Clinical signs
The atypical myopathy is characterized by its suddenness and its usually rapid and fatal outcome.
- Suddenness: Most of the time, the intoxicated equine is found in lateral recumbency whereas the day before, it showed no abnormal behavior.
- Lethality: The high mortality rate for equine atypical myopathy ranges from 43% to 97% depending on years and studies.
- Speed: The affected horses die or are euthanized within 3 to 10 days following the appearance of clinical signs. Surviving horses recover in about 10 days with a range of 1 to 30 days.
The clinical signs of atypical myopathy reflect the massive destruction of muscles involved in posture, breathing as well as the heart muscle. These clinical signs are present in more than 50% of all cases and are represented in order of frequency (in % of cases according to (1)):
- Pigmenturia (e., emission of dark urine; 93%)
- Generalized weakness (85%)
- Stiffness (83%)
- Depression (80%)
- Lateral recumbency (e., horse lying on its side; 78%)
- Maintained appetite (even exacerbated; 72%)
- Tremors (68%)
- Sweating (64%)
Pigmenturia is a discoloration of urine resulting from the release of a muscle protein, called myoglobin, released from muscle fibers damaged by the disease process associated with atypical myopathy. It is the most specific clinical sign of atypical myopathy.
Pigmenturia - Explanations
The color of the urine observed during spontaneous miction or following the emptying of the bladder by the veterinarian (by transrectal palpation or by urinary catheterization) contributes to making the diagnosis of atypical myopathy and to excluding other diagnostic hypotheses (botulism, tetanus, grass disease…). Exceptionally, normal urine can be observed at the very beginning of the intoxication (probably because the myoglobin is not yet released in sufficient quantity to colour the urine) or at the end of the disease (when muscle destruction has ceased).
In this photo, the urine of the same horse was collected during its hospitalization: (a) on the left side, the urine collected at the beginning of the appearance of clinical signs, (b) in the middle, the urine collected after 3 days of hospitalization and administration of infusion and (c) on the right, urine collected on the 10th day after disappearance of clinical signs. Photo credit: D-M. Votion)
The rare clinical signs « warning » of atypical myopathy described by the owners are depression, a decrease in appetite, signs of colic and/or stiffness or even lameness.
Generally, the clinical examination performed by the veterinarian reveals the following changes:
- Increased heart rate: > 45 bpm (79%)
- Congestive mucous membranes (Figure 2; brick red color; 53%)
- Normal rectal temperature: 37–38°C (60%)
- Bladder distension (58%)
Figure 2. Brick red color of mucous membranes. Photos credit: D-M. Votion
Other less common clinical signs
Dyspnea (i.e., respiratory difficulties) and tachypnea (i.e., increase in respiratorye rate over 15 breathes per minute) have been commonly reported. These respiratory signs would be the result of the degeneration of respiratory muscles including intercostal muscles and the diaphragm. When breathing difficulties increase over time and become more severe, the vital prognosis is engaged (2).
Colic signs are sometimes reported by the owners (and are sometimes the reason for veterinary consultation). These signs may result from bladder distension commonly observed in animals suffering from atypical myopathy. Emptying the bladder (by the veterinarian either during transrectal palpation or by urinary catheterization) helps to reduce these signs of colic.
Some clinical signs as dysphagia (i.e., difficulty of swallowing), oesophageal obstruction or motionless standing are more frequently observed in some years.
No abnormal behaviour attributed to central nervous disorders has been reported (the animal is conscient of its environment) except some occasional “pedalling”. This could be a sign of pain and/or anxiety and/or from the desire to get up. The pain score expressed by horses suffering from atypical myopathy could be considered as moderate in comparison with the pain score in horses with exercise-induced myopathy. Nevertheless, some horses show signs of intense pain and that are difficult to relieve with any medication and/or with the emptying of distended bladder. The presence of persistent pain despite the different treatment indicates a negative prognosis in terms of survival rate (2).
Finally, despite the gravity of the intoxication, atypical myopathy horses want to eat (e.g., attempt to grab any hay or straw in their proximity). This behavior is directly related with the action of the toxin that inhibit some energy source as lipids. However, this behavior is not related with the survival rate (2).
Bibliography sources
1. van Galen G, Marcillaud Pitel C, Saegerman C, Patarin F, Amory H, Baily JD, Cassart D, Gerber V, Hahn C, Harris P, Keen JA, Kirschvink N, Lefere L, McGorum B, Muller JM, Picavet MT, Piercy RJ, Roscher K, Serteyn D, Unger L, van der Kolk JH, van Loon G, Verwilghen D, Westermann CM, Votion DM. European outbreaks of atypical myopathy in grazing equids (2006-2009): spatiotemporal distribution, history and clinical features. Equine Vet J. 2012 Sep;44(5):614-20. doi: 10.1111/j.2042-3306.2012.00556.x. Epub 2012 Mar 26. PMID: 22448904. https://pubmed.ncbi.nlm.nih.gov/22448904/
2. Boemer F, Detilleux J, Cello C, Amory H, Marcillaud-Pitel C, Richard E, van Galen G, van Loon G, Lefère L, Votion DM. Acylcarnitines profile best predicts survival in horses with atypical myopathy. PLoS One. 2017 Aug 28;12(8):e0182761. doi: 10.1371/journal.pone.0182761. PMID: 28846683; PMCID: PMC5573150. https://hdl.handle.net/2268/214327
