Managing cats with pruritic skin diseases can be challenging. The diagnosis can be extremely difficult, unless a cat is presented with a heavy flea burden! Some owners, further more, do not believe that their cats are pruritic and they may present them to the practice for apparently spontaneous lesions, such as eosinophilic plaques, and alopecia. As with all pruritic animals (Figure 1), it is imperative to use a systematic approach to diagnosis. This uses the history and clinical signs to give the differential diagnosis. The appropriate tests can then be used to rule out certain conditions and confirm the diagnosis.
Time spent taking a good history is never wasted
History taking is very important as it can help to establish some important points. Table 1 outlines some of the questions to ask. These will help formulate a differential diagnosis list.
| History questions | Typical findings |
|---|---|
| 1. When did the clinical signs start? |
|
| 2. What was the age of onset? | |
| 3. Are the symptoms seasonal? | Fleas, harvest mites and allergies may be seasonal |
| 4. Have any other medications helped or aggravated the clinical signs? | There is usually a good response to steroids in allergic skin disease (including fleas), but steroids may worsen other parasites and infections |
| 5. Are any other in contact animals or humans affected? | Dermatophytosis, fleas, feline respiratory viruses |
| 6. Has there been any change in the environment? | Stress and anxiety |
| 7. Has there been any travel abroad? | Leishmania, tick-borne diseases |
| 8. Does the cat go outdoors? | Fleas, harvest mites, cowpox |
| 9. How many cats and dogs are in the household | Stress and anxiety |
| 10. What flea treatment is used and when was the last treatment (including environmental control)? | Fleas |
| 11. What diet is fed and has a diet trial been performed? | Food allergy, inappropriate diet |
It is also useful at the first consultation to use a scale to map how pruritic the owner perceives the cat to be, if at all. This can then be asked at every appointment to see whether the pruritus is getting better or not. The scale outlined in Figure 2 (modified from Peter Hill and colleagues) can be used:
It is worth remembering that cats can be secretive and may not display any pruritic behaviour in front of their owners. The owners may not therefore see pruritus as a concern and instead present their cat with apparently spontaneous alopecia, eosinophilc plaques or other lesions.
It can be useful to spend some time discussing home life for the cat and whether anything changed before the condition started. If there is more than one cat in the household it is important to determine the number of social units. Generally, cats that engage in relaxed physical contact and feed together can be regarded as one social unit. There should be at least one feeding station, resting area (preferably elevated) and litter tray per social unit. Remember that new cats in the area can upset the routine, even for indoor cats if they can see (e.g. through windows) or smell (e.g. around doors) other cats.
Clinical examination
A thorough clinical examination should be performed, paying particular attention to the skin. Any lesions should be mapped out on a diagram to allow comparison between visits. Pruritic cats commonly present with a variety of easily recognised skin lesions:
Secondary lesions arise from over grooming or more severe self trauma (Miller et al, 2012). Unlike dogs, the distribution and type of lesions are little help in diagnosis. The differential diagnosis for each of these presentations is remarkably similar. Fleas, for example, may more commonly affect the trunk, but cats with a variety of lesion patterns also respond to strict flea control. Recent studies correlate flea allergic dermatitis (FAD) more frequently with lesions on the trunk and food allergies with facial inflammation, but the distribution of lesions is not otherwise associated with any particular diagnosis.
Differential diagnoses for pruritus
There are a number of possible diagnoses of pruritus in cats:
Remember that any one cat can have more than one trigger factor for its pruritus and inflammation.
Diagnostic tests
Equipment needed
Coat combing
Coat combings are very quick, easy and inexpensive to perform (Figure 9). They can be used to look for large ectoparasites (e.g. fleas and lice) and for evidence of flea dirt. Samples collected by this method can either be mounted on a slide with liquid paraffin and examined under the microscope (Figure 10) or a ‘wet paper test’ can be performed. If flea faeces are present, they will leave a reddish brown stain on the wet paper/cotton wool.
Adhesive tape strips (ATS)
ATS are a very quick, easy and inexpensive to perform. To perform, take a piece of adhesive tape (the same size a microscope slide) and apply repeatedly to the area of skin of interest until the adhesiveness is lost and a layer of skin debris has been collected (Figure 11). ATS can be placed directly onto a slide for examination under low magnification for ectoparasites, or can be stained for cytology (looking for microorganisms such as yeasts and bacteria under high magnification). Staining the ATS for cytological examination is an easy process. Once the sample has been collected, the ATS is placed in a loop at the end of the microscope slide (Figure 12). Using a stain such as Diff-Quik® the ATS can be dipped 5–10 times in the eosinophilic stain (pot 2), then in the basophilic stain (pot 3). It must NOT be placed the fixative (pot 1) as this will dissolve the tape. The tape is then gently rinsed with water, placed onto the microscope slide and blotted dry. Alternatively, a drop of the basophilic stain (pot 3) can be placed directly onto the microscope slide, the ATS placed on top, and the excess stain blotted away.
Trichogram
Trichograms are again a quick and simple task to perform (Figure 13). Trichograms can be taken to look for ectoparasites such as Demodex spp or to assess the hair bulb and shaft. The hair bulb (the proximal end) should be checked to see what stage of growth it is in, and the hair tip (the distal end) should be checked for any trauma or damage consistent with self-induced alopecia.
Skin scrapes
Skin scrapes are fairly quick and simple to perform, although if a scrape is needed of a sensitive/painful area, it may be necessary to consult with the veterinary surgeon in charge of the case to discuss sedation. Skin scrapes can be performed to check for ectoparasites such as Demodex or Notoedres spp (Figure 14). Demodex is not as common in cats as in dogs, but it is easily missed and is probably under diagnosed.
Direct and indirect impression smear
Direct impression smears can be taken from a particularly wet/moist skin lesion. A microscope slide can be placed directly onto the lesion, left to dry and stained for cytological examination.
Indirect impression smears are taken by collecting material onto a cotton bud, gently rolling this onto a microscope slide to transfer the material and then staining this once dried.
Woods lamp examination and fungal culture
Wood lamp examination can be performed if Microsporum canis is suspected, but the lamp must be warmed up prior to use. Usually 5 minutes is long enough. A dark room is also required. Microsprum canis should fluoresce an apple green colour due to certain pigments in the hyphae (Carlotti and Pin, 1999). In addition, fungal hyphae and ectothrix spores may be seen on affected hairs on a hair pluck. If unsure, a sample of hair (toothbrush combing) may be submitted to a laboratory for a complete fungal culture.
Skin biopsy
Skin biopsies can be performed with a punch biopsy. Biopsy punches are available in several sizes, although a 6 mm or larger punch is preferred. Sedation is required to perform skin biopsies, but it may be necessary to anesthetise the cat to biopsy a sensitive area (e.g. the nose or interdigital skin). If sedation is used, a local anaesthetic should be infiltrated into the subcutis remembering to use local anaesthetic without adrenaline. Excess hair should be gently clipped away with scissors, ensuring that the skin surface is not disrupted. The skin should not be prepared before taking the biopsy, as this could alter the pathology. The biopsy should be placed in 10% formalin and submitted for histopathology to a histopathologist with an interest in skin disease.
Diagnosis of FAD
A rigorous flea control programme utilising an on-animal adulticide for all in-contact animals and an environmental combined adulticide/insect growth regulator for a minimum of 6–8 weeks is mandatory in all cases of pruritus, as, in the author's experience, fleas and flea dirt may not be found in up to 50% of FAD cases. It can, however, be difficult to control flea exposure in cats that visit other homes, out buildings etc. Remember to treat the house, shed or garage, cat carrier and car that the cat comes to the clinic in! Using selamectin or imidacloprid/moxidectin every 2 weeks initially (this is off-label) for three applications is useful as this protocol also manages Otodectes, Cheyletiella and other ectoparasites/endoparasites. This can then be reduced to the licensed frequency of the product for long-term flea prevention.
The gold standard for diagnosis of FAD is the response to a rigorous flea control programme. Other tests, however, have become popular, partly because they help to improve compliance by highlighting the importance of flea control to the owners. The main allergen is a flea salivary protein. FcεRIα based enzyme-linked immunosorbent assays (ELISAs) and intradermal tests (IDTs) using purified flea salivary antigen are more sensitive than IDTs with whole body extracts, although positive reactions can be seen in healthy animals and levels do not correlate with clinical severity. Recent studies showed that live flea challenge was more sensitive and specific for FAD than either serology or IDT. However, negative test results do not rule out FAD and may undermine flea control efforts if the owner concludes that their cat cannot be flea allergic based on the test results.
Food trials in cats
Once ectoparasites and infections have been eliminated, an exclusion diet trial is necessary to rule out an adverse food reaction in appropriate cases. There is no evidence that food antigen serology is of any clinical benefit in cats.
Feline food trials, compared with dogs, are fraught with difficulty. Diet trials are usually conducted for 6–8 weeks, although some authors recommend 9–12 weeks, and it can be difficult to get some cats to take the same food over this time (the authors normally rotate 2–3 different diets during the food trial). Cats can also be more stubborn than dogs, and there is a risk of hepatic lipidosis (especially in overweight animals) if they are starved for more than a few days (Jackson and Mueller, 2002). Many cats will also access food from other animals in the home or outdoors, and it can be extremely difficult to keep an outdoor cat indoors for a long period. Veterinary surgeons can help owners comply with food trials by allowing the owners to administer 1–2 mg prednisolone once daily for 3–5 days if the cat gets very itchy and distressed — this gives them some control over their cat's clinical signs, but still allows the outcome of the food trial to be assessed.
The previous diet history should be used to select diets with novel ingredients. Home-cooked diets are preferred, as the ingredients can be more easily controlled. In the author's experience most cats will not accept carbohydrate however, so all meat diets may be necessary. Feline diets must also have adequate amounts of taurine and essential fatty acids; so longterm home-cooked feeding is usually not attempted, although a single protein home-cooked diet for 6–8 weeks should be fine for healthy adult cats. There is a limited selection of single protein commercial diets, and as lamb, fish, duck and chicken protein sources are already present in many standard diets, choosing a novel protein can be difficult. Palatability may be a drawback with commercial hydrolysed hypoallergenic diets.
In the author's experience many feline food trials are attempted, but may be difficult to complete. It is still important to attempt, however, as food may be a contributing factor for causing feline pruritus. It is essential to challenge any cat that has an apparent response to a food trial with its normal food to demonstrate cause and effect. Once the cat is stable on its trial food, the owners can add single ingredients one at a time for 2 weeks each to identify the offending food items.
Conclusion
A pruritic cat can be a particularly challenging case. It is imperative to remain methodical in the approach to the dermatological work up to achieve a successful outcome. The cat's home life should also be looked into thoroughly to ensure stress is not a factor.
As it can take some time to achieve a resolution with a pruritic cat, client communication is paramount. The client must be informed from the initial appointment that unfortunately there may not be a quick fix to the condition and they need to be committed to the treatment regimen. Any treatment regimen must be discussed with the owner to ensure that it is practical for them and any hurdles must be promptly and effectively dealt with along the way. Problems and non-compliance can be picked up early with regular contact with owners. Regular telephone and/or email contact during diagnostic food and flea trials with good follow up of treatment can substantially improve compliance, leading to better outcomes for the cat and their owner.