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ATOPIC DERMATITIS IN
THE DOG
by
Stephen Shaw, BVetMed, CertSAD, MRCVS
In 1976, a survey of skin disease
in guide dogs failed to mention atopic skin disease. Since then, there
has been an increased awareness of canine allergic skin disease, both
within The Guide Dogs for the Blind Association and
the wider dog population, and this has been accompanied by an improved
understanding of the causes and complicated factors involved.
This article aims to describe atopic dermatitis and its
treatment in the 1990's, and draws attention to some of the important
questions yet to be answered.
What is Atopic Dermatitis?
Atopic dermatitis is an allergic
skin disease of dogs which is caused by immunological hypersensitivity
to common substances in the environment such as house dust mites.
What is allergy?
The immune system of mammals makes
receptor proteins (antibodies) to substances that are foreign (i.e.
not part of the body), each antibody being specific to a given
substance. Antibodies
are of several types, IgG for instance being involved in protection
against viral diseases after vaccination whereas IgE, involved in
atopic dermatitis, is particularly concerned with protection against
parasites. IgE antibodies coat specialised cells (mast cells) in the
skin where they sit waiting for contact with the parasite proteins to
which the animal is sensitised. If the substance is encountered,
perhaps as a result of a burrowing mite, the mast cell releases
chemicals (mast cell mediators) which try to destroy the invader. In
allergic animals this whole system is oversensitive and the release of
mast cell mediators in the skin occurs inappropriately to apparently
innocuous substances such as pollens, moulds and house dust mites (figure
1).
For allergy to be apparent, dogs need to be first
"allergic" and then be exposed to substances (allergens) to which they
can develop the abnormal immune response. In the UK the main source of
allergens is the house dust mite. These tiny creatures live in all of
our houses, in carpets, beds and other soft furnishings and feed on
skin scales that are constantly falling from people and animals. They
litter our environments with fζcal pellets of half-digested food and
digestive enzymes and it is these minute faecal particles that contain
the most important allergens. Dogs can also become allergic to pollens
and moulds although this is much less common, presumably because of
less exposure.
Other factors known to be important in atopy in man are
certain infectious diseases in the early part of life which modify the
response to allergens. In particular it has been shown that children
who have more respiratory infections early in life, before any allergy
is apparent, have a lower chance of showing signs of allergy. The
effect of such infections is not known in the dog.
Clinical signs
Atopic dermatitis is often first
apparent in the first two years of life. Owners may notice that the
dog grooms excessively, with licking or chewing of the paws, abdomen
and perineum. The ears may be reddened and hot to touch even though
not scratched. The result of this itchiness (pruritis) is that the dog
will often be presented a number of times in the first eighteen months
of life for a variety of seemingly minor skin conditions. Between
these episodes the skin and the coat can look remarkably normal.
Spots, acute moist dermatitis, ear infections and scratching may all
seem to occur independently and it is only in retrospect that a
consistent pattern of disease emerges. As the condition becomes more
severe , pruritus dominates the animals' life and specific anti-itch
therapy becomes necessary. With increasing pruritus, baldness
(alopecia) and redness of the skin become evident and secondary
infections with yeast or bacteria become more common.
Clues to identify unseen itch
Many people scold their dogs for
scratching , almost without realising. Slowly we train our pets to be
quiet and all but the most itchy will choose to scratch and chew in
private. Luckily
there are some tell-tale signs that help us to identify the pruritic
dog. Saliva staining is a commonly-seen feature in these animals. A
red-brown staining of light coloured hair is often seen in allergic
dogs in the groin, armpits(axillae) and between the toes(interdigital
spaces)and can be seen in figure 2. In addition, with long term
problems, the skin itself will also change colour. Instead of being
pink, a black mottling (hyperpigmentation) will slowly develop,
especially if the skin has looked red and angry at the site. This is
most commonly seen on the abdomen.
Diagnosis
At present there is no definitive
test that will absolutely confirm a diagnosis of atopic dermatitis.
Because this is the case, veterinary surgeons may suspect atopy after
examining a patient, but have to make sure that other causes of itch
are not present. Once these have been ruled out, skin testing can be
used a s a pointer to the allergies involved.
So what are these other diseases? Flea infestation and
the allergy are the most important causes of itchiness in dogs in the
UK. Practically all dogs will have fleas at some time during their
lives. The rump and hind end are most often affected. Nibbling and
itching gives a rough feel to the coat and, if severe, pyotraumatic
dermatitis ( wet eczema) or alopecia will result. Very importantly,
dogs with atopic dermatitis are often allergic to fleas as well, so it
is pointless making a diagnosis of atopy without taking rigorous
flea-control measures. Similarly, other parasitic infestations such as
lice or sarcoptic mange may mimic atopy and these should be carefully
ruled out.
Food sensitivity ( often called food allergy) is an
uncommon cause of allergic skin disease, which accounts for a small
percentage of the cases seen by dermatologists. Although a rare
condition, all allergic dogs should undergo food trials before being
committed to long-term drug therapy. Food sensitivity may coexist with
atopy or flea allergy and so partial responses may be seen to food
changes. Bacterial infections are a common cause of pruritus in the
dog and these can be as a result of atopic dermatitis or any other
skin condition that damages the integrity of the skin. Non-allergic
causes of bacterial infection include hormonal problems such as
hypothyroidism and parasitic problems such as demodex infestation.
These are normally non-itchy conditions, but as soon as there is
bacterial involvement this changes and it can be difficult to make the
correct diagnosis.
Skin Testing
Skin
testing is performed to identify the allergens involved in allergic
disease. Under profound sedation an area of hair on the chest is
shaved and small injections of substances known to be possible
allergens made. After 15-20 minutes the reactions are recorded.
Figure 3 shows positive reaction to house dust mite allergens in
an allergic Labrador Retriever.
The diagnostic approach to the pruritic dog is
summarised in figure 4.
Treatment
In
treating atopic dermatitis it is imperative to consider the situation
as a whole. Bacterial infections will make the animal far more itchy
and may even contribute to worsening the allergy through damaging the
skins' protective mechanisms. So any bacterial infections seen as a
rash or pustular spots (Figure 5), need to be treated promptly,
using a combination of shampoos and antibiotics for a minimum of three
weeks, and often longer. Corticosteroid medication is best withdrawn
throughout the period of treatment as steroids can interfere with the
dogs ability to fight infection.
Yeast infection ( caused by the yeast Malassezia
pachydermatis) is another complication. Spots are not seen in
this disease, but instead the organism causes redness, geasiness and a
mousy odour. Dogs can be quite depressed when infected and can be
extremely itchy. Treatment is usually with baths containing
enilconazole, or miconazole in combination with chlorhexidine. Tablet
therapy is also available, but as a surface infection Malassezia is
best treated using baths.
Similarly, fleas and other ectoparasites will make an
atopic dog far more itchy. All allergic animals should have regular
and efficient flea therapy using veterinary preparations to treat both
the dog and the environment. With bacterial, yeast and parasitic
problems under control most dogs will be very much more comfortable
and some may only need minimal therapy using the least potent of the
drugs available.
Specific Therapy
A variety of drugs are now
available for treatment. Generally they are used in combination rather
than alone. Their use is summarised in figure 6.
Essential fatty acids are now widely used for skin
conditions. They are known to have few side effects and will help
about 25% of allergic dogs significantly. Antihistamines potentiate
the action of essential fatty acids (synergy) and so combination
therapy would appear to be valuable. Several veterinary products are
licensed for use.
Antihistamines were widely dismissed as unhelpful in
atopic disease until recently when new studies both in the UK and USA
have shown considerable benefits from their use. No veterinary
products are available and the human drugs , chlorpheniramine,
hydroxyine, and clemastine have all shown to be useful.
Steroids are widely thought to cause side effects which
outweigh their potential for good. Despite this popular view, steroids
are the drug of choice in severe cases of atopic dermatitis and, used
appropriately, when complicating diseases are under control, side
effects are generally minimal.
Hyposensitising Vaccines ( also known as desensitising
vaccines) are prepared from the allergens identified as important at
skin test. By administering these allergens subcutaneously over a long
period the immune response to them is modified and pruritis is
reduced. They are seen to be beneficial in about 60% of dogs, and take
up to nine months to have effect.
Allergen avoidance is useful when house dust mites are
known to be the problem. Exposure to bedrooms should be avoided by
house dust mite allergenic patients to minimise exposure to the
allergen. When pollens and moulds are involved avoidance is
practically impossible as these allergens travel for miles on the
wind, although obviously very large sources of pollens, for instance
hay meadows for grass sensitive individuals should be avoided.
The future
Studies examining the incidence of
atopy in the families where the dam and sire have pruritic skin
disease have shown that around 60% of their offspring will have signs
of allergic disease. When two unaffected animals are bred the
incidence is reduced to 10% and it would appear possible to reduce the
incidence of atopy within a breeding programme by avoiding those dogs
with atopic disease. However identification of the mildly-affected
atopic dog can be difficult because we lack a definitive test for the
disease. Further studies to better predict which dogs will develop
atopy are ongoing and if useful information is forthcoming then there
is hope that we can reduce the incidence of this distressing disease.
The author Stephen Shaw,
is Dermatology Research Fellow at the Animal Health Trust in Newmarket,
Suffolk, England, which involves him in clinical and research work
with The Guide Dogs for the Blind Association (GDBA). The GDBA manages
a breeding stock of about 250 dogs and raises 900 puppies every year.
It also supports programmes aimed at improving the health and welfare
of more than 6000 dogs for which it is responsible, and the quality of
service for over 4000 guide dog owners throughout the UK.
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