Molecular epidemiology of Nigerian and Ghanaian measles virus isolates reveals a genotype circulating widely in western and central Africa

Sub-Saharan Africa is one of the regions of the globe with the highest measles-related morbidity and mortality. Yet only seven virus isolates from this vast region have been phylogenetically characterized on the basis of their nucleoprotein, the last one in 1991. To characterize the prevalent wild-type viruses and to understand their circulation pattern, a large panel (n = 45) of isolates was collected in Ghana and Nigeria in 1997 and 1998. On the basis of their nucleoprotein sequence, the viruses clearly belong to clade B but a reshuffling of the structure of this clade was proposed, tentatively extending the number of genotypes from two to three on the basis of quantitative criteria. The sequences revealed the co-circulation of at least two distinct viruses in the cities of Lagos and Ibadan, suggesting that the number of susceptible individuals seems to be high enough to support endemic circulation of at least two distinct viruses. The endemic co-circulation of several viruses may well be a characteristic of communities with low vaccination rates. One of these viruses was also found in Accra in 1998 as well as in a 1994 case linked to distant Kenya, suggesting that clade B viruses are prevalent in sub-Saharan Africa while non-B viruses seem to dominate the south of Africa.