The hair coat and skin harbor many organisms – good and bad. There is a normal "environment" of the skin, in which "good" bacteria occupy regions or "niches" preventing "bad" bacteria from infecting. When the skin is abnormal (e.g. genetics or disease), the normal milieu of the skin changes. "Bad" bacteria and yeast can grab hold and colonize the skin, producing symptoms and/or worsening other conditions. Most skin infections are secondary problems, developing from underlying allergy, hormonal or metabolic abnormalities, ectoparasites, or immune-mediated dermatoses. Unless and until the primary skin disease is identified and resolved, secondary infections will continue and recur. Identifying the underlying cause of the skin infection as well as resolving the infection is the role of the veterinary dermatologist.
There are three main categories organisms that cause skin infections: bacteria, yeast, and fungi (a fourth, viral, is a very rare cause of dermatitis in companion animals).
Bacterial Skin Infection
Bacterial skin infections (pyoderma) are very common in the dog, common in the horse, and uncommon in the cat. As mentioned earlier, most bacterial skin infections are actually secondary developments from another underlying cause. Symptoms of bacterial skin infection include pustules, crusts, hair loss (often in "paintbrush-like" clumps), and redness. The classical lesion of bacterial skin infection in the dog is the epidermal collarette. This lesion is circular, often with a central area of hair loss and a border of redness and crusting (envision a pebble tossed onto smooth water and the gradually expanding rings that form.) A rancid odor may be present as the over abundance of bacteria on the skin break down oils into rancid fats.
Treatment of pyoderma requires the appropriate use of antibiotics (type and treatment length), topical antiseptic therapy, and identification and treatment of the underlying cause for the infection.
Yeast Skin Infection
Infections of the skin due to yeast organisms occur less commonly than to bacteria. More often, yeast infections occur in skin folds (face, armpits, tail), but can spread over the entire body. Yeast infections may look different than bacterial infections. Yeast infections cause more greasiness, more redness, and more thickening of the skin than bacterial infections. They can also be much more odiferous. However, the only way to distinguish a skin infection due to bacteria or to yeast (or both), is to examine debris from the skin under a microscope. The veterinary dermatologist is expertly skilled in interpreting these laboratory preparations.
Yeast dermatitis is treated by the use of anti-yeast medications and topical anti-yeast therapy. Frequent bathing is important for both bacterial and yeast skin infections to cleanse the skin and reduce the number of organisms on the skin. With proper treatment – which must include identification and control of the underlying cause – the normal environment of the skin can be reestablished.
Fungal Skin Infection
Fungal infections are rarely a cause of itching in the dog and cat, and are not associated frequently with allergy. There are many fungal organisms that cause infections on, in, or beneath the skin. Thankfully, most are uncommon to rare. However, one type of fungal skin infection, ringworm, is more common – and potentially contagious to human beings. Ringworm – dermatophytosis – is the infection of the top, dead layers of the skin and of the hair shafts. The name, "ringworm," describes the lesions in human beings. Human lesions are typically slowly expanding "rings" of redness and scaling. The lesions look considerably different in the dog and cat than they do in human beings.
There are three main fungal organisms – dermatophytes – that cause ringworm in dogs and cats. The two less common, Trichophyton mentagrophytes and Microsporum gypseum, prefer soil, and more most often contracted from digging or rodent burrows. The most common dermatophyte, Microsporum canis, is more adapted to animals, and can particularly reside on cats without obvious signs.
Ringworm lesions in dogs and cats can look like the classical round lesions of human beings; more often they don't. Ringworm has been called one of the "great imitators." It must always be included on the list of possibilities for hair loss and scaling in the dog and cat. (Sometimes the silly phrase, "if it looks like ringworm, it isn't. But if it doesn't look like ringworm, it could be," is very true.)
To diagnose ringworm, the veterinary dermatologist will identify the pattern of lesions and select appropriate samples of hair and scale to imbed into special ringworm culture test media. This media is specifically designed to encourage the growth of dermatophytes and discourage the growth of other fungi and of bacteria. The media also includes a dye that will change color if a dermatophyte grows. A simple color change in the test media is not diagnostic of ringworm. The veterinary dermatologist must assess the appearance of the fungal colony growth, and then examine the organism under a microscope to determine if it indeed is a dermatophyte and which one it is. This is very important for the determination of treatment.
Treatment of ringworm requires oral antifungal medications, topical antifungal preparations, and environmental cleaning. Microsporum canis is particularly hearty, and can live in dander and hair for up to two years in the house – enabling re-infection of companion animals and people.