J . small Anim. Pracl. (1974) 15, 27-32
Spontaneous pneumomediastinum in the racing Greyhound B . R . J O N E S , * M . L. B A T H
AND
A. K . W . W O O D
Department of Veterinary Clinical Sciences, University of Melbourne, Veterinary Clinical Centre, Princes Highway, Werribee 3030, Victoria, Australia
ABSTRACT This paper describes the clinical and radiological features of spontaneous pneumomediastinum in three racing Greyhounds. The clinical signs were characterized by dyspnoea, subcutaneous emphysema and increased bronchial tones. The diagnosis was confirmed by thoracic radiographs. T h e major structures of the anterior mediastinum which are normally not visible in thoracic radiographs, are easily seen with the presence of large quantities of air in the mediastinum. No definite cause of pneumomediastinum was found in these dogs but all of them recovered after 5 days’ kennel confinement and a 4 week break from training. INTRODUCTION Pneumomediastinum is an entity in which free air is found within the mediastinum (Munsell, 1967). It has been recognized in dogs (Walker, 1959; Reed, 1965; Dixon, 1972; Burrows et al., 1972; Olsson, 1973). Air may enter the mediastinum via the deep fascia1 planes of the neck, the adventitia of the musculature of the lung, by perforation of the trachea, ~bronchii,or oesophagus, or rarely by dissection from the retroperitoneal space (Rogers et al., 1972). Pneumomediastinum can be recognized radiographically. In thoracic radiographs of normal dogs the anterior mediastinum is detectable in the lateral radiograph as a soft-tissue shadow, ventral to the trachea, extending from the thoracic inlet to the cranial border of the cardiac silhouette. Usually it is impossible to see anatomical structures apart from the trachea within the anterior medias* Present address : Department of Veterinary Clinical Sciences, Massey University, Palmerston North, New Zealand. C
27
28
B . R. J O N E S ct
al.
tinum. In pneumomediastinum the air acts as a contrast medium outlining the major blood vessels. The purpose of this paper is to present the clinical signs and diagnostic features of spontaneous pneumomediastinum in three racing Greyhounds examined at the University of Melbourne’s Veterinary Clinical Centre. The possible pathogenesis of the condition is briefly discussed. H I S T O R Y AND C L I N I C A L F I N D I N G S Case 1. The first case concerns an 18-months-old male Greyhound. I t had pulled u p in respiratory distress after running 300 metres, but appeared to recover after several hours. When walked the following morning it quickly became dyspnoeic. There was no history of a previous illness and the dog had won a trial-race a week earlier. On examination the dog was in good condition and clinical abnormalities were confined to the respiratory system. Auscultation of the thorax revealed increased bronchial tones which were audible over the left and right lung fields. The dog was admitted to the hospital for observation. The day after admission subcutaneous emphysema was palpated at the thoracic inlet and in the axillae. Lateral and ventro-dorsal radiographs were taken of the thorax and in the lateral radiograph air was seen to be present in the anterior mediastinum, outlining the brachiocephalic artery, left subclavian artery and the anterior vena cava. Air had migrated through the thoracic inlet and subcutaneous emphysema was present in the cervical region and extended back along both sides of the rib-cage. There was also a left-sided pneumothorax with partial collapse of the left diaphragmatic lung lobe. Case 2.
A 29-months-old male Greyhound was presented for examination with a history of poor racing performance. On examination it was found to be in good condition and the only significant clinical finding was subcutaneous emphysema at the thoracic inlet. No abnormalities were detected on auscultation or percussion of the thorax. A diagnosis of pneumomediastinum was made from thoracic radiographs. O n the lateral view, air was present in the anterior mediastinum, surrounding the trachea and outlining the anterior vena cava and the left subclavian artery. Subcutaneous emphysema was present in the ventral neck region at the level of the sixth and seventh vertebrae and along the right rib cage. The ventro-dorsal radiograph showed a unilateral pneumothorax with partial collapse of the right diaphragmatic lobe. Case 3. This Greyhound, a 17-months-old male, was presented with a history of weightloss over the previous 2 weeks. T h e day before examination it had slowed during a trial gallop, and after finishing was dyspnoeic for 30 minutes.
29
P N E U M O M E D I A S T I N U M IN G R E Y H O U N D S
O n examination, clinical abnormalities were confined to the respiratory system. There was an obvious abdominal expiratory effort and increased bronchial tones could be auscultated over the left and right lung fields. The heart was audible over both lung fields. The dog was admitted to hospital and next morning subcutaneous emphysema was detected at the thoracic inlet. Radiographs of the thorax revealed pneumomediastinum. Air was present in the anterior mediastinum, surrounding the trachea and aortic arch. The brachiocephalic artery was clearly visible. The air had migrated anteriorly, and subcutaneous emphysema was present at the thoracic inlet. There was no evidence of pneumothorax. T R E A T M E N T AND O U T C O M E I n each case, 5 days’ kennel confinement and a 4-week break from training was the recommended treatment. Radiographs taken after 4 weeks showed no evidence of pneumomediastinum in any of the dogs. Two of them, cases 1 and 2, returned to racing and were placed a t race meetings. DISCUSSION The clinical and radiological findings of pneumomediastinum in three racing Greyhounds are summarized in Table 1. TABLE 1. Summary of clinical and radiological findings in three Greyhounds with spontaneous pneumomediastinum
Case 1 Case 2 Case 3
Dyspnoea with exercise
Subcutaneous emphysema
+ +
+ + +
Increased Radiographic bronchial evidence of tones pneumomediastinum
+ +
+ + +
Radiographic evidence of pneumothorax
+ +
-
I n man, this condition is referred to as spontaneous pneumomediastinum as the source of air within the mediastinum is obscure (Rogers et al., 1972). I n each of our cases the presence of air in the mediastinum was confirmed radiographically. T h e lateral thoracic radiograph was of most value in the diagnosis of pneumomediastinum. The air in the mediastinum acted as a contrast agent and structures of the mediastinum became clearly visible (Fig. 1). T h e brachiocephalic artery, the first large artery from the aortic arch, passes forwards on the right-hand side to the level of the first rib or first intervertebral space. The left subclavian artery rises from the aortic arch at the level of the third
30
B . R . J O N E S et
al.
left intercostal space and runs forwards towards the thoracic inlet. Major veins in the anterior mediastinum are the anterior vena cava and azygos vein. The azygos vein runs forwards between the vertebral bodies and the aorta to join the anterior vena cava at its termination in the atrium. In pneumomediastinum these major blood vessels are surrounded by air and become easily detectable radiographically.
FIG.1. Lateral radiograph of the thorax of Case 1. Pneumomediastinum-the anterior vena cava (A), the brachiocephalic artery (B), the left subclavian artery (C) and the trachea (D) are surrounded by air. There was evidence of pneumothorax in the ventrodorsal radiograph.
Due to the presence of large quantities of air in the anterior mediastinum, the radiographic features in our cases were more spectacular than those reported by other authors. Dixon (1972) reported on a single case of pneumomediastinum in a dog in which the anterior vena cava and common carotid artery were outlined by air. Pneumomediastinum in a 2-year-old female Cairn Terrier was described by Olsson (1973). I n thoracic radiographs there were streaks of air in the anterior
PNEUMOMEDIASTINUM IN GREYHOUNDS
31
mediastinum and the air had extended around the trachea and into the neck. Also, the diaphragm was flatter than normal. Burrows et al. (1972) described pneumothorax and pneumomediastinum in a dog in association with infestation with Filaroides osleri and suggested that the presence of pneumomediastinum indicated a tracheal or major bronchial air leak. In his discussion of pneumothorax in the dog Walker (1959) stated that interstitial emphysema must be considered in the differential diagnosis. He said that interstitial emphysema developed when alveolar rupture occurred at the lung hilus, with air tracking along the interstitial tissue to the mediastinum and the subcutaneous tissue of the neck. The main symptoms observed in these cases of pneumomediastinum were dyspnoea and discomfort from the presence of air in the mediastinum. T h e radiographic features of pneumomediastinum were not discussed. T h e pathogenesis of pneumomediastinum has been discussed by Macklin & Macklin (1944), Morere et a1 (1966) and Rogers et a1 (1972). Respiratory obstruction provides the basic causal mechanism of spontaneous pneumomediastinum, The obstruction, which is usually acute and transient, but may be recurrent, results in increased intra-alveolar pressure and alveolar distension. A pressure differential is required to produce alveolar rupture and, when the pressure in adjacent alveoli is equal, the inter-alveolar walls remain intact. However, should the alveolar pressure exceed that found within the pulmonary vasculature, the perivascular alveoli may rupture, resulting in interstitial emphysema within the adventitia of the pulmonary vessels (Macklin & Macklin, 1944). The air dissects along the vessels to the mediastinum, and thence into subcutaneous tissues around the thoracic inlet. The theory of increased intra-alveolar pressure leading to alveolar rupture and pneumomediastinum proposed by Macklin & Macklin (1944) has been challenged by Morere et a1 (1966), with the proposal that a reduction of pulmonary capillary pressure may be responsible for the pressure imbalance and result in alveolar rupture. Pathological processes causing weakening of the alveolar walls would be expected to enhance the likelihood of pneumomediastinum (Rogers e t al., 1972). Burrows et a1 (1972) suggested that the high airway pressures necessary to move air in and out of an obstructed bronchus in their case were sufficient to rupture intercartilaginous membranes damaged by the development of Filaroides nodules. The cause of the spontaneous pneumomediastinum in our dogs was unknown. There was no apparent history of respiratory obstruction, evidence of primary respiratory disease or of any condition that would reduce the pulmonary capillary pressure. I n cases 1 and 2 pneumothorax was also seen radiographically. Macklin & Macklin (1944) contend that if a pneumothorax and pneumomediastinum coexist, the former always arises from the latter and the reverse does not occur. Air in the mediastinum may rupture into the pleural sac but the converse is not possible. Subcutaneous emphysema at the thoracic inlet was the most significant clinical
32
B. R. J O N E S
et al.
finding in these Greyhounds. Subcutaneous emphysema secondary to pneumomediastinum may take several hours to develop. In man, pneumomediastinum is associated with dyspnoea and chest and neck pains which tend to subside with the onset of subcutaneous emphysema. This requires at least 12 hours to develop and may not be obvious until 24 hours or more after rupture (Rogers et al., 1972). I n cases 1 and 3 subcutaneous emphysema was not evident until at least 24 hours after the onset of dyspnoea. I n experimental pneumomediastinum in dogs Sehic et al. ( 1971) found that the air disappeared from the mediastinum in approximately 10 days. Walker (1959) stated that pneumomediastinum was rarely dangerous and that the air would be resorbed once the leak stops. Reed (1965) recommended the removal of air by aspiration with a syringe if a large volume collected subcutaneously. This was not necessary in any of our cases. All the dogs in this series were rested for 4 weeks and radiographs taken at the end of this period indicated complete resorption of the air. Rogers et al. (1972) considered the condition in man to be a self limiting disease that resolves spontaneously and specific treatment is rarely required. However they stated that if sufficient air is allowed to accumulate in the mediastinum the condition could conceivably proceed to circulatory collapse, pulmonary oedema and death due to compression of the mediastinal and pulmonary vessels and interference with respiration. Spontaneous pneumomediastinum may occur in racing Greyhounds more often than is commonly thought and may be a cause of respiratory distress or poor racing performance. Respiratory conditions are often neglected as possible causes of poor racing performance and careful clinical examination of the respiratory system and thoracic radiography should be completed in dogs with respiratory difficulty or a poor racing history. REFERENCES BURROWS, C.F., O’BRIEN,J.A. & BIERY,D.N. (1972) 3. small Anim. Pract. 13, 613. DIXON,R.T. (1972) Aust. vet. Pract. 2 [3], 10. MACKLIN, M.T. & MACKLIN, C.C. (1944) Medicine 23, 281. MORERE,P., FLEURY, J. & VAUDOUR, P. (1966) Presse Med. 74, 1653. Cited by Munsell, W.P. (1967) 3. Am. Med. Ass. 202, 120. MUNSELL, W.P. (1967) 3. Am. Med. Ass. 202, 120. OLSSON, S.E. (1973) The Radiological Diagnosis in Canine and Feline Emergencies Lea and Febiger, Philadelphia p34. REED,J.H. (1965) Canine Surgery, American Veterinary Publications; 1st Archibald edn, Illinois p412. ROGERS, L.F.,PUIG,A.W., DOOLEY,B.N., & CUELLO, L., (1972) Am. 3. Roentgenol. 115, 163. SEHIC,M., ELIJAS,B. & GOMERCIC, H., (1971) Vet. Arhio. 41, 163. WALKER,R.G. (1959) Vet. Rec. 71, 859.
Spontaneous pneumomediastinum in the racing Greyhound B . R . J O N E S , * M . L. B A T H
AND
A. K . W . W O O D
Department of Veterinary Clinical Sciences, University of Melbourne, Veterinary Clinical Centre, Princes Highway, Werribee 3030, Victoria, Australia
ABSTRACT This paper describes the clinical and radiological features of spontaneous pneumomediastinum in three racing Greyhounds. The clinical signs were characterized by dyspnoea, subcutaneous emphysema and increased bronchial tones. The diagnosis was confirmed by thoracic radiographs. T h e major structures of the anterior mediastinum which are normally not visible in thoracic radiographs, are easily seen with the presence of large quantities of air in the mediastinum. No definite cause of pneumomediastinum was found in these dogs but all of them recovered after 5 days’ kennel confinement and a 4 week break from training. INTRODUCTION Pneumomediastinum is an entity in which free air is found within the mediastinum (Munsell, 1967). It has been recognized in dogs (Walker, 1959; Reed, 1965; Dixon, 1972; Burrows et al., 1972; Olsson, 1973). Air may enter the mediastinum via the deep fascia1 planes of the neck, the adventitia of the musculature of the lung, by perforation of the trachea, ~bronchii,or oesophagus, or rarely by dissection from the retroperitoneal space (Rogers et al., 1972). Pneumomediastinum can be recognized radiographically. In thoracic radiographs of normal dogs the anterior mediastinum is detectable in the lateral radiograph as a soft-tissue shadow, ventral to the trachea, extending from the thoracic inlet to the cranial border of the cardiac silhouette. Usually it is impossible to see anatomical structures apart from the trachea within the anterior medias* Present address : Department of Veterinary Clinical Sciences, Massey University, Palmerston North, New Zealand. C
27
28
B . R. J O N E S ct
al.
tinum. In pneumomediastinum the air acts as a contrast medium outlining the major blood vessels. The purpose of this paper is to present the clinical signs and diagnostic features of spontaneous pneumomediastinum in three racing Greyhounds examined at the University of Melbourne’s Veterinary Clinical Centre. The possible pathogenesis of the condition is briefly discussed. H I S T O R Y AND C L I N I C A L F I N D I N G S Case 1. The first case concerns an 18-months-old male Greyhound. I t had pulled u p in respiratory distress after running 300 metres, but appeared to recover after several hours. When walked the following morning it quickly became dyspnoeic. There was no history of a previous illness and the dog had won a trial-race a week earlier. On examination the dog was in good condition and clinical abnormalities were confined to the respiratory system. Auscultation of the thorax revealed increased bronchial tones which were audible over the left and right lung fields. The dog was admitted to the hospital for observation. The day after admission subcutaneous emphysema was palpated at the thoracic inlet and in the axillae. Lateral and ventro-dorsal radiographs were taken of the thorax and in the lateral radiograph air was seen to be present in the anterior mediastinum, outlining the brachiocephalic artery, left subclavian artery and the anterior vena cava. Air had migrated through the thoracic inlet and subcutaneous emphysema was present in the cervical region and extended back along both sides of the rib-cage. There was also a left-sided pneumothorax with partial collapse of the left diaphragmatic lung lobe. Case 2.
A 29-months-old male Greyhound was presented for examination with a history of poor racing performance. On examination it was found to be in good condition and the only significant clinical finding was subcutaneous emphysema at the thoracic inlet. No abnormalities were detected on auscultation or percussion of the thorax. A diagnosis of pneumomediastinum was made from thoracic radiographs. O n the lateral view, air was present in the anterior mediastinum, surrounding the trachea and outlining the anterior vena cava and the left subclavian artery. Subcutaneous emphysema was present in the ventral neck region at the level of the sixth and seventh vertebrae and along the right rib cage. The ventro-dorsal radiograph showed a unilateral pneumothorax with partial collapse of the right diaphragmatic lobe. Case 3. This Greyhound, a 17-months-old male, was presented with a history of weightloss over the previous 2 weeks. T h e day before examination it had slowed during a trial gallop, and after finishing was dyspnoeic for 30 minutes.
29
P N E U M O M E D I A S T I N U M IN G R E Y H O U N D S
O n examination, clinical abnormalities were confined to the respiratory system. There was an obvious abdominal expiratory effort and increased bronchial tones could be auscultated over the left and right lung fields. The heart was audible over both lung fields. The dog was admitted to hospital and next morning subcutaneous emphysema was detected at the thoracic inlet. Radiographs of the thorax revealed pneumomediastinum. Air was present in the anterior mediastinum, surrounding the trachea and aortic arch. The brachiocephalic artery was clearly visible. The air had migrated anteriorly, and subcutaneous emphysema was present at the thoracic inlet. There was no evidence of pneumothorax. T R E A T M E N T AND O U T C O M E I n each case, 5 days’ kennel confinement and a 4-week break from training was the recommended treatment. Radiographs taken after 4 weeks showed no evidence of pneumomediastinum in any of the dogs. Two of them, cases 1 and 2, returned to racing and were placed a t race meetings. DISCUSSION The clinical and radiological findings of pneumomediastinum in three racing Greyhounds are summarized in Table 1. TABLE 1. Summary of clinical and radiological findings in three Greyhounds with spontaneous pneumomediastinum
Case 1 Case 2 Case 3
Dyspnoea with exercise
Subcutaneous emphysema
+ +
+ + +
Increased Radiographic bronchial evidence of tones pneumomediastinum
+ +
+ + +
Radiographic evidence of pneumothorax
+ +
-
I n man, this condition is referred to as spontaneous pneumomediastinum as the source of air within the mediastinum is obscure (Rogers et al., 1972). I n each of our cases the presence of air in the mediastinum was confirmed radiographically. T h e lateral thoracic radiograph was of most value in the diagnosis of pneumomediastinum. The air in the mediastinum acted as a contrast agent and structures of the mediastinum became clearly visible (Fig. 1). T h e brachiocephalic artery, the first large artery from the aortic arch, passes forwards on the right-hand side to the level of the first rib or first intervertebral space. The left subclavian artery rises from the aortic arch at the level of the third
30
B . R . J O N E S et
al.
left intercostal space and runs forwards towards the thoracic inlet. Major veins in the anterior mediastinum are the anterior vena cava and azygos vein. The azygos vein runs forwards between the vertebral bodies and the aorta to join the anterior vena cava at its termination in the atrium. In pneumomediastinum these major blood vessels are surrounded by air and become easily detectable radiographically.
FIG.1. Lateral radiograph of the thorax of Case 1. Pneumomediastinum-the anterior vena cava (A), the brachiocephalic artery (B), the left subclavian artery (C) and the trachea (D) are surrounded by air. There was evidence of pneumothorax in the ventrodorsal radiograph.
Due to the presence of large quantities of air in the anterior mediastinum, the radiographic features in our cases were more spectacular than those reported by other authors. Dixon (1972) reported on a single case of pneumomediastinum in a dog in which the anterior vena cava and common carotid artery were outlined by air. Pneumomediastinum in a 2-year-old female Cairn Terrier was described by Olsson (1973). I n thoracic radiographs there were streaks of air in the anterior
PNEUMOMEDIASTINUM IN GREYHOUNDS
31
mediastinum and the air had extended around the trachea and into the neck. Also, the diaphragm was flatter than normal. Burrows et al. (1972) described pneumothorax and pneumomediastinum in a dog in association with infestation with Filaroides osleri and suggested that the presence of pneumomediastinum indicated a tracheal or major bronchial air leak. In his discussion of pneumothorax in the dog Walker (1959) stated that interstitial emphysema must be considered in the differential diagnosis. He said that interstitial emphysema developed when alveolar rupture occurred at the lung hilus, with air tracking along the interstitial tissue to the mediastinum and the subcutaneous tissue of the neck. The main symptoms observed in these cases of pneumomediastinum were dyspnoea and discomfort from the presence of air in the mediastinum. T h e radiographic features of pneumomediastinum were not discussed. T h e pathogenesis of pneumomediastinum has been discussed by Macklin & Macklin (1944), Morere et a1 (1966) and Rogers et a1 (1972). Respiratory obstruction provides the basic causal mechanism of spontaneous pneumomediastinum, The obstruction, which is usually acute and transient, but may be recurrent, results in increased intra-alveolar pressure and alveolar distension. A pressure differential is required to produce alveolar rupture and, when the pressure in adjacent alveoli is equal, the inter-alveolar walls remain intact. However, should the alveolar pressure exceed that found within the pulmonary vasculature, the perivascular alveoli may rupture, resulting in interstitial emphysema within the adventitia of the pulmonary vessels (Macklin & Macklin, 1944). The air dissects along the vessels to the mediastinum, and thence into subcutaneous tissues around the thoracic inlet. The theory of increased intra-alveolar pressure leading to alveolar rupture and pneumomediastinum proposed by Macklin & Macklin (1944) has been challenged by Morere et a1 (1966), with the proposal that a reduction of pulmonary capillary pressure may be responsible for the pressure imbalance and result in alveolar rupture. Pathological processes causing weakening of the alveolar walls would be expected to enhance the likelihood of pneumomediastinum (Rogers e t al., 1972). Burrows et a1 (1972) suggested that the high airway pressures necessary to move air in and out of an obstructed bronchus in their case were sufficient to rupture intercartilaginous membranes damaged by the development of Filaroides nodules. The cause of the spontaneous pneumomediastinum in our dogs was unknown. There was no apparent history of respiratory obstruction, evidence of primary respiratory disease or of any condition that would reduce the pulmonary capillary pressure. I n cases 1 and 2 pneumothorax was also seen radiographically. Macklin & Macklin (1944) contend that if a pneumothorax and pneumomediastinum coexist, the former always arises from the latter and the reverse does not occur. Air in the mediastinum may rupture into the pleural sac but the converse is not possible. Subcutaneous emphysema at the thoracic inlet was the most significant clinical
32
B. R. J O N E S
et al.
finding in these Greyhounds. Subcutaneous emphysema secondary to pneumomediastinum may take several hours to develop. In man, pneumomediastinum is associated with dyspnoea and chest and neck pains which tend to subside with the onset of subcutaneous emphysema. This requires at least 12 hours to develop and may not be obvious until 24 hours or more after rupture (Rogers et al., 1972). I n cases 1 and 3 subcutaneous emphysema was not evident until at least 24 hours after the onset of dyspnoea. I n experimental pneumomediastinum in dogs Sehic et al. ( 1971) found that the air disappeared from the mediastinum in approximately 10 days. Walker (1959) stated that pneumomediastinum was rarely dangerous and that the air would be resorbed once the leak stops. Reed (1965) recommended the removal of air by aspiration with a syringe if a large volume collected subcutaneously. This was not necessary in any of our cases. All the dogs in this series were rested for 4 weeks and radiographs taken at the end of this period indicated complete resorption of the air. Rogers et al. (1972) considered the condition in man to be a self limiting disease that resolves spontaneously and specific treatment is rarely required. However they stated that if sufficient air is allowed to accumulate in the mediastinum the condition could conceivably proceed to circulatory collapse, pulmonary oedema and death due to compression of the mediastinal and pulmonary vessels and interference with respiration. Spontaneous pneumomediastinum may occur in racing Greyhounds more often than is commonly thought and may be a cause of respiratory distress or poor racing performance. Respiratory conditions are often neglected as possible causes of poor racing performance and careful clinical examination of the respiratory system and thoracic radiography should be completed in dogs with respiratory difficulty or a poor racing history. REFERENCES BURROWS, C.F., O’BRIEN,J.A. & BIERY,D.N. (1972) 3. small Anim. Pract. 13, 613. DIXON,R.T. (1972) Aust. vet. Pract. 2 [3], 10. MACKLIN, M.T. & MACKLIN, C.C. (1944) Medicine 23, 281. MORERE,P., FLEURY, J. & VAUDOUR, P. (1966) Presse Med. 74, 1653. Cited by Munsell, W.P. (1967) 3. Am. Med. Ass. 202, 120. MUNSELL, W.P. (1967) 3. Am. Med. Ass. 202, 120. OLSSON, S.E. (1973) The Radiological Diagnosis in Canine and Feline Emergencies Lea and Febiger, Philadelphia p34. REED,J.H. (1965) Canine Surgery, American Veterinary Publications; 1st Archibald edn, Illinois p412. ROGERS, L.F.,PUIG,A.W., DOOLEY,B.N., & CUELLO, L., (1972) Am. 3. Roentgenol. 115, 163. SEHIC,M., ELIJAS,B. & GOMERCIC, H., (1971) Vet. Arhio. 41, 163. WALKER,R.G. (1959) Vet. Rec. 71, 859.
