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3. Differential Diagnoses

Based upon the history provided by Junior’s owner, you believe that the bumps and scabs are signs of a secondary superficial pyoderma, and you begin to formulate a list of differential diagnoses in your mind that could predispose a dog to develop superficial pyoderma.




Why do certain diseases/disorders predispose to infection?

1.  Pruritic disorders:

When a dog has itchy skin disorders such as allergic or parasitic diseases, the scratching, chewing, and/or biting will mechanically remove the stratum corneum, which is a very important skin barrier.  Moreover, the process of chewing will result in the inoculation of Staphylococcus intermedius (which lives normally in the mouth, nares, and anal ring of dogs) into the skin.

2. Hormonal disorders:

Hypothyroidism and hyperadrenocorticism (i.e., Cushing’s disease) are not pruritic diseases, but are often complicated by secondary skin infections that can become pruritic.  The lack of thyroid hormone, which is very important to maintain the normal metabolism of almost all cells in the body, will result in abnormal function of the skin immune defense.  In Cushing’s disease the state of chronic excess cortisol secretion will lead to skin atrophy (the “brick wall” becomes very thin), and to suppression of the normal immune defense.

3. Keratinization disorders:

One of the main functions of the skin is to produce keratin and special lipids to form the stratum corneum, which is composed of completely keratinized epithelial cells (also known as corneocytes) and an intercellular lipid lamellae.  A brick wall is a perfect analogy for the stratum corneum, where the brick is the corneocyte and the mortar is the lipid lamellae.  Moreover, a brick wall is a barrier against the entrance of various invaders, just as the stratum corneum is a barrier against microbial, chemical, and physical insults.  Keratinization disorders can be primary or secondary and are associated with abnormalities of the stratum corneum.  Therefore, animals with keratinization disorders are very prone to develop secondary skin infections.  Keratinization disorders are non-pruritic disorders, but pruritus often follows from the secondary infections.


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What can predispose a dog to develop superficial pyoderma?

Several diseases/disorders can predispose animals to developing superficial pyoderma.  This predisposition is often due to conditions that impair defense mechanisms of the skin.  The skin has innate defense mechanisms which help defend against chemical, physical and microbial insults.  Some examples of these defense mechanisms include the stratum corneum, cells that resemble a brick wall; fatty acids that have a bacteriostatic effect; defensins, which are small antimicrobial peptides; and resident bacterial flora, which will compete with the pathogenic flora for space and nutrients.  The skin normally defends itself well; therefore, it is smart to always consider a bacterial skin infection secondary until it is proven otherwise.