Why did my horse get bumps on his back?

[Matthew]

Hiya. In the summer my horse, a 17.3hh Belgian Warmblood, began to develop loads of bumps over his back and sides. I understand that this may be caused by insect bites, hives or the heat but i was wondering whether anyone could narrow the cause down and help me with how to prevent them.

Hopefully these links will work for a picture, they aren't great but will hopefully suffice. I think more also developed afterwards.
https://www.facebook.com/photo.php?fbid=...e2c2c29ae6
https://www.facebook.com/photo.php?fbid=...2271aa0778

During the winter he hasn't had them but has recently started developing them again.

Thanks

[Charlotte]

You'll have to become your own detective to track down the cause of these lumps.
Does he get them when he's stabled full time (no turn out)?
Try turn out with a lightweight turn out sheet on - does he still get them? If not then you've managed to prevent what ever is causing them from coming into contact with him.
http://www.equinesuperstore.co.uk/horse-...-rugs.html
Do they still appear if you use a fly sheet turn out - like for sweet itch affected horses?
http://www.equinesuperstore.co.uk/advanc...x=114&y=28
Do they still appear when you treat him with a long lasting contact insect repellent/killer?
http://www.petdispensary.co.uk/shop/fly_...opers.html
Will he only get them if out overnight or will they appear if he's out for just an hour late morning?
Do they only appear when the weather is warm/ sunny/cloudy, etc. ? obviously he's not getting them in the Winter so what is different about your Summer & Winter routines? Do they appear starting in a particular month? They could be linked with the appearance of a particular plant which he likes to eat.
Are different turnout fields affecting him in the same way?
If you try to keep a diary of your routines & when the lumps appear/disappear you may be able to track down the cause but there is no point in us guessing when you have all the information! lol Just call yourself 'Sherlock!'

[leegalen22]

They are probably hives if they; only happen when he's been out in the pasture, are not painful when pressed and go away in approximately half an hour to 45 minutes after you bring him back in. He's just one of those horses that are mildly allergic to bug bites.
You can try a much stronger fly spray but of course after he rolls a lot of it is no longer effective. Here in the US there is only 1 fly spray brand I've found that really works but it's expensive. It's called Tri-Tec and comes in silver bottles. You can also try turning him out in a fly sheet.