2. Primary lesions are directly associated
with the disease process.
 They are not pathognomonic, but give a
valuable clue as to the type of disease
process occurring

3. Macules are flat areas of discoloration up to
1 cm (0.4 in) in diameter,
whereas patches are larger than 1 cm in diameter.
Pathogenesis
Changes can involve increased blood flow
(erythema), extravasated blood (hemorrhagic
petechiae and ecchymoses), or pigment changes.
Erythema can be distinguished from hemorrgage by
blanching on digital pressure

5.  Papules are small, solid, elevated lesions
up to 1 cm (0.4 in) in diameter associated
with cell infiltration and/or proliferation, in this
case a mast cell tumor.

6.  A plaque is a flat, solid, elevated lesion of
more than 1 cm (0.4 in) in diameter, again
associated with cell infiltration and/or
proliferation.
 The lesions illustrated are eosinophilic
plaques

7. A nodule is a solid elevation of the skin
greater than 1 cm (0.4 in) in diameter,
again associated with cell infiltration and/or
proliferation.
 The nodule illustrated is a mast cell tumor
on the abdomen of a dog.
 A tumor is a large nodule,although not
necessarily neoplastic.

9. A tumor is a large growth. A lipoma on the
flank of a dog is illustrated.

10. A pustule is a small circumscribed skin
elevation containing purulent material.
This consists of degenerate inflammatory
cells (most commonly neutrophils) with or
without microorganisms or other cells (e.g.
acanthocytes in pemphigus foliaceus).
Pustules and vesicles rapidly rupture in
dogs and cats, leaving epidermal
collarettes and crusts

12.  A vesicle is a circumscribed elevation of the
skin
 up to 1 cm (0.4 in) in diameter filled with serum.
 The illustrated vesicle occurred on this veterinary
nurse’s arm within minutes of a flea
 bite.
 A bulla is a vesicular lesion greater than 1 cm
 (0.4 in) in diameter.

13. A wheal is an irregular elevated
edematous skin area that often changes in
size and shape.
The wheals in this case were acute,
transient, and of unknown etiology.

14. A cyst is an enclosed cavity with a
membranous lining that contains liquid or
semi-solid matter.
The illustration is of a cystic basal cell
tumor on the head of a dog.

15. Secondary lesions are a result of trauma,
time, and degree of insult to the skin.
Often, primary lesions evolve into
secondary lesions.
Thus papules become pustules, which
become focal encrustations, ofter
hyperpigmented.
Secondary lesions are much less specific
than primary lesions.

16. Comedones are the result of sebaceous and
epidermal debris blocking a follicle. They may
be seen in many diseases, but are often very
prominent in cases of hyperadrenocorticism,
as in this instance

17. results from accumulations of superficial
epidermal cells that are dead and cast off from
the skin. In this case there is an epidermal
collarette surrounding a postinflammatory patch
of hyperpigmentation. This presentation is
frequently seen in cases of superficial pyoderma
and other pustular diseases.

18.  Crust is composed of cells and dried exudates.
 It may be serous, sanguineous, purulent, or mixed.
 This cat has pemphigus foliaceus.
 Erythema

19. Erythema is reddening of the skin due to
increased blood flow (see under macules
p.10).The pattern of erythema may be diffuse
togeneralized (e.g. atopic dermatitis) or
macular–papular (e.g. pyoderma or
ectoparasites). In this Springer Spaniel the
erythema is due to Malassezia pachydermatis
infection.

20. An erosion occurs following loss of the
superficial part of the epidermis (i.e. down to but
not including the basement membrane), as on the
face of this dog with discoid lupus
erythematosus. Erosions heal without scar
formation.

21. An ulcer is deeper than an erosion.They occur
following loss of the epidermis and basement
membrane and exposure of the deeper tissues of
the dermis. Lesions may scar, as in this decubital
ulcer overlying the bony prominence of the hip.

22. A sinus or fistula is a draining lesion.
This dog has panniculitis and there are draining
fistulas on the flank.The term sinus is usually
reserved for an epithelialized tract that connects a
body cavity and the skin surface

23. An results from self-trauma.
In some cases, often in cats, the damage
can be extensive, as in this Persian cat with
food allergy.

24. A scar results from the abnormal fibrous tissue
that replaces normal tissue after an injury, such as
a burn as in this case.

25. A fissure forms when thickened, usually
lichenified or heavily crusted, skin splits.
The illustration shows the foot of a dog with
necrolytic migratory erythema.

26. Lichenification occurs following chronic
inflammation, as in this case of M. Pachydermatis
infection.There is thickening of the skin associated with
accentuation of normal skin markings. Histopathologically,
lichenification consists of thickening of the epidermis
(acanthosis) and the stratum corneum
(hyperkeratosis).

27. Hyperpigmentation, an increase in cutaneous
pigmentation, usually occurs after chronic inflammation, as
with atopic dermatitis.
Hyperpigmentation may also be seen in cutaneous changes
associated with an endocrinopathy.

28. Hypopigmentation, a decrease in cutaneous pigmentation,
occasionally follows inflammation, as in this case where it occurred after
a superficial pyoderma. It is more commonly associated with immune-
mediated inflammatory dermatoses. Vitiligo, a rare non-inflammatory,
though possibly immune-mediated condition, is characterized by
symmetrical hypopigmentation. There are also a number of hereditary
conditions associated with complete or partial loss of pigmentation (e.g.
albinism).