Ringworm

Etiology

• Trichophyton verrucosum  Trichophyton spp zoophilic, by far the most common isolate from cases of dermatophytosis in cattle, fomites are important in transmission
• Trichophyton mentagrophytes found in soil though nor geophilic, wild animal reservoir in foxes and rodents
• Microsporum canis zoophilic uncommon.
• Trichophyton equinum zoophilic uncommon.
• Microsporum nanum geophilic.
• Microsporum gypseum geophilic.

Predisposing factors

General

• Particularly calves under 1 year of age.
• Housed animals most at risk with darkness, and moisture both predisposing factors.
• Groups of calves loose-housed in covered yards characteristically affected.
• Long-lived nature of dermatophyte spores leads to infection of susceptible stock year on year.

Pathophysiology

• Spores are picked up from other infected cattle by direct contact or by contact with fomites.
• Spores may be present in soil or via wild rodents and other animal species eg T. mentagrophytes.
• Having been acquired, fungi penetrate to the hair follicles and the stratum corneum.
• Infection is restricted to keratin-bearing tissues.
• Fungi multiply rapidly leading to congestion of the skin by perivascular accumulation of monocytes.
• Skin becomes inflamed and swollen and hair stands up on end.
• Arthrospores are formed along the hair shaft (ectothrix) and also inside the hair shaft (endothrix).
• Hairs break off and small circular alopecic patches appear.
• Alopecic patches become red and inflamed as inflammation progresses, skin becomes moist and scales and crusts appear, composed of epithelial cells, serum exudate and fungal hyphae.
• Crusts eventually fall off and as the infection never reaches the mitotic region of the hair follicle once the old dead hair is shed, hair follicles produce new hairs.

Timecourse

• Initial incubation period is 7-14 days.
• Clinical signs start at days 14-28.
• Crust formation days 28-45.
• Regression days 45-60.

Epidemiology

• Youngstock are most susceptible not as a result of age per se but due to a lack of immunity. Hence older stock may show signs of disease if they are naive to infection.
• Sale yards, introduction of new unquarantined stock may both predispose.
• Winter group housing of youngstock in overcrowded, humid, dark conditions predisposes to infection.
• Poor hygiene between successive groups allows infection to persist from year to year.
• Spores may be viable within the environment for up to 5 months and may remain viable in skin scale for up to 4 years.