- Apr 1, 1999
You’ve probably heard horror stories like these: A breeding stallion completes servicing a mare, slides off, goes down to the ground, gasps a few times, and dies. A 20-year-old pony being ridden by his young owner staggers sideways, falls to his knees, and dies moments later. The trail horse starts lowering his head, stumbles a bit, and drops dead. The Belgian draft horse is found dead in the pasture. The racehorse fatally collapses during a gallop two days after a race.
These tales aren’t hypothetical, but the real thing. They are a few of the case histories seen by Bill Johnson, DVM, Diplomate ACVP (American College of Veterinary Pathologists), associate professor, clinical diagnostic pathology, University of California, Davis. Necropsy proved that all of these horses were killed by the same cause: an aortic rupture.
The aorta is the main artery that carries blood from the left ventricle of the heart (the major pumping chamber) to all the other arteries except the pulmonary artery. Its job is to bring oxygenated blood from the lungs (which enters the heart through the left atrium, flows through the bicuspid, or mitral, valve into the left ventricle, then is pumped into the aorta) to the arteries of the body.
However, sometimes a spontaneous tear or break occurs in the wall of the aorta, causing a condition known as an aortic rupture. "Aortic ruptures usually occur very close to the junction of the aorta with the heart," explains Janice Bright, DVM, Dipl. ACVIM (Specialty of Cardiology), associate professor of cardiology, Colorado State University. "This may result in acute hemorrhage into the pericardial sac. Because there is such a large volume of blood that flows under very high pressure within the aorta, aortic ruptures close to the heart often result in very serious clinical consequences, including sudden collapse and sudden death."
Adds Johnson, "What happens is blood fills the pericardial sac, compressing the heart to the point where it can’t release and fill to eject more blood. So essentially, the heart stops."
As blood fills the horse’s pericardial sac, death usually occurs within a few seconds to a few minutes.
Sometimes, though, the aorta can rupture inside the heart. Says Virginia Reef, DVM, Diplomate ACVIM, professor of medicine, Wagner Hospital, New Bolton Center, University of Pennsylvania, "An intracardiac rupture of the aorta usually ruptures either into the right atrium or into the right ventricle. Often when that happens, blood dissects down the intraventicular septum, and many of those horses will actually present with acute colic and sustained ventricular tachycardia (excessively rapid heartbeat)."
Horses suffering an intracardiac rupture don’t necessarily face immediate death. "If the aorta ruptures into another chamber of the heart, those horses can survive for a while, depending how large the shunt is," states Reef. "In this situation, the aorta is now communicating with either the right atrium or the right ventricle. If it’s a large communication, the horse is going to go into heart failure relatively quickly. If the rupture disrupts the tricuspid valve apparatus or ruptures in the left ventricle and disrupts the mitral valve apparatus, the horse will go into congestive heart failure more quickly."
For most horses, diagnosis is postmortem. Says Harold Schott II, DVM, PhD (exercise physiology), Diplomate ACVIM, assistant professor, equine medicine, Michigan State University, "If the complaint was collapse and death, usually the first thing an autopsy finds, when the chest cavity or thorax is opened, is it’s filled with blood. After the blood is removed and the heart is examined, you can see where there had been a rupture in the area of the aorta."
In those few horses which have an intracardiac rupture and survive the initial event, owners might observe vague clinical signs that include shortness of breath, exercise intolerance, weakness, reluctance to move, looking at their flanks, anxiety, pawing, or even symptoms that present similarly to colic. Explains Bright, "Assuming the horse doesn’t hemorrhage and succumb to sudden death, acute signs may be subtle—just signs of distress or pain. If that rupture into the heart leads to congestive heart failure, which is a common sequela, the congestive heart failure might take a couple of days after that to develop. Signs of congestive heart failure include exercise intolerance, edema or swelling of dependent tissues like the front leg and pectoral regions, enlargement of the jugular veins, difficulty breathing, and rapid heart rate."
Cardiac arrhythmias—either too fast, too slow, or very irregular—also can develop.
An examination by the field veterinarian of horses which survive the initial insult would likely turn up evidence of some sort of cardiac problem. "The fast or irregular heart rate, the swelling of tissues, the increased respiratory rate, sometimes the presence of a heart murmur would strongly suggest to the examining veterinarian that something might be wrong with the heart," says Bright. "Perhaps not a diagnosis for heart failure, but very strongly suggestive of heart failure as one of the differentials. The veterinarian would have a high index of suspicion that the horse has developed heart disease and perhaps heart failure, and would likely call someone who is experienced in imaging the heart and great vessels with an ultrasound (echocardiography)."
Management And Prognosis
There are no medical or surgical cures for aortic ruptures in horses. "In people, doctors can go in and do an aortic replacement with a prosthetic aorta," says Reef. "But that’s not an option in horses."
Instead, management consists of rest and medical therapy to help relieve symptoms. "The first thing you have to do is correct the ventricular tachycardia," says Reef. "Some horses get ventricular tachycardia and actually are able to correct it on their own, so after acute dissection is over, they’ll go back to a normal heart rhythm. But many of them have a sustained ventricular tachycardia that has to be corrected with antiarrhythmia drugs.
"Once you’ve corrected that, the horse’s acute distress seems to be relieved. If there is not a large intracardiac shunt, many horses appear fine, clinically, to the owners, looking indistinguishable from how the horse looked before the event occurred. Many of these horses can, if the shunt isn’t really big, live for weeks to months in relative comfort until they show signs of congestive heart failure.
"But as the shunt gets worse, then it’s going to volume-overload the heart. The heart will start to fail, and they’ll develop signs of congestive heart failure."
Management of these cases usually includes diuretics such as Lasix and antiarrhythmics such as quinidine, says Bright.
Unfortunately, congestive heart failure is a progressive condition for which drugs and rest can only provide temporary symptomatic relief.
"It is not the tear that worsens," Bright explains, "but the congestive heart failure that results from the increased workload on the heart as a result of the extra volume of blood that it is forced to pump."
Prognosis depends on the size and site of the rupture, but in most cases is very grave or poor, Bright says. "Once a horse goes into heart failure, it’s a very poor prognosis. Even with supportive treatment, these animals usually don’t do well. Most horses with an aortic rupture that produces secondary heart failure as opposed to sudden death have only a few weeks to maybe a month or two."
Reef agrees, adding, "It’s pretty rare for a horse to survive for several years, although it has occurred before."
Not much is known as to why aortic ruptures happen.
"I wonder why it is occurring," ponders Johnson. "No one has really described a recurring event. When we microscopically look, postmortem, at those vessel walls, we’re not finding a change in the wall that would explain why it occurred. That really bothers me. Now if the rupture happens at the site of an aneurysm, which is a focal area of dilation in a blood vessel, that’s really understandable because that aneurysm will have a thin wall. But in only one case have I seen a thin wall; in the others, it just happened."
Still, Reef states many believe that a congenital aneurysm might be one possible link, as well as some sort of degenerative disease process that weakens the aorta over time, predisposing it to rupture. "But this is something that is very difficult to diagnose before the event actually occurs," she says.
Another possible cause that’s been suggested is migrating strongyles. Says Schott, "The one species of migrating strongyles, strongylus vulgaris, has a migration phase through the blood vessel walls, and they will actually migrate all the way up the aorta and, in some cases, into the horse’s heart. So that could be something that would cause structural weakness to the wall and predispose to either an aneurysm formation or a rupture, although that would be a minor cause."
Adds Bright, "The aorta can rupture at other sites as a result of parasite migration in the wall of the aorta. That’s usually a rupture down in the abdominal aorta."
However, Johnson states that while migrating strongyles have been suggested as a minor cause, no one has proven that microscopically through examining the vessel walls.
Some people also think there might be a relationship between copper deficiencies and aortic ruptures. "There’s been a situation in the past where mares have ruptured a uterine artery, and it was linked with a copper deficiency," says Johnson. "A low copper situation could weaken blood vessels because copper is required by one of the enzymes in the body that makes strong crosslinking bonds between the elastin and collagen that are in the walls of blood vessels. But a toxicology resident here at UC Davis studied more than 100 necropsied horses to get an idea of what copper levels might be, and we never found a copper deficiency in any of the horses we checked. We also run copper levels on all the horses we necropsy that have a ruptured vessel, and have yet to find low copper."
Nor has Johnson found any significant predisposition among gender, age, breed, or work. "In our database, we showed these occurred in horses between four and 20 years of age, most of them typically about eight years of age," says Johnson.
Nevertheless, Schott points out that they’ve observed aortic ruptures occurring more frequently in breeding stallions 15 years and older. "As they breed, they have an increase in their workload, their heart rate goes up, and it will happen some time during or after breeding a mare," he says.
Adds Bright, "Older horses are predisposed, especially older stallions. For reasons that aren’t clear, it does tend to occur more commonly in the spring after the stallion services the mare. But while this is more common in older breeding stallions, this is a problem that can occur in younger horses and in mares, as well."
Since the cause of aortic ruptures is unknown, there is little anyone can do to prevent them from occurring. "Good basic care of the horse—good deworming, good nutrition, and preventive medicine—will help, but they are not specific against preventing an aortic rupture," says Schott. "I guess the other thing would be as the horse matures to have an annual physical examination and a good listen to the heart as a way to screen for development of heart murmurs. A heart murmur might indicate an ultrasound examination should be performed. If the ultrasound shows an aneurysm (although the aneurysm will not result in a murmur unless it ruptures), then we would make a recommendation that the horse should be retired to perhaps lessen the chance of a rupture occurring. Theoretically, it could reduce the risk a little, but probably not a whole lot."
Low Risk=No Research
Currently, there is no research ongoing concerning the causes of aortic ruptures in horses. "Aortic ruptures don’t occur with nearly the frequency in horses as they do in humans," Reef says. "There really hasn’t been research into the pathophysiology of why it occurs because the incidence of these diseases is relatively low."
Johnson says that at UC Davis, they usually only see about one aortic rupture in the 325-350 necropsies they do a year.
Sudden collapse and death don’t automatically mean an aortic rupture has occurred.
"There are other causes that are equally common," Bright says. "Many of these other causes are arterial ruptures. The uterine artery can rupture in pregnant mares and cause sudden death. The carotid artery can rupture in horses of any age, secondary to guttural pouch infection. Some horses that have pelvic fractures can have the bone fragments lacerate the arteries in the pelvic region. And there are other causes including other vascular problems, lightning strikes, and acute toxicosis due to ingestion of poisons."
So although there is little to be done to prevent or manage an aortic rupture, a horse owner can at least take heart that the risk of this happening is slight
About the Author
Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.