Ecological Modelling 222 (2011) 2093–2102 Contents lists available at ScienceDirect Ecological Modelling journal homepage: www.elsevier.com/locate/ecolmodel New hydroepidemiological models of indicator organisms and zoonotic pathogens in agricultural watersheds Graham B. McBride a,∗ , Steven C. Chapra b a National Institute of Water and Atmospheric Research (NIWA), P.O. Box 11-115, Gate 10, Silverdale Road, Hamilton, Waikato 3216, New Zealand b Department of Civil and Environmental Engineering, Tufts University, Medford, MA 02155, USA article info Article history: Received 20 January 2011 Received in revised form 11 April 2011 Accepted 12 April 2011 Available online 12 May 2011 Keywords: Campylobacter E. coli Faecal indicators Waterborne pathogens Best management practices Hydroepidemiology Stream River abstract Simple analytical models are derived to assess how a series of cattle animal farms affect the transport and fate of an indicator organism (Escherichia coli) and a zoonotic pathogen (Campylobacter) in a stream. Separate steady-state mass-balance models are developed and solved for the ultimate minimum and maximum concentrations for the two organisms. The E. coli model assumes that the organism is ubiqui- tous and abundant in the animals’ digestive tracts. In contrast, a simple dose–response model is employed to relate the Campylobacter prevalence to drinking water drawn from the stream. Because faecal indica- tors are commonly employed to assess the efficacy of best management practice (BMP) interventions, we also employ the models to assess how BMPs impact pathogen levels. The model provides predictions of (a) the relative removal efficacy for Campylobacter and (b) the prevalence of Campylobacter infection among farm animals after implementation of BMPs. Dimensionless numbers and simple graphs are developed to assess how prevalence is influenced by a number of factors including animal density and farm spacing. A significant outcome of this model development is that the numerous dimensional input and parameter variables are reduced to a group of just four dimensionless Campylobacter-related quantities, charac- terizing: animal density; in-stream attenuation; animal-to-animal transmission; and infection recovery. Calculations reveal that for some constellations of these four quantities there can be a greater-than- expected benefit in that the proportional reduction of stream Campylobacter concentrations post-BMP can substantially exceed the proportional reduction of concentrations of E. coli in that stream. In addition, a criterion for system sterility (i.e., the conditions required for the farm infection rate to decrease with downstream distance) is derived. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Zoonoses can dominate reported human diseases in countries with agricultural economies. For example, they have comprised 65–80% of New Zealand’s reported notifiable disease cases in recent years, much of which is campylobacteriosis (Till and McBride, 2004)—a major cause of enteric illness worldwide (Cotruvo et al., 2004). Given that water is an effective vector for zoonotic pathogens deposited by animals on the landscape, one can expect that dis- rupting this (landscape) component of the pathogens’ ecological cycles could be effective in reducing a nation’s disease burden- even if, as in the case of Campylobacter, the majority of human exposures appears to be via contaminated food. Such interventions, described as BMPs (best management practices), could include reducing direct animal input into streams by fencing of ripar- ian stream areas, retirement of stream riparian zones, providing bridge crossings, using deferred irrigation, or provision of “stand- ∗ Corresponding author. Tel.: +64 07 856 1726; fax: +64 07 856 0151. E-mail address: g.mcbride@niwa.co.nz (G.B. McBride). off” areas (Cotruvo et al., 2004; Collins et al., 2007; Monaghan et al., 2008). Their potential advantages are so significant that they can be enjoined by agricultural industries in association with govern- ment agencies. For example the large New Zealand Fonterra Dairy Company has promoted the “Dairying and Clean Streams Accord” to its dairy farm suppliers (NZMFE, 2003). Commonly, the efficacy of BMP interventions in freshwater environments is assessed using a faecal indicator, most notably Escherichia coli. But in so doing an important question arises: “If the interventions result in a reduction in E. coli loads by X%, what is the concomitant reduction in the zoonotic pathogens?” Given that animals can always be expected to shed E. coli, but only some will shed zoonotic pathogens for some of the time, there is some prospect that the reduction of loads of the pathogens could exceed X%—a heartening result for the funders of the BMPs. One could par- ticularly expect that to be the case if in fact the interventions have the effect of reducing the prevalence of the pathogens among the agricultural animals. We have constructed two simple mathematical models to examine this question: one for E. coli and one for Campylobac- ter jejuni (as a typical zoonotic pathogen, and the most prevalent 0304-3800/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2011.04.008