Linear-Nonlinear Rhythmometric Signal Processing of Gastrointestinal Infections by Giardia Lamblia

The potential public health impact of giardiasis (GD) has become increasingly evident in recent years. Preventive health educational interventions are indicated. The timing of such endeavours may be adjusted to the changes in the incidence of GD if such should be predictable insofar as they are rhythmic. Accordingly, monthly totals of detected cases of GD in Minnesota (USA) in 1983, 1984 and 1985 were first fitted by linear least squares. The least squares harmonic analysis shows components with a period statistically significant at 8766 and 1461 hours. The rhythm parameters thus obtained are then used as initial values for the nonlinear least squares method, allowing for the estimation of the period as well as the rhythm-adjusted mean, amplitude and acrophase of each component. Results from the nonlinear approach give estimates and 95% confidence intervals for the period [8765.7, 7757 6,10223.4)], the rhythm-adjusted mean [44.2, 22.3,66.2)], the amplitude [24.8, (-6.26, 55.8)], and the acrophase [-284/spl deg/, (-359/spl deg/,-204/spl deg/)]. This circannual component is the only one that stands out as statistically significant in, the nonlinear analysis. The nonlinear approach, as a complement to the linear least squares approach, combined with biologic considerations, represents an important step forward: it renders the analysis more reliable. This approach may eliminate undue focus on spurious peaks in the spectrum and directly estimates the period along with its confidence interval. Having established the predictable circannual variability in the incidence of GD, the timing of preventive interventions by health education, in the context of other logistic considerations, should be a readily exploitable factor.