ALLERGIC DERMATITIS

GENERAL

Animals suffering from allergic dermatitis are primarily itchy. When we say itchy this does not mean that the animal scratches. Itchy dogs and cats may display this as rubbing, chewing or biting. Many parts of the body can be affected; generally the armpits, groin, abdomen, paws, ears and face are involved, but this is often dependent on the type of allergy involved.

Often the primary allergy is disguised by secondary infection and can be confused with other skin diseases caused by self trauma, which has occurred as a result of the itching. It is therefore important to clean up any secondary skin disease in the process of diagnosing the primary problem.

I generally divide allergies into several groups as follows (remember that scratching can lead to secondary infection (pyoderma) which is, in itself, itchy & requires separate treatment)

1                    Insect related allergies (including sarcoptic fox mange)

2                    Food allergies

3                    Immediate Inhalant / Percutaneous Hypersensitivities (Atopy)

4                    Delayed Hypersensitivities (contact allergies)

Many factors are important in dividing your animals problem into one of these groups. Sometimes it cannot be done without testing or trialling your pet to rule in / out one or more of these allergies.

The history of your pets complaint, any treatment previously offered and the response to that treatment will be invaluable in interpreting the likely possibilities and also deciding on the best course of action to take in finding the primary cause of the problem.

It is also possible that your pet may suffer from more than one type of allergy or hypersensitivity. It is quite common for atopic allergy suffers to concurrently suffer from insect related allergies.

The most common type of allergy that we see in general practice is the insect related allergy, principally flea related. With accurate treatment most of these can be resolved without the excessive use of drugs.

ATOPIC ALLERGIES

The second most common form of allergy is the atopic dermatitis group. These are immediate type hypersensitivity allergies in which a reaction to the allergen is seen straight away. The tendency to display this type of allergy occurs as a result of inhalation or contact with the particles to which the animal is allergic. Hence the inherited tendency is greatly affected by the individuals environment.

The allergens, which cause these reactions, are the same as those, which cause reactions in many people and include pollens, dusts, moulds, wool, etc. These substances are often freely available in the air and the general environment and hence the removal of the source of irritation is often difficult if not impossible.

PRESENTATION

The most common sign is itching, whether in the form of chewing, biting, licking or scratching. In some animals the only sign may be brown saliva staining of the hair around the toes where the animal has chewed its itchy feet. Others will only present with ear or eye disorders.

Dandruff or scale formation, redness, hair loss, increased pigmentation of the skin and infection can all be present as a result of damage the animal does to itself.

DIAGNOSIS

The best way to confirm an individual immediate hypersensitivity of this nature is to perform an intradermal skin test. A blood test can also be performed to look for antibodies to the allergens. These tests are not easily interpreted as the IDST and can be overly sensitive in giving positive reactions. There are some benefits in blood testing in that these can be performed whilst the animal is still on treatment. Your vet will discuss which test is most appropriate for your animal.

As one can imagine there are literally hundreds if not thousands of potential allergens in the environment. The selection we use in the clinic is aimed at isolating the most commonly occurring pollens, moulds, and other allergens in our environment. It covers both summer & winter pollinators as well as some of the factors, which are likely to affect your animal all year round. In this manner we hope to be able to isolate a major group of irritants in your dogs environment.

The skin testing requires that we clip an area of your pets chest then prick the skin with extracts of purified allergens. We then observe the skin for the next thirty minutes and record the reaction at each site.

A number of drugs will affect the skin test, hence it is important that strict withdrawal times for drugs are observed. The withdrawal times vary depending on the drugs being used. Your vet should be able to help you with this.

TREATMENT

If you were asked what do all the people you know who suffer from hay fever do when they have problems you can probably answer this question yourself. Typically in humans antihistamines are used. Unfortunately in dogs these drugs are often not very effective. With antihistamines it is very much a matter of trial and error. In cats they are much more effective. More commonly we use corticosteroids to treat the allergies and the results can be marvellous. In cases of seasonal allergies, where the individual only requires treatment for less than two months of the year this is quite acceptable. However, the importance of using these drugs properly cannot be over emphasised. Used in small quantities the side effects will often not be important, but animals who are receiving cortisone for long periods can suffer very sever side effects.

Typically, corticosteroid side effects include increased appetite, increased thirst, and as a result increased urination and sometimes incontinence. Some animals become quite lethargic and rapid breathing has also been observed. Long term side effects can include liver damage, hair loss, increased susceptibility to infection, muscle weakness and skin thinning. Hence in young dogs long term corticosteroid therapy should be avoided. In some older dogs (10yr plus) it may be the only suitable treatment and economic alternative.

The preferred method is hyposensitisation.

HYPOSENSITISATION

Hyposensitisation requires that a vaccine made up using the results of the intradermal skin test and the history which is repeatedly injected into the patient to decrease the animals sensitivity to the allergens included in the vaccine.

As with long term usage of cortisone, whereby the animal will gradually get used to the drug and then begin to require more of the drug to achieve the same result. The principal behind hyposensitisation is for the animal to develop more tolerance to the allergens involved.

A series of bi-weekly injections for seven and a half weeks will bring the animal up to maintenance dosage and then the injections can be reduced to a three weekly basis, depending on the individual. The aim is for these injections to be administered at home by you the owner, in close liason with your vet.

Approximately eighty five to ninety percent of animals will be improved with hypersensitisation. No improvement is expected until a maintenance level is reached and in some animal signs of improvement may not be seen until they are six months into the course. This is a long term treatment program and those animals that improve in the earlier stages are expected to continue to improve as the two year course continues.

Some animals will experience increased itching for several days around the vaccination and sometimes there may be a temporary loss of appetite or some depression at this time. These side effects are not common and in any event are much safer than those of the alternative cortisone.