Approach to pruritus

Question 1

Pruritus overview

Answer 1

= unpleasant sensation that elicits the desire or reflex to scratch (rub, lick, chew)
- most common dermatological presentation in dogs, 2nd most common in cats
- important welfare and economic concern

Question 2

Pruritus clinical presentation

Answer 2

• scratching, chewing
• licking feet (dogs)
• Scratching, chewing
• licking feet (dogs)
• Feather-plucking (birds)
• Overgrooming (=‘self-induced alopecia’) (cats)
• Also tufts of hair in house, vomiting furballs, hair in faeces
• One of the four feline reaction patterns

Question 3

What are the 3 main groups of causes for pruritus?

Answer 3

• Hypersensitivies
• Parasites
• Microbial infections
  (- others: e.g. contact irritant, autoimmune (PF), neoplasia [e.g. epitheliotropic lymphoma, MCT])

Question 4

Causes of pruritus - hypersensitivities

Answer 4

• Environmental AD / FASS*
• Food-induced AD / FFA*
• Parasite-induced hypersensitivities (esp fleas in dogs & cats)*
• microbial hypersensitivity
• contact allergy
• drug reaction

Question 5

Causes of pruritic - parasites

Answer 5

• Endoparasites (e.g. hookworm in dogs)*
• insects*
• fleas*
• flies
• lice
• arachnids*
• mites (burrowing, surface, demodex [variably pruritic])

Question 6

Causes of pruritus - microbial infections

Answer 6

• bacterial pyoderma
• malassezia dermatitis
• Dermatophytosis (variably pruritic)

Question 7

Approach to pruritic pt

Answer 7

1. formulate initial ranked ddx list (from signalment, history, CE (general and dermatological)
2. eliminate ectoparasites and microbial dysbiosis/infections
3. distinguish between environment and food-induced AD

Question 8

What species is Sarcoptic mange in?

Answer 8

• dogs

Question 9

What species is pyoderma/malassezia dermatitis more common in?

Answer 9

• dogs

Question 10

Age of onset for demodicosis

Answer 10

• young (<1y) and old -> immunocompromise

Question 11

Age of onset for other ectoparasites

Answer 11

• any age

Question 12

Age of onset for environmental AD

Answer 12

• 6m-3y

Question 13

Age of onset for food induced AD

Answer 13

• any age but 30-50% <1y

Question 14

Age of onset for endocriopathies (& why they cause pruritus)

Answer 14

• middle-aged-older
• secondary infection due to immunocompromise -> pruritus

Question 15

Age of onset for neoplasia

Answer 15

• older

Question 16

Breed predisposition for demodicosis

Answer 16

• Staffies
• Shar pei

Question 17

Breed predisposition for canine AD

Answer 17

• Terriers
• Labs
• Frenchies

Question 18

History questions

Answer 18

• Current ectoparasite regime (remember to ask about all aspects: drug, frequency, which animalstreated, environmental treatment)
• Disease progression
  – Which area first affected?
  – Which came first – pruritus or lesions?
  -> Allergic disease – pruritus commonly precedes obvious lesions
  -> Immunosuppression/endocrinopathies – no pruritus until secondary pyoderma lesions develop
• How has the disease progressed?
• Evidence of contagion/zoonosis?
  – Contagion: consider esp dermatophytosis, fleas, sarcoptic mange, cheyletiellosis, lice
  NB care re diseases with hereditary component in related dogs, e.g. CAD, demodicosis!
  – Zoonosis – as above but not lice
• Evidence systemic disease?
• Seasonal?

Question 19

General exam

Answer 19

• Evidence of systemic disease?
  – Dog with dull coat, lethargy and bradycardia? -> hypothyroidism
  Dog with diffuse thinning of coat, PUPD, polyphagia? -> hyperadrenocorticism

Question 20

With what diseases do you see pustules?

Answer 20

• Usually: superficial pyoderma (staphylococcal folliculitis)
• Uncommonly: demodicosis, dermatophytosis, pemphigus foliaceus (PF)
• Rarely: other sterile pustular diseases

Question 21

With what diseases do you see epidermal collarettes and crusts?

Answer 21

• Usually: superficial pyoderma
• Occasionally: PF

Question 22

What is your top ddx for lesions affecting the caudal and dorsal regions in the dog?

Answer 22

• FAD

Question 23

How to rule out parasites

Answer 23

• coat brushing/combing
• trichogram
• skin scrape
• acetate tape strips

NB investigations for fleas, Sarcoptes, Cheyletiella has low sensitivity – don’t believe a negative test -> thorough treatment trial if not on good (ideally isoxazoline) regime already

Question 24

How to rule out microbial dysbiosis/infection

Answer 24

• cytology
• Wood’s lamp
• dermatophyte direct microscopy/culture

Question 25

After eliminating ectoparasites & infection, if pruritic resolves fully what does it imply? Give examples

Answer 25

• implies non-pruritic underlying cause
• e.g. immunosuppressive disease (including endocrinopathy), therefore investigate as appropriate
• e.g. ectoparasites, if not previously on good ectoparasite control, therefore continue ectoparasite control

If unsure which has occurred and client/you don’t wish to investigate underlying health without further justification, continue ectoparasite control and investigate underlying health if problem recurs

Question 26

After eliminating ectoparasites & infection, if pruritic remains what does it imply? Give examples

Answer 26

• implies other pruritic underlying disease
• usually atopic dermatitis (environmental or food-induced)

Question 27

How to distinguish between environmental and food-induced AD

Answer 27

• Exclusion diet trial i.e. rule out food as the cause
• Exclusion diet needs to be fed for 8 weeks minimum but:
  –use of prednisolone or oclacitinib can shorten time to diagnosis
  –Treat for initial 2-3 week, then stop to observe response:

if pruritus recurs -> resume prednisolone/oclacitinib -> repeat cycles of treatment/observation to end of trial

if no pruritus for 2 weeks -> rechallenge with old diet

Question 28

Environmental atopic dermatitis diagnosed by exclusion, what next?

Answer 28

• always: symptomatic control
• sometimes: identify allergens: intradermal tests/IgE serology

Question 30

What is more common: food induced AD or environmental AD?

Answer 30

• Environmental AD