19.4.2: Equine dermatology

Question 1

Examples of Type I hypersensitivity

Answer 1

Type I hypersensitivity
* IgE mediated involving release of histamine by mast cell
* Aggravated after serial exposure
* e.g. Insect-bite hypersensitivity a.k.a. Sweet itch

Question 2

Examples of Type II hypersensitivity

Answer 2

Type II hypersensitivity
* IgG mediated cytotoxic response associated with complement binding
* e.g. Pemphigus complex

Question 3

Examples of Type III hypersensitivity

Answer 3

Type III hypersensitivity
* Immunocomplex deposition on endothelial beds
* Neutrophil activation leads to vasculitis
* e.g. Pastern leucocytoclastic vasculitis, Purpura haemorrhagica, Lupus

Question 4

Examples of Type IV hypersensitivity

Answer 4

Type IV hypersensitivity
* T cell mediated
* Insect bite
* Drug eruptions

Question 5

Answer 5

Sweet itch
(Insect bite hypersensitivity)

Question 6

Answer 6

Pemphigus foliaceus

Question 7

Answer 7

Vasculitis/ purpura haemorrhagica

Question 8

Lesion distribution around the mane, tail and ventral line may be suggestive of…

Answer 8

Insect-bite hypersensitivity

Question 9

Lesion distribution around the base of the tail may be suggestive of…

Answer 9

• Oxyuris equi
• Insect-bite hypersensitivity
• Lice infestation (this is normally more truncal)

Question 10

Lesion distribution around the neck and sometimes face may be suggestive of…

Answer 10

Fungal infections

Question 11

Lesion distribution around the back, chest, rump and sometimes face may be suggestive of…

Answer 11

Rain scald

Question 12

Lesion distribution around the pastern and canon bone area may be suggestive of…

Answer 12

• Mud fever: bacterial ± fungal
• Mite infestation: Trombiculidiasis, chorioptic mange

Question 13

Lesion distribution around the truncal area, neck and occasionaly head may be suggestive of…

Answer 13

• Urticarias
• Atopia
• Lice infestation

Question 14

Lesion distribution around the back and girth area may be suggestive of…

Answer 14

• Contact dermatitis
• Fungal infections

Question 15

What might you use coat brushingsto detect?

Answer 15

Ideal for ectoparasites
Remember to use sealed containers if delayed examination under microscope/ magnifying lens!

Question 16

What should a hair pluck show you? What might you detect?

Answer 16

• Should show a similar number of hairs in anagen and telogen
• Fractured shafts -> suggests self-inflicted trauma e.g. pruritus, pain
• Swollen frayed shaft with fungal spores -> dermatophytosis
• Exclamation mark shaped hair bulbs -> Alopecia aureata

Question 17

Answer 17

Feather mite

Question 18

Answer 18

Biting louse

Question 19

Answer 19

Sucking louse

Question 20

What might you detect on skin scrape? Which direction do you move the scalpel blade?

Answer 20

• Move in the direction of hair growth (with the grain)
• Ideal for mites and dermatophytes
• Mites - add 2 drops of mineral oil to microscope slides
• Dermatophytes - blue staining on slide, qPCR collected in sterile containers

Question 21

What can you use skin biopsies to detect?

Answer 21

• Can be excisional or punch skin biopsies
• Ideal for nodules and deep pyodermas
• You can perform culture and histology

Question 22

Answer 22

Oxyuris equi

Question 23

Causative agent of rain scald

Answer 23

Dermatophilosis congolensis

Question 24

What forms of allergy testing are available in horses and which are most reliable?

Answer 24

• Serum allergy testing (SAT) - not reliable, poor specificity and sensitivity. Interpret alongside clinical exposure and always test during flares.
• Intradermal (IDT) - better diagnostic yield but poor repeatibility. Most reactions are to mites, dust, insects, plants. Useful to establish immunotherapy treatments.

Question 25

What hormones drive hair coat shedding in horses?

Answer 25

• Hair coat is shed dependent on daylight hours
• This process is melatonin and prolactin driven
• Short days -> increasing melatonin and declining prolactin -> winter coat growth