Brendan M. Corcoran, MVB, Dip Pharm, PhD, MRCVS
Hospital for Small Animals, The University of Edinburgh, Easterbush Veterinary Centre Roslin
Idiopathic pulmonary fibrosis is a clinical condition that is described in humans, and more recently it has been suggested that a similar condition might exist in dogs. In particular, it has been noted that a clinical condition with many of the characteristics of IPF appears to be particularly prevalent in the West Highland white terrier, and there are on-going studies in Scotland and the United States that are attempting to confirm this is the case and to fully characterise the clinical features of the disease, its diagnosis and methods of treatment. A similar condition has also been described in the cat, but as for the dog there is limited pathological confirmation that the condition definitely exists, or is at least analogous to the condition in humans.
Dealing specifically with the WHWT, there is a recognised clinical entity that has many of the characteristics that could be attributed to lung fibrosis of unknown cause. Much of our thinking on this disease is extrapolated from what is known in human IPF, but such extrapolation has to be undertaken with caution. The underlying cause of the disease is unknown, but because of the close association between environmental and occupational pollutants with a wide range of interstitial and fibrotic lung conditions in humans, it is tempting to suspect that an environmental cause is implicated in dogs. However, the disease in dogs appears to be highly breed specific implying a genetic basis to the disease. The cause needs to be determined, but this will depend on an accurate phenotypic description of the disease.
IPF in the WHWT typically presents as a slowly progressive onset of clinical signs referable to the respiratory system. Dogs are middle to old aged, and the deterioration noted by the owner is often attributed to aging. They will present with exercise intolerance, tachypnoea and dyspnoea during exercise or exertion, and as the disease progresses, coughing will become apparent. Cyanosis may be apparent at rest or after exertion, and is a sign of advanced disease and usually seen towards the terminal stage of the disease. Syncope or pre-syncope may also occur, and while presumed to be a consequence of intermittent and transient hypoxia due to respiratory impairment, might be due to sick sinus syndrome, which is also recognised in this breed. Throughout, the dogs remain healthy and apart from the respiratory disability, which eventually becomes severe, are bright, alert and responsive, with normal appetite. Death occurs as a consequence of respiratory failure, although euthanasia is usually carried out once the respiratory difficulty becomes intolerable. The course of the disease is quite long. It can be 12 months before the owner even considers presenting the dog for assessment, and survival beyond the time of diagnosis can be up to 36 months. The more likely survival is between 8 and 15 months.
The classic feature of IPF in WHWT, apart from the natural history of the disease, is the presence of inspiratory crackles on chest auscultation. This can vary in distribution and severity, and the more diffuse and louder the crackles, the worse the disease and the shorter the survival time. As crackles are a common feature of chronic bronchitis and pulmonary oedema, these are the main differential considerations, and of the two, it is particularly important the presence of chronic bronchitis is identified or excluded. In some individuals both IPF and chronic bronchitis can be found to co-exist, but what relationship exists between the two diseases is unknown.
The definitive diagnosis of IPF would rely on lung biopsy, but this is not feasible for a variety of reasons. Firstly, the exact pathological features of IPF in the WHWT have not been fully characterised, and secondly the patchy distribution of the pathological changes would make accurate lung biopsy difficult, except in the very advanced cases, where the benefit of such procedures to the patient could be questioned. The tentative diagnosis is made on the basis of the breed, clinical history, detection of crackles on auscultation, identification of an interstitial pattern on thoracic radiography, exclusion of chronic bronchitis on bronchoscopy, and exclusion of any other possible explanation for the clinical signs. In addition the use of computed tomography, to confirm the type of radiographic lung changes, their distribution and severity, would appear to be diagnostically beneficial. On radiography, an increased diffuse interstitial pattern in seen, which can be so severe as to obliterate most of the detail within the chest. There may be evidence of right sided cardiomegaly, but this is usually of no consequence as there is little evidence to suggest right-sided heart failure is a complication of the disease. Bronchoscopic evidence of chronic bronchitis may be found in some cases, and it is difficult to know how to interpret this with respect to IPF. If the chronic bronchitis changes are severe, it would be reasonable to suppose the lung interstitial changes are a consequence of this, and the dog does not have IPF. High resolution computed tomography might allow us to better answer this question, in that it is more sensitive in identifying the presence of interstitial fibrosis and lung honeycombing, and also the presence of traction bronchiectasis, which is readily recognised in human patients with IPF. Studies are on-going evaluating the use of HRCT in IPF dogs.
The medical treatment of IPF in the dog relies on glucocorticosteroids (prednisolone) and bronchodilators. There is anecdotal clinical evidence that this drug combination may be beneficial in some cases, but exact figures or data from controlled studies are not available. However, it is possible that the efficacy of prednisolone in the control of the clinical signs in IPF dogs is due to the fact that they have concurrent chronic bronchitis. In the authors' experience, dogs with strong evidence of IPF alone (no chronic bronchitis) appear to respond poorly, if at all, to prednisolone. Work on the effectiveness of prednisolone is ongoing. Nevertheless, all suspected cases should be given prednisolone on a trial basis as it may improve quality of life and extend survival, but will have no effect on the eventual outcome of respiratory failure. In human IPF prednisolone is widely used and appears to be beneficial in a number of cases, particularly those that have the more active desquamative interstitial pneumonitis form. However, in the more common usual interstitial pneumonitis form of IPF, where there is extensive fibrosis, and to which canine IPF is probably most similar, glucocorticosteroids are less effective. Additional approaches to drug therapy in human patients include the use of immunosuppressive and cytotoxic drugs, such as azathioprine and cyclophosphamide. However, convincing data that their use in combination with prednisolone in human IPF results in a better outcome compared to prednisolone alone is lacking, and there is no information as to what effect such drug combinations would have in canine IPF. The use of the antifibrotic drug colchicine has been advocated for both the human and canine form of the disease, but human studies do not support that suggestion. The use of colchicine in dogs with IPF has not been evaluated. However, if the disease could be identified in the early stages it is tempting to think that colchicine might arrest or slow the development of full-blown IPF.
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