The nerve disease historically known as roaring restricts air flow and can cause a horse to tire soonerRoaring is an antiquated horsemans term that refers to the characteristic sound a horse with a partially paralyzed larynx makes when he exercises. The noise may be only a whistle or a wheeze, but whatever the sound it indicates that the cartilage (almost always on the left side) surrounding the larynx has collapsed into the horses airway. Breathing is restricted. That means the exercising horse gets less oxygen. And that has serious implications for his racing ability. The modern term for the problem is recurrent laryngeal neuropathy (RLN).

Critical oxygen
For hundreds of years, Thoroughbred trainers have searched for ways to get horses to run faster. Sooner or later, every one of them comes up against the same limiting factoroxygen, or lack thereof.
As a horses muscles fire while running, they burn fuel for energy in a complex biological process known as the Krebs Cycle. Oxygen is critical to the energy-producing phases of that cycle. It is really very simple. No oxygen, no energy.
When the energy cycle has plenty of oxygen, the horse is said to be working aerobically. When he uses up all of his available oxygen, his muscles can still produce energy for awhile, but the energy cycle begins to work anaerobically, without oxygen. A by-product of that anaerobic fuel burn is lactic acid. As lactic acid builds up in the muscles, they become sore and tired. The horse slows down, and may even shut down completely.
There are conditioning programs designed to train the equine body to use oxygen more efficiently, feed supplements which intimate they can keep horses working aerobically a few critical seconds longer during a race, and medications like Lasix that try to keep horses lungs clear so they can absorb more oxygen from the air they breath.
Such efforts will all be for naught if the horse is physically unable to inhale the maximum amount of air because of an RLN-related obstruction. Retired Tufts University veterinarian Dr. W. Robert Cook, who has studied the problem extensively and written about it in his book, Specifications for Speed in the Racehorse: The Airflow Factors, believes as much as 95% of the Thoroughbred population suffers from some degree of RLN. He estimates that RLN can cut a horses oxygen supply by as much as 50%.

The problem is nerves, not lungs
Noises such as roaring or whistling are only an audible symptom of a chain of breathing difficulties that start when the nerves that control the muscles which open and close the larynx start to lose their covering of fatty tissue. The trigger that starts this degeneration process is unknown, but Cook believes there is a strong genetic link among roarers, and others are becoming convinced.
As the nerve fibers degenerate, the muscles that open and close the larynx (RLN occurs primarily on the left side of the larynx) become paralyzed. No longer supported by muscles, the cartilage that forms the walls of the larynx collapses inward, restricting the flow of air. Depending on the degree of collapse, there may or may not be that characteristic roaring or whistling noise. With less oxygen, the horse tires sooner and loses his desire to run. Cook and others believe that roaring and bleeding (technically called exercise-induced pulmonary hemorrhage or EIPH) are related problems stemming from the same nerve disease. Labored breathing stresses lung tissues, contributing to bleeding, and further limiting the horses ability to absorb oxygen.

Short term fixes
RLN is an incurable condition. Some trainers and their horses learn to live with it by adjusting racing expectations to the horses ability to breathe. The horse may race in a lower class, at shorter distances, or only when track conditions suit him. Surgical intervention is another option to try to improve the horses breathing. Drs. Norm Ducharme and Richard Hackett of Cornell University took a look at the available surgical options for treating RLN and made some estimates of how successful they might be.
Ventriculectomy involves surgically suturing cartilage at the back of the larynx to cartilage of the thyroid in the hope that the two will fuse. First used around the turn of the century, ventriculectomy reduces breathing noises 20%-to-80% of the time but does not significantly improve airflow.
Laryngoplasty involves insertion of synthetic materials that replace the function of the paralyzed muscles that hold the larynx open. Veterinarians commonly combine laryngoplasty with ventriculectomy. The combination of procedures improves airflow in 70%-to-80% of the horses treated. One side effect from this, however, is coughing due to inhalation of food particles. Some 40% of horses cough after the surgery and 5%-to-10% develop a chronic cough.
More recently, veterinarians have tried renervation. A nerve, or a nerve plus some of its surrounding muscle, is transplanted to the larynx from another part of the body, typically the neck. While the procedure has shown some success in horses with experimentally induced RLN, it has not yet been proven on horses with congenital RLN. The lengthy recovery period of up to a year after surgery poses a major drawback to using the procedure for Thoroughbreds in training.

Long-term cures
Cook postulates that inbreeding within Thoroughbred lines is responsible for the high incidence of breathing difficulties and subsequent lung hemorrhage among racehorses. The best long-term cure for the problem, then, would seem to be judicious breeding.
Some years ago, Cook devised a slap test as a diagnostic tool for RLN. Slapping the horse in the saddle area on one side of the body produces an involuntary twitch in the muscles of the larynx on the opposite side of the body. Veterinarians can compare the reflex from side-to-side and note obvious differences.
Cook carried the use of the slap test a step further when he developed a portable oscilloscope to measure the length of time it takes in milliseconds between the slap stimulus and the response by the muscles of the larynx. Fast speeds indicate healthy nerves while slower speeds indicate nerves affected by some degree of RLN. With his machine, Cook can grade horses on a numerical scale that indicates the degree to which they suffer from RLN.
By selecting individuals showing the least degree of RLN and mating those which would also provide the greatest degree of genetic diversity, Cook believes the incidence of RLN can be decreased. Cook claims using these breeding criteria would benefit not only the breed but also breeders, trainers, and racing fans, since his studies demonstrate a positive correlation between racing ability and grades of RLN.
Not everyone is as convinced as Cook that roaring and bleeding are directly related conditions or that a genetic disease of the nervous system is completely to blame for their prevalence among Thoroughbreds. Still, any serious breeder could do worse than grade the horses in his barn and consider the findings when deciding on the sires and dams of next years foal crop. Collecting data on those offspring and correlating that with their race records could begin to build a database that could be a valuable tool for future breeding programs.