For the Health of It
Insect Bite Hypersensitivity in Horses
Reprinted with permission from Equine Disease Quarterly, Volume 27, Number 3
A large percentage of horses a
affected with seasonal pruritic der-
matitis are hypersensitive (allergic)
to the bites of insects. Culicoides
spp (gnats), black flies, stable flies,
and horn flies are the most com-
monly implicated insects although
any biting insect may contribute to
insect bite- induced hypersensitiv-
12 • Walking On
ity (IBH). IBH is characterized by
intense pruritus (itching) that often
leads to excoriation (abrasive skin
damage), extensive hair loss, sec-
ondary infections, and chronically
to hyperkeratosis and lichenifica-
tion (thickened skin). Many horses
develop IBH in middle age or later,
although horses with atopy may ex-
hibit clinical signs as early as 1 year
of age. Clinical signs often progress
in each subsequent year.
Diagnosis is made from the sig-
nalment, history, clinical signs, and
ruling out of other possible diag-
noses. The distribution of lesions
on an a affected horse with IBH is
dependent on the biting charac-
teristics of the insect(s) responsi-
ble. Since there is still much to be
learned about the identification
and feeding habits of many insects
implicated in allergic dermatoses, it
may not be possible to identify the
exact etiological agent(s). IBH typi-
cally improves and exacerbates sea-
sonally, whereas clinical expression
of allergies in atopic horses may
occur at any time of the year. Urti-
caria (hives), commonly found in
atopy, is not a diagnosis but a cuta-
neous reaction pattern that may be
induced by a wide variety of causes,
both immunologic and non-im-
munologic. Rule outs for urticaria
include drug and vaccine reactions;
stinging and biting insects (such as
wasps) and arachnids; infections;
contact irritation; vasculitis; and
cold, stress, or exercise-induced
lesions. Other diseases that exhibit
pruritus are Oxyuris (pin worm)
infestation, onchocerciasis, and
mite (Chorioptes) and tick infes-
tations. Diseases that occasionally
are associated with pruritus include
ringworm and pemphigus, an auto-
immune disease.
In all cases of pruritic dermatitis,
managerial procedures to decrease
exposure to exciting agents as well