In multivariate analyses, viral diarrhea was associated with the number of holding tank septic systems in the 640-acre section surrounding the case residence [adjusted odds ratio (AOR), 1.08; 95% confidence interval (CI), 1.02-1.15; p = 0.008], and bacterial diarrhea was associated with the number of holding tanks per 40-acre quarter-quarter section (AOR, 1.22; 95% CI, 1.02-1.46; p = 0.026).
The septic system density surrounding each case and control residence was determined for three geographic scales corresponding to conventional land survey units: section (640 acres, 259 hectares), quarter section (160 acres, 64.75 hectares), and quarter-quarter section (40 acres, 16.19 hectares).
Bacterial diarrhea was independently associated with the number of holding tanks in the quarter-quarter section (40-acre block) of the residence (Table 4).
Bacterial diarrhea was marginally associated with the number of holding tanks per quarter-quarter section (13 cases, 111 controls; AOR, 1.12; 95% CI, 0.99-1.46; p = 0.07) but remained associated with entering a calf hutch or pen (AOR, 10.6; 95% CI, 2.75-40.81; p < 0.001).
For bacterial diarrhea, the threshold was > 1 holding tank per quarter-quarter section ([greater than or equal to] 75th percentile of the density distribution of the controls), resulting in an AOR of 2.17 (p = 0.145).
This geographic coding system specifies the location of property by section (640 acres, which is also one square mile), quarter section (160 acres), and quarter-quarter section (40 acres).
Bacterial diarrhea was associated with holding tank density only at the quarter-quarter section level, with the risk increasing by 22% with each additional tank.