Pruritus is defined as an unpleasant sensation within the skin that provokes the desire to scratch. It is the most common dermatologic problem in both small and large animals.
Pruritus may be well or poorly localized. It may manifest as a sharp or diffuse, burning sensation. Although the skin is richly innervated, there are no known specialized pruritus receptors. The sensation of itch is transmitted via a specialized set of afferent fibers. Myelinated fibers that conduct sensations at 10–20 m/second carry the well-localized pricking itch sensation. In contrast, the sensation of burning itch is transmitted via nonmyelinated fibers that conduct sensations at 2 m/sec. Both of these fibers enter the dorsal root of the spinal cord, ascend through the dorsal column, and cross into the lateral spinothalamic tract. From there they go to the thalamus and on to the sensory cortex.
The mediators of pruritus are controversial and may vary depending on the species. These putative mediators include histamines (released from mast cell degranulation), proteolytic enzymes (proteases), and leukotrienes. Proteases are released by fungi, bacteria, and mast cell degranulation, and during antigen-antibody reactions. Leukotrienes, prostaglandins, and thromboxane A2, which are broken down from arachidonic acid, are pro-inflammatory.
Pruritus is a clinical sign and not a diagnosis or specific disease. In general, the most common causes of pruritus are parasites, infections, allergic skin diseases, and miscellaneous causes (eg, cutaneous neoplasia). Many diseases that are nonpruritic (eg, endocrinopathies) become pruritic when the animal develops secondary bacterial or yeast infections.
A thorough dermatologic history and physical examination should be performed. Parasitic causes of pruritus, including Demodex, fleas and ticks, contagious mites, and lice, should be excluded, because they are most common. Skin scrapings or hair trichograms can exclude (or include) various mite infestations, including Demodex. However, some mite infestations (eg, Sarcoptes, Cheyletiella, Psoroptes, Chorioptes) might be missed on skin scrapings.
If a mite infestation is suspected, a response to therapy trial should be undertaken. The most commonly used drug in large animals is ivermectin, which should be used concurrently with a topical product such as lime sulfur. In small animals, the best option is a species-approved isoxazoline.
The next most important group of pruritic diseases to exclude is infectious causes of skin disease. These include bacterial infections (primarily staphylococcal infections, and Malassezia overgrowth). Concurrent bacterial and yeast infections are increasingly recognized as a common cause of pruritus in dogs, cats, and large animals. Bacterial pyoderma and yeast overgrowth is underdiagnosed in cats, and a response to therapy trial may be needed to exclude or include it.
Infectious causes of pruritus commonly induce clinical signs of hair loss, scaling, scales piercing hairs, odor, and/or greasy seborrhea. Marked pedal pruritus and facial rubbing are common in animals with concurrent yeast and bacterial infections. Before pursuing allergies as a cause of pruritus or performing skin biopsies or other more expensive and/or invasive diagnostic testing, a concurrent bacterial and yeast infection should be excluded. Topical antimicrobial shampoo therapy (2% chlorhexidine/2% miconazole) daily or every other day can be used. Oral antibiotic therapy should be used only with evidence of an infection confirmed on culture and susceptibility testing. The number of yeast organisms found on cytology are not relevant in pruritic animals because it is a hypersensitivity to Malassezia that causes the pruritus. Systemic antifungal drugs include ketoconazole, 5 mg/kg orally once a day (dogs only) and itraconazole, 5 mg/kg orally once a day (cats or small dogs). Fluconazole and terbinafine should not be used.
It is possible that the initial trigger has long passed or is seasonal. However, if the animal’s pruritus is unchanged or only somewhat better, the most likely underlying cause is allergic (assuming parasitic and infectious causes have been excluded). The most common causes of allergic pruritus are insect bite hypersensitivity (eg, flea allergy, mosquito bite allergy, fly bite) and atopic dermatitis. Food allergy is less common as a sole cause of pruritus. Flea allergy dermatitis and insect bite hypersensitivity are excluded based on response to insect control.
Animals that do not have insect bite hypersensitivity but are seasonally pruritic most likely have atopic dermatitis. Animals with year-round allergic pruritus have atopic dermatitis and/or food allergy. Food allergy is excluded or included based on response to a diet trial and provocative challenge. Merely changing diet is not adequate, and hydrolyzed diets are the standard of care. Atopic dermatitis is a clinical diagnosis; in vitro allergy testing and intradermal skin testing show only antigen exposure patterns. These tests are used to determine the contents of an immunotherapy vaccine.
Successful therapy depends on identification of the underlying cause. Animals with idiopathic pruritus or those in which treatment of the underlying disease does not eliminate the pruritus (eg, atopic animals) require medical management of pruritus. Currently, evidence-based reviews of antipruritic therapy do not support the use of antihistamines to control pruritus.
Glucocorticoids are the most effective drugs in the management of pruritus. However, they cannot be used safely for longterm management because of adverse effects (eg, suppression of adrenal function, risk of development of diabetes mellitus, risk of secondary urinary tract infections). In addition, owners can rarely tolerate the common adverse effects (polydipsia, polyuria, polyphagia, and panting) for long periods. Anti-inflammatory dosages of prednisone or prednisolone ranging from 0.5–1 mg/kg/day, PO, for 5–10 days and then every other day are recommended. Topical spray formulations of triamcinolone acetate are highly effective and good alternatives to oral steroids.
Cyclosporine modified is a highly effective nonsteroidal drug for control of pruritus. The only formulation that can be used is modified cyclosporine. The dosage is 5 mg/kg for dogs and 7 mg/kg for cats. Maximal benefit can take as long as 30 days to observe. Once efficacy is established, dose tapering to every other day can be attempted. Common adverse effects include vomiting and diarrhea (common) and gingival hyperplasia (less common). Another drug for control of pruritus in dogs is oclacitinib, a Janus kinase inhibitor that provides rapid relief from pruritus and inflammation in dogs with short- and longterm allergic skin disease. Monoclonal antibodies for pruritus in dogs are beneficial.
Pruritus is the most common dermatologic problem of animals.
Parasites are the most common cause of pruritus and, often, response to treatment for 3-4 months is the only way to rule out parasite infestations.
Infections are common complications of pruritus and should be addressed. Systemic drugs should be used conservatively because the first line of treatment is topical antimicrobial therapy.