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
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 fæcal 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.
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 , 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.
