Effects of community water fluoridation on child dental caries in remote Northern Territory, Australia: a difference-in-difference analysis

Objectives: Community Water Fluoridation (CWF) is a cost-effective in reducing dental caries at population level. This study used difference-in-difference (DiD) analysis to measure outcome (dental caries) in the children exposed to Community Water Fluoridation (CWF, treatment group) and children not exposed to CWF (the comparison or control group) both before and after intervention.
Methods: The oral health data obtained for Northern Territory (NT) Department of Health (DoH) contains 64,399 person-year observations during the year 2008-2020 from 24,546 children aged 1-17 years old. The drinking water quality data for fluoride levels was collected for 50 remote communities held by Power and Water Corporation. Two control groups were used in study. The control 1 group included observations that had naturally occurring fluoride in drinking water supplies, at levels at or above the DoH policy threshold of 0.5mg/L.  The control 2 group included observations from communities that had naturally occurring fluoride in drinking water supplies below the level recommended by the DoH policy (<0.5 mg/L). The data were grouped into time-periods prior to 2013 (before) and after 2013 (after).
Results: The result shows dental caries in treatment group significantly decreased at greater magnitude than control groups after the intervention period.  Overall, the treatment group shows decline in number of teeth affected by caries by an average 0.28 (p=0.001) affected teeth after intervention when compared with non-treatment groups. A greater post-intervention decline, of 0.30 (p=0.002) dmft/DMFT was observed for the treatment group when compared to the control-2 group. Notably, children of ages 7-10 years and 11-17 years in the treatment group experienced significantly greater post-intervention declines in average dmft/DMFT by 0.32 (p=0.051) and 0.40 (p=0.012) fewer affected teeth respectively.
Conclusions: The rates of child dental caries for Aboriginal children in remote and very remote NT are still some of the highest in the country. Our study demonstrates the application of the DiD method in public health policy evaluation. The study findings suggest that the longstanding policy position of the NT DoH on CWF has supported improvements in oral health amongst child populations that experience high levels of dental caries in remote communities of Northern Territory.
Keywords: Aboriginal, oral health, fluoridation, dental caries, Northern Territory, Australia.