THE MERCK VETERINARY MANUAL
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Principles of Topical Therapy

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Also see Systemic Pharmacotherapeutics of the Integumentary System.

Topical therapy is an important part of veterinary dermatology. It is often beneficial in improving the cosmetic appearance or odor of the animal, pending the final diagnosis. It can be beneficial as an adjunct to systemic therapy. Finally, it may be the preferred method of treatment for some diseases, eg, flea infestations.

The following are some basic guidelines to consider when prescribing topical therapy: 1) As much of the hair coat as possible should be removed when treating skin diseases. Good grooming practices facilitate topical therapy and can significantly help shorten the course of disease. 2) The cooperation of the owner (and animal) should be evaluated before any topical therapy is prescribed. 3) Animals tend to groom off topical products and may vomit after ingestion. The risk of toxicity is a constant worry for clients. Local ointments, gels, and sprays are best used sparingly, under occlusion, and for specific diseases. Such medications often sting when applied to the skin, especially many of those instilled into the ears. Many agents also may mat the hair. 4) Tepid water is the temperature of choice for bathing animals. 5) The old adage, “If it's wet, dry it and if it's dry, wet it,” has some truth to it; however, this advice should not be carried to extremes. Exudative lesions, eg, areas of pyotraumatic dermatitis, heal faster if they are kept clean and covered with an antibiotic ointment or gel (previous recommendations suggested aggressive astringent use). Dry, lichenified skin is often pruritic, and the judicious use of emollients may be beneficial. 6) The animal should be monitored closely for possible development of irritant or allergic contact dermatitis from topical agents. Many topical agents have very similar bases or ingredients, and changing from one to another may only exacerbate the problem. 7) Owners should be given careful and thorough instructions on how to administer the therapy.

Shampoos are the most commonly used topical treatments. There are three broad classes of shampoos: cleansing, antiparasitic, and medicated. Cleansing shampoos remove dirt and excess oils from the coat. These products include over-the-counter dog grooming shampoos, flea shampoos, and many mild products for people. These products lather well and must be rinsed from the coat. Antiparasitic shampoos are “flea shampoos.” In most cases, the amount of insecticide in these products is not adequate to kill all of the fleas in a severe infestation. However, these products are excellent routine cleansing products. Medicated shampoos include antimicrobial and antiseborrheic products. The most widely used antibacterial shampoos contain chlorhexidine or benzoyl peroxide .  Antiseborrheic shampoos contain some combination of tar, sulfur, and salicylic acid—ingredients that are keratoplastic and keratolytic. Tar is recommended for oily seborrhea, and sulfur and salicylic acid are recommended for scaly seborrhea. Most animals benefit from products that contain all three agents; however, tar products are contraindicated in cats.

When a medicated shampoo is used, the animal should be washed in a cleansing shampoo before the medicated shampoo and rinsed well. Medicated shampoos often are not good cleansing agents, do not lather well, or do not work well in the presence of organic debris. The medicated shampoo should be applied evenly to the hair coat after being prediluted in water. Prediluting the shampoo will facilitate it being rinsed from the coat and minimize the potential for irritant or allergic contact dermatitis. Depending on the shampoo, the concentration of shampoo to water will vary between 1:3 and 1:4. If possible, the medicated shampoo should be allowed to have a contact time of 10 min with the skin and then rinsed thoroughly from the coat. Shampoo residue is a common cause of irritant reactions. Finally, the medicated shampoo should be used often, usually 2–3 times/wk during the early stages of therapy.

The use of medicated antimicrobial shampoo therapy is increasing because of concerns over the development of methicillin-resistant staphylococcal infections.

Last full review/revision June 2013 by Karen A. Moriello, DVM, DACVD

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