Equine Sporadic Exertional Rhabdomyolysis (SER)

In this episode of Current Vet, Dr. Lottie talks about equine sporadic exertional rhabdomyolysis, how it’s different from other forms of rhabdomyolysis, which management factors can predispose to the condition and what signs to watch out for in horses 

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Timeline:
00:00 Intro
00:30 Case
01:10 Aetiology & Pathogenesis
03:21 Clinical Signs
04:07 Diagnosis
05:52 Treatment
07:54 Prevention
09:13 Key Points
10:21 Outro

Recommended Reading:

• Equine Sports Medicine and Surgery (Hinchcliff, Kaneps & Geor)
• Large Animal Internal Medicine (Smith)
• AAEP Proceedings: Diagnostic Approach to Muscle Disorders (Valberg, 2006)
• Diagnosis and management of equine rhabdomyolysis (in ‘In Practice’ journal)

References:

• Valberg, S.J., 2006. Diagnostic approach to muscle disorders. In: Proceedings of the 52nd Annual Convention of the American Association of Equine Practitioners, San Antonio, Texas, December 2–6, 2006. American Association of Equine Practitioners, pp.327–334.
• Hinchcliff, K.W., Kaneps, A.J. and Geor, R.J., 2013. Equine Sports Medicine and Surgery. 2nd ed. St. Louis: Elsevier Saunders.
• Valberg, S.J., 2025. Sporadic and recurrent exertional rhabdomyolysis. Veterinary Clinics of North America: Equine Practice, 41(1), pp.111–124. doi:10.1016/j.cveq.2024.11.003.
• Reed, S.M., Bayly, W.M. and Sellon, D.C., 2018. Equine Internal Medicine. 4th ed. St. Louis: Elsevier.
• Keen, J., 2011. Diagnosis and management of equine rhabdomyolysis. In Practice, 33(2), pp.66–73. doi:10.1136/inp.d454.
• Ememe, M.U., 2025. Exertional rhabdomyolysis in horses: a review. IOSR Journal of Agriculture and Veterinary Science, 18(4), pp.12–19. doi:10.9790/2380-1804011219.

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Any clinical cases discussed in this podcast are fictional, and are designed to reflect typical or likely clinical scenarios for educational purposes. They do not represent specific real-life cases, clients, or animals.

While every effort is made to ensure accuracy and alignment with current evidence at the time of publication, veterinary medicine is a rapidly evolving field, and recommendations may change over time.

Creators and Guests

Host

Dr. Lottie Wilkinson

Creator and host of the Current Vet podcast

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In each episode, Dr. Lottie breaks down clinical conditions, cases, and concepts across species, focusing on pathophysiology, decision-making, diagnostics, and what actually matters in practice. It’s the kind of context that makes your knowledge finally click.

Every month, we’ll also have honest conversations with guests about the incredible variety of veterinary medicine, what you can do with a vet degree and how to think bigger about your career.

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Intro

Hello, hello, and welcome back to a brand new episode of Current Vet. If you're new here, this is this is the podcast that makes vet med simple. I'm your host, Dr. Lottie, and today's topic is known as Monday morning disease, or more formally, sporadic exertional rhabdomyolysis, which doesn't really roll off the tongue, so I may mix it up a bit in today's episode

But otherwise, let's jump straight into our case.

Case

It's a beautiful, sunny Monday, and you're responding to a call out to see Jack, a 10-year-old cob gelding who had been out on A steady hack this morning.

About halfway round, his rider noticed that he became reluctant to move forward, started sweating heavily, and then stiffened up. They walked him slowly home, and now he's standing in the yard with hard, painful hindquarter muscles and looking very uncomfortable. On exam, Jack is anxious, sweaty, and reluctant to move.

His hind limb muscles are firm and painful on palpation. He's tachycardic with an increased respiratory rate, and you can see patches of dark brown urine in the stable

Aetiology & Pathogenesis

Okay, let's talk about what exertional rhabdomyolysis is. So ER or SER as I'm now gonna call it in the rest of this episode is the breakdown of skeletal muscle fibers due to exercise the goes beyond the horses capacity .

Breaking down the word a little bit just to explain this, so myo refers to the muscles, rhabdo means rod-like or striations, so this means it's referring to specifically striated muscles, so the skeletal muscles, and lysis means breakdown. So altogether, rhabdomyolysis means breakdown of striated muscle.

Damaged muscle cells release CK, creatinine kinase, and AST enzymes, as well as myoglobin into the blood, and myoglobin is what is giving that urine the dark brown color. Common risk factors for ER include overexertion, inadequate fitness levels, dietary imbalances, electrolyte disturbances, and dehydration.

Historically, it was called Monday Morning Disease because draft horses were rested on Sundays, but still fed their normal big grain-filled meals. They would then return to work on Monday, and the combination of high glycogen levels in the muscles and sudden exercise triggers muscle breakdown. It is important to note that there are different forms of exertional So sporadic ER, the one we're focusing on here, is often a one-off or occasional problem, typically in leisure horses, as they aren't routinely exercised like racehorses are. Recurrent ER and another condition called Polysaccharide Storage Myopathy, or PSSM, is seen more in racehorses like Thoroughbreds and linked to underlying metabolic or genetic problems.

All of these conditions are different forms of rhabdomyolysis, they appear very clinically similar, but the underlying pathologies are different and therefore management and long-term preventment-- approaches are gonna be different too

Clinical Signs

Okay, let's talk about the clinical signs that we're typically going to see. So common signs include muscle stiffness, reluctance to move or even refusal to move forward at all, sweating, tachycardia, tachypnoea, firm painful muscles on palpation, especially the gluteals and the hamstrings, and then also pigmenturia due to that myoglobin in the urine.

The severity of clinical signs varies from some horses just feeling a little bit stiff and sore versus others becoming recumbent, distressed, and at risk of complications like acute kidney injury that can occur from those high myoglobin levels

Diagnosis

Okay, so how are we going to diagnose this condition?

A few differentials to consider for this presentation are things like laminitis, colic, possible neurological disease, but for most cases, as with this one, it's quite clear from the history of, like, sudden exercise or big meal and the clinical exam that you're looking at rhabdomyolysis. So diagnosis in the field is typically presumptive based on these clinical signs and then the features in the history.

We can also use blood work to confirm muscle breakdown, so looking at an increase in CK levels quickly after exercise. CK only has a half-life of a couple of hours in horses, so if your sample is taken more than around 12 hours after muscle breakdown has occurred, the levels may already be back to, normal."

The other marker for muscle damage that we can use is AST, and these levels rise more slowly and stay elevated for a couple of days or even weeks afterwards. So this is gonna be a much more reliable finding on hematology if the sample is taken later on. A urinalysis may also be useful to confirm myoglobinuria.

We need to make sure that we're differentiating this from hematuria, which is when there is true blood in the urine, and we can do this by centrifuging or spinning down a urine sample. So hematuria will separate, and the red blood cells collect as a sediment at the bottom of the sample, whereas myoglobin will not separate out of the sample, and it will stay unchanged with that reddish-brown pigment

Treatment

Okay, so we've assumed Jack has SCR based on the fact that he has no recent history of any conditions. He has typical signs of tying up, and we now know that he had a midnight feast in the feed shed, which the owner mentioned to you a couple of minutes ago. Treatment approaches to SCR depend on the severity of the case.

So for mild to moderate cases like Jack, we need to make sure we're stopping exercise immediately and keeping the horse calm. We should provide box rest until that muscle stiffness has resolved, and then we can use NSAIDs such as flunixin or phenylbutazone to reduce pain and any inflammation.

Encouraging oral or IV fluids if dehydration or myoglobinuria is present is also useful, and it helps to protect the kidneys. Also just to point out that NSAIDs are nephrotoxic, so they can exacerbate renal damage if it has already occurred due to that myoglobinuria. So make sure that the horse is really well-hydrated and on fluids.

But if renal damage is a serious concern, opioid analgesia is a good alternative In severe cases, hospitalization may be needed for IV fluids, intensive monitoring, and pain control. Renal parameters should be monitored very, very closely due to that potential for AKI with the high myoglobin levels, and sedatives can be given to make sure that the horse is more relaxed and resting, but they can also help to relieve muscle spasms which might worsen muscle breakdown Most horses with sporadic ER recover fully with rest and supportive care.

The prognosis is good for one-off episodes. However, recurrent cases may need deeper investigation for underlying causes. Maybe there is that PSSM or a recurrent ER syndrome. Which would need further diagnostics to uncover

Prevention

The best method of preventing SCR is good management. So ensuring consistent exercise, and avoiding suddenly working a rested grain-fed horse, adjusting the diet to reduce those levels of soluble carbohydrates, making sure we're getting balanced electrolyte levels, and making forage the foundation of the diet.

Provide adequate warm-up and cool-down periods so that muscle strain isn't happening immediately as we start exercising, and ensuring hydration is adequate, especially in hot weather or if they're doing prolonged work. Owners should also be advised that if episodes are recurring, further investigations, like we said before, that potentially a muscle biopsy, genetic tests, or diet analysis should be considered.

If you want further reading, a couple of resources you can check out are the Equine Sports Medicine and Surgery textbook, the Large Animal Internal Medicine textbook by Bradford-Smith And the American Association of Equine Practitioners, AAEP, they have a really good overview on the diagnostic approach to muscle disorders in horses and I'll link those all in the show notes if you're interested

Key Points

Okay, what are the key takeaways that you need to know about equine sporadic exertional rhabdomyolysis. So SER is skeletal muscle breakdown caused by exercise. It's also known as Monday morning disease and it's common in leisure horses and is typically triggered by management or dietary factors. The classic clinical signs are going to be stiff, unwilling to move horse with painful sore muscles, especially in the hind limbs.

They might be sweaty, they might be tachycardic and tachypnoeic, just generally very uncomfortable. Diagnosis is typically based on history and clinical signs, and then muscle enzymes on biochemistry can also be used

Treatment focuses on supportive care, so rest, analgesia, and good fluids, but severe cases may need hospitalization. And finally, consistent exercise and good dietary management are key for prevention.

Outro

And that is sporadic exertional rhabdomyolysis. Thank you so much for listening to this episode of Current Vet.

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