L3 - Skin Infections

How does the stratum corneum provide a physical barrier?

Inert and impermeable

How does the temperature and pH of skin provide a physical barrier?

Too cold and unfavorable pH for optimal growth

What are the chemical barriers of skin?

• FA → bacteriostatic

• Inorganic salts → high salt concentration

• Transferrins → bind iron

Skin normal flora is comprised of…

Resident and Transient flora

What is resident flora? Examples.

• Live and multiply on the skin (obligate parasites)

• Can’t be eliminated → permanent population

• Harmless

Ex. coagulase-negative Staphylococcus, alpha-hemolytic Streptococcus, Micrococcus spp.

What is transient flora? Examples.

• From environment/mucus membranes

• Do not multiply well on skin

• Can be removed/eliminated

• May be involved in pathologic process

Ex. Coagulase-positive Staphylococcus, E. coli, Proteus

Coagulase-positive Staphylococcus is what kind of flora?

Transient

Coagulase-negative Staphylococcus is what kind of flora?

Resident

What are primary skin infections?

• Bacteria causes most of the pathology → single species dominant

• Predisposing factors either aren’t there or play a minor role

• Characteristic disease pattern

• Antibacterial therapy is effective → underlying problem less significant

Examples of primary bacterial skin infections

Greasy pig disease and Dermatophilosis

What are secondary skin infections?

• Bacterial infections associated with other infections or conditions

• Predisposing conditions

Examples of predisposing conditions in secondary bacterial skin infections?

• Skin infection with parasites, viruses, fungi etc.

• Local factors (i.e moisture, skin folds etc.)

• Systemic disease (hypothyroidism, Cushing’s)

• Physical/Chemical trauma

• Immunosuppression

Diagnosis of bacterial skin infections

1. Distinguish between colonization and infection

2. ID bacteria in primary and secondary infection with consideration to normal flora

Colonization vs. Infection

Colonization: present, but not causing dz

Infection: present, associated with pathology

How do we determine if a bacterial infection is present and significant?

• History (predisposing/underlying factors)

• Clinical signs (postules, exudate, crusts)

• Baterial isolates (virulence)

What are the goals of bacterial infection diagnosis?

1. Diagnose the underlying problem

2. Determine the type of bacteria

When sampling bacterial cultures, what shouldn’t you do?

Don’t sample open tracts or erosions (use unopened pustule)

Punch biopsy of lesion

What are gram stains?

• Looks at numbers and bacteria types

• Presumptive ID

What is another name for Wright stains?

Diff-Quik Stains

When should we biopsy?

If dermatosis is not responding to treatment for 3 weeks or any recurrent dermatosis

What shows the severity of skin infections/pyodermas?

• Depth of skin infection

• Deeper = more likely to be a serious problem

What type of pyoderma is in the epidermis only?

Surface pyodermas

What type of pyoderma is in the skin and hair follicles?

Superficial pyoderma

What type of pyoderma is in the SQ layer?

Deep pyoderma

What staphylococcal sp. is likely to cause skin lesions in dogs?

S. pseudintermedius

What are the causes of deep pyodermas? Are they common? Hard to treat?

• Almost always secondary to other contributing factors

• Not common

• VERY hard to treat!

What are the causes of superficial pyodermas? Are they common? Hard to treat?

• Pustules (mini abscesses)

• Usually secondary to other diseases

• Recurrence is common → hard to manage long-term

What are the causes of surface pyodermas?

• Consequence of self-trauma or allergic skin dz

• Hot spots, early skin fold dermatitis

What type of stain determines the presence and type of inflammatory cells?

Wright Stains

What is evidence of infection in Wright Stains?

Neutrophils with bacteria within the cells