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The question whether or not there is evidence for a non-HLA-B*27 protective effect is still open to debate.1-3 The likelihood of finding cases of ankylosing spondylitis (AS) depends on assumptions on the risk for HLA-B27 positive persons to develop the disease. Prevalence figures as low as 1.3% and 1.4% have been reported for population based studies in Zoetermeer (the Netherlands) and Busselton (Australia). These figures are based on populations where most people underwent pelvic radiography.4 5 Estimates based on completion of questionnaires and random sampling provide higher figures. For example, in northern Norway AS may develop in 6.7% of HLA-B27 positive people.6 Studies that have reported that as many as 20% of HLA-B27 positive subjects develop AS are indeed striking, because as much as 6% of HLA-B27 negative controls were found to have radiographic sacroiliitis. This raises the possibility of systematic over reading of the films because of low specificity of the observers.7 Later, the same group reported a much lower risk (1.9%) studying HLA-B27 positive relatives of healthy blood donors.8 Recently, the prevalence of all spondylarthropathies, including AS, was studied among HLA-B27 positive and negative blood donors in Berlin.9 AS occurred in 6.4% of HLA-B27 positive persons; of the nine patients there were seven men and two women. The latter is compatible with the usually reported sex ratio (male: female) of about 3:1. Therefore, although there are clearly proponents and opponents for each extreme, it is fair to say that the true risk for HLA-B27 positive people in the general population lies somewhere between 1% and 7%, but that there might be regional or geographical variations in risk. Probably, non-HLA-B27 genetic and environmental factors might modify the risk of developing AS for those who possess HLA-B27.
As mentioned in the letter by van der Linden and van der Heijde, Jurgen Braun’s excellent recent paper describing a survey of blood donors by questionnaire, clinical, and magnetic resonance imaging examinations revealed a prevalence of ankylosing spondylitis in B27 positive blood donors (6.4%)1-1 very similar to that reported by Gran et al(6.7%).1-2 It is probable that some of the differences in reported prevalence of ankylosing spondylitis by the various studies are because of methodological differences.
None the less, the rarity of spondyloarthritis in West African natives seems to be real and not explained either by the frequency of B27, or the B27 subtypes present. While cases of ankylosing spondylitis have been reported in B*2703 positive persons,1-3 1-4no case has yet been reported from the Gambia despite a B27 prevalence in some ethnic groups as high as 7.8%.1-5 Why not?
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