Pneumonia is a multifactorial disease condition caused by multiple pathogens with synergism of stress inducing factors. It is the most common cause of morbidity and mortality in farm animals across the world. Different biotic and abiotic stress factors either alone or in combination play important role in the development of pneumonia in immune-compromised hosts. Viruses as a primary infectious agents damages local pulmonary defense mechanism thereby increasing susceptibility of host to secondary bacterial pathogens leading to fatal RDC. Therefore, the present study was undertaken to investigate the etio-pathology of RDC in cattle, buffaloes, sheep and goats. Some viral (BRSV, BPI-3, BHV-1-I, BVDV, BCV, PPRV), bacterial (P. multocida, M. hemolytica, Klebsiella pneumoniae) and mycoplasma were investigated in a total of 184 fallen ruminants (86 cattle, 12 buffaloes, 20 sheep and 66 goats) having either history of respiratory signs or gross pulmonary lesions during necropsy.

Histopathological analysis was carried out for a total of 164 samples and based on the histopathological lesions and principle constituents of the exudate, the lungs lesions were categorized into different types of pneumonia and other miscellaneous conditions. Interstitial pneumonia (44.51%, 73/164) was the highest, consisting of acute (28.04%, 46/164), chronic (11.58%, 19/164) and sub-acute (4.8%, 8/164%) interstitial pneumonia. Bronchointerstitial pneumonia (21.9%, 36/164) was the second most common type and was subdivided into chronic (10.9%, 18/164), acute (6.09%, 10/164) and sub-acute (4.8%, 8/164) types. Bronchopneumonia was diagnosed in 14.02% (23/164) cases, and sub classified into acute (4.2%, 7/164), fibrinous (3.6%, 6/164), suppurative 1.8% (3/164) and chronic 1.8% (3/164) types. Caseonecrotic or granulomatous pneumonia was diagnosed in 2.4% (4/164) animals. Besides, other miscellaneous conditions were diagnosed in 19.5% (32/164) animals, which were further categorized into congestion and hemorrhage in 10.3% (17/164), emphysema in 5.4% (9/164) and oedema in 3.6% (6/164) of animals.

A total of 184 tissue samples of fallen animals were screened by PCR for the detection of targeted pathogens including viruses (BRSV, BPI-3, BVDV, BHV-1, and BCV), bacteria (P. multocida, M. hemolytica, K. pneumoniae and Mycobacterium) and Mycoplasma. Further, 86/184 samples were also screened for the detection of PPRV. Etiological agents were detected and single or multiple pathogens could be detected in 61.41% (113/184) cases by PCR. Single and multiple infections could be observed in 30.43% (56/184) and 30.9% (57/184) animals, respectively, while none of the target pathogens could be detected in 38.5% (71/184) cases.

In cattle, the etiology of RDC could be established in 54.65% (47/86) animals, of which, single and mixed infection could be detected in 33.72% (29/86) and 20.93% (18/86) animals, respectively. Mixed infections with two pathogens were observed in 13 animals, whereas mixed infection with three and four pathogens could be detected in 5 animals. No pathogens under study could be detected in 45.34% (39/86) animals. In buffaloes, the etiology of pneumonia could be established in 83.3% (10/12) animals, 5 animals had single infection and 5 had multiple infections among which infection with 2 pathogens detected in 2 animals and with 3 or more pathogens in 3 animals. None of the target pathogens could be detected in 2 animals. In sheep, the etiology of pneumonia could be established in 65% (13/20) animals, of which, single and mixed infection could be detected in 30% (6/20) and 35% (7/20) animals, respectively. In 3 animals, infection with 2 pathogens was detected and infection with more than 2 pathogens was detected in 4 animals, whereas none of the target pathogens could be detected in 35% (7/20) animals. In goat, the etiology of pneumonia could be established in 65.15% (43/6 animals. Single and mixed infection could be detected in 24.24% (16/65) and 40.91% (27/65) animals, respectively.

Mixed infection with two pathogens was observed in 14 animals, whereas mixed infection with three and more pathogens could be detected in 13 animals. None of the target pathogens could be detected in 34.84% (23/66) animals. Out of 113 samples observed positive for one or multiple pathogens, 30.43% (56/184) and 30.9% (57/184 samples were found to be positive for single and multiple pathogens, respectively. A total of 38 different combinations of pathogens were observed in cases of mixed infection. Single infections of BRSV, PPRV, BCV, BVDV, BPI3, Mycoplasma, Klebsiella, BHV-1, Mycobacterium, P. multocida and Mannheimia were observed in 33.6%, 29.06%, 22.22%, 11.9%, 9.5%, 10.3%, 5.9%, 5.4%, 2.1%, 1.6% and 0.5%, respectively.

Present study revealed higher prevalence of viruses followed by mycoplasma and bacteria in respiratory disease complex in ruminants either as single or co-infection. However, the etiology of many cases of RDC could not be established and therefore, it is required to increase the number of pathogens in the panel for routine investigation of RDC in ruminants.

Further, systematic molecular prevalence studies need to be carried out over larger spatial and temporal dimensions in order to establish the exact prevalence of different respiratory pathogens. Isolation and characterization of different pathogens and experimental studies in the laboratory models as well as natural hosts needs to be carried out. Further, development of user friendly diagnostics for different respiratory pathogens for the early diagnosis of the etiological agents of RDC is recommended. Research works targeting development single or polyvalent vaccines for the respiratory pathogens is recommended. It is also suggested to prepare the strategies for the routine investigation and vaccination programs for the ruminants in the country.