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.
The free nerve endings in the skin that detect pruritus are in the dermis, subepidermis and deepest layers of the epidermis. These tree-like networks of fibres link up to form sensory nerves, which transmit the pruritic signal to the dorsal root of the spinal cord, it then ascends in the dorsal column, via the lateral spinothalamic tract, to the thalamus and then to the sensory cortex; at this level the impulse can be modified by competing cutaneous sensations or emotional factors, for example, pain can override the feeling of itch and, according to human studies, is a preferable sensation hence the reason why many pruritic individuals scratch till they bleed (Anzelc and Burkhart, 2020). In addition, factors such as stress or excitement can elicit or enhance pruritus, and owners of pruritic dogs often report how their pets have a scratching session just after greeting their owner or a guest in the house. In humans, it is known that competing sensations can modify the degree of perceived pruritus; for example, when the brain is relaxed and undistracted, for example when one is about to sleep, the sensation of pruritus is heightened as other sensory inputs are low. This is known as the gate-control system of pruritus.
These sensations are mediated by a number of physical and chemical stimuli in both humans and animals, and histamine, cytokines, leukotrienes and prostaglandins are believed to be important (Tater, 2021). Proteases are also thought to be important in humans, but have been less thoroughly investigated in domestic species, and may receive more attention in the future, particularly as more is learned about the role of bacterial and fungal organisms such as staphylococcal bacteria and Malassezia spp. yeasts in pruritus. Another area of interest is the role of peptides in pruritus, particularly the neuropeptides and opioid peptides, as they form the link between the emotional state and pruritus. These are currently the subject of many research projects in human medicine, and in time more should be known about these mediators and their clinical relevance for veterinary patients, as at present veterinary professionals can only surmise how their patients are feeling psychologically.
The pruritic ‘threshold’
It is worth emphasising that pruritus in a patient can be the result of several co-existing pruritogenic processes combining together to push the individual over what is known as the pruritic ‘threshold’. This concept is based on the fact that, in common with painful stimuli, an individual's tolerance to pruritus varies and some dogs/cats with, for example, chronic allergic skin disease, may ‘cope’ with their itch to differing degrees at different times. If an additional pruritogenic factor develops, however, such as an acquired flea or other ectoparasitic infestation, or an imbalance in the cutaneous microbiome, then the relatively stable allergic patient can cross this threshold and begin to exhibit signs of pruritus. This phenomenon is particularly important to bear in mind when the attending veterinary surgeon prescribes medications for pruritic patients, as failing to address these additional factors can lead to overall treatment failure, resulting in extra cost and distress to the client and their pet.
Signs of pruritus
Dogs and cats have similarities and differences in the way they exhibit signs of pruritus and occasionally owners are unaware that these signs reflect that their pets are ‘itchy’. Dogs, for example, will often scratch at their face, neck, ears, axillae and flanks with their hind legs; a sign that no owner would be likely to miss or interpret. However, many clients do not appreciate that excessive foot, ventral abdominal, anal or vulval licking can be a sign that their animal is pruritic, often commenting that they believe they are behaving so as a sign of boredom, or just as a ‘habit’. This is even more prevalent in cat owners, who are often surprised when it is suggested that their pet's over-grooming is as a result of feeling itchy, as opposed to a habitual or stress-linked behaviour. While there are undoubtedly cases of psychogenic alopecia, it is important to attempt to rule out all pathological, as opposed to psychological, causes of pruritus, because most of the anxiolytic/mood-altering drugs that are used are unlicensed for veterinary use and they may have unpleasant or unexpected side effects.
Approach to the pruritic patient
When first presented with a pruritic dog or cat, in common with any medical or surgical condition, it is imperative to take a thorough history, as this can provide many clues as to the root cause of the problem, and is therefore a vital part of the work-up (Miller et al, 2013). It is useful to take the time to quiz the owner in detail about the patient's general health, lifestyle, exercise routines, nutrition etc, in addition to the dermatological problem concerning them; however this is a difficult task to squeeze into a 10–15-minute consultation and this may be the reason that some long-term skin cases prove difficult to manage for both veterinary surgeon and client. If possible, once a veterinary surgeon or veterinary nurse realises that a particular patient is being re-presented for an ongoing skin problem, the ideal solution would be to schedule a double appointment slot for that client, to enable them to go into more detail about their pet's issues. A questionnaire could be given to them ahead of that visit, to help save time during the consultation, and also to assist the veterinary surgeon/nurse in remembering all the important information that needs to be gathered (see Figure 1 for an example of this). After taking a detailed history, a thorough clinical examination should be conducted, comprising both the general body systems and then the skin. On occasion, a definitive diagnosis of the cause of the pruritus can be arrived at immediately, if there are gross signs of ectoparasitic infestation, e.g. with fleas, lice or surface-living mites such as Otodectes cynotis (ear mites) in the ear canals or Neotrombicula autumnalis (Harvest Mites) in the cutaneous marginal pinnal pouch (aka ‘Henry's pocket’) or interdigitally. Again, a problem with short consultation slots is the lack of time to be able to perform these tasks satisfactorily, and as the skin can be also affected as part of several other concurrent, systemic diseases, the latter may be missed as the cause of the skin problem, if there is insufficient time to conduct thorough general clinical and dermatological examinations.
Once the history has been taken and the patient has been examined, the veterinary surgeon should have an idea of the differential diagnoses that need to be considered and then investigated. The ‘pattern’ of the affected areas (i.e. symmetrical versus non-symmetrical) can provide a clue as to whether the disease is likely to be coming from ‘within’ the animal (e.g. in the case of an allergic skin disease; symmetrical), or from the exterior (e.g. an acquired parasitic or microbial infection; asymmetrical). It should be determined whether there is a ‘seasonal’ variation to the patient's level of pruritus and if so, if this is predictable (as one might expect with seasonal atopic dermatitis), or if the level of pruritus is gradually worsening with time (which may suggest an uncontrolled parasitic or infectious cause). It is useful to know if the patient has been affected by pruritus in similar areas in the past (typical of allergic skin disease), or, particularly in an older patient, if it has developed very suddenly, with no prior history of itching, which is strongly suggestive of a recently acquired parasitic infestation, especially in the absence of recent, reliable ectoparasiticidal therapy. Conversely, if regular, prescription-grade ectoparasiticides are being used, this points to a non-parasitic cause of the pruritus. The owner should also be quizzed with respect to possible signs of contagion, such as if other animals or humans in the household are similarly affected, as this may be typical of an ectoparasitic or fungal infection (e.g. dermatophytosis aka ‘ringworm’).
Once the major differential diagnoses have been established, the attending veterinary surgeon will suggest appropriate diagnostic tests and, in dermatological patients, the majority of these can be performed by the veterinary nurse. In the case of a pruritic patient, these will generally be sampling techniques designed to look for evidence of ectoparasitic and/or microbial elements, requiring good microscopy skills (Macfarlane, 2018), or allergen identification. The various tests will be discussed in detail in part 2 (Allergic causes of pruritus) and part 3 (Ectoparasitic and microbial causes of pruritus) of this series of articles, but it is worth veterinary nurses familiarising themselves with the practical aspects of them as a broad overview:
- Skin scraping — generally used for the harvesting of ectoparasites, the epidermal layers of the skin can be gently removed by repeatedly grazing the surface of a fixed area of the skin with the edge of a size 10 scalpel blade, in the direction of the hair growth in that area (Figure 2). A small amount of liquid paraffin is dropped onto a microscope slide (Figure 3), into which the material dislodged from the surface of the skin is mixed and a cover slip placed over the top of the sample. Microscopic examination at x40 and x100 magnification can then be undertaken (Macfarlane, 2014).
- Tape stripping — adhesive tape can be gently and repeatedly pressed onto the surface of the hair or skin, to collect samples of scale and crust (Figure 4). The tape can then be fixed, sticky side down, to a microscope slide and examined at x40 and x100 powers. This technique is particularly useful for detecting surface ectoparasites, such as Cheyletiella spp. mites, or lice. If the sample is stained before inspection (e.g. with DiffQick®) then microscopic organisms such as bacteria and yeast may be visualised at x100 and x1000 (oil immersion) powers of magnification (Paterson, 2019).
- Coat brushing — a simple tests is to place the animal over or next to a piece of white paper towel and then to vigorously brush the hand back and forth through the coat, to dislodge scale or other debris. The material can then be collected and examined grossly and microscopically. Flea droppings and flea ova are visible to the human eye and the former can be more easily discriminated from general debris by the addition of a small amount of water, which causes them to leach semi-digested blood, which is clearly visible as a red-brown streak on the white paper. Microscopy may also reveal this faecal material, plus oval, pearlescent flea ova or evidence of Cheyletiella spp., mites or ova.
- Hair plucking (aka trichogram) — hairs can be plucked from the skin, placed in liquid paraffin and covered with a cover slip, before being examined microscopically at x40 and x100 powers, to check for evidence of louse or Cheyletiella spp. mite ova (which are attached to the hair shafts), follicular Demodex spp. mites (which may be dragged out of the follicle with the hair during its removal) or fungal spores/hyphae within the hair shaft, as seen in cases of dermatophytosis. Additionally, pets that are ‘secret’ groomers/scratchers can be identified by the presence of roughly fractured hair shafts, caused by self-trauma (Figure 5).
- Cerumen microscopy — a sample of cerumen can be obtained using a cotton bud, smeared on a microscope slide, and examined directly under a cover slip, in a small amount of liquid paraffin. This may enable the detection of mites such as Otodectes cynotis or, less commonly, Demodex spp.. Microbes can also be visualised if the sample is dried and stained using, for example, DiffQuik® stains and the specimen is examined at higher powers/under oil immersion.
- Serological testing — in pruritic patients, two major blood tests are most frequently performed; the first is used to check for the presence of canine anti-Sarcoptes mite IgG antibodies in cases of suspected canine sarcoptic mange. This test is fairly reliable but can have some false negative and occasionally false positive results, so it is important to consider the results along with the patient's history and clinical signs. Serum can also be submitted from both dogs and cats to screen for elevated IgG and IgG antibodies to a range of common environmental allergens, once a provisional diagnosis of canine or feline atopic dermatitis has been made. The results of these tests can then be used to help formulate a course of allergen-specific immunotherapy (ASIT). At the present time, the author does not recommend using serological testing for the identification of potential dietary allergens, or for the selection of appropriate restriction diet ingredients, as these tests have been shown to be unreliable for this purpose (Mueller and Olivry, 2017).
Conclusions
The ability to recognise the signs of pruritus in various species is an important first step in working towards its cause. Many of these can be highlighted by taking a detailed history and the relatively simple, non-invasive tests that can be performed by both veterinary surgeons and nurses can, in many cases, yield results that help to determine the specific cause (e.g. in the case of an ectoparasitic infestation) or a contributory cause (e.g. a secondary cutaneous microbial imbalance which can augment the pruritus). In parts 2 and 3 of this article, the commoner causes of pruritus seen in canine and feline first opinion practice will be discussed in more detail and in addition to reviewing the appropriate investigative tests, general recommendations for treatment will be described.
KEY POINTS
- Pruritus is a common presenting sign in veterinary patients and the ability to recognise its various clinical manifestations is important.
- Allowing time to take a thorough history from a dermatological patient is crucial to the organisation of an appropriate work-up.
- The veterinary nurse can be involved in harvesting samples from patients with skin disease.
- Many causes of pruritus can be investigated and determined ‘in-house’ within minutes, if the appropriate laboratory tests are performed and the veterinary surgeon and nurse has the appropriate skills to detect the causal/contributory agents, e.g. ectoparasites and skin microbes.