EQUINE | Equine Disease Quarterly
Equine Disease Quarterly –
July 2018
COMMENTARY
Immunotherapies on the Horizon for Insect Bite
Hypersensitivity?
The season is upon us when approximately 10
percent of horses worldwide are faced with one of the
most frustrating skin diseases in the equine industry,
insect bite hypersensitivity (IBH), more commonly
known as “sweet itch” or “summer eczema.” IBH
is an allergic, recurrent, seasonal dermatitis caused
when the immune system overacts or becomes
”hypersensitive” to allergens in the saliva primarily of
biting midges (Culicoides) and other biting insects.
Dr. Susan White in her article provides an overview
of current diagnostic and management practices
to combat signs of IBH. Although there is no
cure for IBH, there is ongoing research to assess
immunotherapies as future treatment options for
horses suffering with IBH.
While IBH has a multifactorial cause, it is
acknowledged that the immune system plays a role
in its development. Hypersensitivity reactions are
classified into four types, and IBH is a combination
of Type I and Type IV. Type I hypersensitivity
reactions develop when antigen specific allergens
bind to IgE and cross-link receptors on mast cells,
basophils and also involve eosinophils. These cells
release vasoactive amines, such as histamine, that
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affect blood vessel size and leakage and produce
inflammatory cytokines and other mediators, which
result in inflammation and pruritus. Type IV reactions,
often referred to as delayed-type hypersensitivity
reactions, involve cell-mediated responses particularly
of activated Th-2 lymphocytes which produce IL-5
cytokine and large numbers of eosinophils, both
contribute to the clinical manifestations of IBH.
Immunotherapy treatment is based on a century-old
concept that the immune system can be desensitized
to specific allergens that trigger hypersensitivity
reactions. Allergen immunotherapy (AIT) involves
inoculation with gradually increasing doses of an
allergen to which the individual is allergic. The
incremental increases in exposure to the allergen
cause the immune system to become less sensitive
to the substance, which reduces the allergy signs
when the substance is encountered in the future.
Data are conflicting as to whether or not traditional
AIT therapies are successful in treating IBH. Research
is now exploring whether IBH can be prevented
by vaccination with recombinant Culicoides
antigens versus whole antigen preparations to more
successfully modulate the immune response, in
particular, IgE response. Another interesting area
under current investigation is oral immunotherapy,
whereby transgenic barely expressing allergens are
administered orally to prevent hypersensitivity to
Culicoides.
Finally, given the important role of eosinophils in
• Equine Health Update •