If not for a prepurchase exam, no one would have known there was anything unusual about Frosty.
The 7-year-old Quarter Horse mare is deaf, a congenital condition linked to her extensive overo pattern, but that is not remarkable. What sets Frosty apart is that she is able to carry a rider and romp in her pasture like any other horse despite having a cardiac condition that, in theory, should greatly limit her activity if not lead to congestive heart failure.
“The cardiologists are scratching their heads with this one,” says Philip Johnson, BVSc, of the University of Missouri. “Frosty isn’t following the typical pattern of what we’d expect to see. Obviously, she hasn’t read the textbooks.”
Frosty was a homebred foal, born in 2008, produced specifically to be sold. “We owned her mother, who had distinguished Western pleasure lineage, and her sire was a world champion halter producer,” says Johnell McLean of De Soto, Missouri. “We were hoping to produce an all-around horse that we could sell the following year.”
But the timing couldn’t have been worse. The economic downturn of 2009 hit the horse industry hard, and no buyers appeared for the yearling. Fortunately, McLean was perfectly happy to keep the filly at home for a few more years. “We were in no rush,” she says. “She’s a great mare to work with.”
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Frosty was trained to respond to visual cues and was eventually broken to ride. She was shown successfully in both halter and Western pleasure classes. Eventually, the market picked up, and in the spring of 2015, a buyer emerged who was interested in putting Frosty to work on a cattle operation, riding fences and checking calves.
As a routine part of the transaction, the buyer arranged for a prepurchase exam at the clinic of Frosty’s veterinarian. “Frosty had had regular veterinary care her entire life and had always been healthy,” McLean says. “So we weren’t anticipating finding anything usual. The prepurchase exam was just a formality; the buyer had even already made shipping arrangements to pick her up afterward.”
However, one of the first things the examining veterinarian did was to place a stethoscope against Frosty’s rib cage and listen to her heart. “I remember he stepped back after a few minutes and said, ‘Her heart’s not beating right,’” says McLean. “He sounded as surprised as I felt.”
At the time, Frosty was in training for the upcoming show season, being ridden regularly and kept very fit. “The veterinarian asked us to longe her hard so he could watch her reaction to exertion,” says McLean. Frosty showed no signs of distress. The veterinarian listened to her heart again after she worked, and it still wasn’t beating normally. None of the local veterinary clinics had the equipment for a full cardiac examination, so the veterinarian ended the prepurchase exam and recommended that McLean ship her mare to the University of Missouri for a full workup.
“The sale was off, understandably,” says McLean. “But at that point, I was more concerned about what was wrong with Frosty.”
Johnson became involved in Frosty’s case when she arrived at the university a few days later. “We examined her and conferred with our cardiologists, and we were all a bit mystified,” he says.
It was immediately obvious that the mare had an arrhythmia, which is a disturbance in the rhythm of the heartbeat. In a normal heart rhythm, the two upper chambers (atria) contract simultaneously to push blood downward to fill the two lower chambers (ventricles); as the atria relax, the two ventricles contract simultaneously to push blood outward to the lungs (from the right ventricle) and the rest of the body (from the left ventricle). These actions create the familiar “lub-dub.” Any deviation from that steady, two-beat cycle is considered an arrhythmia.
Heart arrhythmias aren’t unusual in horses, says Johnson. In fact, he says, “There are certain types of arrhythmias that are quite common and even normal. One common arrhythmia is an atrioventricular (AV) block, in which electrical signals sent from the atria to the ventricles are delayed or interrupted.
“In many horses these are what we call physiological arrhythmias,” says Johnson. “They are normal, particularly for an athletic horse. Human athletes tend to develop slower heart rates while at rest, but horses do it a bit differently. Their heart rates are naturally slow, so as they get fitter they appear to develop these arrhythmias as a way of regulating blood flow by skipping beats.”
Johnson says that an experienced veterinarian can usually identify physiological arrhythmias simply by listening, but to be certain, the test is to exercise the horse. “Arrhythmias that are purely physiological and considered normal resolve themselves when the horse’s heart rate increases through exercise or excitement,” he says. “At times when it’s critical for the heart to be pumping correctly, it does.”
Just as the referring veterinarian had done, the Missouri team worked Frosty hard on the longe, then listened to her heart again. Her arrhythmia was still there.
Next, the team hooked up an electrocardiogram (EKG) to generate a graphic printout of the electrical activity controlling Frosty’s heartbeats. The readout enabled the team to pinpoint her diagnosis to “intermittent periods of high grade 2nd degree block and 3rd degree block.” In other words, Frosty’s ventricles were consistently receiving delayed impulses from the upper chambers, and sometimes those impulses didn’t come at all. The EKG also showed evidence of “ventricular escape,” a coping mechanism that causes the ventricles to generate electrical impulses on their own.
In most cases, a horse with this type of arrhythmia would be extremely exercise-intolerant. “These horses will lose steam, not maintain power and fatigue quickly,” says Johnson. These arrhythmias can also progress to congestive heart failure, characterized by coughing, accumulation of fluid in the chest and episodes of fainting. However, Frosty had never shown any hint of any of these signs.
The team then performed an echocardiograph, to look at the mare’s heart via ultrasound, but they found no structural abnormalities that would account for the arrhythmia. They also drew blood to look for underlying disease or other inflammatory markers that can indicate heart disease—but again, Frosty appeared to be completely healthy.
Still, the veterinarians recommended a two-week course of the corticosteroid prednisolone plus an omega-3 fatty acid supplement and vitamin E as anti-inflammatories. That, they said, would address any inflammation that might be the cause of the problem. They also asked McLean to rest the mare completely, with no forced exercise that could, in theory at least, lead to her fainting and injuring herself or a rider. They also wanted to see Frosty back at the university for a recheck in four to six weeks.
Still an enigma
McLean noticed nothing unusual as she monitored Frosty over the course of the month. “We watched her closely but she was absolutely fine,” she says. The mare cavorted and roughhoused with her pasturemates, showing no signs of any trouble keeping up.
In June, McLean trailered Frosty back to the university where an EKG confirmed that the same arrhythmia was still present. This time, however, the veterinary team offered one more suggestion. Sometimes, cardiac arrhythmias can be permanently corrected, or “converted,” by administering a dose of medication that temporarily speeds up the heart. The procedure can be risky–atropine, the drug they proposed using, can also induce colic. But McLean agreed it was worth a try.
Frosty received the atropine slowly, via an intravenous drip, while the veterinarians monitored her heartbeat closely using an EKG. As expected, the mare’s heart rate began to soar as the drug took effect and, for a few short minutes, the arrhythmia disappeared. When the dose tapered off, however, the abnormal rhythm returned.
The last option the veterinarians offered for correcting Frosty’s arrhythmia was to implant a pacemaker. But the placement of a pacemaker would be an extreme measure for a young, healthy horse who didn’t seem bothered by the problem. “With Frosty not being adversely affected at all, something so drastic and invasive didn’t seemed called for,” says McLean.
The veterinary team couldn’t fix the problem or even tell McLean what was wrong with her mare—because, says Johnson, maybe nothing was wrong at all. “It is entirely possible that this is a normal physiological state for this horse. This could simply be how her heart functions. This could be a physiological arrhythmia that we’ve simply not seen before. That doesn’t mean there aren’t other horses out there with it—although I wouldn’t expect there are many—being ridden and doing fine. This is just the mare that we happened to find it in,” he says.
“It’s a quandary when horses don’t fit the profiles we expect,” he adds. “But sometimes you have to let them not fit. Frosty has been thoroughly examined twice with many tests done by talented cardiac specialists, and she’s always had this condition. The jury is still out, but the longer this is present, the less likely it is to go away, and it’s probably just part of who Frosty is.”
Still, the veterinary team was understandably reluctant to give McLean the OK to resume training on a horse with an unusual heart arrhythmia they didn’t fully understand. “Anytime you ride a horse that has something not quite right with his heart, we always worry that the horse will collapse,” says Johnson. But retiring Frosty permanently didn’t seem right, either.
McLean and Johnson discussed the options, and McLean agreed to keep Frosty out of training for six months, then bring her in for another follow-up visit. That would allow enough time for any underlying heart disease to progress to a point where it could be identified. If, at that time, the arrhythmia is unchanged and there are no new indicators of heart trouble, McLean says she may consider putting Frosty back in training.
“At 72 years old, I’m not going to ride her myself,” says McLean. “But I have a young friend who is an accomplished rider and had worked with Frosty for years before this. She is ready and eager to get back on. She understands the risks; any horse can fall with you at any time, and as of right now we have no reason to expect Frosty will do that.”
McLean has also dropped the plan to sell Frosty. However, the potential buyer whose prepurchase exam sparked the entire investigation still keeps in touch. “She called just the other day to ask how Frosty was doing,” McLean says. “I was happy to tell her that she’s just fine.”
This article first appeared in EQUUS issue #460, January 2016.