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The course of a self-induced infection with about 100 Ancylostoma duodenale larvae was followed by twice-weekly blood and stool examinations. After a small initial increase, no further change in eosinophil counts occurred until the 33rd week of infection when they began to increase sharply. Seven weeks later, eggs were first observed in stools. Thus, in this infection, the prepatent period was five times the expected. It is suggested that a strain of A. duodenale exists which either 1) has an inherent, abnormally long prepatent period or 2) has a prepatent period which is susceptible to change depending upon the environmental conditions experienced by the free-living larvae. In either case, the third stage larvae probably enter a hypobiotic state within the host which lasts for about 8 months.
Present address: University of Pennsylvania, School of Veterinary Medicine, Department of Pathobiology, 3800 Spruce Street, Philadelphia, Pennsylvania 19174.