Hi, welcome to forum. Horsefly is known to cause two types of reactions namely Local reactions and Systemic reactions. They are capable of inducing allergic reactions and/or transmitting infectious diseases. The swelling suggest local reaction to the Flies bite.
Reduction of local edema may be induced with cooling (ice or cold pack). Topical lotions such as calamine decreases pain and pruritus. Topical anesthetic and antihistamine preparations should be avoided as they can sensitize the skin following sun exposure and induce allergic contact sensitivity. Hence, Systemic antihistamines (injection is preferred) and antipruritics such as hydroxyzine or cetirizine against prescription may be helpful. Even addition of corticosteroids has possible benefits.
I suggest you to consult dermatologist for further evaluation and treatment. Take care and regards.
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