Concurrent administration of a combined clostridial vaccine or ivermectin suppresses the antibody response to foot-and-mouth disease vaccine in sheep

Foot-and-mouth disease (FMD) is a highly contagious viral disease affecting cloven-hoofed animals and poses severe economic losses to the sheep industry. In intensive sheep farming, simultaneous administration of the FMD vaccine with other vaccines or anthelmintics is frequently practiced to reduce handling stress and labor costs. However, whether such combined administration can influence the immunological efficacy of FMD vaccine still lacks sufficient research evidence. In this study, 96 healthy sheep were randomly allocated into three groups (n=32 each). Group A received FMD vaccine alone (control); Group B received FMD vaccine concurrently with a quadrivalent clostridial vaccine; and Group C received FMD vaccine together with ivermectin. Serum samples were collected at 7, 14, 21 and 28 days post immunization, and liquid-phase blocking ELISA was applied to detect FMDV serotype O-specific antibody titers and the immune protection rate was determined based on whether the antibody titer exceeded 1:128. At day 28, the protection rates in Group A were 37.50% (12/32), while those in Groups B and C were only 0% (0/32) and 6.25% (2/32), respectively, with the ivermectin group showing significantly greater suppression than the clostridial group. In addition, both co-administration groups showed significantly lower titers at all time points. These findings demonstrate that concurrent administration of FMD vaccine with either quadrivalent clostridial vaccine or ivermectin severely impairs the humoral immune response to FMD in sheep, yielding inadequate protective levels. This study provides important field-relevant evidence for rational vaccine and anthelmintic application strategies in intensive sheep farming.