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

DISCUSSION
The major features that make measles a target for eradication is that there is a single
serotype and the only natural host is man. Sequence analysis of the measles virus genes has
enabled the differentiation of measles virus isolates from geographically different origins into
8 clades and 21 genotypes (WHO, 2001). Although these viruses may have initially been
more restricted geographically, the advent of modern travel has led to a more global
distribution of the different genotypes. This is more evident in countries where there has been
intensive vaccination programs and the only subsequent measles cases were due to the
importation of the virus. This was the case in the South American continent (de Quadros et
al., 1996) and is still the situation in the USA (Bellini & Rota, 1998).
The African continent is one of the greatest challenges to the eradication campaign. As
it represents one of the largest reservoirs of endemic measles, it is important to establish the
epidemiology of both the transmission within and between epidemics and the virus strains
involved. The B clade viruses are found in regions of Central Africa and in countries from
West Africa to the Sudan. The clades A and D are found on the East coast stretching down to
South Africa (Kreiss et al., 1997 ; Nigatu et al., 2001). Apart from two exported isolated
infections, the B clade viruses have not been reported elsewhere. The first B clade viruses
(B1) were isolated in 1983 in the Cameroon and measles viruses isolated the following year
in the Gabon were sufficiently different to be designated as a B2 genotype. Subsequent
measles virus isolates in countries to the north (Gambia, Ghana, Nigeria : Hanses et al.,
1999 ; Truong et al., 1999 & the present paper) showed the viruses to be more heterogeneous
and has led to the suggested grouping as B3.1 and B3.2.
The vaccines used today whether they are based on the Edmonston strain which was
isolated in 1954 or other isolates are clade A viruses. Although it has been reported that there
are differences in the in vitro efficiency of virus neutralisation by serum from natural
infection and immunised individuals (Klingele et al., 2000 ), it is generally agreed that sera
from vaccinated children have lower levels of neutralising antibody than after natural
infection (Christenson et al., 1994). The efficiency of protection after vaccination varies with
a number of factors including age and time after immunisation (Christenson et al. 1994).
However, there are now a number of well documented studies showing that measles can
propagate in vaccinated populations giving rise to a range of conditions from silent infections
to clinical measles (Edmonson et al., 1990 ; Gustafson et al., 1987 ; Mathias et al,. 1989 ;
Pedersen et al., 1989 ; Whittle et al., 1999). Studies have established that although seroconversion
after vaccination can be > 95%, protection can decline to 66% over 10 years
(Samb et al., 1993 ; Cisse et al.,1999). This would have serious consequences in an
eradication campaign as not only are the children not protected long term, but it could
eventually lead to the immuno-selection of viruses which were less efficiently dealt with by
the present vaccine. Thus, in the present study one of our initial aims was to examine the
change in the endemic viruses over an 18 year period against known vaccination coverage.
However, the viruses we isolated in the Cameroon in 2001 belonged to the B3 genotype and
so had not been derived from the virus isolated 18 years previously, but presumably were
imported from neighbouring countries. Within the epidemic the viruses were extremely
stable diplaying a maximum of 3 nucleotides different in the HA and none in the NP. No
viruses related to the original B1 viruses were isolated.
Amongst possible explanations of our observations are that the B1 viruses isolated in
1983 were not representative of the majority of the measles viruses present in the Cameroon
at this time. Alternatively, after importation, the present B3 strain may have had a selective
advantage. This may be due to its arrival following vaccination campaigns in which the level
of circulating B1 was reduced or this virus strain may have a selective advantage for other
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