Evaluation of water fluoridation scheme in Cumbria: the CATFISH prospective longitudinal cohort study
Authors: Goodwin Michaela , Emsley Richard , Kelly Michael P , Sutton Matt , Tickle Martin , Walsh Tanya , Whittaker William , Pretty Iain A 
Published: November 2022

Background
Water fluoridation was introduced in the UK against a background of high dental decay within the population. Levels of decay have dramatically reduced over the last 40 years following widespread use of fluoride toothpaste.

Objective
The aim of the CATFISH (Cumbrian Assessment of Teeth a Fluoride Intervention Study for Health) study was to address the question of whether or not the addition of fluoride to community drinking water, in a contemporary population, lead to a reduction in the number of children with caries and, if so, is this reduction cost-effective?

Design
A longitudinal prospective cohort design was used in two distinct recruited populations: (1) a birth cohort to assess systemic and topical effects of water fluoridation and (2) an older school cohort to assess the topical effects of drinking fluoridated water.

Setting
The study was conducted in Cumbria, UK. Broadly, the intervention group (i.e. individuals receiving fluoridated drinking water) were from the west of Cumbria and the control group were from the east of Cumbria.

Participants
Children who were lifetime residents of Cumbria were recruited. For the birth cohort, children were recruited at birth (2014–15), and followed until age 5 years. For the older school cohort, children were recruited at age 5 years (2013–14) and followed until the age of 11 years.

Intervention
The provision of a ‘reintroduced fluoridated water scheme’.

Main outcome measures
The primary outcome measure was the presence or absence of decay into dentine in the primary teeth (birth cohort) and permanent teeth (older school cohort). The cost per quality-adjusted life-year was also assessed.

Results
In the birth cohort (n = 1444), 17.4% of children in the intervention group had decay into dentine, compared with 21.4% of children in the control group. The evidence, after adjusting for deprivation, age and sex, with an adjusted odds ratio of 0.74 (95% confidence interval 0.56 to 0.98), suggested that water fluoridation was likely to have a modest beneficial effect. There was insufficient evidence of difference in the presence of decay in children in the older school cohort (n = 1192), with 19.1% of children in the intervention group having decay into dentine, compared with 21.9% of children in the control group (adjusted odds ratio 0.80, 95% confidence interval 0.58 to 1.09). The intervention was found to be likely to be cost-effective for both the birth cohort and the older school cohort at a willingness-to-pay threshold of £20,000 per quality-adjusted life-year. There was no significant difference in the performance of water fluoridation on caries experience across deprivation quintiles.

Conclusions
The prevalence of caries and the impact of water fluoridation was much smaller than previous studies have reported. The intervention was effective in the birth cohort group; however, the importance of the modest absolute reduction in caries (into dentine) needs to be considered against the use of other dental caries preventative measures. Longer-term follow-up will be required to fully understand the balance of benefits and potential risks (e.g. fluorosis) of water fluoridation in contemporary low-caries populations.

Limitations
The low response rates to the questionnaires reduced their value for generalisations. The observed numbers of children with decay and the postulated differences between the groups were far smaller than anticipated and, consequently, the power of the study was affected (i.e. increasing the uncertainty indicated in the confidence intervals).

Study registration
This study is registered as Integrated Research Application System 131824 and 149278.

Funding
This project was funded by the National Institute for Health and Care Research (NIHR) Public Health Research programme and will be published in full in Public Health Research; Vol. 10, No. 11. See the NIHR Journals Library website for further project information.