It appears that there are several types of pulmonary disease affecting sheep in this country. These are loosely classified as "sheep pneumonias"; though the etiological agents in some cases have not been identified clearly and the picture is somewhat obscured by the presence of secondary infections.
A pneumonia in sheep has been encountered in the Dubbo District (no doubt it is also present in other sheep-raising districts of the State) affecting, in the great majority of cases, sheep of the British breeds. Border Leicesters and, to a lesser extent, Dorset Horns, seem to be affected most commonly. However, owing to the preponderance of the above-mentioned breeds in the Dubbo District, compared with other British breeds, this breed incidence may not be satisfactorily significant. Merinos, although constituting a high proportion of the sheep carried in the District, do not appear to be affected. This could be accounted for in the case of stud sheep in that the Merino studs generally are run in a much superior fashion to those of the British breeds.
The disease is much more serious in studs, where sheep are housed and forced into closer contact, than in sheep running under open range conditions, indicating the contagious nature of the disease.
Numerous outbreaks were encountered during the excessively wet years of 1955 and 1956, and although odd cases were seen in paddock stock; it was only in stud stock kept under confined conditions that the disease reached troublesome proportions.
The disease may be acute or chronic. The acute form is characterised by rapid prostration, elevated temperatures-106 F. plus. There is respiratory distress, together with a painful cough. A purulent nasal discharge is usually present and there is complete anorexia. Death occurs in three to seven days from the onset of symptoms.
A more common form is the sub-acute extending to chronic. In the sub-acute form, the above symptoms are not so severe, although partial or complete anorexia is a fairly constant sign and body temperatures vary from slightly above normal to 106 F. Respiratory distress is sometimes not so well marked, and it is amazing on occasions that there is so little respiratory distress compared with the amount of lung involvement found on post-mortem examination. The nasal discharge is fairly characteristic; being present in copious quantities, of muco-purulent type and usually forming encrustations around the nostrils. The presence of this nasal discharge in Border Leicester sheep is not pathognomonic of pneumonia; in fact it is rare to see a mob of sheep of this particular breed in which some degree of nasal discharge is not present even in the absence of pneumonia.
A harsh, chronic cough is present and may be evident even when the animal is at rest. Deaths usually do not occur under two to three weeks.
Chronic cases may live for months. Pathetic animals in poor condition, lethargic and exhibiting a "graveyard" cough.
On post-mortem examination, lesions generally are restricted to the thoracic cavity.
In acute cases, red hepatisation is well marked. The apical lobes affected almost always and usually the ventral portions of the mediastinal and cardiac lobes, with extensions to the diaphragmatic lobes in severe cases. As a rule, both lungs are involved. In an affected lung, the stage of hepatisation is uniform throughout the affected tissue. The lung is quite firm and the affected portion has a gland-like appearance. When squeezed, small amounts of creamy pus exude from the bronchioles. There may be an excess of fluid in the thoracic cavity. Regional lymph nodes are enlarged and juicy.
In sub-acute cases, the appearance is similar except that lesions of pleurisy usually are more marked. In early cases there is a yellow, fibrinous deposit on the surface of the lungs and as the disease progresses, pleuritic adhesions occur.
Chronic cases often are marked by one or more abscesses in the substances of the affected tissue. At times, the pus in these abscesses is yellow and creamy in consistence: while at others the contents can be described only as a mixture of cheesy, inspissated pus and necrotic lung tissue in a foul-smelling, discoloured watery fluid. These abscesses undoubtedly are caused by secondary invaders.
Histological examination of the affected tissues shows a typical picture of broncho-pneumonia. There is congestion of pulmonary vessels, numerous foci of polymorph infiltration and some fibrosis in older cases.
NATURE OF THE ORGANISM.
This complaint appears to be similar to, if not identical with, a type of pneumonia in sheep found in other parts of the world and variously described as "Enzootic Pneumonia", "Laikipia Lung Disease". "Jaagsiekte", "Montana Sheep Disease" and "Pulmonary Adenomatosis".
The causative organism has not been isolated successfully, but it is suspected strongly as being a filterable virus.
With the primary pathogen, usually there are associated various secondary organisms; notably Cornyebacterium pyogenes and Pasteurella spp.. Indeed, it is held by some workers that these secondary organisms are common and normal inhabitants of the upper respiratory tract of sheep.
Considerable difficulty has been experienced in transmitting the disease under controlled experimental conditions. South African work has shown that although it attacks clean sheep introduced into infected flocks (once again proving its infectious nature), when clean susceptible sheep are sprayed intranasally under high pressure with an emulsion of freshly-diseased sheep lung, they resist the infection; while equally, it does not appear possible to reproduce the disease in suitable experimental sheep by the intra-tracheal inoculation of similar lung emulsions. The disease has been reproduced in Kenya sheep by the subcutanous inoculation of such emulsions; with an incubation period of four to six months. Also, recent work shows that sheep may be infected fairly readily when fragments of freshly-diseased lung are incorporated in an agar plug, which is then introduced into the lung by the jugular vein route. Here again, the incubation period is about four to six months.
In one outbreak in the Dubbo District which had been treated with Penicillin, it was found that, after treatment, although pipettes and sections of tissues were found to be free from bacteria, the pneumonic lesions were still very much in the active state. This could indicate that although the Penicillin had been effective in eliminating the secondary invaders, it had not killed the primary pathogen.
1. "FRIEDLANDER INFECTION".
An acute, rapidly fatal pneumonia of sheep; due to infection with Klebsiella pneumoniae, was recorded by Hindmarsh et al in 1931.
The disease was marked by sudden onset of severe pneumonia with early death; the pneumonia being characteristically croupous in type.
The organism responsible was isolated from affected sheep and transmission experiments were successful in some cases, the organisms again being recovered from the lesions set up experimentally. In cultures, the organism rapidly lost virulence.
It is fairly common to find odd cases of broncho-pneumonia in sheep set up as a result of faulty drenching or careless dipping, and on occasions severe losses have occurred.
The pneumonias resulting from these causes are termed also "gangrenous pneumonia", and usually are marked by the presence of well-defined abscesses and varying degrees of necrosis of lung tissue.
3. LUNG WORM INFESTATION.
Moderate to heavy infestations with lung worms in young sheep may cause collapse of lung tissue supplied by the occluded bronchioles and this may lead to the development of a broncho-pneumonia. On examination of the bronchioles, the offending parasites are seen readily.
4. POISON PLANTS.
Crown Beard (Verbesina encelioides) and Yellow Daisy (Wedelia asperrima) may cause symptoms and lesions simulating pneumonia when eaten in sufficient quantities. Lesions in these cases comprise congestion and oedema of the lungs with pleuritic effusion.
Department of Veterinary Services Report, Kenya (1957)-East African Agricultural Journal, 31:207.
Hindmarsh, W. L., McGrath, T.T. and Belschner, H. G. (1931)-N.S.W. Department of Agriculture, Veterinary Research Report, 6:11.
Hurst, E. (1942)— "Poison Plants of N.S.W".