Molecular epidemiology of foot-and-mouth disease virus in West Africa

Abstract

The economy of West African countries is dependent mainly on agriculture. Livestock production is a vital source of providing dietary protein for the rapidly growing human population and it is therefore important to define strategies for controlling infectious diseases that are undermining the livestock industry. Although the foot-and-mouth disease (FMD) virus causes one of the most devastating economical diseases, it has been mainly ignored in West Africa due to low mortality rates in the face of other diseases that cause significant mortalities. This may explain the lack of interest for studying FMD infections in the region. However, the eradication of other diseases such as Rinderpest together with an increase in the number of outbreaks of FMD in recent years has caused a renewed interest in understanding the epidemiology of the disease. Foot-and-mouth disease is a highly contagious disease of cloven-hoofed animals. The causative agent, FMD virus, has a high rate of genetic variation in its single-stranded RNA genome. The genetic characterization of the surface capsid protein gene, VP1, is the most informative technique for studying the molecular epidemiology of FMD. The genetic profile of different serotypes of FMDV isolated across West Africa was investigated in this study using manual and automated nucleotide sequencing. A total number of 21 type O isolates from Ghana, Burkina Faso and South Africa (1992-2000), 23 SAT-1 viruses from Niger and Nigeria (1975-1981) and 30 SAT-2 viruses from Mali, Ivory Coast, Ghana, Nigeria, Liberia, Senegal and Gambia (1974-1991) were investigated. The sequence data was used to establish the phylogenetic relationships between the west African strains and those previously characterized from East, central and southern Africa as well as other regions of the world in the case of serotype O. Viruses from West Africa formed a single genotype while the isolates from South Africa clustered with the Pan-Asian topotype (Bangladesh 1997&Japan 2000). Sequence identity of 99 % and 95 % were found between Ghana-Burkina Faso and South Africa-Bangladesh type O viruses, respectively. Within SAT-2, the viruses characterized were isolated over 27 years from seven countries in West Africa and two indigenous topotypes (> 97 % sequence identity in the cluster) were identified. Of interest was the clustering of viruses Nigeria from 1982 and Eritrea in 1998, which has provided the first evidence of virus transmission between West and East Africa. For SAT-1, two distinct lineages (I-II) were identified. Lineage I consisted of viruses isolated between 1975-1976 from neighboring countries Niger and Nigeria, while lineage II was composed of viruses recovered from outbreaks between 1979-1981 in Nigeria. Furthermore, viruses from the latter lineage shared > 98 % sequence identity across the VP1 gene providing a clear indication of a long circulation of virus in the field in West Africa. For the serotypes investigated in this study viz. serotypes O, SAT-2 and SAT-1, it was shown that the year of isolation is more important in the epidemiology of FMD in West Africa than country of origin. The phylogenetic analysis demonstrated that viruses from each serotype grouped according to year of isolation rather than their geographical origin. This is in contrast of what was reported previously for FMDV strains in southern Africa. Results further revealed that FMD viruses from West Africa are evolving independently from viruses elsewhere on the continent and clustered in discrete genotypes. The genetic distinctiveness of west African FMD isolates is likely to be reflected antigenically and has implications in the selection of regionally appropriate field strains for use in vaccines to assist in the control of the disease.