16.3 TopicalAntifungals
A wide variety of topical agents belonging to different classes of antifungals are available as creams, ointments, gels, lotions, powders, shampoos, and other formulations, which may differ widely in their efficacy.
Of note, topical antifungal compounds alone can reduce the transmission of spores (Brammer et al. 1990).Topical antifungals can be applied to the skin, nails, or mucosa to kill or inactivate fungi. Regardless of the actual mechanism of action of the drug, or the viscosity, hydrophobicity, and acidity of the formulation, the drug's ability to penetrate or permeate deeper skin layers is an important property impacting the therapeutic efficacy of topical antifungals (Trommer and Neubert 2006; Gupta and Cooper 2008; Sugiura et al. 2014).
Natamycin is an allylamine antifungal agent licensed for veterinary use only for cattle and horses. It has a moderate activity against dermatophytes, yeasts, and Aspergillus. It can also be used for environmental decontamination (Rochette et al. 2003). For a successful treatment, twice whole-body spray (0.1%) treatment of natamycin in an interval of 3 days is recommended in cattle naturally infected with T. verrucosum (Oldenkamp and Spanoghe 1977; Oldenkamp 1979a). Natamycin is used topically in the horse and is reported as being effective in successful elimination of dermatophytosis within 4 weeks. After treatment the recovered animals did not show any evidence of reinfection for up to 6 months (Oldenkamp 1979b).
Clotrimazole, miconazole, and enilconazole are external imidazoles. Because of a high first-pass effect, the topical antifungals show minimal systemic bioavailability and are therefore confined to topical use (Conte et al. 1992). Miconazole, chlorhexi- dine, and selenium sulfide were components of the treatment protocol for Malassezia dermatitis in dogs in several randomized and controlled trials.
Miconazole in combination with polymyxin B and prednisolone is licensed for use in dogs and cats and is included in an otic preparation for treatment of otitis externa (Cornelissen and Van den Bossche 1983). A shampoo containing 2% miconazole nitrate + 2% chlorhexidine, when used twice a week for 3 weeks, has been demonstrated to be effective for treatment of Malassezia dermatitis in dogs (Negre et al. 2009).Clotrimazole has been reported to be effective for the treatment of localized dermatophytosis (McCurdy et al. 1981) and sinonasal aspergillosis in dogs and cats (Peeters and Clercx 2007; Furrow and Groman 2009). Like miconazole, it is commonly used against Malassezia otitis in dogs and cats in combination with an antibiotic and an anti-inflammatory agent (Bensignor and Grandemange 2006).
Enilconazole is an imidazole with broad-spectrum activity against dermatophytes (Microsporum sp., Trichophyton sp.). It has activity against Aspergillus spp., Peni- cillium spp. and yeasts, including Candida spp. In veterinary medicine, enilconazole is used as a topical antimycotic (4 mg/kg bw) against dermatophytes in cattle (Dekeyser 1981), horses (Mayer 1983), dogs, and cats (Hnilica et al. 2000). Enilconazole can be applied topically for treatment of dermatophytosis and aspergillosis. It has been used safely in cats, dogs, cattle, horses, and chickens and is prepared as a 0.2% solution for treatment of fungal skin infection. Treatment consists of four whole-body applications at 3-4-day intervals. Enilconazole can be applied topically for sinus irrigation to treat canine nasal aspergillosis (Benitah 2006). In several countries, it is also used as a fungicide formulation (smoke generator or emulsifiable concentrate) for the disinfecting of farm buildings including poultry houses and rabbit farms, which showed highly effective against spores of dermatophytes and Aspergillus spp. (Van Cutsem et al. 1985, 1988).
The anthelmintic thiabendazole is also an imidazole with potent efficacy in the treatment of bovine dermatophytosis caused by T. verrucosum and superficial yeast infections in animals. Thiabendazole has been licensed for use in dogs and cats as ingredient of topical antifungals (Gabal 1986).