Parasitology

Research Article

How will public and animal health interventions drive life-history evolution in parasitic nematodes?

PENELOPE A. LYNCHa1a2 c1, UWE GRIMMa1 and ANDREW F. READa2a3

a1 Department of Mathematics, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

a2 Institutes of Evolutionary Biology & Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, UK

a3 Current address: Centre for Infectious Disease Dynamics, Departments of Biology & Entomology, The Pennsylvania State University, University Park PA 16802-5301, USA

SUMMARY

Infection caused by parasitic nematodes of humans and livestock can have significant health and economic costs. Treatments aimed at alleviating these costs, such as chemotherapy and vaccination, alter parasite survival and reproduction, the main selective pressures shaping life-history traits such as age to maturity, size and fecundity. Most authors have argued that the life-history evolution prompted by animal and public health programmes would be clinically beneficial, generating smaller, less fecund worms, and several mathematical models support this view. However, using mathematical models of long-lasting interventions, such as vaccination, and regularly repeated short interventions, such as drenching, we show here that the expected outcome actually depends on how mortality rates vary as a function of worm size and developmental status. Interventions which change mortality functions can exert selection pressure to either shorten or extend the time to maturity, and thus increase or decrease worm fecundity and size. The evolutionary trajectory depends critically on the details of the mortality functions with and without the intervention. Earlier optimism that health interventions would always prompt the evolution of smaller, less fecund and hence clinically less damaging worms is premature.

(Received September 20 2007)

(Revised November 27 2007)

(Revised January 16 2008)

(Accepted January 17 2008)

(Online publication March 10 2008)

Correspondence:

c1 Corresponding author – pennymath@lynch-fm.demon.co.uk

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