The apparent immune-suppressive effect of ultraviolet radiation (UVR) has suggested that this environmental exposure may influence the development of immune-related disorders. Self-reported prevalence rates of type 1 diabetes mellitus, rheumatoid arthritis (RA), eczema/dermatitis, and asthma, from the 1995 Australian National Health Survey, were therefore examined by latitude and ambient level of UVR. A positive association of type 1 diabetes mellitus prevalence was found with both increasing southern latitude of residence (r = 0.77; p = 0.026) and decreasing regional annual ambient UVR (r = -0.80; p = 0.018); a 3-fold increase in prevalence from the northernmost region to the southernmost region was evident. In contrast, asthma correlated negatively with latitude (r = -0.72; p = 0.046), although the change in asthma prevalence from the north to the south of Australia was only 0.7-fold. For both RA and eczema/dermatitis, there were no statistically significant associations between latitude/UVR and disease prevalence. These ecologic data provide some support for a previously proposed beneficial effect of UVR on T-helper 1-mediated autoimmune disorders such as type 1 diabetes. The inverse association of type 1 diabetes prevalence with UVR is consistent with that previously reported for another autoimmune disease, multiple sclerosis, in Australia, and also with type 1 diabetes latitudinal gradients in the Northern Hemisphere. The finding also accords with photoimmunologic evidence of UVR-induced immunosuppression and may suggest a beneficial effect of UVR in reducing the incidence of such autoimmune conditions. In light of this study, analytic epidemiologic studies investigating risk of immune disorders in relation to personal UVR exposure in humans are required. Key words: asthma, Australia, autoimmune disease, ecologic analysis, eczema/dermatitis, immune disorders, latitude, rheumatoid arthritis, type 1 diabetes, ultraviolet radiation.
Autoimmune diseases such as type 1 diabetes mellitus, multiple sclerosis (MS), and rheumatoid arthritis (RA) are immune system disorders that share common features of self-reactive T cells and the presence of auto-antibodies; as a group they affect some 5% of the population (Davidson and Diamond 2001). Although their precise etiologies are unknown, these autoimmune disorders are generally agreed to reflect interactions of polygenic traits with various ill-defined environmental factors (Cantorna 2000; Dahlquist 1998; Hayes et al. 1997; Karges et al. 1995; Weinshenker 1996). Descriptive epidemiology may further elucidate the role of environmental factors in the etiology of the autoimmune diseases MS, type 1 diabetes, and RA, as well as other immune-related disturbances such as asthma and eczema/dermatitis.
MS, type 1 diabetes, and, to a lesser extent, RA in the Northern Hemisphere, particularly in Western Europe and North America, display a latitudinal gradient in disease frequency, with the prevalence of these disorders increasing at higher latitudes (Cantorna 2000; DERIG 1988; Hayes et al. 1997; Karvonen et al. 1993). MS exhibits a similar prevalence gradient in the Southern Hemisphere, in Australia and New Zealand (Hammond et al. 2000; Miller et al. 1990). In Australia, however, where the opportunity exists to study gradients in rates across a large-area population that is less ethnically and genetically diverse than across Europe, analyses for other immune-related disorders have not been done previously.
Ultraviolet radiation (UVR) reaching the earth's surface varies inversely with latitude; UVR is thus a prominent latitude-related environmental factor. Recent photoimmunologic work shows that UVR downregulates cellular immunity (Damian et al. 1998; Kelly et al. 1998), attenuating T-helper (Th)1 T-cell-mediated immune responses (Clydesdale et al. 2001). These responses are thought to be significantly involved in some autoimmune disorders …