Timing of florfenicol administration determines the protective efficacy of an Actinobacillus pleuropneumoniae live attenuated vaccine in pigs.

backgroundActinobacillus pleuropneumoniae (APP) is a major cause of severe respiratory disease in pigs and is associated with substantial economic losses. During outbreaks, the urgent use of antibiotics in infected herds often conflicts with emergency vaccination of at-risk animals, as antibiotics may impair the effectiveness of live vaccines. This study investigated how the timing of florfenicol administration affects the protective immunity induced by a live attenuated APP vaccine.

methodsThirty pigs were immunized with a live attenuated APP vaccine (HB04M strain). Animals were divided into six groups (n = 5 per group). Four groups received a single therapeutic dose of florfenicol at the time of immunization (0 dpi), or at 1, 3, and 5 days post-immunization (dpi). The remaining groups served as controls, including a vaccine-only group without florfenicol treatment and an unvaccinated blank control group. Nasal colonization by the vaccine strain was monitored by qPCR. Humoral immune responses (ApxII/III antibodies, IL-4) and cellular immune responses (IFN-γ, IL-6) were measured by ELISA. At 7 dpi, all pigs were challenged intranasally with a lethal dose of mixed heterologous APP serotypes 1 and 5. Clinical signs, survival, and lung pathology were evaluated for 7 days after challenge.

resultsFlorfenicol significantly reduced nasal colonization by the vaccine strain, and this reduction was associated with a marked, time-dependent inhibition of both humoral and cellular immune responses. In the G1 (florfenicol at 0 dpi), seroconversion was almost completely abolished, and protective efficacy was lowest, with a survival rate of 20%. Clinical scores and lung lesions in this group were comparable to those in the blank control group, in which survival was 0%. Protection improved as antibiotic treatment was delayed. Survival rates were 40% in G2 (1 dpi/florfenicol) and G3 (3 dpi/florfenicol) groups, and 60% in G4 (5 dpi/florfenicol). The G4 group also exhibited milder clinical signs and significantly less severe lung pathology than the early-treatment groups (P < 0.05). The vaccine-only group (G5) showed the highest level of protection, with an 80% survival rate.

conclusionsThe inhibitory effect of florfenicol on the protective efficacy of a live attenuated APP vaccine is strongly dependent on the timing of administration. To preserve vaccine-induced immunity, florfenicol should be avoided during the first 3 days after vaccination. Delaying treatment until at least 5 days post-vaccination may provide a better balance between necessary antibiotic therapy and the developmen of protective immunity, offering a practical approach to APP outbreak control.