Pruritus in dogs and cats part 1: what is pruritus and how do we approach the pruritic patient?

Pruritus is defined as an unpleasant sensation that provokes the desire to scratch, but in common with most biological functions, despite the discomfort it causes, it serves a physiological and protective purpose. Just as pain helps to remove us from noxious stimuli, preventing further harm, the pruritic response undoubtedly evolved to help an animal detect and then attempt to shed or remove parasitic or fungal organisms from the surface of its skin. This response is seen not only in mammals but in other species such as fish which ‘flash’ when they are pruritic, i.e. dart around in the water as if trying to dislodge something attached to them.

Two forms of pruritus have been described, both of which are detected by nociceptors which also transmit pain sensations. Epicritic itch is a focal, pricking pruritus, the sort that may be experienced at the site of an insect bite, for example, and this sensation is transmitted along myelinated A delta fibres at high velocity of 10–20 m/second. The second is known as protopathic itch and is a more generalised, burning type of pruritus such as that may expected in chronic atopic dermatitis or, as a human example, in cases of actinic damage, i.e. sunburn. These impulses are carried by unmyelinated C fibres and are slower as a result, being transferred at 2 m/second.