You can help your horse more extensively:

- avoid letting your horse graze in the early morning and evening hours.  (mosquitoes are very active at this time of  day)

- avoid pastures situated close to slowly flowing or standing waters. (here is a very  high population of mosquitoes)

- avoid an oversupply with protein. (try to find out if the quantity of fodder your horse gets is really necessary)

- you should prefer pastures that are used extensively. (a low content of protein in the grass and valuable herbs)

- you should prefer pastures where there is wind or which are situated in an higher situated area. (lower population of mosquitoes)

Not all this advice is feasible of course, but every additional measure you take is an additional help for your bothered horse. But we do agree about one thing. A horse with summer itch suffers.