Scanning electron microscopic study of the lower respiratory tract in cattle

The surface characteristics of the bovine lower respiratory tract were studied with the use of the scanning electron microscope (SEM). The first step in the investigation was to become familiarized with the methods of SEM. Thereafter two different projects were designed and performed. The first project was to assess the pattern of ciliated cells in normal one week old calves and compared this with the pattern in adult cattle. Two groups of animals, one calves and the other adult cattle, were studied and none of them had gross morphological evidence of pulmonary disease. The trachea was examined as well as bronchi, bronchioles and alveoli in both the cranial and caudal lobes of the right lung. In general in both groups, the lumenal surface of the large airways was completely covered by cilia apparently forming an efficient "mucociliary escalator". However, in the adult cows some areas of ciliated cells were found devoid of cilia, and these were considered to be abnormal. The non-ciliated cells in this part of the lower respiratory tract were not easily identified unless they were discharging secretion. In small bronchi, non-ciliated cells v/ere more evident and based on the fact that these cells were present sometimes as frequently as the ciliated cells, they were thought likely to be epithelial secretory cells, either mucous or serous cells. The bronchioles had many non-ciliated cells and almost no significant ciliated cells capable of forming a complete ciliary carpet. Type I and Type II alveolar epithelial cells, as well as alveolar macrophages, were identified in both groups of animals. Pores of Kohn were found in the alveolar walls in all the animals and considered to be normal. No brush cells were found. Distinctive respiratory bronchioles were not seen and there was a relatively sudden transition from terminal bronchioles to alveolar ducts. After the normal pattern of the surface morphology was established for normal bovine lower respiratory tract, a second investigation was designed. This was done to assess the changes that could be observed with SEM on the surface of the respiratory tract of calves infected with Dictyocaulus viviparus and calves vaccinated against lungworm and experimentally infected with D. viviparous Ten Friesian cross calves were divided into three groups which received different treatments. Group 1 comprised two calves vaccinated with Dictol as recommended by the manufacturers. Group 2 comprised four calves experimentally infected with approximately 5,000 infective larvae of D. viviparus and Group 3 comprised four calves vaccinated with Dictol at the same time as Group 1 and challenged orally with the same dose of D. viviparus larvae at the same time as Group 2. One calf from Group 1, Group 2 and Group 3 was killed on 15, 25, 35 and 45 days after challenge. The changes observed in the luminal surface of the trachea, as well as the bronchi, bronchioles and alveoli of both the cranial and the caudal lobes of the right lung were recorded on the pleural surface of the lungs. In Group 2 the infected calves appeared with a variety of pathological changes as the infection progressed. These changes were mainly described as adult parasites and eggs in the bronchi, a severe cellular infiltration of the lung with worm eggs and aspirated larvae occupying the lumena of the alveoli. Intercurrent infection was diagnosed occurring on the surface of the conducting airways, where microorganisms were found colonising the tips of the cilia. In addition, areas devoid of cilia and extruded cells were observed and considered to have resulted from viral infection. The relative proportions of epithelial ciliated cells and non-ciliated cells were also affected at these levels. The surface of the conducting airways of the calves in Group 3 were slightly modified. The lung showed infiltration of cells, lymphocytic nodules implanted in the lung parenchyma in close relation in close relation to bronchioles and small bronchi, was the most relevant finding. The SEM results produced three dimensional pictures which strikingly illustrated the modified epithelium and the reactions present during the course of prepatent and patent lungworm infection in susceptible and immune calves