Pruritus Flashcards
Define pruritus
Unpleasant sensation that elicits the desire or reflex to scratch
What does pruritus look like in a dog?
Scratch
Rub
Nibble
Lick
Temperament changes
What does pruritus look like in a cat?
Often secretive
Overgrooming
Hair plucking (vomit hairballs, tufts of hair around house, hair in faeces)
What does pruritus look like in a horse?
rubbing
Stamping (itchy feet)
biting
scratching
bucking/kicking
temperament changes
What does pruritus look like in livestock
Rubbing
What does pruritus look like in birds
Feather plucking
What does pruritus look like in small furries?
Scratching/barbering
What are the welfare issues linked with pruritus
Distress
Discomfort
Effect on appetite
Quality of life
What are the economic considerations linked with pruritus
loss of body condition
damage to hide/fleece
What is pruriceptive pruritus
pruritus due to stimulation of peripheral receptors in skin (health nervous system) - usually due to skin disease
What is neuropathic pruritus
Pruritus generated in the CNS in response to:
- anatomical lesions of PNS or CNS
- circulating pruritogens (stimulate nerves)
- pharmacological mediators
What type of fibres mediates pruritus
unmyelinated slow-conducting C-fibres and A-delta fibres
What acts as the main itch receptor in skin (pruriceptors)
Epidermis with free C-neuron receptors
What are the main chemical mediators of cutaneous itch
various proteases/leukotrienes/neutropins
Peptides
Prostaglandins
Cytokines e.g., IL31
histamine
Where do chemical mediators for cutaneous pruritus originate?
keratinocytes
Mast cells
Skin leucocytes: T cell (IL31), eosinophils
How do chemical mediators of cutaneous pruritus act?
Direct stimulation of intraepidermal nerve fibres (IL31)
Stimulate mast cells
Stimulate other mediators/transduction cascades
Why is IL-31 an important chemical mediator for cutaneous pruritus
It is the basis of main therapeutic intervention of pruritis
Drugs to block JAK1 pathway (oclacitinib)
Monoclonal antibody therapy against IL-31 (lokivetmab)
Why do antihistamines not work very well in dogs to treat pruritus?
Histamine is only one of the mediators of acute inflammatory itch in allergic dogs and it is only present for a short time at the start of allergic pruritus
How does IL-31 cause pruritus
produced by activated T cells in allergic skin disease
Binds to receptors on surface of neurons in skin -> activation of Janus kinase (JAK) enzymes -> stimulate pruritic nerve impulse to brain
What is the neural pathway that causes pruritus?
Sensory afferent from C-neurones in skin
Dorsal nerve root to spinal cord
Dorsal horn - synapse with spinal interneurones
Cross and ascend in lateral spinothalamic tract
Thalamus -> internal capsule
Sensory cortex
What factors can suppress/exacerbate pruritus
Physical factors
Mechanical factors (rubbing, scratching causes brief suppression)
Distraction
How can the behaviour of a dog with pruritus display the severity of the case?
A dog that stops eating to scratch or scratches in the practice is a serious case
How does scratching suppress pruritus?
Scratch -> fast conducting A-beta neurons -> activation of inhibitory neuronal circuits -> widespread surround inhibition
How does distraction inhibit pruritus?
Distraction -> increased activity in descending pathways from reticular formation -> activation of inhibitory circuits in dorsal horns of spinal cord -> closes gated mechanism -> diminishes afferent itch messages
Explain how peripheral sensitisation changes in chronic pruritus
Scratching -> increased local inflammation -> production of pruritogens by inflammatory cells -> increased C-fibre responsiveness
Explain how central sensitisation changes in chronic pruritus
chronic inflammation of skin -> altered perception of gentle mechanical/other stimuli -> perceives as pruritus (allokinesis) -> pruritus incited by only minor stimuli
How does heat enhance pruritus?
lowers threshold of receptors to pruritus
Explain the relevance of the pruritic threshold
Multiple sources of pruritus summate to exceed the pruritic threshold which lead to clinical pruritus
What are some therapeutic interventions for pruritus?
Reduction of skin inflammation - corticosteroids
Blocking peripheral inflammatory mediators - lokivetmab block IL-31, antihistamines
Moisturisers
Topical cooling preparations
