Download Skin Diseases Refresher Urticaria

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EQUINE VETERINARY EDUCATION / AE / August 2007
Skin Diseases Refresher
Urticaria
R. C. PILSWORTH*
AND
D. KNOTTENBELT
*Greenwood Ellis and Partners, 166 High Street, Newmarket, Suffolk CB8 9WS; and Philip Leverhulme Hospital,
University of Liverpool, Leahurst, Neston, Cheshire CH64 7TE, UK.
Keywords: horse; urticaria
Disease profile
The horse is more prone to urticaria than other species, and
this one of the commonest skin conditions of horses. Urticaria
is a clinical sign rather than a disease.
All types and ages of horses are affected, but it is
particularly common in Thoroughbreds. The aetiology in many
cases is unknown or cannot be established, but food
sensitivities have been strongly implicated (cereal foods are
probably responsible for most cases). A single insect bite can
cause generalised urticaria in sensitised horses.
Drug reactions are the commonest known causes of
urticaria; penicillin, clenbuterol and especially phenylbutazone
are often implicated, but reactions can develop to the carriers
and preservatives rather than the drug itself.
Larger plaques of oedema can develop and coalescence
can cause extensive lesions (Fig 2). The lesions should always
‘pit on pressure’ with a cotton wool bud or a fingertip. Some
forms have a gyrate or ‘doughnut’ form (Fig 3) and others are
diffuse and exude serum (angio-oedema). Mild pruritus may
be present but more usually the lesions are nonpruritic.
Clinical signs
Multiple, raised, oedematous plaques of varying size
(0.5–10 cm diameter) over the body surface (Fig 1).
Lesions may affect localised areas of the skin and limb
involvement is less common than the body trunk and head.
Fig 1: Multiple diffuse lesions of urticaria produced following a
single injection of phenylbutazone, as part of an induced
anaphylactoid reaction.
Fig 2: Urticaria produced on the thigh of a horse following
recumbency in the paddock and presumed contact with nettles.
Fig 3: These circular urticarial ‘doughnuts’ were part of a
suspected atopic dermatitis linked to feed. The lesions resolved
spontaneously on an exclusion diet of lucerne cubes, but
developed again following test-feeding on the original diet.
EQUINE VETERINARY EDUCATION / AE / August 2007
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Fig 4: The head of a horse showing an anaphylactoid reaction
of unknown aetiology. This resolved completely following a
single i.v. injection of dexamethasone.
Fig 5: A case of dermatophyte infestation. This horse was
misdiagnosed (R.C.P.) as an urticaria several days previously
when multiple raised plaques were seen across the body, but
no alopecia was present. Treatment with dexamethasone, an
immunosuppressant, hastened the development of severe,
widespread dermatophyte infestation.
Recurrent episodes are frequently encountered often with
increasing severity and reducing response to therapy. Urticaria
can develop as part of an anaphylactoid response (Fig 4) but
the skin lesions are seldom the main clinical focus.
Investigations
1) Clinical recognition.
2) Skin scraping (to eliminate dermatophytosis if in doubt).
3) Skin biopsy (in recalcitrant or recurrent cases).
Differential diagnosis
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Dermatophyte infection.
Eliminate by culture and biopsy.
Insect bites.
Careful clipping of a single lesion may identify a
haemorrhagic focus in the centre of the wheals.
Erythema multiforme.
Contact hypersensitivity. Very rare in horses and seldom
causes wheals.
Infectious and immune mediated vasculitis.
Focal or diffuse angio-oedema with evidence of cutaneous
necrosis and/or purpura.
Confirmation of diagnosis
A history of sudden onset and the clinical appearance is
characteristic. Skin biopsy of fresh lesions confirms the
existence of dermal oedema but specific aetiology is seldom
identified.
Cases that do not respond to cortisone therapy, and other
conditions with clinically similar lesions that either do not
respond to corticosteroid therapy (see below) or lesions that
do not pit on pressure may be identified by biopsy.
Urticaria-like lesions sometimes occur with dermatophyte
infection; this may be due to an allergic response in the skin
to the fungi. Corticosteroid administration usually causes a
significant exacerbation of the disease (Fig 5). Confirmation of
the diagnosis by biopsy or culture will preclude such
inappropriate treatment.
Dermatographism is urticaria that is induced by skin
pressure. This is an urticarial response to pressure. An urticarial
wheal should follow writing on the skin with a relatively blunt
tipped object, within a few minutes. Development of urticaria
in contact sites with tack is suggestive of this form.
Cold induced urticaria can be tested for by application of
ice cubes directly to the skin, which should be followed by a
wheal of the same shape within half an hour of removal of the
ice cube.
Management
Acute onset allergic urticaria may resolve spontaneously
within 24–48 h so delay in treatment is a sensible option for a
first episode.
The best treatment is avoidance of the allergen but it is
seldom possible to identify it.
A careful investigation involving dietary and environmental
restrictions should be performed in recurrent cases or those
that fail to respond fully to corticosteroid injections.
Radio-allergosorbent tests (RAST) for specific IgE types is
sometimes used to identify putative allergens but they are
expensive, unreliable and seldom helpful.
Lesions may resolve following vigorous exercise and
sweating or swimming: this may be useful if the use of drugs
precludes competitive work. Almost all cases respond rapidly
to i.v. injection of dexamethasone (0.04 mg/kg bwt).
Oral prednisolone has much less (or no) effect in many
cases. If the urticaria is part of an anaphylactoid reaction,
adrenaline,
nonsteroidal
anti-inflammatory
drugs,
corticosteroids and supportive therapy may also have to be
considered (Fig 4).