A Biometrical Study of the Oocyst of Eimeria necatrix , a Parasite of the Common Fowl

SUMMARY. Measurements of the length and width of a total of 6,915 unsporulated oocysts of Eimeria necatrix from 11 infected chickens revealed a length range of 12.1‐28.9 μ, width range of 10.8‐23.8 μ, and a shape‐index (length/width) range of 1.0‐1.64. The average size range of the samples of 25‐100 oocysts (mean, 66 oocysts) constituting this total and taken on the various days of the patent periods was 15.6–23.9 μ× 13.9–19.5 μ. The lengths, widths and shape indexes of the oocysts constituting such samples usually described unimodal, near‐normal frequency distribution curves. The average size of oocysts issuing from an average chicken on an average day of the patent period of the infection was, with standard deviations, 19.7 ± 1.82 μ× 16.7 ± 1.20 μ. The average standard errors for the length and width were 0.237 μ and 0.158 μ, respectively, in samples of the size indicated. A tendency to increase slightly, though significantly, in size after the first three days of the patent period was noted; also, a slight, though significant, linear increase from the first to the seventh day, inclusive. A slight, though significant, tendency to shorten with the passing of the days of the patent period was detected. Analysis of variance revealed much variation in oocyst size from bird to bird on a given day and for given bird and day combinations, but significant day to day variation was demonstrated for width only. By splitting the variance during the first seven days of the patent period into its parts, it was determined that the mean oocyst size during this period was, with standard errors, 19.59 ± 0.238 μ× 16.63 ± 0.200 μ. Further analysis disclosed that a considerable reduction in oocysts measured could be accomplished, with no loss of accuracy, if fewer measurements were taken per bird and more birds were employed.