This study spatially evaluated the MS incidence distribution in Fars province, using Bayesian model. Spatial description of the disease is a useful tool for evaluating the incidence rate data and to generate leads in further public health investigations and interventions . As far as we know, this is the first study to have estimated the Spatio-temporal incidence rate of MS. This study also investigates the association of vitamin D3, smoking, BMI, and alcohol consumption with MS incidence rate, as well as spatial dependence between the neighboring regions. The findings suggest that further studies are warranted in order to confirm the allocations of health resource across different region.
The results of BYM model describes the low incidence rate of MS in south and southeast of the province. Zarindasht county, located in south with hot and dry climate and high temperature, had the lowest incidence rate. This result is in line with previous studies that showed maintaining adequate levels of vitamin D (as one of the environmental factors) have a protective effect, resulting in lower risk of developing autoimmune diseases including MS[29-31]. The results of this study also showed that larger cities, located in the center of province, especially Shiraz and Abadea in northeast had high MS incidence rates. This is in concordance with previous studies which showed MS to be more common among urban dwellers than rural ones. There is no clear explanation as to why, but higher frequencies in these regions might be due to more accurate diagnosis in these regions which is a result of easier access to healthcare and higher-quality diagnostic procedures. Although, high socioeconomic status (SES) index which includes income, education, ethnicity, job and asset is generally associated with better health outcomes, it seemed that regions with higher SES (i.e. urban areas compared to rural areas) had led to increased risk of MS. However, this is just an association and not a cause, but the reason is unknown [32, 33]. Even though some previous studies were in line with our result, some were contrary and reported no evidence of an association between MS incidence and SES or reported an association with low SES [32-34].
Our results showed that the percentage of Vitamin D3 intake and smoking can be associated with reduced risk of MS incidence, which is in line with previous studies that had suggested maintaining adequate levels of vitamin D can have a protective effect by lowering the risk of developing autoimmune diseases including MS[29-31]. When a person has MS, his or her immune system attacks the coating that protects nerve cells. Research suggests a significant positive effect of vitamin D on the immune system.[29-31, 35]. Smoking as a risk factor can influence the course of MS [16, 36, 37]. Both duration and smoking intensity can contributed independently to the increased risk of MS . Studies amongst MS patients and healthy controls, consistently have provided evidence that both active and passive smoking can result in an increased risk of MS and disease progression. Studies have shown that the risk of MS associated with HLA genotypes is increased by smoking status [36, 39].
According to several studies, obesity is one of the potential risk factors of MS. The growing world-wide obesity epidemic has multiple deleterious effects on public health and has also been associated with an increased risk of MS . Increased obesity leads to lower levels of 25-hydroxyvitamin D, which in turn predisposes to MS . The association between obesity and MS is similar amongst men and women, and the observed trend of higher BMI resulted in a higher risk of developing MS . However, in our study BMI was not associated with the incidence of MS.
Although the impact of alcohol, which might directly suppress various immune responses, on the risk of developing MS has been investigated in different studies, the results were inconsistent. In some studies, researchers found no association between alcohol intake and risk of MS , but in some other researchers showed a potential dependency between alcohol consumption and the incidence of MS .
Our study had some limitations, which should be taken into consideration in future studies. The major limitation of the present study was that although the MS incidence rate was longitudinal, covariate information was not available per year between 1991 and 2016; hence, we had the overall information of these covariates during this period. Lack of this information prevented us from using these covariates in spatio-temporal model, but investigated their effect on longitudinal mapping. Another limitation was that in this study we focused only on the statistical test results while assessing the effect size gives additional information about the extent of the effected values. Since the incidence and prevalence rates are different in male and female , further investigations on the impact of other important covariates, such as climate, sunlight, and air pollution should be conducted [13, 17, 44-48].
Conclusion: Spatial analysis of MS showed low incidence rate of this disease in the south and south-east of Fars province. In addition, large cities also had very high MS incidence rate. Effect of different covariates which were investigated in this study showed that vitamin D and smoking among all covaiates might be associated with the incidence of MS.