Challenge exposure of sheep immunized with live vaccine and culture supernatant of Mannheimia haemolytica A1: Effects of revaccination

Vaccination against Mannheimia haemolytica is a routine procedure in sheep farming, being repeatedly performed on the recommendations of commercial veterinary manufacturers or due to outbreaks of respiratory disease. However, the consequences of repeat vaccination after a short period, using live vaccines or culture supernatant (CS), have not been established, and its benefits are unknown. This study aims to evaluate, in a challenge exposure, the effects of revaccinating sheep with this type of biological product. Thus, 20 mixed-breed, 6–9-month-old sheep were divided into four groups, with five animals in each. Animals in group I were inoculated subcutaneously with 2 ml of a 1 × 109 CFU/ml suspension of bacteria in the exponential growth phase. Group II was inoculated with the same bacterial concentration but in the stationary phase. Animals in group III were inoculated with 3 ml of CS and group IV with saline solution, as control. Each group was immunized again 14 days later, following the same guidelines. On day 21, they were intranasally exposed to parainfluenza-3 virus (PI-3). On day 27, they were challenged with a transthoracic injection of live M. haemolytica (2 × 109 CFU/ml). In this experiment, the levels of agglutinating and antileukotoxin antibodies were evaluated through indirect hemagglutination and toxin neutralization assays. During the first 24 h post-challenge, four animals in group I died, three in group II and two in group III, even though they had developed significant levels of agglutinating and antileukotoxin antibodies. Only one animal from the control group died. Thus, contrary to the development of improved protection due to continuous immunizations with live bacteria or CS, a negative effect was observed. This can be explained as the result of a hypersensitivity type III reaction, due to the significant decrease in agglutinating antibodies observed shortly after the challenge, indicating the formation of immune complex, which triggered the release of chemical mediators of inflammation that, finally, promoted edema and pulmonary congestion.