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Genetic Welfare Problems of Companion Animals
An information resource for prospective pet owners
American Miniature Horse
Tracheal collapse
VeNom term: Tracheal collapse (VeNom code: 2003).
Related conditions: respiratory infection, pulmonary oedema, cardiomegaly; allergic respiratory disease; obesity
Outline: Tracheal collapse is a chronic progressive disease affecting the rings of cartilage in the trachea that keep open this airway to the lungs. It is a result of changes to the cartilage that cause it to soften and weaken. Affected horses with the condition have a dry, harsh or “honking” or wheezing cough and noisy, laboured breathing. Factors that exacerbate these signs include obesity, exercise and pregnancy. Horses with the condition exhibit breathing difficulties at rest, which can worsen when horses are distressed, excited, exercising or when the weather is hot and humid.
Tracheal collapse is seen most commonly in middle-aged horses, and in miniature breeds specifically. Equine vets will diagnose tracheal collapse using one or more diagnostic tests and look for a persistent cough, response to tracheal palpation and inflammation and thickening of the lining of the trachea. Management of the clinical signs of tracheal collapse in horses is limited, and the condition may be fatal, in severe cases.
American miniature horses are over-represented in cases of tracheal collapse, and it is therefore suspected to be an inherited trait. With the limited state of knowledge on the causal factors of tracheal collapse in horses, the best advice that can be offered to prospective breeders is to avoid breeding between affected horses or from horses with severely affected relatives (ie grade 2-4), including grandparents, siblings, previous offspring and siblings of parents.
Summary of Information
(for more information click on the links below)
1. Brief description
The trachea, or ‘windpipe’, is a thin-walled tube of tissue through which air is transported to and from the lungs. Rings of cartilage along the trachea give it structure and hold the airway open. Tracheal collapse is a result of cellular changes to the cartilage of these tracheal rings that affects the rings’ structural integrity causing them to soften and weaken. Collapse of the trachea causes obstruction, inflammation and, increased mucus secretion and congestion of the airways (Maggiore 2014). The most common clinical sign that a horse has the condition is a dry, harsh or “honking” cough. Other clinical signs include respiratory distress (ie difficulty breathing), noisy breathing and a persistent cough. Bluish discolouration of the skin may also occur, due to low levels of oxygen in the blood. Once clinical signs develop, the condition is progressive, since persistent coughing can promote changes in the tracheal wall lining and results in an over-production of mucus which, in turn, further exacerbates coughing. Affected horses are at risk of respiratory infections, since the normal mechanisms to remove debris and microbes are impaired.
Tracheal collapse is often diagnosed in middle-aged horses, and it is suggested that it is caused by a congenital condition ie present at or before birth, similar to in dogs (see Yorkshire terrier). Factors that contribute to an exacerbation of symptoms include obesity, respiratory infection, fluid build-up in the lungs (pulmonary oedema), abnormal enlargement of the heart (cardiomegaly) and allergic respiratory disease.
2. Intensity of welfare impact
Tracheal collapse is a chronic progressive disease. Affected horses suffer difficulties breathing and this is likely to cause bouts of respiratory distress, and to inhibit their ability to conduct normal activities. Breathing difficulties can be seen by affected horses at rest and these can worsen when the horse is distressed, excited or when the weather is hot and humid. The complications and associated diseases of tracheal collapse may cause other problems, such as weakness and fatigue. Affected horses that are overweight or obese may suffer more intense symptoms. Individuals in severe respiratory distress with pale or blue gums, or that have collapsed, should be treated as a medical emergency.
3. Duration of welfare impact
Tracheal collapse commonly affects middle-aged horses. The respiratory distress that affected individuals suffer from can be prolonged, and occur for weeks or years before the signs become severe enough for the owner to make the decision to take the dog to the veterinary surgeon.
Management of the symptoms of tracheal collapse is not always effective, and the condition may be fatal, sometimes progressing rapidly after diagnosis, in severe cases.
4. Number of animals affected
Tracheal collapse is considered rare in the equine population in general, but it is more commonly reported in miniature breeds.
There is no reported difference in the distribution of males and females affected with tracheal collapse. However, there may be an increased risk – or exacerbation of symptoms – of tracheal collapse during mid-to-late stage pregnancy in American miniature horses.
5. Diagnosis
Veterinary surgeons diagnose tracheal collapse using one or more diagnostic tests - radiography, fluoroscopy, tracheoscopy and ultrasonography. Signs looked for are a persistent cough, response to tracheal palpation and dynamic collapse of the trachea during respiration, with inflammation and thickening of the tracheal membrane.
6. Genetic
There has been no research to date on the mode of inheritance or genetic mechanisms underlying tracheal collapse in horses. American miniature horses appear to be over-represented in cases of tracheal collapse, and therefore it is suspected to be an inherited trait.
7. How do you know if an animal is a carrier or likely to become affected?
It is not possible to be sure whether an individual is a carrier or likely to be affected as the exact mode of inheritance of tracheal collapse is unknown, however American miniature horses are more commonly affected than other breeds.
8. Methods and prospects for elimination of the problem
With the current limited state of knowledge on the causal factors of tracheal collapse, the best advice that can be offered to prospective breeders is to avoid breeding between affected horses or horses with severely affected relatives (ie grade 2-4), including grandparents, siblings, previous offspring and siblings of parents.
For further details about this condition, please click on the following:
(these link to items down this page)
- Clinical and pathological effects
- Intensity of welfare impact
- Duration of welfare impact
- Number of animals affected
- Diagnosis
- Genetics
- How do you know if an animal is a carrier or likely to become affected?
- Methods and prospects for elimination of the problem
- Acknowledgements
- References
1. Clinical and pathological effects
The trachea, or ‘windpipe’, is a thin-walled tube of tissue through which air is transported to and from the lungs. Regular rings of cartilage along the trachea give it structure and hold the airway open. Tracheal collapse is a result of cellular changes to the cartilage of these tracheal rings that affects their structural integrity and causes them to soften and weaken.
Cartilage is a connective tissue made up of two main components – cells and extracellular material. It is an important structural tissue of the body. It is firm, but is softer and more flexible than bone and in adult animals it gives support and structure to parts of the body that lack bone, such as the external ears and the airways of the lung.
Cartilage is characterized into three types, based upon its degree of flexibility, and consists of specialized cells called chondrocytes surrounded by a dense extracellular matrix of collagen and elastin fibres that are all embedded in a rubbery ‘ground substances’ that fills the spaces between the fibres and the cells. The molecules that form the ground substance, called glycosaminoglycans, are very good at absorbing water such that 90% of the normal extracellular matrix is comprised of water. This water plays an important role in ability of cartilage to resist compression.
In affected animals there are changes to the extracellular matrix and a reduction in normal levels of calcium, and complex carbohydrate molecules (glycosaminoglycans and chondroitin sulfate (Simmons et al 1988, Siger et al 1998), which are involved in the maintenance and support of collagen and elastin. These changes reduce the ability of the cartilage of the tracheal rings to absorb and retain water, leading to weakening and decreased rigidity of the trachea rings so that the rings appear flattened. Collapse can affect the cervical or intrathoracic trachea, or bronchial walls, or it may affect multiple regions. Tracheal collapse can be graded from 1 to 4, in severity, with each grade representing a 25% reduction in the diameter of trachea airway (ie grade 1 is 25% reduction, grade 2 is 50%; Tangner & Hobson 1982).
Collapse of the trachea causes airway obstruction, inflammation, increased mucus secretion and mucus trapping in the airways (Maggiore 2014). The most common clinical sign for which the horse is presented to the veterinarian for is a dry, harsh or “honking” cough upon expiration. Other clinical signs include respiratory distress (i.e. difficulty breathing), noisy breathing and a persistent cough. Bluish discolouration of the skin may also occur, due to low levels of oxygen in the blood. Once clinical signs develop, the condition is progressive, since the coughing can lead to persistent inflammation of mucosal cells lining the tracheal walls, which leads to permanent changes in the wall lining and affects the normal function of the ciliary cells that clear the trachea from mucus and debris. The result is an over-production of mucus which in turn exacerbates coughing. Affected horses are at risk of respiratory infections, since the normal mechanisms to remove debris and microbes are impaired.
Although tracheal collapse is often diagnosed in middle-aged horses, it has been reported in foals (Couëtil et al 2004), and it is therefore possible that it is a congenital condition, which most commonly is diagnosed in middle-aged animals. There are other factors that may cause or exacerbate symptoms of tracheal collapse. Over-weight or obese horses may suffer an increased intensity of symptoms, since there is extra pressure on the airways. Respiratory infections, fluid build-up in the lungs (pulmonary oedema), abnormal enlargement of the heart (cardiomegaly) and allergic respiratory disease can also exacerbate or cause the symptoms of tracheal collapse (White & Williams 1994).
2. Intensity of welfare impact
Tracheal collapse is a chronic progressive disease. Affected horses suffer difficulties breathing and this is likely to cause distress, which may be severe, and to inhibit their ability to exercise normally or eat. Breathing difficulties may be seen in affected individuals at rest, and these can worsen if the horse is in states of emotional distress or excitement, as well as during exercise. Symptoms may also worsen in hot and humid weather conditions. Individuals in severe respiratory distress with pale or blue gums or that have collapsed should be treated as a medical emergency.
There is no cure for the disease, but it can be managed by reducing exercise, stress and avoiding the use of harnesses, which put pressure on the windpipe. Medication to supress coughs, clear mucus and reduce inflammation may also alleviate the symptoms of tracheal collapse. Surgical treatment has been reported with variable success, but serious and persistent complications such as excessive granulation tissue and migration of stents, are common (Epstein 2008).
3. Duration of welfare impact
Tracheal collapse commonly affects middle-aged horses. In one study of 13 American miniature horses with tracheal collapse, the average (median) age of horses at diagnosis was 12 years, and ranged from 3 to 17 years (Aleman et al 2008).
Affected horses may suffer from respiratory distress for a prolonged period. It may be weeks or years before the signs become severe enough for the owner to make the decision to take the horse to the veterinary surgeon. In the study by Aleman et al (2008), 9 of 13 American miniature horses diagnosed with tracheal collapse had clinical signs present for 1 to 5 years and ten died due to sudden death or elective euthanasia.
4. Number of animals affected
Tracheal collapse is considered rare in the equine population in general, but it is more commonly reported in miniature breeds. There are no prevalence data for tracheal collapse in horses but there are several case reports describing tracheal collapse in miniature horses (Siger et al 1998, Couëtil et al 2004, Wong et al 2008). Tracheal collapse was reported in 13 of 231 (6%) of American miniature horses presented at a veterinary referral population in California (Aleman et al 2008).
There is no reported difference in the distribution of males and females affected with tracheal collapse. However, there may be an increased risk – or exacerbation of symptoms – of tracheal collapse during mid-to-late stage pregnancy in American miniature horses (Aleman et al 2008), and this may be due to increased pressure on the thorax, due to the developing foetus, increased metabolic demands or stress associated with pregnancy.
5. Diagnosis
Tracheal collapse is diagnosed with one or more diagnostic tests - radiography, fluoroscopy and tracheoscopy.
Veterinarians diagnose tracheal collapse on the basis of a persistent cough, response to tracheal palpation and endoscopic confirmation of the anatomic tendency to collapse, with inflammation and thickening of the dorsal tracheal membrane. The degree to which the trachea has collapsed is graded on a scale from 1 to 4, with 4 being the most severe.
6. Genetics
Tracheal collapse is rare in equines but most commonly occurs in miniature breeds, suggesting it is an inherited trait. However, there has been no research to date on the mode of inheritance or genetic mechanisms underlying tracheal collapse in horses.
7. How do you know if an animal is a carrier or likely to become affected?
It is not possible to be sure whether an individual is a carrier or likely to be affected as the exact mode of inheritance of tracheal collapse is unknown, however American miniature horses are more commonly affected than other breeds.
8. Methods and prospects for elimination of the problem
Since the genetic mechanisms of tracheal collapse are unknown, the advice that can be given to eliminate the problem is limited (Meyers-Wallen 2003). If, as suspected, tracheal collapse is an inherited defect in the tracheal cartilage, then prevention of the condition is limited to controlling the genetic predisposition of the disorder in the breed. However, the problem is more complex since some horses may not show clinical signs until they are exposed to one or more other contributory factors (e.g. respiratory infection, obesity, pregnancy). With the current state of knowledge on the causal factors of tracheal collapse, the best advice that can be offered to prospective breeders is to avoid breeding between affected horses or horses with severely affected relatives (ie grade 2-4), including grandparents, siblings, previous offspring and siblings of parents.
9. Acknowledgements
UFAW thanks Dr Emma Buckland (BSc, PhD), Dr David Brodbelt (MA VetMB PhD DVA DipECVAA MRCVS) and Dr Dan O’Neill (MVB BSc MSc PhD MRCVS) for their work in compiling this section.
10. References
Aleman M, Nieto JE, Benak J and Johnson LR (2008) Tracheal collapse in American Miniature Horses: 13 cases (1985–2007). Journal of the American Veterinary Medical Association 233: 1302–1306
Couëtil LL, Gallatin LL, Blevins W and Khadra I (2004) Treatment of tracheal collapse with an intraluminal stent in a miniature horse. Journal of the American Veterinary Medical Association 225: 1727–32, 1701–2
Epstein K (2008) Tracheal collapse: Are there other options for treatment? Equine Veterinary Education 20: 91–92
Maggiore A Della (2014) Tracheal and airway collapse in dogs. The Veterinary clinics of North America: Small Animal Practice 44: 117–127
Meyers-Wallen VN (2003) Ethics and genetic selection in purebred dogs. Reproduction In Domestic Animals 38: 73–6
Siger L, Hawkins J, Andrews F and Henry R (1998) Tracheal stenosis and collapse in horses. Compendium on Continuing Education for the Practising Veterinarian 20: 628–635
Simmons T, Petersen M, Parker J, Dietze A and Rebhun W (1988) Tracheal collapse due to chondrodysplasia in a miniature horse foal. Equine Practice 10: 39–41
Tangner CH and Hobson HP (1982) A Retrospective Study of 20 Surgically Managed Cases of Collapsed Trachea. Veterinary Surgery 11: 146–149
White RAS and Williams JM (1994) Tracheal collapse in the dog - is there really a role for surgery? A survey of 100 cases. Journal of Small Animal Practice 35: 191–196
Wong DM, Sponseller BA, Riedesel EA, Couëtil LL and Kersh K (2008) The use of intraluminal stents for tracheal collapse in two horses: Case management and long-term treatment. Equine Veterinary Education 20: 80–90
© UFAW 2016
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http://depositphotos.com/28622549/stock-photo-american-miniature-horse-standing.html ©Depositphotos.com/Zuzule
