Ralf S. Mueller, DipACVD, FACVSc
Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University
Fort Collins, CO 80523, USA
Conférence présentée en 2003 lors des journées du Groupe de Travail Belge en Dermatologie Vétérinaire
Dermatophytosis in small animal dermatology is an infection with Microsporum or Trichophyton spp. Dermatophytes grow in two ways. In endothrix growth, arthroconidia are formed within the hairshaft (the cuticle is not destroyed), in ectothrix growth the fragmentation of mycelium into arthroconidia happens around the hairshaft and may destroy the cuticle. All dermatophytes of veterinary importance are ectothrix.
Hairs are only susceptible to ectothrix dermatophytes during mid to late anagen, hyphae spread from the perifollicular stratum corneum into and around the hairshaft and descend until they reach the border of the keratogenous zone, but they never enter the nucleated zone. As the hair grows outward, so the fungi continue to grow inward. The terminal tuft of hyphae is located in the “Adamson's fringe”, just above this is the hair's weakest point where it often breaks, when plucked. Once the individual hair enters the telogen phase keratin production slows and eventually stops. The dermatophyte relies on actively growing hairs for survival, so it will either be shed and lost with the telogen hair shaft, wait and infect the new growing anagen hair underlying the telogen hairshaft or spread to surrounding anagen hairs. This spread to adjacent hairshafts can naturally occur at any time.
In animals, Microsporum sp. and Trichophyton sp. are most commonly seen. They are classified as either geophilic (soil loving), zoophilic (animal loving) or anthropophilic (human loving). The zoophilic ones will often cause less of an inflammatory reaction. In fact, infection with Microsporum canis in adult cats is rarely associated with inflammation. Young animals as well as young children are more susceptible to infection with dermatophytes and generally exhibit more of an inflammatory response. Similarly immuno-suppressed individuals, such as AIDS patients, patients with cancer or the elderly and cats affected with FIV, FeLV and cancer are more susceptible.
The mechanical barrier of intact skin and mucous membranes and the fungistatic activity of sebum and sweat are the first line of defence. Proteolytic enzymes and other diffusible substances such as toxins produced by the fungus may penetrate the skin and induce an inflammatory response. Trichophyton spp. can produce proteolytic enzymes that lead to destruction of desmosomes and keratinocyte acantholysis. Some M. canis strains produce enzymes associated with more inflammation and pruritus than others. In some dermatophytes, substances such as mannan have been shown to decrease cell-mediated immunity. In healthy dogs or cats, such dermatophyte infections are typically self limiting and clinical signs resolve within several months. If these animals are skintested with suspensions of the dermatophyte, a delayed and immediate reaction is typically seen, the former is not and the latter rarely observed with previously not infected animals.
If such an inflammatory response is not elicited, it is difficult for the immune system to recognise and eliminate the infection. The cell-mediated immune response is the main cellular defence mechanism in fungal infections, antibody levels do not correlate at all with protection. T-cell-mediated immunity induces activation of macrophages and stimulation of epidermal growth and keratinisation. This is seen clinically as increased scale production and in some more extreme cases as adherent hyperkeratotic outgrowths. Neutrophils can attach to the hyphae and pseudohyphae of invasive opportunistic fungi such as Candida and damage them via extracellular mechanisms. Fungal spores may be phagocytosed by macrophages. Increased rate of elimination of a fungus during reinfection may be due to immunocompetent memory cells remaining at that site in the skin after the primary infection.
An apathogenic, modified life vaccine of T. verrucosum has been used prophylactically in cattle in the Soviet Union and Scandinavia. Intramuscular vaccination of calves at one and three weeks of age conferred nearly 100% protection. Localised injection reactions and fever were seen, a few calves had diarrhoea, dyspnoea and anaphylactoid reactions. Protection (only against T. verrucosum!!) lasted 4-5 years. Fungal vaccines have also been useful in controlling endemic dermatophytoses in foxes. The goal of the vaccination is induction of strong cell-mediated immunity. Studies with a killed M. canis vaccine at the University of Wisconsin have yielded high titers of M. canis IgG antibodies and increased lymphoblastogenesis responses to M. canis antigen. Although the antibody titers were as high as those produced with normal infection, the lymphoblastogenesis increase was less pronounced. All of the vaccinated cats showed clinical signs of dermatophytosis and had positive fungal cultures after an M. canis infected cat was introduced to the colony.
A Boehringer Ingelheim product is in development (Insol Dermatophyton). It contains microconidia of a number of Trichophyton and Microsporum spp. including T. mentagrophytes, M. canis and M. gypseum, and thus could be suited for dogs and cats. Two intramuscular administrations are given two weeks apart every 9 months. The intramuscular administration is similar to the bovine vaccine. Adverse effects reported include swelling and pain at the injection site in <3% of dogs and <0.3% in cats and systemic reactions in <0.4% of dogs.