| dc.description.abstract | The impetus for research on sheep pulmonary adenomatosis has been the recognition of its contagious and neoplastic nature and, of a possible relationship with certain cancers.
Literature Review;
A detailed review of the literature revealed that the disease has a world-wide distribution. It has been clearly identified as a serious cause of loss on a number of sheep farms in Kenya,
South Africa, South West Africa, Bulgaria, Scotland and, in Iceland before it was eradicated. Most countries in which the Merino breed (or those they came in contact with) had been imported have recorded
presence of SPA. The important exceptions are Australia and New Zealand. Despite this wide distribution there have been gaps in our knowledge of the aetiology, pathology and pathogenesis
of the disease.
Aetiological Studies:
Chicken embryos, bacterial media and cell cultures from SPA lungs were used in attempts to identify the aetiological agent. Serological studies against Chlamydial antigens were also carried out. Infective lung materials from 3 natural and 3 experimental
clinical cases of SPA were inoculated via the yolk-sacs into embryonating eggs. Two specimens from the natural cases and one specimen (lung fluid) from experimental cases were passaged 18, 17 and 27 times, respectively. Constant lesions produced by these 3 specimens were haemorrhages under the skin of embryos and in visceral organs particularly the liver and heart in those dying 3-5 days after inoculation. The specimen passaged 27 times revealed presence of Chlamydiae.
Ten out of 14 lung tumours and fluid from clinical cases of SPA yielded Mycoplasma when cultured on Tryptose Serum Agar (TSA) and in Newing's Tryptose Broth (NTB). More than one isolate were encountered in some cases. Their biochemical and serological behaviour
showed that they were different from bovine pleuropneumonia vaccine strain and a strain of caprine pleuropneumonia used for comparison. Four samples did not yield Mycoplasma on culturing. Macrophage cultures from 4 adenomatous lungs maintained for 5-7 days formed many giant cells. Both macrophages and giant cells
had no intracellular inclusion bodies.
Although the aetiological studies failed to demonstrate the presence of clear-cut infectious agent, a virus is still considered to be the organism involved. Mycoplasma spp. encountered may be opportunists that take advantage of fluid produced by the adenomatous
lesions to proliferate and, not involved in the causation of SPA. Serological investigation showed that Chlamydial infections• are widespread in many sheep flocks in Kenya. High antibody titres were found in sera from healthy sheep and, no titres in sera from some sheep with clinical SPA.
Transmission Experiments:
Four transmission experiments were carried out. In the first one, a 25% inoculum was prepared from adenomatous lungs of freshly killed sheep using a pestle and mortar. Eight sheep were each inoculated with 10 ml., 4 intratracheally and 4 into the lungs through the thorax. Three of the sheep at days 65, 118, and 251 after inoculation had lung lesions typical of SPA but no clinical signs. They had been inoculated intratracheally. Sheep inoculated
intrathoracically and one intratracheally and, killed 255 days after inoculation had no lung lesions.
Ultra-sonic vibration was used in preparing a 57% inoculum for the second experiment. The infective material was adenomatous
lung tissue from freshly killed sheep. Ten sheep were all inoculated intratracheally, 7 of them showed clinical signs (dyspnoea, increased respiration, lung discharge, moist rales, cough) of the disease between 107 and 260 days after inoculation. They all had lung lesions of the disease when examined post-mortem. An 8th sheep without clinical signs also had lung lesions of SPA.
In the 3rd and 4th experiments, infective materials passaged in embryonating eggs and, mixed Mycoplasma isolates were used respectively.
Inoculations carried out intratracheally in 8 and 5 sheep respectively produced no lung lesions of SPA. In Experiment 3 the sheep were observed for a year and in Experiment 4 for between 151 and 215 days.
It was demonstrated in Experiment 2 that reproduction of typical lesions and clinical disease of SPA was not as difficult as previously thought. This also showed that the aetiological agent was contained in the tumour cells. The reproduction of clinical SPA would appear to be dose-and susceptibility-dependent
and not time-dependent. The incubation period would appear to be between 6 and 8 months.
Electron Microscopic Studies:
Eight freshly killed sheep with SPA provided adenomatous
lung tissue for investigation. Several abnormalities in the
tumour cells were observed. These were an enlarged polymorphic nucleus, enlarged Golgi complex, an increase of free ribosomes. Dilatation into vesicules of endoplasmic reticulum was evident.
There was also irregularity of cytoplasmic membranes in some adenomatous cells. No intracellular inclusion bodies were seen.
In all, the ultrastructural changes of the adenomatous cells had similarity to carcinomas in human beings.
Pathology and Pathogenesis:
The gross lesions encountered in 7 of the sheep in Experiment 2 with clinical SPA revealed that infection is normally via inspired air. Anterior lobes and antero-ventral aspects of the diaphragmatic
lobes are the ones which showed the grayish adenomatous tissue typical of SPA. This distribution shows that SPA behaves
differently from any other lung cancer.
The histopathological lesions clearly revealed the multicentric
origin and the progressive nature of this adenomatous cancer. Within the same lung and even the same section, could be seen adenomatous foci of less than 10 tumour cells and sheets of tumour cells. The earliest adenomatous foci were often located in "centres" of thickened interalveolar walls. This suggests that epithelium of microatelectatic alveoli is the first to be involved in transformation and proliferation in sheep that are affected. From these early beginnings adenomatous foci increase in size until they fuse with adjacent ones to form continuous tumour tissue. Although no metastases were found in the pulmonary lymph nodes of 8 sheep of Experiment 2 with typical lesions, metastases have been found in natural cases by several workers.
To date the author has encountered 4 cases with metastases.
This study indicates the need for more research on the aetiology of SPA and, the involvement of infectious agents
in the causation of tumours. | |
