In horses, bleedling in the lungs (or pulmonary hemorrhage) is a common complaint which can be very serious for a horse's long-term health. It is found in most racehorses at some point, but as many as 10% of non-racehorses are thought to suffer from the condition at some point according to Laura Phelps of Today's Horse.
Exercise-Induced Pulmonary Hemorrhage (EIPH) seems, as the name suggests, to be caused by a horse indulging in strenuous physical exercise, ie running. But there is much disagreement as to the causes of the bleeding.
A popular theory is that horses are just not supposed to run as much as humans make them in racing. That from an evolutionary standpoint, they are supposed to be placid grazing animals that walk around the countryside and only opt for a spot of cantering, galloping or sprinting when panicked or evading a predator. In other words, by sitting on them and making them run round racecourses, humans are pushing their equine friends beyond their biological limits.
Whatever the background, the immediate cause of EIPH is blood pressure. High blood pressure in the pulmonary capillaries in the horse's lungs can build up during strenous exercise. Vets think the problem could be that the heart does not have a chance to properly contract, relax and fill while it is sprinting. Alternatively, certain performance feeds have been blamed. It has been suggested that alfalfa hay, which is often fed to racehorses due to its high protein levels, leads to high levels of a nitrogen by-product called urea which is expelled in the horse's urine. In other words, horses are inhaling ammonia fumes from their own urine (especially when being transported in horseboxes) and this could be enflaming their lungs and trachea which then leads to bleeding during exercise.
In any case, this high blood pressure can cause the capillaries to rupture and blood to flow into a horse's airways. Bleeding lungs in horses can be quite difficult to confirm without resorting to an endoscopy, unless it is a very severe case, when the horse may end up bleeding from the nostrils as the blood flows up its trachea (windpipe).
Although it is rare for a horse to die from bleeding lungs, it is a serious condition in spite of its frequency. Treatment is generally via the diuretic Lasix, which reduces blood pressure but can leave a horse dehydrated. A horse can take up to six weeks to recover fully from a bleeding episode, so will rest is part of the treatment also.
Because some countries have laws stating that horses which have suffered bleeding lungs must not race again, it is important to try and prevent an episode of EIPH recurring (or from happening in the first place). Good steps to prevent the condition include making sure the horse's accommodation is well-ventilated and dry. Muck out the horse area regularly, to prevent irritation from ammonia in the urine, and make sure the horse is out in the fresh air as often as possible. And make sure that the horse's training fits its health and energy levels - do not risk a recurrence of bleeding lungs by over-training the horse.