Shipping fever is one of the most economically important infectious diseases of ruminants with a wide prevalence throughout the continents. The disease is characterized by an acute febrile course with severe fibrinous bronchopneumonia. Infected animals may die within a few days of the onset of clinical signs, but those which survive the acute attack may become chronically infected. Both Mannheimia and Pasteurella species are commensally resident in the respiratory tract of healthy ruminants and are capable of causing infection in animals with compromised pulmonary defense system. Bovine respiratory disease (BRD) is the most common and costly problem encountered in stocker or feedlot calves. BRD also called “shipping fever”, accounts for major economic losses to the producer by reducing average daily gain, feed efficiency, and overall performance of beef calves. The aim of present study was isolation of M.haemolytica from cattle. The identification of organism was performed through biochemical tests and confirmation by polymerase chain reaction. The nature of disease was evaluated through gross and microscopic lesions.
A total of 50 tissue samples (25 lungs and 25 pharynx) were collected from Punjab Agriculture and Meat company Lahore and brought to the Department of Pathology UVAS, Lahore and were analyzed for biochemical and molecular detection of M .haemolytica. For studying the pathogenesis of the disease, experimental infection was given to rabbits in Department of Pathology, UVAS Lahore. Rabbits were randomly divided into Group A, Group B and Group C with nine rabbits (n=9) in each group. Experimental infection of field isolated M. hemolytica was given intratrachealy to the rabbits. Rabbits of group A and B were infected with 0.5 mL bacterial inoculum having 103 and 106 CFU/mL respectively. The rabbits of Group C served as control group. Rectal temperature of each rabbit was recorded daily. On postmortem,
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gross and microscopic lesions were recorded.
The results showed that rabbits of control group not showed any gross or microscopic change. There was significant increase in rectal temperature of infected rabbits as compared to uninfected rabbits. The gross lesions were specific for the organism which was prominently observed in lungs of rabbits. The microscopic lesions revealed that there was severe consolidation, congestion and fibrin exudation in lungs of rabbits of group A which were given less number of organism and they developed clear signs of disease. The rabbits of Group B showed less prominent signs compared to group A due to early death of rabbits. There were multiple hemorrhages, of varying sizes and hyalinization of myocardial cells in infected rabbits. The severity of changes was significantly more different in Group A, as compared to Group B.
It can be deduced by this study that the rabbit can be used as a model for further studies exploring the pathogenesis of the disease as the lesions resemble to shipping fever caused by M. hemolytica in ruminants. The lesions, which developed, could be descending infection resulting in typical lesions of bronchopneumonia or lobular pneumonia.