Time for topicals: A spot-on guide to treating dermatologic diseases

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Topicals such as pour-ons, shampoos and ointments, provide concentrated treatment for both focal and generalized skin problems while minimizing systemic adverse effects. Although there are many other antiparasitic, antimicrobial, otic, and ophthalmic topicals, this article focuses on ingredients available in the United States.

Cytology

With any dermatologic problem, cytology is the first step for ruling infectious agents in or out.¹ Slides stained with Diff-Quick can identify bacteria and yeast.¹ A trichogram, which is a direct examination of hairs, has some efficacy in diagnosing dermatophytosis.² Superficial or deep skin scrapings can detect mite infestations.³ In general, the more samples that can be obtained, the better the likelihood of making a diagnosis.⁴ The authors recommend sampling from at least 2 or 3 locations on a patient to improve sensitivity. These noninvasive, inexpensive diagnostics can sometimes make the diagnosis or discover the next step to a more expensive and more invasive test. Veterinary technicians can be trained in obtaining and reading cytology to achieve efficiency in busy clinics.

Infectious agents

Staphylococcus

The growing prevalence of methicillin-resistant Staphylococcus species in companion animals necessitates judicious systemic antimicrobial use in veterinary medicine and emphasizes the need for topicals as sole-agent therapeutics for superficial and local pyodermas as frequently as possible.⁵ Chlorhexidine 2% to 4% is effective for Staphylococcus and comes in multiple convenient formulations, including shampoos, sprays, wipes, and mousses. For focal lesions, twice-daily application for 10 to 15 minutes of contact time would be ideal. For generalized lesions, bathing twice weekly with chlorhexidine 4% shampoo has an efficacy similar to systemic amoxicillin-clavulanate administration.⁶ Because the turnover of skin takes about 3 weeks, this study recommends the duration of topical treatment be similar to that of systemic antibiotics for superficial pyodermas: a minimum of three weeks, with one week beyond clinical resolution.^5,7

Another effective treatment for superficial pyoderma is a 5.25% solution of bleach. It has been shown that the minimum bactericidal concentration (MBC) may require a 1:32 dilution (ie, ½ c bleach to 1 gal of water), although anecdotally a 1:50 dilution (ie, ⅛ cup bleach to 1 gallon of water) may still be clinically effective.⁹ The dilution loses stability after 24 hours. There may be limited published clinical evidence in veterinary medicine.⁵ Bleach is commonly used in human medicine to decolonize or treat resistant bacterial skin infections.^9,10

Povidone-iodine can be bactericidal, but it can also be drying and irritating and cause staining.¹ Benzoyl peroxide can be effective by lowering the skin pH but can be drying.³ To attenuate the drying adverse effects of these ingredients, consider pairing with conditioners and/or emollients. Mupirocin ointment can be applied to focal infections. However, a concern for growing resistance has led some countries outside the United States to withhold it from veterinary medicine.^3,5 Silver sulfadiazine cream can also help with focal infections, but may best target gram-negative bacteria.³

Malassezia

Malassezia yeast dermatitis can be treated topically with twice-daily administration of combination shampoos containing 2% chlorhexidine and 2% miconazole.¹¹ Clinics may elect to carry only 3% chlorhexidine products for both bacterial and yeast dermatitis, although there is less evidence supporting this as a sole agent compared with the miconazole-chlorhexidine combination.¹¹ Lotions containing antifungal ingredients such as clotrimazole or miconazole administered twice daily may also be considered in focal infections.¹¹

Demodex and Sarcoptes

In recent years, infestations by either Demodex and Sarcoptes mites have often been treated with the isoxazoline class, although none is labeled specifically for this indication.³ Many isoxazolines are oral, but in the United States there are topical spot-ons of fluralaner for dogs and cats and of sarolaner for cats.

Benzoyl peroxide shampoos administered twice weekly can be an adjuvant treatment for demodicosis by effectively flushing out mites, but they can be drying.¹ Weekly application of a moxidectin/imidacloprid spot-on has demonstrated safety but also potentially lower efficacy.³

Amitraz dips are the only FDA-approved treatment for canine demodicosis (labeled every 14 days but perhaps more effective weekly), but their use is decreasing due to toxicity risks in both humans and animals.³

For cats, 2% lime sulfur dips once to twice weekly have been used for both Demodex cati and Demodex gatoi, but efficacy was a variable. The idea of bathing cats may be a nonstarter for many clients.

Dermatophytosis

For dogs and cats, dipping in lime sulfur (8 oz diluted in 1 gal of water) twice weekly along with oral antifungal administration and environmental decontamination can be the fastest clinical resolution, in about three weeks.² Lime sulfur must be left on for about an hour until dry and should not be rinsed off. There is a rotten-egg smell associated with this product, and animals with white coats in particular man be temporarily stained yellow following administration.² Young or small animals are susceptible to hypothermia during the drying process, and ingestion may (rarely) lead to oral ulceration, pharyngitis or gastrointestinal upset.²

If lime sulfur is not available or owners request a different option, then biweekly use of a shampoo containing 2% miconazole and 2% chlorhexidine may be effective, but not to the same degree as lime sulfur. The topical should be combined with oral antifungal medications and environmental decontamination.² Chlorhexidine as monotherapy is not effective for the treatment of ringworm.² For focal lesions, clotrimazole and miconazole can be considered. But due to limited data on efficacy, they should not be used as sole therapy. There is limited in vitro evidence demonstrating some efficacy of shampoos containing other antifungals, such as climbazole, terbinafine, and ketoconazole. Similarly, accelerated hydrogen peroxide formulated as a leave-on rinse may be a future adjuvant, but in vivo studies are needed to confirm safety and efficacy.²

Keratinization disorders

Disorders of the superficial layers of the skin, such as ichthyosis, sebaceous adenitis, or primary seborrhea, can cause the skin to be either too dry or too greasy, depending on the condition, resulting in a compromised skin barrier function.¹ Topicals used to address these problems are generally classified by their keratoplastic or keratolytic effect. Keratoplastic activity restores normal rate of keratinocyte division and skin layer turnover, whereas keratolytic activity eliminates excessive layers of skin for scale removal.

When degreasing is needed, once- to twice-weekly benzoyl peroxide shampoos are effective options.¹ In the past, coal tar or selenium disulfide have been used, but they can cause a rebound increase in seborrhea and/or skin irritation when discontinued; both can be toxic to cats. Regardless of the topical of choice, long-haired dogs will need to be shaved to have appropriate shampoo contact with the skin.¹

For disorders leading to excessive drying, emollients that soften and lubricate the skin or moisturizers that increase the water content of superficial skin layers are beneficial. These are also indicated following degreasing therapy to balance and restore the protective barriers of the skin. A wide range of ingredients, such as propylene glycol, mineral oil, lactic acid, urea, and glycerin, can carry out these functions.¹ The authors’ first choice is 0.1% phytosphingosine administered as a shampoo, mousse, and/or spray. Alternatively, spot-on products containing essential oils can be applied to the skin between the shoulder blades once a week for 2 months, then every 2 weeks for maintenance.

There is some evidence that topical polyunsaturated fatty acids, like omega-3 and omega-6, may restore the skin’s barrier. However, they are typically an additional ingredient in some veterinary products, rather than the sole therapy for keratinization disorders.¹² Coconut oil may be popular among owners, either as a topical or oral supplement, but is not recommended because there is no scientific evidence to support its use and it can aggravate secondary infections.

Pruritus

Clinicians should always address the underlying causes of pruritus, such as allergy or infection, but topicals may provide short-term relief to focal and mild lesions. For moderate-to-severe or multifocal-to generalized pruritus, oral anti-inflammatory doses of steroids are more appropriate.¹³ The chronic administration of topical steroids, like OTC hydrocortisone, or even such a soft steroid like mometasone furoate can lead to focal skin atrophy, hyperpigmentation, or delayed wound healing (if applied to traumatized skin).¹³

Nonsteroidal agents, such as colloidal oatmeal or polyunsaturated fatty acids, have only mild efficacy in relieving pruritus.^12,13 They typically provide relief for 24 to 48 hours, do not have any antimicrobial action, and should be offered only as adjunctive therapy for anything beyond mild pruritus.¹² Clinicians may consider offering oatmeal-based shampoos for long-term maintenance. Dogs with atopic dermatitis should be bathed once every other week with this shampoo, which will help remove environmental allergens and support skin barrier function.¹³

Instructing the owner

Efficacy of treatment largely depends on owner and patient compliance. Topical therapies can be more labor-intensive for owners and may not be feasible for some patients. Slow introduction and positive reinforcement is essential for any patient, but particularly for those with a history of anxiety or aggression. It is important to provide clear instructions and to troubleshoot if an owner is having difficulties with topicals. Here are some of the topics you should discuss with the pet owner about treatment.

• Contact time: Many topicals need to be left on the animal for 10 to 15 minutes before rinsing (chlorhexidine, benzoyl peroxide, bleach) or may require up to one hour to completely dry on the animal without rinsing (lime sulfur, amitraz). Animals will need to be monitored during this time or even wear an Elizabethan collar to prevent licking.
• Staining: Bleach, benzoyl peroxide, or lime sulfur may permanently stain towels and furniture before drying. Similarly, chlorhexidine and lime sulfur may temporarily stain the patients’ fur.
• Pet safety: Owners should be instructed to avoid the animal’s eyes and face while bathing or applying topicals. Lesions on the face should be treated only with a cotton ball or makeup pad soaked in lukewarm water to avoid burning or overheating the animal in warmer months or inducing hypothermia in colder months. Drying should be done only with fans or low-temperature hair dryers, as heated air may cause burns and/or inactivate some topicals like bleach.

A handout from your clinic may be helpful to walk a client through the preparations and supplies needed for bathing and the instructions for contact time, rinsing, and drying.

Conclusions

For many superficial dermatologic diseases, veterinarians have the unique advantage of practicing “external medicine” with topicals. If owners are willing to overcome the minor cost and increased time and labor required with topicals, their pets can benefit from a safe, inexpensive, and often effective sole or adjunctive treatment.

Reference

1. Miller WH, Muller GH, Scott DW, Griffin CE, Campbell KL. Muller and Kirk’s Small Animal Dermatology. 7th ed. Saunders; 2013.
2. Moriello KA, Coyner K, Paterson S, Mignon B. Diagnosis and treatment of dermatophytosis in dogs and cats. Vet Dermatol. 2017;28(3):266-e68. doi:10.1111/vde.12440
3. Mueller RS, Bergvall K, Bensignor E, Bond R. A review of topical therapy for skin infections with bacteria and yeast. Vet Dermatol. 2012;23(4):330- 41, e62. doi:10.1111/j.1365-3164.2012.01057.x
4. Albanese F. Canine and Feline Skin Cytology A Comprehensive and Illustrated Guide to the Interpretation of Skin Lesions via Cytological Examination. Springer International Publishing; 2018.
5. Morris DO, Loeffler A, Davis MF, Guardabassi L, Weese JS. Recommendations for approaches to meticillin-resistant staphylococcal infections of small animals: diagnosis, therapeutic considerations and preventative measures. Vet Dermatol. 2017;28(3):306-e69. doi:10.1111/vde.12444
6. Borio S, Colombo S, La Rosa G, et al. Effectiveness of a combined (4% chlorhexidine digluconate shampoo and solution) protocol in MRS and non-MRS canine superficial pyoderma: a randomized, blinded, antibiotic-controlled study. Vet Dermatol. 2015;26(5):339-344.doi:10.1111/ vde.12233Hillier A, Lloyd DH, Weese JS, et al. Guidelines for the diagnosis and antimicrobial therapy of canine superficial bacterial folliculitis (Antimicrobial Guidelines Working Group of the International Society for Companion Animal Infectious Diseases). Vet Dermatol. 2014;25(3):163- e43.doi:10.1111/vde.12118
7. Pariser M, Gard S, Gram D, Schmeitzel L. An in vitro study to determine the minimal bactericidal concentration of sodium hypochlorite (bleach) required to inhibit meticillin-resistant Staphylococcus pseudintermedius strains isolated from canine skin. Vet Dermatol. 2013;24(6):632- e157. doi:10.1111/vde.12079
8. Mueller RS, Rosenkrantz W, Bensignor E, Karaś‐ Tęcza J, Paterson T, Shipstone MA. Diagnosis and treatment of demodicosis in dogs and cats. Vet Dermatol. 2020;31(1):4-e2. doi:10.1111/vde.12806
9. Creech CB, Al-Zubeidi DN, Fritz SA. Prevention of recurrent Staphylococcal skin infections. Infect Dis Clin North Am. 2015;29(3):429-464. doi:10.1016/j. idc.2015.05.007
10. Bond R, Morris DO, Guillot J, et al. Biology, diagnosis and treatment of Malassezia dermatitis in dogs and cats. Vet Dermatol. 2020;31(1):73-77. doi:10.1111/vde.12834
11. Blaskovic M, Rosenkrantz W, Neuber A, SauterLouis C, Mueller R. The effect of a spot-on formulation containing polyunsaturated fatty acids and essential oils on dogs with atopic dermatitis. Vet J. 2014;199(1):39-43. doi:10.1016/j.tvjl.2013.10.024
12. Olivry T, Deboer DJ, Favrot C, et al. Treatment of canine atopic dermatitis: 2015 updated guidelines from the International Committee on Allergic Diseases of Animals (ICADA). BMC Vet Res. 2015;11:210. doi:10.1186/s12917-015-0514