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Mannheimia sp. strains obtained from bovine nasal exudates of either clinically healthy (n = 1902) or infected with pneumonia animals (n = 189) were isolated and characterised to estimate the prevalence of isolated serotypes and to identify some factors associated to prevalence in dairy farms in Mexico, by means of a transectional descriptive study. Strains were isolated and typified through conventional in vitro culture methods, biochemical and immunological tests. Chi square or fisher statistical tests were applied, as well as odds ratio calculation and logistic regression analysis to evaluate the association and effect of some variables on Mannheimia sp. isolation. Isolates were composed in 48% by serotype A1, in 2.4% by A6 and 49.3% were non-typable. The apparent prevalence rates of Mannheimia haemolytica were significantly higher in diseased bovines (OR = 2.54; p< 0.05), as well as in bovines younger than 1 year of age (OR = 2.26; p< 0.05). Health condition and age were the variables that remained in the logistic regression model. Serotype A1 showed the highest prevalence, even when most isolates were not-typable. Bovines younger than one year of age and those with respiratory disease were the groups with the highest frequency of M. haemolytica isolates.