Purpose: The objective of this work was to measure time-dependant changes in populations of oocysts from the surrogate species Eimeria papillata following their inoculation onto field lettuce and to use the data in the development of a predictive mathematical model.
Methods: Known concentrations of E. papillata sporulated oocysts in water were sprayed onto four-week-old Romaine lettuce plants in field plots at two locations (BC and NS) in three separate trials. Lettuce samples were collected on 0, 1, 2, 3, and 4 weeks post-inoculation, and oocysts eluted and enumerated using standard parasitological methods. Changes in log numbers were plotted against time, non-linear regression was performed and data were fitted with a Weibull decline function.
Results: Oocyst concentrations declined from 2 log CFU/g to <1 log CFU/g over four weeks but there was between-trial variation in the time required for the first log reduction and the Weibull shape parameter. A bootstrapping technique was applied to develop cumulative distributions used to simulate additional datasets (25), which then were fitted with the Weibull function to derive a predictive model. Data points generally fell within the 95% prediction limits defined by the model, and in a few cases exceed this range. Hence the model adequately predicted the fate of E. papillata oocysts in field lettuce.
Significance: The proposed model will be useful for the development of quantitative microbial risk assessments for protozoan oocysts in leafy vegetable field crops.