Please keep in mind as we go through this information, PHF is fairly uncommon in our region. There is a higher concentration in central New York (near Syracuse), and a much higher concentration in the Potomac River Valley, including Pennsylvania, Maryland, Virginia. While we still watch for it in this area, we don't see or hear of many cases.
Recent work by Dr. John Madigan of UC Davis has proven that horses become infected when they ingest the flies that are infected with the bacteria. (Read more about it here.) Under experimental conditions, they were able to cause infection in a horse by feeding just 8 (yes, 8!) caddisflies. Other methods including feeding the snails or feeding the organism directly did not lead to transmission of the disease. It's important to remember that since the fly is the vector, that means the horse doesn't need to drink directly from a river or pond. The disease can come to the horse and land in its water bucket.
Side note: While oxytetracycline is available over the counter (for use in cattle), it's very important NOT to attempt treatment of your horse without a veterinarian. Oxytetracycline is poorly absorbed orally in horses, and must be given IV, diluted in sterile fluid, slowly through an IV catheter. Don't take this on alone!
There is a vaccine, but it's of variable effectiveness. We do vaccinate horses in our region who seem to be at higher risk (those horses who live in a paddock with a pond or stream in it, or who commonly trail ride in wet areas, or drink from ponds or streams). Since the vaccine isn't particularly protective (only reaching about 78% protectiveness in experimental studies, and certainly less than that in actual infections), and the effect of vaccination wanes quickly (reaching about 50% protective at 6 months, and about 33% after 9 months), if you're going to vaccinate, it's best to do so about a month before cases start to emerge in the area. For the northeast, we start to see cases in July/August, so vaccinating in June makes the most sense. There is some evidence that the vaccine is waning in effectiveness over the past several years, and in some region it seems to be much less effective than in others. This lends support to the idea that we are actually seeing several different strains, and the vaccine may be effective against some, but not others. It's also believed that since the disease is contracted orally, vaccines may need to be reformulated to provide better protection.