Miliary dermatitis in cats

Introduction

Miliary dermatitis is a common presentation of dermatologic disease in cats. Historically this condition was called “miliary eczema”; the word “miliary” is defined as “resembling or suggesting a small seed or many small seeds” 1, and the term found usage as it describes the feeling encountered when examining the coat of an affected cat. Lesions are often felt easier than they are seen, due to their small size and distribution within an often otherwise normal-appearing coat. In more specific dermatologic terms, miliary dermatitis can be described as clusters of small, typically crusted, papules. Miliary dermatitis is not a diagnosis, but rather a reaction pattern with many different potential causes; the differential diagnoses are numerous and include hypersensitivity to flea bites; cutaneous adverse food reactions; atopic dermatitis; ectoparasites; bacterial, viral, yeast or dermatophyte infections; cutaneous adverse drug reactions; pemphigus foliaceus; dietary imbalances; feline hypereosinophilic syndrome; and urticaria pigmentosa 2. This article will review the clinical signs, typical presentation and etiologies of feline miliary dermatitis, as well as focusing on the diagnostic approach and treatment options for this common skin condition.

Clinical presentation

Miliary dermatitis can be localized or generalized, and it may be the sole presenting sign or it can occur with other primary or secondary dermatologic lesions. Most commonly, groups of small papules are found on the head and neck, limbs, trunk or dorsal lumbar region (Figure 1). Lesions that may be concurrently detected in patients with miliary dermatitis include alopecia, excoriations, erosions and ulcers. Miliary dermatitis may also be found in cats with lesions consistent with the feline eosinophilic granuloma complex (EGC) such as indolent ulcers, eosinophilic plaques and eosinophilic granulomas. Although pruritus is common in affected patients, it is not a consistent finding. This may in part be due to the discrete grooming practices of cats; owners may not visualize pruritic behavior, or may not understand what constitutes abnormal grooming behavior.

In an attempt to concisely evaluate outcome measures of therapeutics in cats affected by miliary dermatitis and various other skin lesions, a novel scale, known as SCORFAD, has been developed 3. Using this scale, 10 body regions are identified, namely the head; neck; dorsal and lateral thorax; rump and tail; flanks; sternum and axilla; abdomen; perineum; forelimbs and paws; and the hindlimbs and paws. Using this scoring system, miliary dermatitis may be classified as very mild in patients with 10 or fewer papules in one body region; mild in patients with more than 10 papules in one body region; moderate in patients with 10 or fewer papules in more than one body region; and severe in patients with more than 10 papules in more than one body region 3.

Etiology

The potential causes of miliary dermatitis are many, so for the purposes of this discussion they will be subdivided into larger categories.

Hypersensitivity

Hypersensitivity responses to insects (especially fleas), environmental allergens, food allergens and drugs have all been known to cause miliary dermatitis. Flea bite hypersensitivity is the most common cause of feline miliary dermatitis and should be considered in every case occurring in flea-endemic regions 2 4 5 as flea saliva contains many irritating substances that can lead to a hypersensitivity reaction. In contrast to dogs, where intermittent exposure to fleas seems to cause more hypersensitivity reactions than continuous exposure, cats who are exposed to fleas regularly are at the same or greater risk to develop flea bite hypersensitivity than cats exposed intermittently 2. Cats with flea bite hypersensitivity are pruritic and frequently have lesions on the head, the dorsal lumbar region, the tail and the ventral abdomen (Figure 2a) (Figure 2b) 5. In one multicenter study it was found that 35% of cats with flea bite hypersensitivity had miliary dermatitis as at least one of their presenting signs 5.

Hypersensitivity to environmental allergens (non-flea, non-food induced hypersensitivity dermatitis or feline atopic syndrome) is another common cause of miliary dermatitis. Along with head and neck excoriations, self-induced alopecia and EGC lesions, miliary dermatitis is a common clinical reaction pattern 6. The literature shows that 18-34% of cats with feline atopic syndrome will present with miliary dermatitis 5 7 8, with the head and ventral abdomen most commonly exhibiting lesions and pruritus 5.

Cutaneous adverse food reactions (food hypersensitivities) are reported as the primary cause of pruritus in 12-17% of non-seasonally pruritic cats 2 5. 42% of cats with pruritus and concurrent abnormal gastrointestinal symptoms of chronic vomiting or diarrhea had confirmed food hypersensitivities 9. One study found that 20% of cats with food-induced hypersensitivity presented with miliary dermatitis 5. Cats with adverse food reactions commonly have lesions and pruritus around the head and neck as well as the ventral abdomen 5.

Other hypersensitivities can also create lesions of miliary dermatitis. For example, mosquito bite hypersensitivity may cause miliary dermatitis on the bridge of the nose and pinnae, and cutaneous adverse drug reactions may cause pruritus and subsequent miliary dermatitis in some patients 10.

Infection

In addition to fleas, other ectoparasites such as Trombiculae spp. (Chigger mites), Cheyletiella spp., Otodectes cynotis, Sarcoptes scabiei, Felicola subrostratus, Notoedres cati and Demodex spp. can occasionally cause miliary dermatitis (Figure 3) 11 12 13 14. Location of lesions is dependent on the preferred habitats for the parasite in question, e.g., miliary dermatitis would be more likely to occur on the trunk in a patient infested with Cheyletiella spp. but more commonly on the head and around the ears in a cat with Otodectes cynotis.

One study found that 29% of cats diagnosed with pyoderma had evidence of miliary dermatitis on examination 15. The majority of cats in the study were pruritic and lesions were often multifocal (face, neck, limbs, ventral abdomen and dorsal trunk).

Cats with marked pruritus due to dermatophytosis may present with miliary dermatitis. Most often these cases are caused by Microsporum canis. Dermatophytosis is usually a minimally pruritic condition in cats, so when evidence of miliary dermatitis and pruritus is found in affected individuals it is suggested to investigate for concurrent bacterial infection, ectoparasitism or allergy 16. Malassezia yeast overgrowth or hypersensitivity may also cause miliary dermatitis. Rarely, cats affected by feline immunodeficiency virus (FIV) (Figure 3) can present with generalized miliary dermatitis 17.

Other

Feline pemphigus foliaceus presents as a focal or generalized crusting dermatosis, and could be considered as a differential diagnosis for miliary dermatitis, particularly if there are crusted papules on the head, face and ears 10. Urticaria pigmentosa is a manifestation of mastocytosis and may present as a crusted papular rash in some cats; Sphinx cats may be more severely affected 18.

Diagnostic approach

Signalment and history

When a patient presents with signs of miliary dermatitis it is important to consider all differentials, take a detailed history and have a methodical approach in order to properly diagnose and treat the individual. Signalment and history are often some of the most important details when attempting to differentiate between etiologic causes of miliary dermatitis. One of the most valuable pieces of information is to ascertain whether or not the cat goes outdoors, or if there are any other pets in the household who go outdoors. This will help to determine the likelihood of exposure to fleas, parasites, mosquitos, and other sources of irritation. Finding out if there have been any new pets introduced to the household or if there are any other pets or humans in the household that are affected may help in differentiating contagious etiologies.

The presence or absence of pruritus, distribution of pruritus and any history of seasonal pruritus is helpful in determining hypersensitivities such as flea bite allergy, adverse food reactions and feline atopic syndrome. As noted above, owners may not perceive certain feline behaviors as pruritus, and it may help to ask if the cat is licking, biting, rubbing, rolling or grooming more, or if there are any patches of hair loss, or tufts of hair around the house. Owners may not identify overgrooming but they may report their pet has more trichobezoars due to increased hair ingestion from excessive grooming.

It can be helpful to know how long the miliary dermatitis has been present, and if this is a recurring issue or a new problem. If the cat has recently been exposed to new medications or treatments an adverse drug reaction should be considered. Concurrent symptoms of gastrointestinal abnormalities may increase the suspicion of an adverse food reaction.

The breed and age of the patient can be helpful clues when determining an etiology. As noted above, Sphinx cats may be more severely affected by urticaria pigmentosa, whilst Siamese cats are reported to have a higher incidence of adverse food reactions than other breeds 2. Most cats with feline atopic syndrome will present with initial symptoms between 6 months and 2 years of age 2.

Clinical examination

A general physical exam should be followed by a thorough dermatologic exam. The crusted miliary dermatitis lesions are often very small, so it can be helpful to feel the skin by massaging the patient’s hair coat to detect the papules (Figure 4). An otoscopic exam may detect the presence of concurrent abnormalities that may give clues to the underlying etiologic cause, and the paws and claws should be examined for any evidence of disease.

Examine the skin for evidence of other types of lesions as well. Miliary dermatitis is one of four common clinical reaction patterns found in cats with hypersensitivity disorders (Figure 5), the others being head and neck excoriation, self-induced alopecia, and EGC lesions 6. One study found that 30% of feline atopic syndrome cases presenting with miliary dermatitis had concurrent EGC lesions, whereas only 4% had miliary dermatitis lesions alone 8. Cats who develop pemphigus foliaceus will often have purulent debris around the claw folds in addition to crusting on the rest of the body, particularly the face. Fractured hair shafts, erythema and alopecia can occur with dermatophytosis. Large white scales or dander can accompany Cheyletiella. Otitis externa may occur in cats with hypersensitivities or Otodectes cynotis infestations.

Diagnostic tests

Hairs and debris collected by combing should be examined for evidence of fleas or flea dirt. This can also identify parasites such as Cheyletiella spp. or Felicola subrostratus. Skin scraping is advised to look for evidence of parasites such as Demodex spp., Notoedres cati, and Sarcoptes scabiei. A treatment trial may be necessary to rule out parasitic infestations; in particular, flea bite hypersensitivity should be excluded by using appropriate fast-acting and effective flea adulticides such as spinosad, nitenpyram or fluralaner. It is essential to understand the flea life cycle (and explain this to owners) as flea eggs will not be affected by most adulticidal products, and cats will need to be protected against fleas that will hatch over a period of weeks to months in the environment. It is equally important to treat all in-contact animals when attempting to rule out flea bite hypersensitivity.

 

Fecal flotation can be a helpful adjunctive test when ruling out parasites. Otodectes, Notoedres, Cheyletiella mites, Demodex gatoi (Figure 6), Lynxacarus radovskyi and Chigger mites can all be found on fecal flotation 19. The presence of Dipylidium caninum tapeworms on fecal analysis may increase the suspicion for the presence of fleas, which serve as an intermediate host 20.

All cases of miliary dermatitis should have a cytological examination. Cytology is invaluable for diagnosing and monitoring the progress of bacterial and yeast infections as well as aiding in the diagnosis and monitoring of conditions like pemphigus folliaceus and dermatophytosis. If bacterial infection is found, bacterial culture and sensitivity may be indicated in order to guide the use of systemic antimicrobials.

Dermatophyte fungal culture, Wood’s lamp examination, direct microscopic hair shaft examination, dermoscopy and dermatophyte PCR testing can all be helpful for diagnosing dermatophytosis. Of course, none of these tests are 100% sensitive or specific, and any results must be interpreted in light of clinical lesions, physical exam and history.

Once infectious causes have been ruled out or treated, additional diagnostic tests may be warranted. Tissue biopsies and dermatohistopathology can be helpful to rule out pemphigus foliaceus, cutaneous adverse drug reactions, urticaria pigmentosa and neoplasia. They can also be useful to support a diagnosis of hypersensitivity. Blood work and urinalysis can help to exclude systemic diseases such as hyperthyroidism or retroviral infections that may predispose to infection or poor wound healing.

If the patient has a history of non-seasonal pruritus contributing to the miliary dermatitis, once flea bite hypersensitivity has been ruled out, an elimination diet trial with a limited ingredient novel protein or hydrolyzed protein diet is advised to rule out a cutaneous adverse food reaction. In most cases, at least some improvement in symptoms will be noted within 6 weeks of starting an elimination diet trial in affected cats, but it may take up to 12 weeks to see full resolution of symptoms 2. Elimination diet trials should be followed by provocative re-challenge in order to confirm a diagnosis of adverse food reaction.

Intradermal skin testing and serum allergy testing may be helpful to direct treatment once a diagnosis of feline atopic syndrome has been made by ruling out differential diagnoses (such as cutaneous adverse food reaction, flea allergy dermatitis, pediculosis, etc.) in cats with supportive signalment, history and clinical symptoms. Allergy testing should not be used to diagnose feline atopic syndrome.

Treatment

Effective resolution of miliary dermatitis requires treatment of any infection as well as identification and treatment of all underlying causes.

If found on cytological examination, bacterial infection is most effectively treated with a combination of systemic and topical antimicrobials. Good options for empirical systemic antibiosis include amoxicillin with clavulanic acid and cefovecin, and treatment should continue for one week beyond clinical resolution of the infection 21. Topical therapy, including chlorhexidine, benzoyl peroxide, silver sulfadiazine and fusidic acid 22, may be used alone in cases of localized infected lesions or to speed resolution of more widespread lesions. It is important to restrict patient grooming after topical products are applied; this may require an Elizabethan collar or distraction with play until the product has dried or been absorbed.

All cases of generalized dermatophytosis should receive topical therapy. Effective options include twice weekly 2% lime sulfur dips, 2% miconazole plus 2% chlorhexidine rinses or 0.2% enilconazole 23. If systemic therapy is used, itraconazole and terbinafine are good options for cats. Therapy should be continued until 2-3 negative fungal cultures taken at weekly intervals are obtained. It is important to remember the contagious nature of dermatophytosis and affected animals should be separated or all in-contact individuals should be treated topically for the duration of treatment of the affected individual. Environmental control is important when attempting to clear dermatophyte infections, as infective spores can remain viable for up to 18 months 16. Clients should be instructed on how to reduce environmental contamination by vacuuming and using electrostatic cleaning cloths to pick up hairs, followed by disinfection with 0.5% sodium hypochlorite or accelerated oxygen products 16.

Parasitosis should be treated with appropriate anti-parasiticides depending on which parasite is suspected or identified. Repeated dosing with a broad-spectrum parasiticide such as selamectin or 10% imidacloprid with 1% moxidectin may be used to rule out most parasites in cats. Infestations with D. gatoi may be particularly difficult to resolve, however, and will often require six weekly dips with lime sulphur for the affected individual and all in-contact cats 12. There is also evidence that weekly application of 10% imidacloprid with 1% moxidectin may be effective to treat D. gatoi 24.

Flea bite hypersensitivity is the most common cause for chronic miliary dermatitis in cats worldwide and should be considered in every case presenting in flea endemic regions. A treatment trial with flea adulticides should be considered as described in the section on diagnostic testing, and affected individuals may need to be treated for 2-3 months to see full resolution of signs. When ruling out flea bite hypersensitivity it is important to remember to treat all in-contact animals and the environment in order to reduce exposure of the affected individual to flea bites.

If a cutaneous adverse reaction to food was diagnosed with improvement on an elimination diet trial and worsening upon provocative re-challenge it is recommended that patients undergo individual ingredient challenges to determine the specific food allergens so that avoidance can be practiced for treatment. Alternatively, a well-balanced limited ingredient novel protein diet or hydrolyzed protein diet can be fed long term.

Cats with feline atopic dermatitis may benefit from a variety of treatment options. These can include allergy testing and allergen-specific desensitization; symptomatic control with medications such as cyclosporine, corticosteroids and antihistamines; and supportive care with diets that contain products to help the skin barrier, and long-chain polyunsaturated essential fatty acids, predominantly the omega-3-family. Topical therapy with antimicrobial products, corticosteroids and local anesthetics like pramoxine may benefit patients with feline atopic dermatitis.

The causes for feline miliary dermatitis are many, and effective treatment of miliary dermatitis depends on determining and treating all infectious and underlying etiologies. Flea bite hypersensitivity is the most common cause of feline miliary dermatitis and should be considered as a differential in any cat presenting with symptoms.