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 could lead to further public health investigations and interventions . As far as we know, this is the first study that has estimated the spatio-temporal incidence rate of MS in Fars province. This study also investigates the association of vitamin D supplementation intake, 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.
In the current study, the average of Female/male ratio was 4.25 and women had higher risk than men. Among the age groups, the highest prevalence belonged to range 20-30 (362.10 per 100,000-person), and then 30-40 years old (233.00 per 100,000-person). Our results are similar to Sahraian et al.’s study (2010) on 8000 cases of MS in 2010. They reported that the most cases of MS were in the age range of 20-39 years old and Iranian women have higher risk of MS. The results of spatio-temporal model describe the low incidence rate of MS in south and southeast of Fars province. Zarindasht county, located in south with a rather 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. They result in lower risk of developing autoimmune diseases including MS[35-37]. The results of BYM model (without time effect) in this study showed that larger counties, located in the center of Fars province, especially Shiraz and Abadeh 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 Dehghani et al. 2017 introduced urbanization as a major risk factor in incidence rate of MS in Iran. In addition , Sahraian et al. 2017b indicated that the impact of air pollution, unhealthy lifestyle, exposure to industrial solvents, low exposure to sunlight, infectious agents and smoking have increased incidence rate of MS among urban dwellers
Our results showed that the percentage of vitamin D supplementation intake and smoking can be associated with the reduced risk of MS incidence which is in line with previous studies that had suggested that maintaining adequate levels of vitamin D could have a protective effect by lowering the risk of developing autoimmune diseases including MS[35-37]. 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.[35-37, 41]. Smoking as a risk factor can influence the course of MS [16, 42, 43]. Both the duration and intensity of smoking can contribute independently to the increased risk of MS . Consistent studies on MS patients and healthy controls 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 [42, 45].
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 among men and women, and the observed trend of higher BMI resulted in a higher risk of developing MS . However, in our study BMI had no association 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 the risk of MS . But some studies showed a potential dependency between alcohol consumption and the incidence of MS .
Our study was a long-term study in an almost uniform population but it had some limitations which should be taken into consideration in future studies. The major limitation in such studies is the incomplete registration of information due to lack of diagnosis facilities and access to neurologist, especially in rural areas. However, The various models in disease mapping, such as Bayesian hierarchical models, for smoothing disease rates and solving this problem have been suggested. Bayesian hierarchical models try to improve the estimates of log risk by using neighbor regions information in a spatial structured component as well as region variation in an unstructured component. Despite these models, government supports for the complete recording of information could increase the accuracy of the estimates.
Another limitation is related to risk factors. We needed covariates in each area during study for spatio-temporal modeling. However, some covariates such as climate, sunlight, air pollution industrial solvents and so on, were not available per year between 1991 and 2016; hence, we had the information of four covariates during this period. We are hoping, by improving the quality of data registration in Iran in recent years, to investigate environmental causes by statistical analysis very soon.
Conclusion: Bayesian spatio-temporal analysis of MS incidence rate revealed that trend is less steep than the global trend during time in the south and south east of Fars province, which is due to the effects of the higher percentage of vitamin D supplementation intake and the lower percentage of smoking. The effects of different risk factors, which were investigated in this study, showed that smoking and low vitamin D among all risk factors might be associated with the high incidence of MS.