problems.
What causes them?
A cat's immune system has a number of different modes, or arms. One of these utilises a special kind of antibody called IgE, and a chemical called histamine, to help fight off internal parasites. However, it sometimes make a mistake and overreacts to the wrong things - that's what an allergy is. IgE detects and responds to the presence of a foreign protein, and tells the mast cells to release histamine, which causes itching, swelling and inflammation. This then sensitises the area to IgE even more, so a negative spiral develops.
What cats are at risk?
Any cat may develop an allergy, but they are most commonly diagnosed for the first time in young adults. It's also important to realise that a cat cannot become allergic to something the first time they're exposed to it - it takes a while for their system to develop an excessive response to any new chemical they come into contact with, either natural or artificial.
What are the symptoms?
It depends, of course, on the allergy in question! The most common feline allergic condition is Flea Allergic Dermatitis (FAD), where the cat become sensitive to flea saliva, but there are many, many other possibilities, including responses to pollen, dust mites, food, or washing materials. In cats, we generally recognise three common presentations of allergies. The most common of all is Allergic Dermatitis, or Allergic Skin Disease. This can be caused by any type of allergen (yes, even food allergies!) and mainly causes itching, self trauma, and a thing called Miliary Dermatitis - where small lumps form inside the skin, often with scabs on top. Other possible presentations, however, include Allergic Rhinitis (runny noses and snuffling, often caused by dust or pollen allergies); and Contact Dermatitis, where a specific patch of skin becomes sore and inflamed due to local contact (with, for example, a detergent or washing powder).
How are allergies diagnosed?
Well, telling that your cat has an allergy is pretty straightforward (inflamed, itchy patches or noses are pretty diagnostic!). However, determining exactly what the cause is can be much harder. Vets will usually try to rule out Flea Allergies first, because they're so much more common than other types, by starting a very strict flea control programme, and perhaps using a short term medication (like a steroid) to damp down the itchiness. If that isn't sufficient, they'll investigate other options, either by allergen exclusion (avoid the suspicious substance, for example, changing to a hydrolysed protein diet that cannot cause allergic reactions to rule out a food allergy), or direct testing (with blood tests for IgE levels, and/or skin prick tests for histamine release sensitivity).
How can they be treated or managed?
Allergies in cats can be very challenging to manage effectively. The ideal management technique is to exclude the allergens from the cat's diet and environment that they're sensitive to - for example, excellent and strict flea control in cats with FAD. However, this isn't always practical, so other techniques may also be needed. Immunotherapy (a type of vaccine given to desensitise the immune system) isn't widely used in cats, but there are now labs who can supply the appropriate vaccines; however, drug therapy is often required. Antihistamines are quite variable in how effective they are - some cats respond really well, but for most, it's pretty hit and miss - and often have marked side effects (they are essentially sedatives!). Steroids (as injections, tablets, creams or sprays) are often used, but if used in the long term, the side effects to be quite marked. In some very severe allergies, a drug like cyclosporine may be needed to reduce the cat's immune response. In most cases, however, a balanced management programme incorporating multiple different strategies is needed to keep them comfortable.
Can they be prevented?
No - cats will develop allergies when they develop them!