Estimating the force of infection; Mycobacterium bovis infection in feral ferrets Mustela furo in New Zealand


Peter Caley, Landcare Research Ltd, Private Bag 11052, Palmerston North, 5301, New Zealand (fax + 64 6355 9230; e-mail


  • 1The force of Mycobacterium bovis infection (λ) in feral ferret Mustela furo populations in New Zealand was estimated, by fitting candidate models to age-specific disease-prevalence data. The candidate models were constructed from a set of a priori hypotheses of how M. bovis infection is transmitted to ferrets, and model selection used to assess the degree of support for each hypothesis.
  • 2The estimated force of M. bovis infection ranged between five sites from 0·14 year−1 to 5·8 year−1, and was twofold higher in males than in females.
  • 3The data most strongly supported the hypothesis that transmission of M. bovis to ferrets occurs from the ingestion of M. bovis-infected material from the age of weaning, as modelled by the force of infection being zero up to the age of weaning, and constant thereafter. Other candidate transmission hypotheses (e.g. mating, suckling, routine social interaction) and combinations thereof were unsupported in comparison, and hence it was concluded that transmission from these postulated mechanisms must be insignificant compared with dietary-related transmission.
  • 4The preferred transmission hypothesis was nearly equally supported regardless of whether disease-induced mortality was included or not, although omitting disease-induced mortality resulted in a lower force of infection estimate. The dietary transmission hypothesis (omitting disease-induced mortality) could be easily represented by a generalized linear model, enabling simple analysis of critical experiments designed to identify the source of M. bovis infection in feral ferrets.