Does anyone have a horse with asthma and if so, what are the symptoms?  Recommended treatments? Thanks for your input. 

Nebulizing can go a LONG way and treat it directly. Try and minimize dust , wet hay if you can. 

Have your horse scoped by a good equine vet to make sure it is asthma or is your horse have breathing complications from something else before you spend alot of money on supplements that probably won't work. Nebulizers are great but you need to find out the root of your horses problem. 

Heat and exercise intolerance, heavy breathing are symptoms. You need to scope and do a BAL to know what's causing the symptoms. 
there is a 10 day treatment called Aservo that's been very effective for COPD but you need to find out for sure what's going on first.

Loaded question.

Have you taken the horse to the vet yet? If not, make that your first step. It would likely be a good idea to do a BAL (which will analyze the cells) as well as a scope (to look at the airways) and I would also suggest allergy testing.

Based on those results, then your vet can come up with an intial game plan for your horse. Like with anything, it is going to be trial and error to figure out what is going to work for your horse.

I have one with asthma. We started having issues last year when we had dense wildfiresmoke coming down from Canada. I haven't been fully able to figure out her triggers but so far, one does seem to be when it gets warmer in the spring and she still has her winter coat. Could also be the mold out in the pastures that sat all winter, since the grass hadn't gotten green yet. Waiting to see how she does with summer if we have a hot/humid day.

I have a myriad of medications from my vet to use with certain symptoms: dexamethasone (both for nebulizer and IM shots), albuterol, ipratropium. And I also have done 2 rounds of Alpha2 in her nebulizer. I've got her on allergy shots, and we're also doing kenalog IM shots every 4 - 6 weeks to help control the inflamatory reponse. Knock on wood, she seems to be doing okay right now and we made a run over the weekend, and she felt the best she has.

Also figured out last year that my mare is a bleeder and so now I run her on LASIX as well.

It's a battle and it's not fun, but you are going to need to work closely with your vet.

What state do you live in?

 I live in Texas, and the symptoms occur all year round.  Working with vet and have had a BAL (nothing abnormal), dynamic endoscopy (normal), tried neb treatments for 6 months, antibiotics, etc.  We're scratching our heads on what is going on if anything. Horse looks and feels great.  Symptoms include:  a few deep breaths, occasionally jerking head down, sometimes blowing nose, & some head tossing ONLY during an easy lope at home or warming up at an event.  And some reluctance in the alley to enter the arena for a run.  No bleeding, no shortness of breath, or long recovery after a run.   Thank you to everyone for your input!

I asked because I had one that couldn't breathe 8 months out of the year. Turns out she was allergic to Bahia grass. She found a great home in Kentucky. Stepped off the transport breathing perfectly.