by Allison Hu
Tuberculosis (TB) is a major global public health problem – it affects millions of people annually and ranks as the second leading cause of death from an infectious disease worldwide (World Health Organization (WHO) 2013) (Onozuka et al. 2014). The WHO estimated that there were 8.6 million new TB cases and 1.3 million TB deaths in 2012. The worldwide TB incidence rates peaked in 2004 and have decreased at a rate of less than 1 % per year since then. Thus, the overall worldwide burden continues to rise as a result of the rapid growth of the world population. TB is a leading cause of death in people in the most economically productive age groups. Furthermore, with growing concerns about global climate change, many studies have focused on associations between weather variability and the fluctuations of infectious diseases and have suggested that weather factors play an important role in their incidence, indicating the possibility of multiple functional pathways.
Although seasonal variation in the incidence of tuberculosis (TB) has been widely assumed, few studies have investigated the association between extreme temperatures and the incidence of TB. Therefore, Onozuka et al. examined the possible relationship of extreme temperatures with the incidence of TB cases using surveillance data collected in Fukuoka, Japan, from 2008 to 2012. Time-series analyses was used to assess the possible relationship of extreme temperatures with TB incident cases, adjusting for seasonal and interannual variation. Analysis revealed that the occurrence of extreme heat temperature events resulted in a significant increase in the number of TB cases and the occurrence of extreme cold temperature events resulted in a twenty-three percent increase.
Onozuka et al. speculate that extreme temperatures are associated with behavioral patterns that lead to increased contact among people, thereby facilitating the spread of TB infection. Extreme temperatures are also associated with patients presenting for evaluation, rather than being associated with TB incidence. Therefore, it is possible that the explanation for the association observed is that patients are less comfortable when temperatures are high and low, causing them them to seek medical attention. Additionally, vitamin D is an important determinant of adaptive and innate immunity.
There are a few limitations to this study. First, it is possible that TB surveillance data might not capture all of the cases in a community. This under-reporting of infections can occur anywhere in the reporting chain, from the initial decision of a patient to not seek health care to the failure to record cases in the disease registry, due to the mildness or lack of symptoms. Second, TB can often remain undiagnosed for some time if a patient does not seek care or if a clinician does not immediately recognize TB. In these cases, the actual onset of disease would have been prior to the week the case was reported. Lastly, a short period of time was analyzed (5 years) so a longer study period can better determine seasonal patterns of TB.
This study provides quantitative evidence that the number of TB cases increased significantly with extreme heat and cold temperatures. The results may help public health officials predict extreme temperature-related TB incidence and prepare for the implementation of preventive public health interventions (Onozuka et al.).
Onozuka, D., & Hagihara, A., 2014. The association of extreme temperatures and the incidence of tuberculosis in Japan. International journal of biometeorology, 1-8.