Domestic mammals have guard hairs produced by central and lateral primary hair follicles. These produce the guard hairs and determine coat colour. Secondary hairs produce the undercoat or wool hair. Secondary follicles have neither apocrine glands or arrector pili muscles.
Hair follices have a permanent upper part of infundibulum and isthmus (with tricholemmal keratinization). The lower part develops and has an upper suprabasalar part and the bulb.
Primary alopecia is when there is a follicle targeted disease.
A primary inflammatory alopecia is a follicularly targeted disease. It could be infectious or immune mediated. See the folliculitis furunculosis sebaceous adenitis pattern of dermatitis
Typically these are bilaterally symetrical, seldom head and paws, and seldom motheaten or with pruritis.
The animal in this circumstance is born with alopecia.
Viral infections - en utero Pestivirus
These diseases typically occur in young animals
Congenital failure of organogenesis
Impaired formation of hair
follicular and hairshaft issues
These diseases are typically in older animals.
Endocrine
Idiopathic
telogen effuvium
Anagen alopecias
Secondary alopecia is when there is a disease that does not target the follicles but result in alopecia.
Folliculitis and furunculosis
Self trauma
Wounds and burns and scarring