By Catherine Rudenko
Tying-up or ER (exertional rhabdomyolysis) is a problem that every yard will encounter at some point in time with reports of 5-7% of the thoroughbred population being affected. ER is the general term used to cover two main forms of tying-up, acute or recurrent. ER by definition relates to the breakdown of striated muscle fibres following exercise. These fibres connect to the bone allowing movement of the skeleton. Damage causes anything from mild stiffness to the inability to move.
With much still unknown about the condition, the focus falls on reducing risk and ongoing management of those affected with recurrent form. The main area for intervention and management relates to feeds and feeding practices, an area that can be directly controlled by the yard and adjusted as needed for the individuals most affected.
Acute Exertional Rhabdomyolysis
The acute form is typically caused through factors external to the muscle rather than their being an intrinsic muscle defect.
Most commonly seen when the horse is adapting to a new level of work and the intensity or duration is too strenuous. Where speed work is concerned, the most likely cause is a depletion of cellular high energy phosphates, the muscles’ energy supply, combined with lactic acidosis. Where endurance work is concerned, depletion of intracellular glycogen—the stored form of glucose often combined with over-heating and electrolyte imbalances—is the common cause.
The other key factor for an acute episode is dietary energy intake being excessive to the current level of work. The use of high starch feeds to supply energy for horses in training is a common practice with grains, traditionally oats, forming the basis of such feeds. In the early stages of fitness work, an over-supply of energy relative to need, particularly when starch forms a large part of the diet, is a risk factor.
Recurrent Exertional Rhabdomyolysis
This form of ER, where episodes are frequent and often seen even at low levels of exercise, has led to the suggestion that much like humans, there is an inherited intrinsic muscle defect. Such defects would predispose the horse to ER. Documented defects relevant to thoroughbreds include a disorder in muscle contractility or excitation contraction coupling, whereby muscle fibres become over-sensitive and normal function is disrupted.
Risk factors for ER in horses with the recurrent form include stress or high excitement during exercise, periods of jogging (10-30 minutes), infrequent exercise and over-feeding of energy in a high starch format relative to need.
Dietary Considerations for ER
The amount of energy fed and the type of energy fed are important considerations whether looking to avoid an acute feed related episode or considering the management of a horse with the recurrent form.
Other nutrients often talked about when managing ER include vitamin E, selenium and electrolytes. Historically the inclusion of vitamin E and selenium were considered important for the prevention of further episodes, however there is no evidence to support such use. A case of deficiency in either of these nutrients may well put the horse at a disadvantage and could perhaps create a state where occurrence is more notable; however, with the advent of fortified and balanced complete bagged feeds, such nutrients are normally supplied in more than adequate amounts. Their role as antioxidants which function to ‘mop-up’ damaging free radicals generated through training is where their use can benefit any horse at this level of work. The use of additional vitamin E is also recommended when increasing the fat content of the diet—a common practice when feeding horses with recurrent ER.
Electrolytes do play an important role in normal muscle function, and any deficiency noted in the diet should be corrected. Identifying a need in the diet is more easily done than determining if the individual horse has a problem with absorption or utilisation of the electrolytes. A urinary fractional excretion test (FE) will highlight issues, and subsequent correction through the diet to return the horse to within normal ranges may offer some improvement. However, it is important to note that for horses with recurrent ER, where an intrinsic muscle defect is present, the research to date has shown no electrolyte imbalances or differences between such horses and unaffected horses.
Quantifying ‘Low Starch and High fat’ Feeding
The recommended practice for management of ER is a reduction in starch and an increase in fats. This practice has two ways of benefiting the horse: a reduction in ‘spookiness’ or reactivity and a positive effect on muscle damage as seen by lower CK (creatine kinase) levels following exercise.
Positive effects on lowering CK levels were found when a higher proportion of the energy fed came from diets higher in fats and lower in non-structural carbohydrates (starches and sugars). The effect was noted when fed at 4.5kg/day—an amount easily reached and normally surpassed when feeding horses in training. The beneficial diet provided 20% of energy from fats and only 9% from starches and sugars compared to the more traditional sweet feed diet providing 45% of energy from starches and sugars and less than 5% from fats.
Top dressing of oils will increase fat in the diet - with a normal intake of up to 100 mls per day. Although the horse can digest higher amounts, palatability usually restricts a higher intake. Pelleted or extruded fat sources are increasingly popular as alternatives to oils for their convenience of feeding and palatability. Straight rice bran and blends of materials such as rice bran, linseed and soya are available from most major feed companies. Oil content will typically range from 18-26% providing 180g-260g of oil per kilogram as fed.
Racing feeds will also provide oil in the diet; content is quite varied typically from 4-10% providing 40g-100g per kilogram as fed. Hay and haylage also contains oil at a low level, typically 2% providing just 20g per kilogram on a dry matter basis.
Traditional feeding based on oats and other whole grains will have a higher starch content than feeds using a combination of grains and fibres. Levels of starch found in complete feeds and straights have a broad range from as low as 8% in a complete feed, specifically formulated to have a low starch content, and up to in excess of 50% for straights such as barley and naked oats.