There are some things that may legally be done to help the situation otherwise you may just have to accept the situation for what it is - especially if the animals are well fed and sheltered even if its not to your standard.
OP, just as an FYI -- where I am we have a line fence act that basically says a shared fence between two property owners is the financial responsibility of both if they are keeping livestock. If you going to check out zoning and animal housing requirements with your county, you may also wish to ask them if they have something like that on the books.
I was finishing reading the posts, and yes, even the flies are worse. I've never had issues before, but I had to buy a fly mask this year and went through an entire gallon of concentrated fly spray for my one mare(which normally lasts about 2 years for two horses).