A stochastic model for the aggregation of Onchocerca volvulus in nodules

H. P.  DUERR  a1 a2 c1, K.  DIETZ  a1, D. W.  BÜTTNER  a3 and H.  SCHULZ-KEY  a2
a1 Department of Medical Biometry, University of Tübingen, Germany
a2 Institute of Tropical Medicine, University of Tübingen, Germany
a3 Bernhard Nocht Institute of Tropical Medicine, Hamburg, Germany

Article author query
duerr h   [PubMed][Google Scholar] 
dietz k   [PubMed][Google Scholar] 
buttner d   [PubMed][Google Scholar] 
schulz-key h   [PubMed][Google Scholar] 


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 μq = 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 μq 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.

(Received December 21 2000)
(Accepted February 12 2001)

Key Words: Onchocerca volvulus; savanna and forest strains; aggregation; modelling; onchocercomata; nodule formation.

c1 Corresponding author: Department of Medical Biometry, University of Tübingen, Westbahnhofstrasse 55, 72070 Tübingen, Germany. Tel: +49 7071 2978259. Fax: +49 7071 295075. E-mail: