Immune responses to ectoparasites of horses, with a focus on insect bite hypersensitivity.

The research focuses on the immune responses of horses to various arthropod ectoparasites, in particular, the insect bite hypersensitivity (IBH). Ectoparasites initiate a variety of responses in horses, including inflammatory responses and adaptive immune reactions. This study specifically highlights the role of mast cells and IgE antibodies in overseeing these immune responses and the resultant pathological reactions.

Various Parasitic Responses

• Life cycles of different parasites on horses vary. Lice stay on the horse for their entire lifespan, ticks feed for several days, while other insects visit the host briefly for nourishment.
• Ectoparasites invoke various responses in horses. These involve innate inflammatory responses and adaptive immune reactions.
• These different responses are connected through immune mechanisms directed by mast cells and IgE antibodies, which play a critical role in horse resistance to ectoparasites.
• Although immune responses are beneficial, they can also lead to severe pathological reactions, impacting the horse’s health significantly.

Insect Bite Hypersensitivity (IBH)

• One of the best-known examples of such pathological outcomes is insect bite hypersensitivity (IBH).
• IBH in horses is an IgE-mediated type 1 hypersensitivity to the saliva proteins of Culicoides spp. This hypersensitivity is associated with the production of IL4 and IL13 by T-helper-2 cells.
• Interestingly, all horses exposed to Culicoides have an expanded population of Culicoides antigen-specific T cells with this pattern of cytokine production.

The Role of Regulatory T Cells

• Despite the hypersensitivity, some horses remain healthy due to the presence of FoxP3+ CD4+ regulatory T cells.
• These regulatory T cells express IL10 and TGF-beta, which suppresses the IL4 production by Culicoides antigen-activated T cells, reducing the inflammatory reaction and helping maintain health in horses exposed to Culicoides.