I have lots of experience in this area.....
One of my horses is having a bizzare breakout of lumps, mainly on his neck. I first noticed this about a month ago. I immediately started applying a "calming gel" to it (I can't remember the brand name off hand), along with Fly Block fly spray. The lumps decreased in size and disappeared after a few days. Not long after that, they became apparent once again. I have been fighting this for about two weeks now. They will go down in size, disappear in some spots, then return. This morning he had very minimal TINY bits on the left side of his neck, however on the right side there were lumps ontop of lumps, clusters if you will.
I am covering him in Pyranna Fly Spray twice a day (before he goes outside and just before he goes into his stall at night), I clean the stall daily, and spray the stall walls with fly spray. All the wet food, along with the dry grains, are covered so that they wont attract bugs as much.
I wondered if he was allergic to the spray, BUT I wasn't spraying his before all this started. Nothing dramatic has changed in his feed regimen. He has been at this barn off and on for the last 6 years, and has NEVER had this sort of thing happen. All my other horses are fine. I'm almost positive its bug bites that are causing this.
Your vet can draw blood and send it off to a lab to be checked for allergies and they are very detailed tests. We had this done for a horse with similar problems, he was having the same issues and it kept getting worse and worse. The test results came back showing he was allergic to black flies, deer flies and mosquitoes.
How do you avoid all those when you are a horse?
He was put on a year long allergy shot program to hopefully desensitize him. It started out a shot just under the skin every 3rd day I think and gradually went down to once a month then stopped. It did improve him but he still gets the hives and lumps sometimes. He is also sensitive to certain brands of pine shavings and has broken out at shows I think from the bedding.
By the way, the vet told me to be sure and not feed him any feeds with molasses in it because horses with allergies tend to be very sensitive and frequently are allergic to the molasses and their owners dont know it.
Also when he is body clipped he gets bumps and itchy. I always try to clip him with no more than a #10 and give him a good bath afterward. The vet gave me a bottle of coal tar shampoo -- it is in a plain bottle with no label on it, looks home-made, actually, but it works great it is very soothing and helps a lot. Vet also gave me a couple of emergency shots to keep in the first-aid kit -- cannot remember the name offhand -- if he has a bad allergic reaction to something the shot instantly takes down the hives/lumps and something like that would be good to have in case he is exposed to something that causes a really nasty reaction.
I have been told oil-based fly sprays can make allergies like this worse and Pyrana (the one with the bright yellow and black bottles or cans) now makes a more gentle and natural water-based fly spray that I use on him instead and it seems to work.
A fly sheet helps a lot and is very lightweight -- not hot in the summer -- I noticed at Shetland Congress that I think Star Lake Tack had some for $20 and I was going to buy a couple and forgot. I think she also had fly hoods too for horses that are really sensitive on the neck. Hopefully she still has them at Nationals.
Another thing I haven't done but I know someone who did, she got one of those pop up screen gazebos and put it up in her horses stall so he has a totally fly and bug free stall area. I have on my list of things to do is put up screen doors on the barn so that I can keep both ends of the barn open but spray the inside really well and hopefully keep bugs and flies out. Keeping the horse inside during dusk when the bugs are at their worst might be a good idea, around here by about 10PM the bugs are gone for the night.
My kids have bantam chickens now and those annoying little beggers really eat the mosquitoes and flies in and around the barn because I have noticed since we got them that we hardly have any flies at all out in and around the barn and mosquitoes are better too. I have not had to put up fly traps or worry about spraying much at all. I even saw one of the chickens catch a mouse the other day which surprised me.
He has been at this barn off and on for the last 6 years, and has NEVER had this sort of thing happen. All my other horses are fine. I'm almost positive its bug bites that are causing this.
Our horses previous owner AND the breeder that he came from both say he never had a problem like this before. Our vet did say horses can develop allergies when they get older (our horse is 12) and I do know the first year we had him he was ok and then the past three years he got worse and worse until we finally had him tested.
The allergy shot program didnt totally eliminate it but it is a lot better than previous years.