In the present study the overall isolation rate of pasteurellosis was found to be 76.2% (95% CI = 65.7–84.8). The current finding in the prevalence of pasteurellosis is relatively higher than reports of previous studies in the country. Thus, , ,  , and , who 50.2, 40.8, 39.2, 13, and 8.7%, respectively. This might be due to the difference in sampling that this study was conducted on cases and suspected of pneumonia, study area, time of sampling and farm management .
Analysis on sex related susceptibility showed that pasteurellosis was higher in male (84.1%) than female (67.5%) calves, in that the odd of being positive was 1.64 times higher in the former (OR = 1.64; 95% CI = 0.52–5.2). The possible explanation for sex related susceptibility is that less colostral immunoglobulin absorbed in male than female during neonatal life which leads to disease in male calves . It is also worth mentioning that male calves are not as valuable to the dairy operation as females and therefore may not receive an attention the heifers do have, possibly accounting for the higher infection in males [29, 30]. Meanwhile, the prevalence was significantly higher in younger calves (< 16 weeks) (82.5%) than older one (≥ 16 weeks) (63.0%) with younger calves have 3.1 times the chances of being affected than older (OR = 3.1; 95% CI = 1.0-9.6; p = 0.04). The possible explanation for the age-related susceptibility might be due to failure of passive immunity in hand feeding practices of young calves being able to predispose for bacterial infection and other predisposing etiological agents . For instance, calves less than one month of age lack sufficient postruminal digestive enzymes to break down most sugars and are limited in their ability to utilize starch, maltose, sucrose, or dextran . Partial or complete failure of passive transfer of maternal antibodies is an important host factor related to development of pneumonia in young calves [7, 32].There was significant association (p < 0.05) between pasteurellosis and breed of calves, with higher prevalence in cross breed (82.5%) than local (63%) calves. This deference might be linked to variation in environment adaptability. Therefore, local breed has high disease resistant capability. Similarly, the difference in the isolation rate of the two breeds might be due to the difference in feed access of the calves. Local breeds feed relatively less in amount and quality than cross breed calves which consume much amount and quality feed. So local breeds being less exposed to infection and therefore, have lower isolation rate than cross breed .
This study compares the level of isolation rate between animals kept under intensive and semi intensive management systems. The infection rate was higher in calves kept under intensive system (80%) compared to semi intensive one (70.6%). An intensive management system is mainly associated with confinements and predisposing calves are frequently contact to accumulations of urine and other wastes. This situation is likely to favors the spread of Pasteurella species among animals . Similar observation was reported previously by [34; 35]. Moreover, the occurrence of pasteurellosis varied significantly (p = 0.035) among livelihood, in that it was higher in urban (82.8%) than mixed crop (61.5%) production system.
The overall proportion of Mannheimia haemolytica and Pasteurella multocida was 46.4% and 28.8%, respectively, indicating M. haemolytica was the major causative agent involved in calve pneumonic pasteurellosis in the study area. Although the infection rate varies, this finding is consistent with previous reports of . Though Mannheimia haemolytica is a normal flora of the upper respiratory tract, suppressors of the host immune system favor the multiplication of Pasteurella species, leading to bronchopneumonia in purely pneumonic animals .The reasons for increased susceptibility to M. haemolytica infection in stressed animals are primarily attributed to the breakdown of innate pulmonary immune barriers by stressors [37, 38]. Although the percentage of isolation was relatively low (28.8%) the possible role of P. multocida in the etiology and pathogenesis of calve pneumonia should not be under estimated .
Concerning the isolation rate of Pasteurella species in relation to the health status, it was higher in animals with respiratory illness (93.6%) than no respiratory illness (54.1%), among which Mannheimia haemolytica and Pasteurella multocida were isolated from 57.4% and 36.2% of calves with respiratory disease symptoms, while they were isolated from 32.4% and 21.6% of calves with no respiratory illness, respectively. In this study significant variation was observed in prevalence of Pasteurella species in calves with and without respiratory illness. This suggests the possible involvement of these bacteria in the genesis of pneumonia.