The antigenic relationships between foot-and-mouth disease virus isolates from some susceptible wild and domestic animal species in Kenya
Sangula, Abraham Kiprotich
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Foot-and-Mouth Disease (FMD) is a highly contagious infectious viral disease of livestock, which exhibits antigenic variation and is of great economic importance worldwide. Wildlife particularly the African buffalo (Syncerus caffer) is important maintenance hosts for foot-and-mouth disease virus (FMDV). The determination of prevalent antigenic types in domestic animals and wildlife; their relationship with vaccine strains is important in vaccine selection and success of vaccination programs. Serological methods, such as virus neutralization test (VNT) and the enzyme-linked immunosorbent assay (ELISA) have been used to subtype FMDV strains for epidemiological and vaccine suitability determination. In this study, sixty-eight FMDV isolates held at the FMD Laboratory, Embakasi, from some susceptible wild and domestic animal species were compared serologically with seven vaccine strains produced by the Kenya Veterinary Vaccines Production Institute (KEVEVAPI) and currently in use for FMD control in Kenya in order to determine vaccine suitability in cross-protection. Isolates were passaged in baby hamster kidney (BHK-21) mono-layer cells and sero-typed by ELISA. Rabbit and guinea pig anti-sera for use in the ELISA tests were produced locally using purified antigens of the seven vaccine virus strains grown in BHK-21 mono-layer cells. Bovine vaccinal sera (BVS), against the seven vaccine strains were selected from those obtained during routine potency assay experiments. Liquid-phase-blocking ELISA (LPBE), and VNT were used to antigenically compare eleven serotype A, twenty-four serotype O, eleven SAT 1 and twenty-nine SAT 2 isolates. The simple one-way relationship ratio (r1) values (measure of the antigenic disparity of the isolates) were determined by comparing the ratio of the BVS titre against the isolate to the titre against the vaccine strain. All the four serotypes were identified among the sixty-four cattle isolates as follows; O (21), A (11), SAT 1(8) and SAT 2(24). Serotypes O (1), SAT 1(2) and SAT 2(6) were found among the wildlife isolates but not serotype A. Five isolates (6.7%) from wildlife (1) and livestock (4) had mixed infections of two virus serotypes each. The antigenic relationships established by the two tests were similar (correlation of 0.88-0.95). The antigenic comparisons indicated that the isolates from the same species and which were isolated from the same ecological zone during the same chronological period wee antigenically closely related to vaccine strains (r1 values ³ 0.6). Six wildlife strains were antigenically unrelated to the vaccine strains (r1 values £ 0.2). Vaccine strains OK77/78, SAT1T155/71 and SAT2K183/74 that were more than 20 years old showed low antigenic closeness (r1 values <0.6) to current outbreak isolates and were unlikely to offer protection. The findings of this study suggest that it is necessary to constantly establish the antigenic relationships of any FMD outbreak strain to vaccine strains in order to select appropriate vaccine strains.
- MST-Zoological Sciences