New research from the University of Queensland, Australia, has confirmed once again, the incidence of multiple sclerosis increases the further you move away from the equator.
To date, many studies in the northern hemisphere have observed that the further north you live from the equator, the greater chance you have of developing MS. Likewise, in the southern hemisphere, the further south you live from the equator, the greater chance you have of developing MS.
The majority of this type of data has derived from the northern hemisphere. Researchers will estimate incidence of MS based on field surveys of selected geographical areas. The major pitfall in this method is that the selected geographical regions may not represent the overall population perfectly.
Here in this study, the researchers led by Dr Samantha Hollingworth approached the incidence of MS pharmacologically. In Australia, drugs to help manage relapsing-remitting MS (RRMS) are subsidized by the government. On the other hand, clinically isolated syndrome and progressive MS are not.
Individuals who receive subsidized drugs to manage RRMS are part of the nation’s Pharmaceutical Benefits Schemes program, and a record of their use of subsidized RRMS drugs are recorded by the Schemes program. The researchers looked at the data of the program and found the following:
- Overall they found a prevalence of 31.1 persons per 100,000 take drugs to manage RRMS in Australia.
- The territory closest to the equator (Northern Territory) had the lowest prevalence, with 7.5 persons per 100,000 taking drugs to manage RRMS.
- The territory furthest from the equator (Tasmania) had the highest prevalence, with 53.2 persons per 100,000 taking drugs to manage RRMS.
- Overall, there was a significant linear relationship between increasing prevalence of drugs taken to manage RRMS and increasing south latitude (r = 0.957, p = 0.0002).
The strength of this kind of study is how comprehensive it was in collecting data on RR-MS rather than having to rely on field data. This was made possible by nation-wide records of individual’s use of subsidized RR-MS drugs.
The biggest limitation is that the researchers couldn’t collect data for the overall prevalence of MS. People with progressive MS do not receive subsidized drugs, thus they were not taken into account in this study. Also, not all patients with RR-MS choose to take drugs to manage their disease. It is estimated that RRMS accounts for 68% of all MS cases, and that of people with RRMS, only 65% choose to take drugs to manage their disease. So, the incidence rates in this study likely only account for 45% of the overall incidence of MS.
The authors conclude,
“The present data have the advantage of comprehensiveness and being more or less simultaneous but the disadvantage of being restricted to one type of MS, albeit the most common type. Nevertheless, the fact that the different methodologies have led to similar conclusions reinforces the finding of the latitudinal gradient in MS prevalence in Australia.”