Dermatophytes in Horses: Infectious Etiology, Diagnosis, and Intervention
Rachna poonia1, Sumnil Marwaha2
Introduction
Dermatophytosis (ringworm) is a superficial cutaneous fungal infection caused by keratinophilic fungi that are able to invade the stratum corneum of the skin, the hairs and other keratinized structures.
Etiology
Fungi of the genera Microsporum and Trichophyton cause animal dermatophytoses. The most common equine dermatophyte species isolated from horses are Trichophyton equinum, M. canis, M. equinum, M.gypseum, T. mentagrophytes, T. bullosum and T. verrucosum. Trichophyton equinum and M. canis : ‘ringworm’ in horses, particularly in young animals T. mentagrophytes or M. gypseum :isolated from skin lesions T. bullosum and M. praecox from healthy animals and the surrounding environment.
Transmission
By direct contact : with diseased animals or asymptomatic carriers. Horse (e.g., T. equinum), a cat (e.g., M. canis), cattle (e.g., T. verrucosum), or a rodent(e.g., T. mentagrophytes).Spores can exist on the skin without causing lesions, and up to 20% of normal animals in an infected group will act as ‘carrier animals’.Indirect contact with inanimate objects, particularly bedding, harness, grooming kits and horse blankets and from the environment (e.g., M. gypseum).
Epidemiology
Environmental risk factors: Although dermatophytosis is worldwide in distribution, it is more prevalent in hot humid climates than in cold dry regions. Host risk factors: Depend on immunologic status.Young animals (less than 2year) are more susesptible. Skin ph also plays important role. Acidic pH is less susceptible. Young animals have high pH thus more susceptible. Traumatic injuries to skin also increase the risk.
Pathogen factors: Dermatophytes produce enzymes mainly keratinases (enable invasion of the hair and the stratum corneum Trichophyton equinum produces urease, gelatinase, protease, hemolysins, and keratinase. Stronger the enzymatic activity greater will be the inflammation. E.g., T. mentagrophytes causes severe inflammatory reaction as compared to T. equinum. Hemolytic activity and the ability to induce hypersensitivity reactions are also important virulence factors, especially for Trichophyton spp.
Economic importance
Lesions causes discomfort and are unsighty affecting esthetic appearance. Prevent the horses from working and interferes with their use in polo, racing, and riding because the horse will not be allowed at shows or other events (because it can transmit it to other horses), thus decreasing the cost value of the horse.
Pathogenesis
Fungi chiefly attack keratinized tissues, particularly the stratum corneum and hair fibers. Autolysis of the fiber structure, breaking off of the hair, and alopecia. Exudation from invaded epithelial layers, epithelial debris and fungal hyphae produce the dry crusts – characteristic of the disease. The lesions progress if suitable environmental conditions for myceliagrowth exist (warm humid atmosphere and a alkaline pH of skin). Ringworm fungi are all strict aerobes and the fungi die out under the crust in the center of most lesions, leaving only the periphery active.Centrifugal progression and the characteristic ring form of the lesions.
Immunity to reinfection: A resistance to reinfection occurs. But a local mycotic dermatitis may occur at the reinfection site. Immunity is specific to the fungal species. Lasts up to 2 years.
Site of lesions: Commonly present on the face and neck Dorsolateral thorax Girth (“girth itch”) Legs: less commonly affected. Lesions may be limited to the caudal pastern region (“scratches,” “mud fever,“grease heel”) The mane and tail are rarely affected.
Clinical signs
The initial lesions are often tufted papules, 2-5 mm in diameter. Early lesions may also appear as erect hairs in annular areas of 5-20 mm diameter. Occasionally, an urticarial-like eruption will precede the more obvious follicular dermatosis by 24-72 h. Lesions may be superficial or deep. Classically, a circular area of alopecia and scaling with aerythematous margin is evident. Superficial infections are much more common and are manifested by the development of thick crusts, or more generally a diffuse moth-eaten appearance with desquamation and alopecia. Pruritus is usually minimal to absent In horses with acantholytic dermatophytosis or those with secondary bacterial infections, erosions, epidermal collarettes, suppurative exudate, or rare pustules may bepresent.
Diagnosis
Wood’s lamp, can be used as a screening tool, False negatives are common because not all dermatophytes fluoresce. Most effective as a monitoring tool when a fluorescing strain of M canis (M equinum) is known to be present ia group or on an individual. Fluorescence is best observed during early or active infection, when the entire hair shaft will fluoresce.
- Microscopic examination of plucked hairs may reveal hyphae and arthrospores in 54-64% of the cases.
- Fungal culture: Dermatophyte test medium ,Sabouraud dextrose agar (Marsella, 2014)
- Biopsy: infiltrative lymphocytic mural folliculitis and suppurative luminal folliculitis, pyogranulomatous furunculosis,fungal elements are often most easily found in surface scale,crust, and hair fragments
Treatment
Topical therapy:
Creams and lotions are available for use on focal lesions, and are typically applied every 12 h. Multifocal or generalized skin involvement, antifungal rinses(dips) are indicated. Rinses are preferred because the entire body surface can be treated, rubbing of the hair coat is minimized, and the antifungal agent can be allowed to dry on the skin. Rinses are usually applied daily for 5 to 7 days, and then once or twice weekly until clinical cure. Lime sulfur 2-5% and enilconazole 0.2% are the most effective.
https://www.pashudhanpraharee.com/lumpy-skin-disease/
https://www.msdvetmanual.com/integumentary-system/dermatophytosis/dermatophytosis-in-horses
