Evaluation of different foot-and-mouth disease vaccination protocols in cattle in the Zambezi region of Namibia

Abstract

Trade in beef products in Namibia was limited to geographic areas free from foot-and-mouth disease (FMD) until the inception of commodity-based trade. This study examined the equivalence of various vaccination protocols as part of a commodity-based trade initiative in the Zambezi Region of Namibia by determining whether the proportion of animals protected after vaccination with the Aftovaxpur vaccine (Botswana Vaccine Institute) containing FMD virus (FMDV) SAT-1, SAT-2, SAT-3 and O serotypes in the quarantine camp was significantly different to the proportion protected by field vaccination. Cattle (n = 131) were allocated into four groups. Group 1 (n = 40) consisted of cattle that had been vaccinated once in the field and again on entry to the quarantine camp, Group 2 (n = 29) consisted of cattle that had been vaccinated twice in the field and were not vaccinated in the quarantine camp and Group 3 (n = 39) consisted of cattle that tested positive to FMDV non-structural protein (NSP) antibodies to FMDV and were considered to have been naturally infected with FMDV. The three groups were quarantined for 30 days before slaughter at the Kopano quarantine facility located in Katima Mulilo, Zambezi region of Namibia. Group C (n = 23) was the control group and consisted of naïve cattle from south of the veterinary cordon fence that had never been vaccinated or exposed to FMDV. This group was vaccinated upon entry to the Kopano quarantine camp and again three weeks later. These animals were kept in the quarantine camp for 60 days. FMDV non-structural and structural protein antibody titres in cattle were determined at the start and end of the quarantine period. Cattle that tested positive for FMDV NSP antibodies at the beginning of quarantine were excluded from Groups 1 and 2. FMDV structural protein antibodies were determined with a liquid-phase blocking ELISA (LPBE) and a solid-phase competitive ELISA (SPCE). Cattle were considered protected against FMD if LPBE results were ≥ 1.6 or SPCE results > 50%. At the end of the quarantine period, a Z-test for independent proportions showed no significant differences (p > 0.05) between Groups 1, 2 or 3 and Group C for FMDV SAT-1, SAT-3 and O serotypes. There were, however, significant differences in the cattle protected against FMDV SAT-2 in Group 1 (100%, p = 0.001) and Group 3 (97.4%, p = 0.007) compared to Group C (78.3%). Independent-samples Mann-Whitney U tests compared FMDV antibody titres between Groups 1 or 2 and Group C. There were no significant differences for FMDV SAT-1 and O serotypes (p > 0.05), but significant differences for FMDV SAT-2: Group 1 (median = 2.0, p = 0.005) and Group 2 (median = 1.7, p = 0.044) compared to Group C (median = 1.8); and FMDV SAT-3: Group 1 (median = 2.2, p = 0.004) and Group 2 (median = 2.1, p = 0.003) compared to Group C (median = 1.9). The poorest immunogenic strain of the multivalent Aftovaxpur vaccine was FMDV SAT-2. The mean difference in LPBE titres between the start (day 0) and end (day 30 for Groups 1 and 2, and day 60 for Group C) of quarantine was 0.08 for Group 1, 0.23 for Group 2 and 0.59 for Group C. The most immunogenic vaccine strain was FMDV O serotype, with a mean difference in titres of 0.39 for Group 1, 0.61 for Group 2 and 0.94 for Group C. The results indicated that field vaccination was not equivalent to vaccination in the quarantine camp. There was a significant difference in the proportion of cattle with protective levels of antibodies for FMDV SAT-2 and significant differences in the median titres for FMDV SAT-2 and SAT-3 in field-vaccinated cattle compared to cattle vaccinated in the quarantine camp. There were differences in the immunogenicity of the FMDV strains in the vaccine, with serotype O being the most immunogenic and SAT-2 being the least immunogenic. The difference in protective levels of FMDV antibodies between field—and quarantine-vaccinated cattle depended on the serotype involved. Still, it was most likely due to field-vaccinated cattle receiving a booster and having a more mature immune response to the FMDV vaccine than quarantine-vaccinated cattle, who only received a primary series of vaccinations. The contribution emanating from this dissertation will add to the limited information available to support commodity-based trade, which will significantly help improve the lives of Namibian farmers north of the veterinary cordon fence.