Skin Disease

Diagnostic Aids for Equine Skin Disease

Empirical treatment may often be an attractive and appropriate initial
approach to cases seen in first opinion practice, although it is possible
that this may confuse the results of later tests. Diagnostic tests are
best performed early in the course of equine skin diseases (and
repeated if necessary) to confirm a definitive diagnosis and ensure
specific treatment and management is provided.

SKIN SCRAPINGS are often collected and submitted from equine
dermatology cases without good justification. This is a methodology
primarily useful for identification of mite infestations, which are rare
causes of skin disease in horses with the exception of Chorioptes
which is best diagnosed by brushings (see later).

Superficial skin
scrapings are collected from the skin surface without causing bleeding
and are best for ear mites (Sarcoptes sp., Psoroptes sp.), leg mange
(Chorioptes sp.), Dermanyssus gallinae (poultry mite), forage mites
and harvest mites. Deeper skin scrapings from the intrafollicular space
and superficial dermis should be deep enough to cause bleeding. This
is primarily useful for burrowing mites (Sarcoptes sp. and Demodex
sp.) and also nematode larvae such as Pelodera spp or Strongyloides
sp.. Occasional Demodex mites on facial scrapes may be normal.

SELLOTAPE PREPARATIONS are primarily used for detection of
Oxyuris equi eggs around the anus, vulva and perineum which may
cause pruritus around the tail base. The sellotape test can also be used
for detecting superficial mite infestations such as Chorioptes and also
other avian or free-living forage and hay mites. For Oxyuris eggs the
tape should be pressed firmly in several sites in the perianal area.
Chorioptes mites might be better detected by brushing (eg
toothbrush) the surface skin scales onto the tape. Chorioptes mites are
very mobile and the tape helps stop them running away!

HAIR PLUCKS are indicated whenever there is hair loss, broken hairs,
crusts, scales or altered growth patterns particularly if
dermatophytosis (or dermatophilosis) is suspected. It is best
performed with rubber-covered artery forceps and this is a far better
sample for fungal qPCR, culture and microscopy than scrapes.
Dermatophyte growth is best obtained from hair plucks following
alcohol spraying the skin to reduce contaminants. Microscopic
examination for dermatophytosis based on plucks from the margins
of the lesions offer the highest diagnostic rate. Direct microscopic
identification of dermatophytes can be difficult and negative findings
during direct microscopy does not rule out dermatophytosis.

Definitive findings in dermatophyte infections are hyphae and
arthrospores in the hair shaft but these are seen only in 50-60% of
cases at most. Rapid testing with qPCR is the quickest and most
sensitive means of diagnosing dermatophytosis and detects more
than a third more cases than culture alone. Ideally all suspected
dermatophyte samples should be further cultured to help
confirmation and identification of the species.

IMPRESSION SMEARS can be performed directly from fresh scabs of
suspected cases of dermatophilosis. Place 1-2 drops of saline onto a
slide, clip excess hair from the scab, place underside of scab onto slide
and allow to macerate for a few minutes, then remove scab from slide.
Allow slide to air dry, then send to the laboratory. Demonstration of
gram-positive branching of filaments of parallel rows of cocci
(zoospores) is diagnostic although culture on blood agar can be helpful
in confirming this.

BACTERIAL CULTURE in skin disease in horses is complicated by the
fact that skin is never normally a sterile site and therefore results are
frequently complicated by contaminants and commensals leading to
confusion as to which bacteria are relevant and which are not. It is also
important to realise that primary bacterial skin disease is extremely
rare in horses as virtually all cases of bacterial dermatitis will arise
because of predisposing skin damage – e.g. from wetting, trauma,
photosensitisation etc… Pustules can be punctured and

aspirated/swabbed although these are not commonly encountered in
equine skin diseases. Bacteriology taken from underneath crusts can
be helpful but are also strongly subject to contamination. Skin biopsies
can also be subject to culture. The commonest potential pathogens in

skin cases comprise coagulase positive Staphylococci and beta-
haemolytic Streptococci although both could also be

contaminants/commensals. Interpretation alongside cytologic
examination (preponderance of neutrophils) is helpful for establishing
pathologic relevance.

TRICHOGRAPHY (Stage of hair growth) is also an occasionally useful
part of hair pluck microscopy. The main two phases of the “hair cycle”
are: 1) anagen where there is active growth of the hair within the hair
follicle; and 2) telogen which is the resting phase where the dead hair
is retained in the follicle (catagen is an intermediate phase between
the two). Anagen hairs have rounded and smooth bulbs at the root
that are usually pigmented and soft so the root will usually bend at 90°
to the hair shaft. Telogen hairs are characterised by a uniform shaft
thickness and a slightly clubbed or spear-shaped (rather than
rounded) bulb. They are usually unpigmented, rough and straight. In
normal horses roots will be seen with varying ratios of anagen and
telogen stage hairs and this can vary at different times of year.
However, it is never normal to see all hairs in a telogen phase and if
this is the case then this is consistent with the diagnosis of follicular
arrest (telogen defluxion).

SKIN BIOPSIES can provide useful additional information and are most
valuable if performed early in the course ofdisease. Self inflicted
trauma particularly in pruritic animals can significantly alter primary
lesions into non-specific secondary inflammatory lesions which are
invariably disappointing. Sample a large enough area using a minimum
6mm punch biopsy. It is important not to not shave, clean or scrub the
lesion prior to biopsy as this may remove key superficial diagnostic
features. Infiltrate only a minimum of local anaesthetic or use a ring
block to minimise artefacts caused by local anaesthetic infusion.

ALLERGY TESTS – Allergies in horses are commonly encountered but,
given their prevalence, are poorly understood, often difficult to
investigate, and frustrating to treat. Serum Allergy Testing (SAT) and
Intradermal Testing (IDT) are commonly used to investigate allergic
dermatitis. There is little evidence basis for the use of SAT or IDT in the
diagnosis of respiratory allergies or suspected food intolerance. In
human and veterinary medicine, IDT remains the “gold-standard” for
diagnosing allergic skin disease, although SAT is popular because of its
convenience. In one study of three commonly used equine SATs, all
tests had poor sensitivity and poor positive predictive value when
compared to IDT. The best of the 3 tests had a sensitivity of only 37%
and a positive predictive value of only 48.5%. In one study of horses
with RAO, levels of IgE and IgA in serum were not elevated despite
significant elevations in IgE and IgA in bronchoalveolar lavage fluid.
Although IDT is accepted as the gold standard in dermatology it is far
from perfect. False positive results are common and in most normal
horses a few false positive tests are expected. A positive skin test
reaction indicates that the patient has allergen-specific IgE fixed to
mast cells that causes them to degranulate. Care in interpretation and
experience in reading the tests is therefore important as is thorough
history taking. IDT remains the recommended technique at Liphook
Equine Hospital.

Based on the findings of the IDT recommendations could include
exclusions from the allergen if possible or the introduction of an
Allergen Specific Immunotherapy. The vaccination protocol has been
shown to lead to resolution of signs and cessation of drug therapy in
59% of cases, 9% of cases had corticosteroids removed from their
treatment protocol, 16% needed to stay on medications but improved
and the remainder did not respond.

Further reading
Stepnik, C. T. et al. (2011) Equine atopic skin disease and response to allergen specific
immunotherapy: a retrospective study at the University of California-Davis (1991-2008).
Veterinary Dermatology, 23, 29-e7.