A stochastic model for the aggregation of Onchocerca volvulus in nodules

A model is presented which describes the aggregation of female Onchocerca volvulus in nodules and their distribution in the human population. The basic model is based on a single parameter, the formation probability q, which represents the probability with which incoming larvae form a new nodule. This parameter describes parasite behaviour which cannot easily be recognized in available data without modelling. The estimate for the average formation probability of muq = 0.39 suggests an attraction of the invading infective larvae to already existing nodules or resident worms with probability 0.61. No significant difference in muq was found between the forest and savanna parasite strains. The model can be used inversely to estimate the worm burden of persons from palpation data. The observed variance in the number of nodules per person requires the assumption of a variance-increasing mechanism which was implemented by heterogeneity within the host population (extended model with 2 parameters). Possible reasons for this heterogeneity are presented and its implications concerning the reproductive biology of the parasite are discussed.