SOUCHES AFRICAINES DU VIRUS DE LA ROUGEOLE : ETUDE ...

ABSTRACT
Africa remains one of the major reservoirs of measles infection. Molecular
epidemiological studies have permitted different measles virus isolates to be grouped into
clades and genotypes and the major group which has been identifed as indigenous to Africa is
the clade B. We have analysed the viruses from epidemics in the Gambia (1993) and in the
Cameroon (2001). In both studies, the homogeneity of the virus isolates within the epidemic
as shown by sequence analysis revealed less than 0.2% variation of nucleotides between
isolates. The measles viruses isolated in 1983 in Yaoundé, Cameroon, were designated as the
B1 genotype. However, in 2001 only viruses belonging to the B3 genotype were found in this
city. The viruses in the Gambia (1993) were also of the B3 genotype. However, these viruses
could be distinguished from each other at the antigenic level and by comparative sequence
analysis. The B3 Cameroon (2001) viruses were related to the proposed B3.1 subgroup,
whereas the Gambian (1993) isolates corresponded to the B3.2 subgroup. The geographical
distribution for the period 1993-2001 of these two viruses shows that B3.1 is found from the
Sudan to Nigeria and Ghana extending south to the Cameroon, whereas the B3.2 genotype is
found in West Africa. In Nigeria and Ghana the viruses co-circulate. The identification of
these viruses will permit more meaningful epidemiological studies following the proposed
increased measles vaccination coverage.
INTRODUCTION
Epidemiological studies are essential for the successful planning and application of
vaccination programs. The aim may be simply to reduce the disease burden or target
eradication. Measles ranks as one of the most infectious diseases and although a vaccine
exists for more than 30 years, measles is still responsible for the deaths of approximately 1
million children each year. Under optimum conditions the vaccine has a sero-conversion rate
of over 95%, but this is lower in children under 12 months of age due to the immaturity of the
immune system and the presence of maternal antibody (Gans et al., 1998). Although natural
infection gives a life-long immunity, vaccination has been shown to induce lower immune
responses (Markowitz et al, 1990) and there are a number of reports showing subsequent
infection when confronted by circulating measles viruses (Edmonson et al., 1990 ; Gustafson
et al., 1987 ; Mathias et al., 1989 ; Pedersen et al., 1989 ; Whittle et al., 1999). These may
range from silent infections to clinically diagnosed measles. Despite these reservations,
intensive vaccination campaigns in the South American continent eliminated the endogenous
virus (de Quadros et al., 1996). However, it is probable that re-introduction from outside was
the source of further epidemics in Brazil (Siqueira et al., 2001).
Measles virus is considered to be a single serotype. Over the past 20 years a number of
virus isolates have been obtained from geographically distinct regions (Rima et al., 1995 ;
Rota et al., 1996). Nucleotide sequence analysis of the different viral genes has shown that
most variation is found in the haemagglutinin (HA) and the carboxy region of the
nucleoprotein (NP). Sequence analysis of these two viral genes from different viruses has
provided a classification in which isolates are separated into 8 clades (A-H) each containing
variants or genotypes (WHO, 1998). This molecular epidemiological approach enables the
origins of the measles viruses to be traced.
Only limited information is available on the stability of measles viruses within and
between epidemics. Further the increase in vaccine coverage may impose an immunoselection
on the circulating viruses. El Mulbarak et al. (2002) have shown very little sequence
variation in the HA (0-0.5%) in measles viruses isolated in the Sudan between 1997-2000. In
contrast, Hanses et al. (1999) suggested that there were two measles virus strains cocirculating
in epidemics in Nigeria and Ghana in 1998.
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