Pneumonitis is a multifactorial clinical condition, the causation of which is attributed to multiple pathogens with stressor factors acting synergistically. In farm animals, the cause of morbidity and mortality is commonly attributed to pneumonia and gastroenteritis across the world. Multiple biotic and abiotic agents either alone or in synergy contribute in the development of pneumonia in hosts, especially immuno-compromised individuals. Viruses being primary agents of BRDC, damage local defense mechanism in the airways causing increased susceptibility to commensal and other secondary bacterial pathogens, which may lead to development of fatal pneumonia. The current study was undertaken to investigate the etio-pathology of BRDC in ruminants with special reference to two viral pathogens i.e. BRSV and BPI3V. Research work included a total of 132 fallen ruminants (64 cattle, 6 buffaloes, 22 sheep and 40 goats) having either ante-mortem history of respiratory affections or gross lesions of the lungs during necropsy.

Histopathological examination of formalin fixed paraffin embedded tissue samples from necropsy cases was carried out for 132 samples and based on the histopathological lesions and nature of the exudate; the lesions were classified into various types of pneumonia and miscellaneous conditions. Among different types of pneumonia interstitial pneumonia (39.5%, 52/129) was the highest, which included chronic (0.7%, 1/129), acute (11.6%, 15/129) and subacute (36%, 27/129) forms. Bronchointerstitial pneumonia (36.1%, 48/129) was categorized into chronic (2.3%, 3/129), acute (13.2%, 17/129) and sub-acute forms (21.7%, 28/129) types. The lesions of bronchopneumonia was diagnosed in 6.70% (8/129) cases which included acute (3.1%, 4/129), fibrinous (0.7%, 1/129) and subacute bronchopneumonia 2.3% (3/129). Pyogranulomatous / granulomatous pneumonia were diagnosed in 1.5% (2/129) animals. Besides, miscellaneous conditions were diagnosed in 12.6% (15/129) animals, which included congestion and hemorrhage in 3/129 (2.3%), emphysema in 3/129 (2.3%) and edema in 7/129 (5.42%) cases. Interstitial pneumonia was most common type of penumonia with occurrence rate of 39.5%, followed by bronchointerstitial pneumonia (36.1%).

Nucleic acid extracted from the tissue, and nasal swabs were screened for the detected of target pathogens i.e. BRSV and BPI3V using PCR and/or realtime PCR. Etiology of pneumonia could be established in 43.41% (56/129) cases, out of which BPI3V was detected in 28.68% (37/129), BRSV in 9.3% (12/129) and Mycobacterium in 0.7% (1/129) cases. Besides, mixed infection of BPI3V and BRSV was detected in 4.65% (6/129) cases. Interstitial and bronchointerstitial were present in 38.88% and 61.11% in BRSV positive cases and 58.1% and 38.88% of BPI3V positive cases, respectively. In the current study, the occurrence of BPI3V was observed to be higher than that of BRSV.

Phylogenetic analysis of the sequences of the purified PCR products revealed close homology of the BPI3V isolates with those reported from Japan, China, Egypt and USA. Similarly, the BRSV isolates revealed close homology with the those reported from Brazil, USA, UK and distant relation with those reported from France, Croatia and Italy. Corelation of the gross, microscopic and molecular pathological studies revealed that RDC is primarily associated with BPI3V and BRSV, as they could be detected in 42.64% (55/129) fallen animals. Bacterial pneumonia was detected in only 6.7% (8/129) animals. In many cases of pneumonia, the exact etiology could not be established and therefore it is required to carry out routine investigation of pneumonia for more number of viruses and bacteria. It will be useful in the further development of diagnostics, therapeutics and prophylactics for the management of RDC in domestic ruminants.