Shampoo therapy: making sense of all of the choices (Proceedings)

Shampoo therapy is an important adjuvant therapy in pets for treating a variety of dermatologic conditions. In order to get the maximum benefit of the shampoos, the shampoos need to be used properly. Contact time is of utmost importance when using shampoos. The shampoo needs to be on contact of the skin for 5 to 15 minutes. The contact time allows for proper hydration of the skin but provides sufficient time for the penetration and action of the shampoo ingredients. It is also important that shampoo be thoroughly rinsed off so that no residual shampoo is present which could irritate the skin.

Pet owners prefer a shampoo that smells and lathers well. Therefore, if you have a choice between two shampoos with similar ingredients then it may be best to purchase the shampoo that has a more pleasant smell and one that lathers well.

Depending on the condition being treated, some pets require twice weekly until adequate control of odor, grease and scale are achieved (usually 2 to 4 weeks worth of therapy). After that time, depending on the individual's response to the shampoo therapy, a maintenance shampoo as frequently as once weekly or as little as once monthly may be required. It is important to realize that the maintenance shampoo therapy may need to be adjusted depending on seasonal influences (i.e. variations in heat, humidity) since changes in environmental influences can affect dryness, greasiness, scaliness and the tendency to develop bacterial infections.

One question that is often asked by pet owners is whether they can use their human shampoo to bathe their pet. Several differences exist between human skin and dog and cat skin. The human epidermis is 10 to 15 layers thick and normally displaces a 28 day transit time from the basal to cornified layer. This differs from dogs that have 3 to 5 cell layers and the cat has 1 to 3 cell layers of the epidermis. The skin turnover time for pets is 1 week which is a much shorter time period than humans. The pH also differs between humans and dogs and cats. The pH of human skin is acidic 5.5 whereas dog and cat skin pH is neutral or 7.5. Therefore, bathing with a human shampoo in pets would decrease the surface pH of the skin which would result in potential skin irritation, drying, scaling and changes in the bacterial flora on the skin.

Currently over 100 veterinary shampoos exist. Instead of memorizing all of the different shampoo name brand options, a more logical approach is to become familiar with the active ingredients in shampoos and become educated in their clinical applications and any side effects associated with their use. The different shampoo types to be discussed are moisturizing shampoos, antipruritic, antibacterial shampoos, antifungal shampoos and antiseborrheic agents.

Moisturizing ingredients

Two different types of moisturizing ingredients exist for shampoos and other topical products. These two ingredients are emollients and humectants. Examples of emollients are oils (i.e. almond, corn, cotton seed, coconut, olive, peanut, Persia, safflower and sesame), animal fats (i.e. lanolin) and hydrocarbons (i.e. mineral oil, paraffin, petrolatum). Examples of humectants include: natural moisturizing factors (i.e. carboxylic acid, lactic acid, urea), sodium lactate, propylene glycol, glycerin, and polyvinylpyrrolidone.

Both of these ingredients may accomplish the same objective of moisturizing the skin but they work differently. The emollients moisturize the skin by supplying a layer of oil to coat the skin. The humectants work by drawing the moisture up out of the skin to rehydrate outer surface of the skin.

Antipruritic ingredients

Three ingredients have been added to shampoos because of their antipruritic properties. These three ingredients include: oatmeal, pramoxine and aloe vera. All of these products are safe and have minimal side effects. Although extremely rare and antedoctal, the only reported side effect is skin irritation when aloe is used topically. The way that each of these ingredients differ is by the mechanism of action.

Oatmeal evolved out of folk medication and it has been reported to soothe irritated skin for as long as 2 days. The exact mechanism of action is unknown but it is thought to decrease inflammation of the skin by inhibiting prostaglandins.

Pramoxine is a topical local anesthetic that numbs the skin by decreasing the neuronal membrane's permeability to sodium ions. This decrease in the amount of sodium ions being present on the neuron results in the initiation and conduction of pain impulses being blocked so depolarization of the neuron's impulses are inhibited. Pramoxine starts as quickly as 2 to 5 minutes and lasts for several days.

Aloe vera is from stabilized viscous juice from the inner part of the aloe vera leaves. The two chemical components of aloe vera that produce its antipruritic effects are salicylic acid and magnesium lactate. Salicylic acid inhibits the production of prostaglandin from arachidonic acid by inhibiting cyclooxygenase. The magnesium lactate inhibits the conversion of histidine to histamine in mast cells by an enzyme called histidine carboxylase. Aloe vera also products healing properties by increasing dermal perfusion and decreasing inflammatory mediators (i.e. thromboxane, prostaglandin) in burn lesions. Side effects of aloe vera in pets have not been reported.

Antimicrobial ingredients

Antimicrobial ingredients can be divided up into ingredients which kill bacteria and ingredients that kill fungus. Four shampoo ingredients are commonly found in antibacterial shampoos. These antibacterial ingredients are: benzoyl peroxide, chlorhexidine, ethyl lactate and chloroxylenol. Each of these ingredients has a different mechanism of action and possible side effects.

Benzoyl peroxide

Benzoyl peroxide is an excellent antibacterial ingredient that has degreasing and antiseborrheic (keratolytic) properties. Its antibacterial action results from its ability to oxidize substance and has been shown to last for as long as 2 days. Precautions associated with benzoyl peroxide use are excessive drying and/or irritation of the skin and hair coat, bleaching of fabrics, odor (smells more like chlorine) and lack of latherability. Benzoyl peroxide is difficult to manufacture and breaks down with time so it is important to pay attention to expiration dates on the bottles. The problem with excessive drying may be avoided by using a moisturizing spray or conditioner after benzoyl peroxide application.

Chlorhexidine

Chlorhexidine is a synthetic biguanide with antibacterial, antifungal, and antiviral properties. It exerts its antibacterial effects by acting on the bacterial cell membrane, precipitating intracellular contents, and inhibiting ATP. Chlorhexidine is not inactivated by organic debris (i.e. dirt, scale, crust) and has residual properties (i.e. lasts for up to 2 days. No specific precautions have been documented when using shampoos that are of concentrations of 4% or less. This product is nonirritating, nontoxic and well tolerated.

Ethyl lactate

Ethyl lactate is an antibacterial agent that penetrates hair follicles and sebaceous glands but has no proven benefits in treating deep pyoderma. This ingredient becomes effective by breaking down into two compounds within the skin, lactic acid and ethanol. Lactic acid acts by decreasing the skin's pH and thus inhibiting bacterial lipases whereas ethanol renders fats soluble and decreases the amount of sebaceous secretions. Side effects such as irritation, erythema and pruritus are infrequent and rarely reported.

Chloroxylenol

Chloroxylenol is a modified-chlorinated xylene compound which is a potent antiseptic agent. Chloroxylenol works by causing disruption of the cell membrane potential which blocks the production of adenosine triphosphate (causes the cells to starve). This ingredient has a broad spectrum of activity (i.e. antibacterial, anti-algae, and antifungal). According to some reports, this ingredient has been reported to be effective in treating methicillin resistant Staphylococcal infections (MRS, works within 15 seconds). Chlorxylenol is a mild skin irritant and it may stimulate an allergic reaction in some individuals.

Two antifungal ingredients are commonly used in veterinary medicine. These two antifungal agents are ketoconazole and miconazole. Both of these ingredients are azole medications. Azole antifungal agents work by attacking the synthesis of the fungal cell wall.

These two medications differ in that ketoconazole is also available as an oral medication whereas miconazole is not orally absorbed so it is only available as a topical agent. One report suggests that some cases of Microsporum canis are resistant to ketonazole.

Antiseborrheic ingredients

Seborrhea is the name used in veterinary medicine to describe scaling or abnormal turnover of the skin. The purpose of the antiseborrheic shampoos is to normalize the surface by remove the excess scale (keratolytic) and normalize the basal layer by slowing down the turnover (i.e. keratoplastic).

Several antiseborrheic ingredients are commercially available for pets. In the past, tar shampoos were frequently used. More recently tar shampoos have been discouraged and difficult to purchase. The antiseborrheic ingredients that available are sulfur, salicylic acid, and zinc gluconate.

Sulfur is both keratoplastic and keratolytic. The keratoplastic action of sulfur is through its cytostatic capabilities or its reaction with cysteine in the skin to form cystine and hydrogen sulfide (the building blocks for normal keratinization). Sulfur's keratolytic properties for sulfur are from its ability to produce hydrogen sulfide.

Salicylic acid is also keratoplastic and keratolytic. The salicylic acid causes a decrease in pH which increases the hydration of the keratin and causes swelling of the stratum corneal cells. Salicylic acid also solubilizes the intercellular cement substance that holds the scale together.

Zinc gluconate is important nutrient in wound healing, protein synthesis and supports immune function. Zinc has been shown to regulate the activity of oil glands in the skin and reduces inflammation. This nutrient is thought to help normalize epidermal turnover. Zinc is an enzyme cofactor that protects cell membranes from lysis caused by complement activation and toxin release.

Comments and conclusions

Several different shampoo ingredients exist. Each ingredient is indicated for a specific dermatologic condition. It is important to choose the shampoo ingredient that most closely matches what is occurring on the pet. The frequency of the shampooing will often times depend on the skin condition that is present, environmental factors and how the pet responds to the therapy. The author does not recommend bathing any more often than twice weekly.

Selected readings

Kwochka KW. Symptomatic topical therapy of scaling disorders, in Griffin CE, Kwochka KW, MacDonald JM (eds): Current Veterinary Dermatology, St. Louis, Mosby-Year Book 1993, pp. 191-202.

Ascher F, Maynard L, Laurent J, Gouber B. Controlled trial of ethyl lactate and benzoyl peroxide shampoos in the management of canine surface pyoderma and superficial pyoderma, in von Tscharner C, Halliwell REW (eds): Advanced in Veterinary Dermatology, ed 1. London, Baillere Tindall, 1990, pp. 375-382.

Leyden JJ, McGinley KJ, Mills OH, et al: Effects of sulfur and salicylic acid in a shampoo base in the treatment of dandruff: A double-blind study using counts and clinical grading. Cutis 39: 557, 1987.

Hallowell REW. The use of shampoos in veterinary practice. Vet Annu 36: 127-141, 1988.

Klein AD, Penney NS. Aloe Vera J Am Acad Dermatol 18: 714-720, 1988.

Rees CA. Noninsecticidal Veterinary Shampoo Components. Compendium for Continuing Education Small Animal Practice 21(1): 55-64, 1999.