Literature review of three common equine hoof ailments: Laminitis, Thrush and Navicular disease
August 8, 2017
The health of the equine hoof is closely tied to the animal’s overall health and well-being. Despite this, the hoof remains relatively understudied. Laminitis, thrush and navicular are three common disorders which impact domestic horses and donkeys. Laminitis can present itself in three different forms, acute, mechanical and endocrine (Ramey, 2011). Acute and endocrine laminitis are related to hormone imbalances in the body, specifically related to sugar consumption and insulin production (Morgan, 2015). Mechanical laminitis is most often related to conformation faults at birth, or poor trimming or shoeing methods (Ramey, 2011). Thrush is an aerobic bacterial infection of the hoof by the Fusobacterium necrophorum bacteria in the absence of Dichelobactor neodosus bacteria (Petrov, 2013). This infection often goes untreated as it appears to cause minimal damage, but can often cause extensive pain if left untreated. Navicular disease is the least understood of these three diseases (Yorke, 2014). It is generally described as pain originating from the navicular bone but there are conflicting theories regarding what is causing the pain. Through this research, it can be concluded that a better protocol for diagnosing and treating all of these ailments needs to be further established, by encouraging veterinarians and farriers to work together.
The expression “no hoof, no horse” is very common within equine communities, and yet so many diseases which impact the equine hoof are left under studied. Disorders such as laminitis, thrush and navicular are of particular concern for domestic horses. In a news article by the Inquirer, Andrew van Eps describes that in the early 2000’s veterinarians still believed that all cases of laminitis were the same and related to blood flow problems (Jerardi, 2017). Current research has only just begun to delve into how insulin is a major contributor to problems concerning laminitis.
Thrush has long gone under the radar as it does not appear to be a particularly concerning problem. However, horses can go years without successful treatment that often leads to the more severe white line disease. With current practices encouraging the use of ineffective and drastic shoeing practices many horses never see relief.
The most understudied of these maladies is navicular disease. Without a clear understanding of how to diagnose the disease, many horses are left for years with mystery lameness. Even once the horse has been diagnosed with navicular disease, the causes are understudied and there is minimal action that owners can take to help their horse. This paper intends to explore the diagnosis, causes and treatments for these common hoof ailments, through literature review as well as discussion with hoof lameness specialists.
Laminitis can manifest due to a variety of ailments and has a wide range of acute and severe to chronic and mild cases. Laminitis is a disease that causes the connective tissue in the hoof capsule called the laminae to become inflamed (Ramey, 2011). As laminitis progresses and the laminae become weaker, the P3 or distal phalanx can begin to rotate downward; an action referred to as foundering (Ramey, 2011).
There are three types of laminitis, acute laminitis, mechanical laminitis and endocrine laminitis. Acute laminitis can be caused by excessive intake of sugar at one time or toxins in the form of bacterial infection or colic. Mechanical laminitis is caused by the physical malformation of the hoof or poor trimming or shoeing techniques, in combination with impact over time. Endocrine laminitis can be caused by obesity, hormonal imbalances, acute or prolonged periods of stress or toxins created by liver or a respiratory diseases (Rendle, 2017a). As much as laminitis is a serious disease in its own right, more often than not, it is a symptom to a larger problem. Both acute laminitis and endocrine laminitis are related to the horses’ inability to correctly regulate insulin (Rendle, 2017a).
Within domestic horse populations acute laminitis most often presents itself in the form of sudden increase of excess sugars (Ramey, 2011). This can be in the form of excess grains or other concentrated feeds over time or in an acute setting. However, more commonly the excess sugars are found in grasses in the spring and fall. Fructons are the plant’s storage sugars or oligosaccharides. These sugars are more present in greater amounts in grasses in the spring and fall after frost events, as well as throughout the winter while the grasses are surviving under the snow (Ramey, 2011). The sugars in the grass are problematic at these times because this form of sugar cannot be digested by the horse. The oligosaccharides will pass through the horse’s small intestine and be deposited into the hind gut undigested. This sugar is then processed and consumed by the Streptococcus lutetiensis bacteria (Ramey, 2011).
This causes harmful problems for the horse in two ways, first, the Streptococcus lutetiensis bacteria produces harmful lactic acid in the horses gut as they consume the sugar. Secondly, the disruption in the horses gut triggers the body to produce an increase of insulin; the increase of insulin is related to the inflammation of the laminae. Horses who are out on pasture all the time will have the ability to adjust to the changes in the grass. However, horses who are only turned out part time and are fed supplemental grains and hay will likely have difficulty processing the grass. An event like this may also cause the horse to colic, which can create other toxic side effects which may contribute to the acute inflammation of the laminae.
Although equally as serious, mechanical laminitis is far simpler in nature. When the horse’s heel is left artificially long in order to “enhance” natural movement it can impact the internal structures of the hoof, including the distal phalanx (Ramey, 2011). When the laminae are forced into a weak position due to shoeing or trimming style, the ground force can cause the laminae to become inflamed over time establishing chronic laminitis. The horse may face conformation faults such as a “club foot” which may also force the laminae into a week position (Ramey, 2011). Mechanical laminitis can cause serious pain and discomfort for the horse and in severe cases, mechanical laminitis can develop into a foundering situation. However, mechanical laminitis is the easiest form of laminitis to treat. Often the laminitis can be treated when the hoof shape is corrected through various trimming methods (Ramey, 2011).
Endocrine laminitis is the most complicated form of laminitis to treat and according to a study by Morgan (2015), about 90% of laminitic horses have some form of endocrine laminitis, although to date no study has been conducted to determine which forms of endocrine laminitsis are more common. Endocrine laminitis can be caused by an array of hormonal imbalances in the horses’ body. The two most prevalent being equine metabolic syndrome and pituitary pars intermedia dysfunction (Rendle, 2017a). Although these diseases can have devastating results, evidence from literature suggests that both can be diagnosed and treated at early enough stages before the disease progresses into laminitis (Morgan, 2015).
Equine metabolic syndrome
Equine metabolic syndrome develops when both obesity and insulin resistance are present in an equine. Obesity can be observed as a regional increase of fatty tissues in the crest (top of neck) and rump or general weight gain evenly across body (Morgan, 2015). Insulin resistance can be characterized by excessive insulin production response or insulin dysregulation; this is detected through blood testing (Morgan, 2015). According to literature studied by Morgan (2015) obesity is prevalent in 19 – 40% of domestic horses and insulin resistance occurs in 22 – 29% of domestic horses. The main risk associated with equine metabolic syndrome is that adipose tissues are higher in obese horses.
Adipose tissue behaves as an endocrine organ and secretes adipokins including leptin and adiponection. Adipose tissue also secretes cytokins including tumour necrosis factor A and interleukin -1 (Morgan, 2015). When adipose tissues are present in excess, the chemicals that ae secreted can cause both local and system wide problems for the horse. The exact way that insulin resistance in equines is unknown, yet researches have identifies a combination of symptoms that a horse experiences which can lead to equine metabolic syndrome and laminitis (Morgan, 2015). When adipose tissue is high there is a combination of local and remote inflammation, caused by the increase of chemical secretions; oxidative damages; lipid overspill; and alteration in the release of adipokins. When these factors are present in the horses system it impairs insulin signalling within the fat tissues (Morgan, 2015). The impaired signalling can lead to excessive production and / or dysregulation of insulin within the body.
The high amounts of insulin in the horses’ body will result in the alteration of the form and function of vascular tissues with the body. Equine metabolic syndrome based inflammation has rapid and significant disruption of vascular membranes. The lamina (a vascular tissue) becomes inflamed as it is unable to process the insulin in the blood stream (Morgan, 2015). As previously mentioned, as the lamina become inflamed, the lamellar – hoof wall junction becomes disrupted from its regular form and function (Rendle, 2017a).
A challenge with equine metabolic syndrome is to treat the disease before it manifests as laminitis. The most effective way to diagnose equine metabolic syndrome is through visual body condition scoring in combination with blood tests if further concern is spiked. In many domestic situations, equine obesity is not treated as a serious condition, making early diagnosis rare (Morgan, 2015). If equine metabolic syndrome is diagnosed early enough, the primary forms of treatment are diet, exercise and controlled pharmaceuticals.
It is important the horses’ new diet consist of no concentrated feeds or grains including succulent treats such as carrots or apples. Forage that is low to medium in non-structural carbohydrates with no more than 10% non-structural carbohydrates is important (Morgan, 2015). Non – structural carbohydrates are most concentrated found in the seed of a plant followed by the leaf and stalk, thus the type of grasses in the hay and the cut of the hay will impact the percentage of non- structural carbohydrates (Morgan, 2015). Horses trying to lose weight should consume approximately 1.25% of their body weight in hay, a healthy horse might consume between 2 and 2.5% of their body weight in hay. If more drastic measures need to be taken in order to help the horse lose weight rapidly, soaking the hay is an effective strategy. Soaking hay in cold water for about one hour reduces the sugar content in the hay by about 30% and soaking the hay for 12 to 16 hours can reduce the sugar content by about 50% (Morgan, 2015). It is critical that horses being fed soaked hay are provided with adequate supplements as many of the water soluble macronutrients found in the hay will be leached out (Morgan, 2015).
The horse should take on a new and healthy exercise routine to help with weight loss. This should include regular turn out on a dirt paddock, or grass paddock with a grazing muzzle. If the horse is staring at zero fitness, the exercise regime should start with short trot sets that increase in number of sets and length of sets over time (ie. 10 minutes of walk followed by trotting for 3 minutes then repeated two times; after six weeks this might shift to 10 minutes of walk followed by 5 minutes of trot repeated three times). Studies have shown that exercise, independent of weight loss, can help improve the body’s sensitivity to insulin and reduce symptoms of equine metabolic syndrome (Morgan, 2015).
Pharmaceutical intervention is generally seen as a last resort if diet and exercise are not helping to relieve the symptoms which the horse is experiencing. The primary drug to treat equine metabolic syndrome is an anti – hyperglycaemic drug which improves blood glucose homeostasis (Rendle, 2017a).
Pituitary Pars intermedia dysfunction
Pituitary pars intermedia dysfunction is also known as equine Cushing’s disease, however it is not the same as human Cushing’s disease. This disease is relatively common, as approximately 30% of domestic horses are at risk (Rendle, 2017b). Pituitary pars intermedia dysfunction is characterized by an over active intermediate lobe of the pituitary gland. Including the excessive release of metabolically active proteins and hormones (Rendle, 2017b). The primary proteins and hormones that are released are the adrenocorticropic hormone, endorphins and melanocortins. When these chemicals are released in excess they have the capability to damage the nerve supply to the pituitary gland (Rendle, 2017b).
Pituitary pars intermedia dysfunction is a disease which progresses slowly and primarily in older domestic horses. The symptoms include Hirsutism, abnormal fat deposits, laminitis, muscle wasting, polydipsia and polyuria (Rendle, 2017b).
Hirsutism is the excess growth of hair and lack of hair loss during the summer. This is the most common symptom to identify pituitary pars intermedia dysfunction as it is the only equine disease that will cause the increased hair growth and retention (Rendle, 2017b). Abnormal fat deposits, are particularly in the crest. The dangers of the abnormal fat deposits in the crest area of the horse’s body are the same as discussed earlier in the paper in reference to equine metabolic syndrome. Laminitis is the most severe symptom of pituitary pars intermedia dysfunction. Muscle wasting is an important symptom to pituitary pars intermedia dysfunction as the deconditioning of the horse may impact their energy habits (Rendle, 2017b). Polydipsa and polyuria are when the horse increases water consumption and urination production. Horses who are suffering from pituitary pars intermedia dysfunction may experience a one third increase of water consumption. It is unknown why horse experience polydipsia and polyuria, however it is not the only disease that can present these symptoms (Rendle, 2017b).
Treatments for pituitary pars intermedia dysfunction are seen as both pharmaceutical allopathic medicine and natural long term treatments. Pergolide is a drug that stimulates the dopamine receptors in the equine brain. The drug is able to replace the function of the nerves damaged by pituitary pars intermedia dysfunction. This results in a reduction of hormones, returning them to their previous levels (Rendle, 2017b). Cyprohepatidine is another common drug for pituitary pars intermedia dysfunction. This anti – histamine can have a range of impacts on the body’s hormones; though on its own is not considered effective to treat the disease, it is used in conjunction with pergolide (Rendle, 2017b).
Natural remedies are commonly discounted through the scientific lens, but it is important not to discount these powerful forms of medicine. There are a variety of herbal remedies that can aid recovery including extract from chasteberry (Vitex agnus castus) and other marketed products such as “CUSH – AID”. Marketed products often have a vitamin B complex, Iron, folic acid, garlic oil and fenugreck (Animal health company, 2017). Another important natural remedies can include correctly trimmed barefoot management of horses. Although there is very little scientific evidence supporting the barefoot trim as treatment, I have witnessed success with a variety of horses through Kate Romenanko’s trimming school and apprenticeship hours. Further there are many ways to naturally aid a horse’s natural digestion that will aid in their metabolizing of sugars; for example adding probiotics and prebiotics to “reseed” the gut and adding apple cider vinegar to their water (Rendle, 2017b).
When both equine metabolic syndrome and pituitary pars intermedia dysfunction are caught in the early stages and effectively treated if through natural treatments such as diet and exercise or through pharmaceutical options, laminitis can be prevented.
The causes of laminitis if acute, mechanical or endocrine are relatively well understood. However, during an acute attack, mechanical failure or endocrine example of laminitis, how to fight the swelling of the laminae and foundering of the P3 bone is relatively understudied. Throughout research conducted in this reading course, many theories for improving the care for a horse who is experiencing laminitis were suggested. Yet, confusion and tension remain at the heart of how to treat this painful condition. Veterinarians continue to publish contradicting studies with a huge range of success and failure. Although the success of treatment is dependent on wide range of factors, further research needs to be conducted to help establish a basis of effective treatment protocol.
Thrush is a bacterial infection that can occur on the bottom of the hoof capsule. Thursh can occur on the sole or horn of the hoof capsule. Thrush can be easily identified on the hoof as black foul smelling necrotic material in later stages or in early stages as slight whitening of tissues which is usually only visible during a trim (Petrov, 2013). The anaerobic bacteria that cause thrush tend to thrive in the collateral groves and central sulcus of the hoof (the deep grooves between the frog and the bars and the groove in the center of the frog) (Petrov, 2013).
There is very minimal research regarding the frequency of this disease, however, a study by Abidin (2013) surveyed five Malaysian stables and thrush was present at all five stables, but the horses who lived in stables with poor property management were about 12% more likely to contract thrush. Of the 53 horses who were observed 40% of them suffered from thrush in some degree (Abidin, 2013).
Thrush in horses is slightly different then foot rots in other common domestic animals. In animals such as cows, sheep, pigs and goats foot rot is caused by two bacteria, Fusobacterium necrophorum and Dichelobactor nodosus while equine thrush is caused by Fusobacterium necrophorum in the absence of Dichelobactor nodosus (Petrov, 2013).
This bacteria can manifest on the hoof capsule when poor trimming or shoeing has occurred; when the hoof does not work correctly, it is unable to “pop” dirt and other particulates out of the sole area of the foot and anaerobic bacteria can begin to thrive (petrov, 2013). Another cause for thrush is the lack of exercise; many domestic horses are kept in stalls for a significant portion of the day, this prevents movement which can again encourage dirt and debris to stay in the sole area of the foot
Thrush can also travel up into the hoof capsule underneath the wall. When this travelling occurs, the Fusobacterium necrophorum causes white line disease. White line disease can occur on any part of the hoof capsule, causing the disease to range from very mild to quite severe (Petrov, 2013). Depending where the bacteria is on the foot, ground force may not be severe (quarter areas) or may cause separation of the hoof wall and the lamina (toe area) (Petrov, 2013). Thrush can develop into white line disease as the bacteria make their way up through the foot, or due to a direct trauma which can introduce the bacteria in a location higher up in the hoof capsule (Petrov, 2013).
Thrush can be treated through a variety of store bought remedies such as “Thrush Buster”, or one can use more natural treatments such as apple cider vinegar mixed with tea tree oil. Thrush can also be prevented through correct and routine hoof care (Petrov, 2013). Although this bacteria is relatively easy to treat, too often it goes untreated or unnoticed until it develops into a more severe white line disease. Often shoeing methods are used to “hold the foot together” as a method of treatment. Shoeing the horse is a positive intention which unintentionally can seal the bacteria into the foot causing the progression, not regression of the bacterial infection (Petrov, 2013).
Research needs to be conducted on the effectiveness of treatment methods. Questions such as how a farrier can effectively treat thrush or white line disease without inadvertently aiding in the progression of the disease need to be answered. Research also needs to be conducted regarding the effectiveness of natural treatments such as the application of apple cider vinegar, compared to store bought formulas.
Navicular disease or navicular syndrome is a less common, but very serious condition related to the health of the equine hoof capsule. This disease is characterized through pain or lameness originating from the digital sesamoid bone, also known as the navicular bone (Parks, 2015). The first indication of a problem with navicular bone is that the horse will stand forward at the knee and will not fully straighten at the knee standing or in motion. The second indication that the problem is navicular and not another foot problem is that the pain significantly decreases after a nerve block of the palmar digital nerve (Smith, 2015). A standard diagnosis standard has yet to be established.
Some researchers advocate that the exact cause of navicular disease is unknown, due to the fact that the complex disease is seriously under researched (Yorke, 2014). Although the detection for navicular disease has become significantly more sophisticated, it is critical to remember that often navicular disease has a snowball effect on the horse’s body. Meaning that it is often not a single disorder occurring with the navicular bone and rather a sum of problems (Dyson, 2011).
One common theory is that ossified fragments are often found adjacent to smooth concave areas of the navicular bone cause pain. This identification of ossified bone fragments began as the common theory in 1937 with the use of radiographic imaging on horses who suffered from navicular disease (Yorke, 2014). However, it was later discovered that there is no statistical difference between horses who experience lameness and those who do not in correlation to distal border bone fragments, though this common theory persists (Yorke, 2014). In recent years, MRI is used to image the hoof capsule when navicular is suspected as it can allow more definitive diagnosis. The MRI can identify a variety of potential causes for pain including pathology of navicular bone and the associated ligaments (Park, 2015). The MRI can detect increased uptake of radiopharmaceuticals which can provide indication of injured areas of both bone and soft tissues which would not otherwise be visible on the radiographic images (Dyson, 2011).
One suggested cause of navicular disease is the chronic overloading of the forelimbs (Frevel, 2014). The excess force on the forelimbs will induce dorsiflexion of the hoof capsule and change the amount of force on the navicular apparatus (Frevel, 2014). This change of force on the navicular apparatus then pulls the impar and suspensory ligaments to a taught and almost overextended stance. As this extension occurs over time, the junction between the ligament and the associated bones become under stress. This tension then contributes to forces on the navicular bone, potentially being a source for pain (Frevel, 2014). In more extreme cases, the navicular bone can be subject to an avulsion fracture; meaning that the bone is fractured from the force of the impar ligament (Yorke, 2014).
Another suggested cause of navicular disease is vascular disturbance (Zaag, 2016). The navicular bone has two large arteries which pass through the bone and into the hoof capsule (Belknap, 2017). When functioning correctly, the spindle shaped navicular bone should pivot with every step the horse takes. This pivoting action is important in order to control blood flow into and out of the hoof, acting both as a pump and a limit to the amount of blood in the hoof capsule (Belknap, 2017). A defect can occur during the horses’ fetal stage which can impact the function of the navicular bone. If a defect of this nature occurs, the navicular bone can be prevented from pivoting, causing the arteries to be permanently left “on” or “off” consequently either eroding the bone causing pain or more seriously preventing blood flow to the hoof capsule (Zaag, 2016).
This same vascular problem can occur over time. With a combination of poor trimming or shoeing and / or poor environmental conditions, the horses’ heels can become contracted (Zaag, 2016). As the heels contract the internal structures of the hoof changes and the bars of the hoof will be pushed closer together and higher up in the hoof capsule. When this shift occurs, the bars will then begin to impact the function of the navicular bone (Zaag, 2016). This slow shifting of horn and bone will likely cause the navicular to be turned on for more time than it can shut off. As the bone erodes from the excess blood flowing through the arteries the horse will experience pain.
It is also possible that navicular related pain can be caused by the degeneration of the fibrocartilage (Dyson, 2011). This most often occurs on the palmar aspect of the navicular bone. As degeneration of fibrocartilage occurs and lesions become more significant in the impacted area, the adjacent bone can become damaged (Dyson, 2011). Not only can these lesions impact the surrounding bone, but the lesions may lead to other health problems in the hoof capsule. For example when the lesions reach a certain size they can have a significantly negative impact on the deep digital flexor tendon and therefor the whole function of the leg (Dyson, 2011).
Again due to the under researched nature of this disease, the potential treatments for this disease are relatively unknown. Both in literature and the general public, treatment for navicular disease is often not an option, however the focus is more on pain management (Ruff, 2016). For most horses pain management means that they are subject to a life on a variety of supplements and pain killers.
However, a suggested management tool for navicular disease is the use of Clodrovic Acid (Frevel, 2014). This acid is a bisphosphate which can stop bone resorption. Bone resorption is when osteoclasts in the body break down bone tissues therefore releasing minerals that were previously stored in the bone tissue. This deterioration is problematic on two fronts; firstly the bone deterioration causes mechanical weakness and potential pain in the navicular bone; secondly the minerals that are released can travel through the blood stream and become deposited in other parts of the body (Frevel, 2014). Clodrovic Acid inhibits the formation of hydroxpatite crystals, it is these crystals that allow the osteoclasts to deteriorate the bone tissues (Frevel, 2014).
In order to determine the effectiveness of this treatment 146 horses at different locations in the United States and in Germany underwent observed treatment for 180 days (Frevel, 2014). At 8 weeks of treatment horses who had the treatment of Clodrovic Acid were experiencing statistically significant improvements over those who were not receiving treatment. After 180 days of treatment, it was observed that the effectiveness of the initial treatment was sustained in 60% of horses who experienced success at the 8 week mark (Frevel, 2014). Although this experiment indicated some success, further research needs to be conducted in order to help horses who suffer from the disease.
Through the next steps of understanding and addressing navicular disease a substantial amount of research needs to be conducted. Firstly researchers need to establish which factors or detectable changes to the navicular bone do and do not cause pain for the horse. The confusion surrounding whither or not fragments cause pain needs to be addresses. The relationship between the various causes and how they relate to each other needs to be better understood. How the palmar digital nerve relates to navicular pain needs to be better understood. Further, a strategy for early diagnosis needs to be established. Many horse owners do not know what to look for until the symptoms have become severe and the horse is in a great deal of chronic pain. Once the detection of the disease and the cause of the disease are better understood, only then can effective treatments begin to be formed.
After investigating these hoof ailments, it is clear that a large amount of research has been conducted in order to better understand the disorders which cause our equine partners pain. The recent revelation that laminitis can come in many different shapes and forms continues to encourage me to question common theories and research which does not align with my personal views or experiences.
As a barefoot trimmer, I come across chronic cases of thrush and white line disease which have persisted for years under the care of farriers who choose shoeing to help solve the problem; then after two to three barefoot trims the bacteria no longer impacts the hoof. I can only hope that like laminitis the common understanding of how the ailment thrives and how to care for the hoof can shift to a healthier, preventative scenario allowing all horses to have bacteria free feet. The understanding of navicular disease continues to improve through research, it is critical that veterinarians and researchers are able to use literature to guide them, without getting caught up in simply perpetuating myths regarding the disease. Most importantly researchers need to continue to ask the difficult questions and continue to delve deeper into how to help domestic horses overcome hoof related health concerns.
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Jerardi, D. (2017). Research into laminitis, the disease that doomed barbaro, showing great. The Inquirer: Daily News. http://www.philly.com/philly/sports/other_sports/laminitis-research-barbaro-andrew-van-eps-new-bolton-center-chris-pollitt-20170719.html?utm_campaign=coschedule&utm_source=facebook_page&utm_medium=American+Association+of+Equine+Practitioners