Litter effects on caries in rats and implications for experimental design

The purpose of this study was to obtain quantitative estimates of litter effects on caries development in rats and to examine the implications for design of rat caries experiments. Twelve female Sprague-Dawley rats, aged 60 days, were bred with 4 male rats. Nine of the 12 dams had litters in close proximity. The litters were culled to 10 pups. One pup from each litter was placed with each of the other dams for nursing, leaving 2 pups from each litter with the birth dam. This design allowed the litter effect to be separated into a prenatal component, reflecting the shared genetic makeup and in utero environment of littermates, and a postnatal component reflecting a shared environment from shortly after birth to weaning. Pups were infected with Streptococcus sobrinus and fed Diet 2000 and 10% (w/v) sucrose water for 5 weeks. There was no significant evidence of a postnatal litter effect for smooth surface caries (p = 0.37) or sulcal caries (p = 0.43). The prenatal litter effect was significant for both smooth surface caries and sulcal caries (p<0. 01). When litter effects are present, the statistical power of caries studies is improved if animals from the same litter are divided evenly among experimental groups. In addition, if litter effects are present but not allowed for in data analysis, incorrect statistical inferences may be drawn. Based on our results and other reports of litter effects, we recommend planning for litter effects in the design and analysis of rat caries studies.