"L" is for Lyme disease
You can find pretty much all of the information on Lyme disease in horses on the internet, so I won't repeat everything here. Below are just some observations I have made throughout the about 20 years that I have been encountering Lyme disease since it first showed up in the North Country, as well as some links to more in-depth information.
And, as always, nothing written here is meant to diagnose disease or be construed as treatment advice. Every horse is different and requires a proper diagnosis before treatment. NOTHING should replace the advice of your veterinarian!
1. Oral doxycycline for 2 months - this is the most common treatment because it is well tolerated, affordable (under $300 when I last checked), and moderately effective. In previously untreated horses I would say the success rate is about 65%. However, in horses that have been treated before that percentage is much lower, to the point where I would say I would probably only expect it to "knock down" the titer temporarily but have them rise again later on
2. Oral minocycline for 2 months - similarly well tolerated as doxycycline but a lot more expensive ($1250-1500, but that was last year, I haven't looked into prices lately). However, this drug penetrates better into some of the tissues that we often have problems affecting with doxy (mainly joints and nervous tissue). This too may not be a "forever" cure, unfortunately Lyme is a crafty organism that is very capable of evading treatments and the immune system.
3. Intravenous oxytetracycline for 4 weeks - this treatment I would usually only offer in the clinic, unless you are VERY comfortable maintaining an intravenous catheter ("port"). The other thing about it is that we have to monitor renal function when we treat horses this way, as the drug can impair kidney function. All of this, of course, makes it the most expensive treatment option (somewhere around $2500). All of that said, 4 weeks of oxytet is the gold standard treatment, with the highest (albeit NOT 100%) success rate.