Density-dependent effects on establishment of Necator americanus and Ancylostoma ceylanicum

The relationships between the number of infective larvae (L3s) to which animals were exposed and the establishment of A. ceylanicum and N. americanus in hamsters were examined. There was no evidence of density-dependent constraints on the establishment of A. ceylanicum in the range 10-1000 larvae, but an experiment in which the range was extended to 1500 L3s gave a significant negative correlation between the magnitude of the infecting dose and percentage establishment. Even so the percentage reduction was relatively small, approximating to 0.89% per 100 larvae administered, and in practice density-dependent constrains on establishment are unlikely to affect experiments with this species in which much lower doses (< 250 L3) are generally employed. The range of doses for N. americanus L3s was smaller (10-400). Of the four experiments reported, two gave a significant reduction of establishment with increasing dose and two did not. When the data was split into low doses (< 100 L3s) and high doses (> 100 L3s), falling establishment with increasing dose was only detected in the lower dose range. There was no difference in the establishment when doses of 100 L3s were compared with 250 or 400 L3s. On balance, it was concluded that density-dependent constraints on establishment of N. americanus in hamsters were not marked and would have little significant effect on experiments utilizing fewer than 200 L3s (approximately 7.6% reduction between 10 and 200 L3s). These results are discussed in relation to host regulation of hookworm burdens.