The itchy Horse Flashcards
What are some common and probable causes of Pruritus?
-> Parasites
- Lice
- Mites (chorioptes, sarcoptes, free living mites)
- Oxyuris equi
-> Allergic dermatitis
- Insect bite hypersensitivity
- Atopy (contact, feed)
What are some less common causes of pruritus?
- Dermatophytosis
-Bacterial folliculitis - Coat shedding
- Excessive rugging
- Pemphigus folliaceus
- Malasseia
- Onchocerca
- Pelodera
- Hepatic insufficiency
Describe Insect bite hypersensitivty
Allergy to various insect salivary proteins
Generally caused by Culicoides species
Different Culicoides species:
different geographic locations
different biting distribution on the horse
What is the typical clinical picture of insect bite hypersensitivity?
Chronic pruritus (can be extreme)
Typically mane and tail
Can be elsewhere – eg ventral midline
Secondary trauma
- Acute – scratches/abrasions
- Chronic – keratinisation/lichenification
What predisposing factors to insect bit hypersensitivity?
- Season
- Still water, low wind speed
- Mid afternoon - dusk
- Genetic determinants
- Age of horse when Culicoides first encountered
Detail the genetic determinants of hypersensitivity?
- All horses bitten by flied
- All horses develop IgG repsonse to Culicoides antigens
- Some horses develop IgE responses to the same antigens
What does ti matter age first bitten by a culicoides?
if as a foal -> sweet itch less likely
As an adult -> more likely
what possible implications of sweet itch immunity
adult horses moving from low to high risk sweet itch areas are
at high risk
don’t put fly repellents on foals?
immunotherapy might be applied in foals born to affected
parents?
How to treat/prevent Hypersensitivty ?
Fly-avoidance
repellents
rugs/hoods
stable at times of peak
activity
fans/meshes in stable
change environment???
ponds, ditches, trees
Describe what Atopic dermatitis is
- IgE-mediated allergy to presumed environmental allergens
- Hard to define/strictly diagnose
- Eliminate insect bite hypersensitivity
How does atopic dermatitis present in horses?
As pruritus ANd/OR Urticaria
What does atopic dermatitis require?
A sensitisation period
What two keys things to treat atopic dermatits?
- Identify the allergn(s)
- Avoid the allergen(s)
WHY is allergen ‘guessing’ ineffective?
- allergen may have been present for a long time
- allergy requires a period of exposure and sensitisation (not a new exposure)
- eaten, inhaled, contacted,….?
- many allergen sources are invisible in the locality (not always bright yellow!)
What are the main two ways t go abotu allergen identification?
- Serum allergy testing
- skin testing
Describe Serum allergen testing
- several different methods (none well-validated in horses)
- success depends on several factors:
- is the allergy IgE-mediated?
- is allergen-specific IgE present in the serum in proportion to that in the skin?
- is the analytical method IgE-specific?
Describe skin testing
a) Intradermal testing
b) Skin Prick testing
* tests for tissue-fixed allergen-specific IgE
* maybe limited by availability
What changes can be made if we aren’t sure which allergen is causing it?
- Move to new stable (preferably distant)
- Take the horse, and NOTHING else
- Change bedding, forage, feed, rugs, tack,…………
- Stop putting “stuff” on the horse!
- If improves then gradually reintroduce items
What is ASIT?
Allergen Specific Immunotherapy
* Attempts to “re-educate” the immune system by repeated injection of diluted allergen
* IgE=>IgG
How do we go about ASIT?
- Choose allergens by serum or skin tests?
- Ensure choice of allergens makes sense
- Will take several months (2-12?)
- Cost £ a few thousand
- Owner OK to inject?
- Continue for at least 2 years (or for life)?
Diet allergy?
- Anecdotal
- hard to diagnose
- Feed exclusion trial? - Feed 1 simple dietary element x 4-6 weeks? - no turnout, no other feeds
OR - simplify diet as far as possible for 3-4 weeks
How can we manipulate diet for pruritus relief ?
Increase oil in ration
* Omega-3-rich oils?
* Any vegetable oil?
* improves coat quality and resolves dryness/scurf
* Increase gradually up to 1 mL/kg BWT/day
* May require a few weeks to see benefit
What topical products for pruritus?
-> Moisturise dry skin/occlude skin surface: ( E45, Balneum plus, Oilatum)
-> Antiinflammatories (hydrocortisone, dexamethasone)
- Counter-irritants (menthol,camphor, capsaicin)
What about unlicensed options?
- zinc oxide
- 5% Doxepin (TCA/ antihistamine)
- Tacrolimbus (calcineurin inhibitor)
- Antimicrobials?
Describe use fo AMs for pruritus
- No bacterial species is an unequivocal pathogen
- However, microbiome is different in skin allergy and pruritus
> Staph aureus/epidermidis?
* secrete proteases that may provoke pruritus
* Can induce pruritus experimentally
> Chronic pruritus warrants consideration of treatment of persistent infection
* Topical antimicrobials??
* antibacterial shampoo
What systemic products for pruritus might we use?
- glucoCs (dexamethsaone or pred)
- Antihistamines
- Doxepin (TCA)
- Gabapentin
- Oclactinib ‘apoquel’
- Anti-IL31
What antihistamines might we use?
-> 1° generation (+mild sedation??)
* Chlorpheniramine
* Diphenhydramine
* Hydroxyzine
> 2° generation -> Cetirizine (least potent so try first?)
What does Doxepin do?
- Tricyclic antidepressant – potentiates norepinephrine (and serotonin) effect
- Antihistamine (very potent – 800 x diphenhydramine!)
Gabapentin use?
- Analgesic/anticonvulsant/anxiolytic
- inhibits excitatory neurotransmitter release (substance P, glutamate, norepinephrine)
- May take a few weeks to have effect
Describe use of Apoquel ?
- Janus kinase inhibitor (anti-IL-2,4,6,13,31)
- longer t½ in horses (q24h dosing?)
- 0.25 mg/kg q 24 hours – no effect on pruritus or
dermatitis vs placebo - Anecdotal benefit at 0.25 mg/kg bid??
Describe lice in horses
- Werneckiella equi (biting) & common
- Haematopinus asini (sucking - rare)
What does Lice cause & in who?
Pruritus
-> young, old, hairy, debilitated
Is lice hard to treat?
no do some washes at intervals
What mites to worry about
MANGE MITES
- chorioptes bovis (main)
- Sarcoptes
FREE LIVING MITES
- Dermatophagoides
- Trombicula automnalis
- Dermanysssus gallinae
- Acarus sp
Describe Chorioptic mange?
- Chorioptes bovis
- Common in feathered breeds
- Sores extend up to carpus / tarsus
- Crusting
- Scaling & prurirtic
What diagnostics for chorioptic mange?
-> Sellotape collects scale with mite -> put onto a slide with KOH
->Brushings -> into sealable pot with insecticides - mites quick
Treatment for chorioptic mange?
- Lime sulphur
- Selenium sulphide
- FIPRONIL
- Avermectins
recurrence typical
Describe how horses are affected by sarcoptes scabei?
- Mainly head neck legs
- Easy to miss on skin scrapes
- Contagious
- Responsive to avermectins
Describe Oxyurids equi
- Eggs not tourinely passed in faeces
- eggs laid peri-annaly
- Perianal/tail pruritus (UNDER tail)
- Pre-patent period 5 months
Diagnosis of oxyuris?
Adult worms passed after deworming (beansprouts)
- Sellotape strips to find eggs
TX for oxyuris?
- Only partially responsive to anthelmintics
- Topical/ systemic / higher dose?
- Long term strategy: remove eggs with frequent perineal washes (soap and water)
