My daugters gelding has come up with the same loads of lumps on his underside and nothing in his routeen or diet have changed they look like fly bites but there are so many and quite big. The vets say may be reaction to something. Am tempted to try anti-histamine but am unsure

iv got a sec a mare that is the same if she get bitten by a horse fly the whole one side comes up in those bumps it doesn't affect her in any way usually it is round the neck, underside or bum where the blood supply is close to the skin that transfers it to the the surrounding tissue