and help diagnose the more complex cases. While it sounds invasive, horses tolerate the procedure very well and the samples are so small that no scar is leſt . Sometimes, however, veterinary assistance is not an option and even
if we do collect diagnostic samples, we have to wait days and some- times weeks for the results. In the meantime, we must move forward with treatment to provide our horses with relief. Because of this, it is important that we know the most common causes of dermatitis and how they are treated so we can form an empirical plan to resolve the problem. By learning about common skin diseases, we will discover that not every case of itchy, scabby skin is caused by fungus and that fungal infections are overdiagnosed, which may be why that anti-fungal cream you’ve been using isn’t get ing the job done.
DERMATOPHILOSIS, AKA RAINROT It is a common misconception that rainrot is due to a fungal infec-
tion. T e culprit is a bacteria—Dermatophilus congolensis. T is bacteria exists in zoospores and hyphae, which can oſt en look very similar to fungi under a microscope. Infection with this bacterium is highly dependent on host (i.e. horse) factors. Any condition that weakens the horse’s natural immune system will leave them susceptible to rainrot. Poor general health and nutritional plane play a factor as well as chronic disease states such as Cushing’s Disease in elderly horses. T e other key factor in the development of rainrot is moisture.
Dermatophilus loves warm, moist environments, which is why we see cases of rainrot crop up in the spring and during mild, wet winters. Ad- ditionally trauma to the skin opens the doors to this invading pathogen. Horses that are constantly bombarded by biting fl ies (again, in the moist springtime) are much more likely to be aff ected. Rainrot does have a classic presentation that can help us diagnose it,
though remember, without a culture you can never be sure. It is found on the rump, across the topline and on the face. In horses that are in work, larger lesions can be found under the saddle pad or tack (where the heat and moisture from sweat are concentrated. T e lesions begin as papules that mature to pustules that ooze serum and cause mat ing of the hair around the site. T e mat ed tuſt s of hair can easily be plucked out, bringing the
underlying skin with it in what we call a “paintbrush lesion.” T e area underneath this removed scab is raw, eroded and oſt en bleeds. When these crusted areas are stained and looked at under a microscope, the bacterial hyphae have a “railroad track” appearance making a presump- tive diagnosis easy in these cases. T e key to treatment of rainrot is to keep the animal dry! Oſt entimes
this tactic alone will lead to spontaneous regression of the lesions. We also must consider the general condition of the horse and if there is a nutritional or medical problem that must be addressed. T e skin should be washed with anti-bacterial and keratolyic (designed to soſt en and loosen crusts) shampoo. Products containing benzoyl peroxide are particularly eff ective.
It is important to allow the lather enough contact time (10 minutes) to work and because the skin under the scabs is raw and sensitive, ag- gressive scrubbing is discouraged. Many of these cases can be treated with topical therapy alone but for severe cases and horses who have underlying immune defi ciencies, penicillin or trimethoprim-sulfa antibiotics can help speed recovery.
BACTERIAL FOLLICULITIS T ese crusting lesions oſt en start as papules (solid circular eleva-
tions in the skin) and progress to scales. T ese superfi cial infections are found in the epidermis and upper levels of the dermis and are caused
by Staphylococcal bacteria such as Staph. aureus, Staph. intermedius and Staph. hyicus. As staphylococcal bacterial are ever present on the skin, these infections only take hold once the skin has been damaged. T erefore we see this dermatitis around abrasions, wounds, surgical incisions and cellulitis. Ideally, treatment should be based on the culture and antimicrobial
sensitivity results so that the right drug is picked for the bug. However, in the absence of a sensitivity pat ern, topical therapy should include dilute 0.2% chlorhexidine scrub that is lathered and allowed to sit for 5-10 minutes. It is the length of the contact time between shampoo and bacteria that accounts for the killing of the infectious agent, not the amount of elbow grease put into the scrubbing. Topical creams should be antibacterial, such as silver sulfadiazine
(SSD). In most cases topical therapy will suffice but in severe or protracted cases, systemic (whole body) treatment may be required. Recent veterinary literature recommends trimethoprim-sulfa or a drug from the cephalosporin class as most of the Staph we encounter is resistant to penicillin.
DERMATOPHYTOSIS, AKA RINGWORM Ringworm is the most common fungal dermatitis in horses and
notice it is the fi rst time we’ve mentioned a fungus so far. Dermato- phytosis (not to be confused the bacterial disease dermatophilosis) is caused by fungi of the Trichophyton and Microsporum species and are highly contagious and easily passed from horse to horse. T ese fungi possess keratinases, enzymes that help them penetrate deep into the skin. T ey also have the ability to live in the environment (on brushes, tack and equipment) for up to 12 months. Individual factors play a role in the development of ringworm.
Younger horses (<3 years) are more susceptible to the disease as their immune systems are still developing. Horses with concurrent illness or immunosuppression are also at a higher risk. Like many fungi, ringworm prefers environments with high temperature and humidity. Ringworm forms circular patches of alopecia (hair loss) and crust-
ing. T e areas of hair loss are not red, raw or sensitive. T ese horses may be itchy but not in all cases. Lesions are most commonly seen on the face and in the girth region, corresponding with areas that are contacted by tack or contaminated hands. Diagnosis is typically made based on
WWW.TRAILBLAZERMAGAZINE.US • June 2013 | 69 continued next page
Dermatophilo- sis aka rainrot in a horse fl our- ishes under wet conditions.
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