Download 1. the scabby horse - CK Mobile Equine Services

SPECIFIC SKIN DISEASES
Dr. Cyndi Kasper BSc, DVM
November 2006
1. THE SCABBY HORSE:
- Alopecia, scaling, crusting +/- pruritus, +/- exudation
(overview of only of the more common ones)****
NONPRURITIC LESIONS (USUALLY)
Dermatophilosis
Dermatophytosis
Anhidrosis
Selenium toxicosis
Note/ dermatophytosis and bacterial skin infections may cause pruritis but they are
recognized more commonly as crusting or exfoliative dermatosses
PRURITIC LESIONS
Ectoparasites – mites, lice
Endoparasites
Contact dermatitis
Pemphigus foliaceus
A. NONPRURITIC LESIONS – usually:
a) Dermatophilosis or rainscald
- common crusty dermatosis; superficial and infectious disorder; it is rainscald until
proven otherwise
- lesions are typically crust and matted hair; this results in a paintbrush appearance
- due to Dermatophilus congolensis – a facultative anaerobic actinomycete that exists on
carriers until condition becomes right for infection – usually prevalent during periods of
heavy or persistent rainfall and damage to skin surface.
- the organism cannot penetrate intact healthy skin
- prolonged moisture can damage stratum corneum and becoming more liable to trauma
from rubbing, ectoparasites, grooming etc
- can have outbreaks as it can spread by fomites; bits, ticks etc
- the wet exudative form is frequently found to areas of the body which are persistently
wet or sweaty ie back, dorsum, loin, face, neck, distal extremities and saddle area
- lesions may be painful but rarely pruritic
- in longer haircoat – as it sheds, it may leave long, linear hyperkeratotic scabs; in shorter
haircoat, they often more apparent by palpation.
- interestingly enough – white or non-pigmented coat is more susceptible therefore it may
be commonly seen in distal limbs (especially on wet grass)
- zoospores can be alive in crust for 42 months – the normal habitat of the organism is
unknown
- clinical signs of dermatophilosis can develop within 24 hours; crusted, mats of hair can
resemble small paintbrushes. The skin under the crusts is soft, exudative and yellowtinged.
- low zoonotic potential
- to diagnose – need to smear to see gram positive branch like filamentous bacteria –
railroad tracks. One can make preps from dried or fresh crusts or from direct smears of
exudates.
- May also need to culture and biopsy
- Treatment – important to keep clean and dry; shampoo; improve nutrition and some
cases may need penicillin(some do also respond to trimethoprimsulfa)
NOTE/ Persistent muddy and wet underfoot conditions are associated with skin diseases
called Mud fever; mud rash; pastern dermatitis etc. This occurs primarily in the plantar or
palmar areas of the pastern area/bulbs of the heels. The condition is very typical of
dermatophilosis with moderate to sever dermatitis and exudation. Neglect of these areas
can cause lameness but also can more commonly cause scabs, thickening and cracking of
the area. This will release serum in the area to perpetuate the syndrome – and secondary
pathogens such as Staphylococcus spp will prosper.
b) Dermatophytosis:
- ringworm or girth itch – is zoonotic and is the most common contagious skin disease in
the horse.
- likely most overdiagnosed condition in equine dermatology
- one of few asymmetric alopecias in the horse
- is an infection of the superficial keratinized layers of the epidermis – limited only to
actively growing hair follicles.
- most commonly caused by Trichophyton equinum
- transmitted by direct contact or fomites(indirect); via grooming brushes, blankets,
halters, saddles, girths)
- usually see in young or old, immunosuppressed; risk factors include illness, stressors
and moisture especially sweat.
- sun seems to have a direct inhibitory affect
- incubation can be from one to six weeks
- the spores are notorious for surviving for several years in the environment in a viable
form
- early lesions can be urticarial like – then progress to circular area of alopecia with
stubby hairs at the margin and variable amounts of scaling and crusts
- progression can take 35-55 days where hair growth resumes characteristically starts in
the center of the lesion
- pain and pruritis are variable – but generally non-pruritic
- DIAGNOSIS –a skin biopsy: a fungual culture is preferred and can take up to three
weeks; other tests such as Woods lamp and direct examination may reveal false positives
- TREATMENT – the disease is self limiting and usually spontaneously heals in about
three to six months; however, it should be treated as it is easily transmitted to other
animals and is zoonotic
- many compounds are commonly used and opinions on treatment vary
- lime sulphur and iodophore shampoo; thiabendazole paste and griseofulvin;
chlorhexidine, captan and povidone iodine; spray stall and bedding with 1:10 household
bleach; miconazole/chlorohexidine shampoo; T equinum vaccine and is available
(protects greater than 80%)
B. PRURITIC LESIONS (usually)
a) Insect Hypersensitivity
- one of the most common dermatoses
- one of the most common pruritic diseases
- it is seasonal in colder climates and non seasonal in warmer climates
-any age, breed or sex but uncommon in horses less than 2 years of age. It is found that
Icelandic and Welsh ponies are more susceptible
- culicoides most common (sweet itch, Queensland itch, muck itch etc); also black flies,
ticks, stable flies, mosquitoes, mites and horn flies
- the disorder is attributed to a Type II (cytotoxic) and Type IV (cell mediated or delayed)
hypersensitivity to the salivary antigens of these flies or insects
- often begins with mane and tail rubbing due to pruritus; face frequently involved but
alopecia can be dorsal, ventral or generalized – the varied areas depend on the type of
insect involved
FYI stable flies prefer the lower legs; black fly tends to feed on the head, ears and ventral abdomen; culicoides feeding
si species dependent; horn flies feed on the ventrum around the umbilicus; mosquitoes prefer feeding on the lateral
aspects of the body. Others such as deer flies, horse flies, house flies, bees and wasps bite anywhere on the body and do
not prefer a specific location.
- clinical lesions created by self trauma – as the skin damage becomes more severe, and
over a prolonged period of time; lichenified folds develop
- DIAGNOSIS is made by the history and clinical presentation
- skin biopsy – can see superficial and deep eosinophic perivascular dermatitis indicative
of a hypersensitiviy reaction
- intradermal skin testing??? – may not be necessary as therapy and environmental
control measures are similar for most biting and flying insects
- rule out onchocerca with ivermectin administration
- TREATMENT can be done by insect control (screened in stalls, insect repellant,
environmental management) and medication control
- Medication does not have any effect without insect control – can use prednisolone
(begin at 1 mg/kg PO daily until effective then taper dose to lowest effective dose),
dexamethasone, hydroxyzine ( 200-400 mg orally BID or TID) and antihistamines
NOTE/ Onchocercal cervicalis lesions likely arise from antigens of dying microfilaria.
Adults live in the nuchal ligament. Microfilarial population migrate to the skin where
they are ingested by the intermediate host – Culicoides. Most common sites are ventral
midline(esp the umbilicus), dermis of the face and neck, lower eyelid and lateral limbus
of eye – microfilaria can even migrate through cornea. A bulls eye lesion in center of the
head is very suggestive. It is non-seasonal but may be worse in spring and summer..
Alopecia, ulcerations and depigmentation occur. Diagnosis can be made on biopsy
including impression smears and response to appropriate therapy. Ivermectin at
200ug/kg orally is the treatment of choice but it can worsen signs and will have to repeat
therapy as does not kill adults.
b) Other Ectoparasites:
1. Lice
- primarily biting louse (Damalinia equi) and sucking louse (Haematopinus asini)
- usually winter seasonality as summer causes haircoat temperature to be too high
- even small numbers of biting louse can cause severe pruritus, scurf and alopecia of the
head, neck and dorso-lateral trunk
- severe tail rubbing is seen in sucking louse and can look similar to culicoides
hypersensitivity, oxyuris equi infestation and tail mange
- lice are host specific and complete their entire life cycle on the host – they can live for
2-3 weeks; they can only live a few days off of the host
- all louse infestations can be transmitted by direct contact
- debilitated and immuno-compromised horses are most susceptible. Most commom
pruritis in foals.
- ivermectin at 200 ug/kg every 2 weeks for three treatments is effective for sucking lice.
Biting lice should be teated with medicated dips (lime sulfur and pyrethrin)
2. Mites
i) Chorioptes spp
- found on foot and leg and perinium
- host specific and is not zoonotic/not reportable
- particularly prevalent in winter months
- more common in draft breeds(esp breeds with feathered pastern)
- causes severe pruritis, leg stamping and self mutilation
- more severe forms can lead to greasy heel
- treatment with ivermectin; some resistant strains may need dips
ii) Psoroptes spp
- involve trunk, head, mane and tail
- reportable disease
- highly contagious to other horses but not humans
- live on skin surface/biting mites
- can cause otitis externa
- treatment with ivermectin
iii) Trombicula spp
- live in hay and straw
- infestations usually occur in late summer and autumn
- mainly seen in pastured animals
- small rodents are the natural host
- clinical signs include restlessness and leg stamping
- skin lesions start as papules or wheals
- diagnosis by scraping and microscopic evaluation
- self limiting disease
- treatment with dips/prednisolone mostly for discomfort
3. Ticks
- becoming more common is some US areas; important seasonal ectoparasites in tropical
and subtropical regions
- skin injury is cause by the local effects of the bites and rarely through self mutilation
- can transmit viral, bacterial and protozoal diseases
- severe infestation can cause blood loss with obvious anemia
- Treatments aims at killing ticks on the horse – including sponge dips with pyrethrin and
some respond to treatment with oral ivermectin.
4. Others
- include the stable fly, horse flies, horn flies (can contribute to ventral dermatitis)
2. RAISED LESIONS
- papules, nodules or masses +/- ulcerations
Some of the more common raised lesions include:
- infectious nodules: papillomatosis, habronemiases
- neoplastic nodules: sarcoids, melanoma, scc, lymphoma, fibroma
- fungal:
pythiosis, sporotrichosis
- urticarial lesions
- others:
nodular necrobiosis, systemic granulomatous dz
a) Warts/papillomatosis
- caused by equine DNA papilloma which is host specific
- occurs usually in young horses with presentation commonly multiple lesions on the
muzzle
- can be via direct or indirect contact – caution with use of twitches, bits, bridles etc
- infection requires damaged skin
- incubation can be 2-6 months
- lesions will spontaneously regress after 3-6 months
- the virus appears to be able to survive from season to season
- older horses may be affected with large numbers over the nose and genitalia – need to
differentiate from the verrucose form of sarcoids
- DIAGNOSIS if made by typical appearance and location; biopsy may be performed
- TREATMENT include cryotherapy, autogenous vaccines, surgical excision, and even
Equistim (Propionibacterium acnes) has reported to clear up in 14-16 days, etc
NOTE/ Adult presentation causes aural plaques which are not self limiting and very
rarely can go to other places. These start as small, smooth depigmented papillae and
progress to large, often extensive hyperkeratotic plaques on the inner surface of the pinna
b) Sarcoid
- should be considered with any raised cutaneous lesion
- it is the most common skin tumor – can occur anywhere on the body and often in
multiple forms – often at a site of a previous wound
- it is fibroblastic, locally aggressive, non-malignant or benign
- most contain bovine papilloma virus DNA
- can occur in any age but 70% occurs in horses less than 4 years of age
- may have familial tendencies; with QH at twice the risk than TB; appaloosas and arabs
have increased risk; standardbreds being at low risk
Sarcoids are divided into five types:
1. Occult or flat: circular hyperkeratotic areas found on the neck, mouth, eyes,
and medial aspect of the forearm and thigh.
2. Verrucous or warty: tend to occur on the face, body, sheath and groin areas
3. Fibroblastic or proliferative ; resemble proud flesh and usually are around the
groin, lower limbs, coronet and eyelid
4. Nodular:firm, raised, circular 5-20 mm in diameter that tend to occur around
the sheath/groin area and the eyelids.
5. Mixed: common in long standing lesions or those that have sustained trauma
Diagnosis done by biopsy and histopath – a fibroblastic proliferation of the dermis with
epidermal hyperplasia, whorled pattern of collagen and spindle-shaped cells. NOTE/ a
flat or small verrucous forms may become more aggressive after a biopsy. It is for this
reason that some veterinarians are reluctant to take a biopsy sample of these lesions.
Treatment – many options
1. Scientific neglect
2. Radioactive isotope implants – expensive but effective
3. Surgical excision – 50-65% success; however may change severity and
character of original lesion
4.Cryotherapy – debulk with 2 freeze/thaw cycles to -20C; need to control
hemorrhage; keep wrapped until healed if on distal limb
5. Laser – cosmetic with accurate dissection with minimal damage; but has
regrowth potential and cost of equipment can be inhibitory
6. Immunotherapy – BCG(bacillus Calimette Guerin) – Regressin; it is injected to
promote immune response; best result on periocular sarcoids; difficult to inject; and
anaphylaxis is possible. Equistim can also be given parenterally or intra-lesionally with
some positive results.
7.Cisplatin – requires protective gear and multiple treatments; mixing instructions
with vasoactive modifier are important and there are regulatory concerns
8. Fluorouracil – is a good cream for small lesions or after debulking. Solution is
inexpensive and can be given topically or intralesionally but data is inconsistent
9. Other topicals include podophylin, Aw-3-Lubes, Bloodroot extracts (animex
and Xxterra
10. Others are autogenous vaccines, intratumoral hyperthermia;
electrocheotherapy ECT
NOTE/ It is important to inform the owner that sarcoids will most likely recur.