The skin of the horse, like other organs, is the object of numerous pathological processes: infectious (bacterial and viral), parasitic, fungal, cancerous … but it has, compared to the deep organs, a special situation due to two characteristics: that of being in permanent contact with the external environment, consequently being exposed to multiple aggressions and pathogenic agents; that of being considered as “the mirror of the organism” of the horses because it is immediately visible and likely to easily provide samples while being the seat of lesions which can be the consequence or the expression of deep, “internal” pathological processes. This particular situation gives dermatology an essential and original place within the medical disciplines: if on the one hand, cutaneous semiology (i.e., the study of signs, in this case skin lesions) is apparently simpler and complementary investigations easier, on the other hand, the reasons for dermatological consultation are frequent, the clinical cases being difficult and coming under internal medicine rather than dermatology.
The different types of skin disease in horses
Skin lesions are very varied. Their observation and recognition are the basis of the consultation to identify the pathological process responsible. Their identification is based on observation and palpation or touch. We can meet:
- lesions of discoloration (vitiligo) or pigmentation of the skin (macule). The skin can present extravasations of blood (purpura), large spots with black pigmentation (melanization patches) or be the site of a congestive phenomenon giving a uniform red color (erythema);
- lesions in relief deforming the skin, isolable between the fingers, voluminous or small, single or multiple, firm or deformable, full or containing liquid (lymph, blood) or pus, colored or depigmented, eroded (loss of superficial substance not going beyond the epidermis) or ulcerated (attack of the dermis), dry or wet, exudative, bloody, fatty (seborrhea), malodorous;
- squamous lesions (abnormal and increased production of “films”, epidermal cells attached to each other), scabby (result of a process of desiccation of a liquid on the surface), fibrous or scarring, old, chronic or lichenified (very thickened, hairless, wrinkled skin);
The collection of memorabilia (history of the animal, in-depth knowledge of its past and lifestyle), of the anamnesis (history of the disease: age, evolution, previous treatments…) and of the previously described cutaneous lesions and of the possible general symptoms allow the veterinarian to consider clinical hypotheses which he will confirm (or deny) by appropriate complementary examinations. The practitioner is indeed confronted with many causes or factors responsible for skin diseases, which constitute the etiology.
A common skin disease in horses: dermatophilosis
Dermatophilosis is a bacterial dermatosis characterized by the production of greenish pus agglutinating the hair and evolving to alopecia; it is a pyoderma caused by a filamentous bacterium Dermatophilus congolensis and the conjunction of several factors.
It occurs in horses exposed to repeated rainfall over several weeks or to excessive humidity: the long persistent coat favors a certain amount of water retention, especially on the upper body line, the site of the skin lesions. It is contagious in that it occurs in several individuals living together and subject to the same conditions. In fact, the presence of biting insects (hematophagous flies: horse flies, “spider flies”, mosquitoes…), which are abundant in the spring or during the humid summer, provokes the appearance of bite wounds in exposed horses, which are the gateway to the bacteria.
The lesions are quite characteristic: clumps of hair, which after falling off, leave a greenish, thick, sticky area of suppuration, located on areas where water is retained (croups, backs, upper parts of the flanks, cannons of horses trained or kept in areas with wet, long, and thorny grass). These lesions do not cause itching or rubbing sores and the general condition of the animal is preserved (unless the condition was previously so profoundly degraded as to lead to a secondary complication of dermatophilosis). They then evolve towards the formation of crusts and cracks. The diagnosis is easy and can be confirmed by microscopic observation of the filamentous bacteria.
Dermatophilosis is an example of a multifactorial dermatosis, the rigorous analysis of which makes it possible to define the rules of treatment and protection: return of the animals to stalls or grazing areas with shelter, shearing of the upper regions of the body, cleaning with non-irritating diluted antiseptic solutions, general administration of antibiotics in the case of very extensive or very old lesions.
Summer scabies” or recurrent summer dermatitis in horses
Is a pruritic, crusty, alopeciating, sometimes disabling, summer dermatitis, recurrent and affecting certain horses from certain bloodlines: it is the result of allergy mechanisms induced by a hematophagous midge of the Culicoides genus. This insect bites the animal to make its blood meal. To do this, it inoculates “a saliva” that induces allergy phenomena, i.e., exacerbated, spectacular skin reactions, caused by substances that are a priori harmless but repeatedly inoculated, thus provoking pathological reactions from the immune system. The treatment implies to put the animal in the box and to use corticosteroids.
Important steps to take in the presence of skin disease in horses
When confronted with skin lesions in a horse, it is useful to follow some basic rules: cut the hair, clean when the skin is broken with soap or a non-irritating diluted antiseptic, make sure that the harness is not injuring or responsible for the lesion, check the validity of the tetanus vaccination, do not apply ointment or cream containing corticoids, seek medical advice at the slightest doubt or when the initial lesion extends or does not heal.
Contrary to what we hear every day in consultation: “veterinary medicine is difficult because patients do not speak and cannot say where they hurt”, this apparent handicap is in fact an advantage because the silent expression of our patients is often much more significant and useful than some long speeches, often tinged with anthropomorphism and whose arguments bring little or nothing to the establishment of a diagnosis. Although there may be a few cases of behavioral disorders in animals, our patients do not suffer from imaginary, artificial, simulated, or even self-inflicted illnesses, as is the case in humans!
