Bovine tuberculosis: Within-herd transmission models to support and direct the decision-making process

Abstract

Use of mathematical models to study the transmission dynamics of infectious diseases is becoming increasingly common in veterinary sciences. However, modeling chronic infectious diseases such as bovine tuberculosis (bTB) is particularly challenging due to the substantial uncertainty associated with the epidemiology of the disease. Here, the methodological approaches used to model bTB and published in the peer-reviewed literature in the last decades were reviewed with a focus on the impact that the models’ assumptions may have had on their results, such as the assumption of density vs. frequency-dependent transmission, the existence of non-infectious and non-detectable stages, and the effect of extrinsic sources of infection (usually associated with wildlife reservoirs). Although all studies suggested a relatively low rate of within-herd transmission of bTB when test-and-cull programs are in place, differences in the estimated length of the infection stages, sensitivity and specificity of the tests used and probable type of transmission (density or frequency dependent) were observed. Additional improvements, such as exploring the usefulness of contact-networks instead of assuming homogeneous mixing of animals, may help to build better models that can help to design, evaluate and monitor control and eradication strategies against bTB.

Introduction

Although mathematical models of disease dynamics have been extensively used in human medicine since the eighteenth century and particularly from the 1900s onward (Keeling and Rohani, 2008), their use in veterinary medicine has been quite limited until the end of the twentieth century. However, in recent decades, the number of animal disease mathematical models published in the peer-reviewed literature has substantially increased. In the field of animal health, different types of disease models of varying complexity have been useful tools for development of policy, design and evaluation of surveillance systems and the prediction of consequences due to introduction of new diseases and the expected impact of control strategies (Garner, Hamilton, 2011, Willeberg et al, 2011a, Willeberg et al, 2011b). In the case of bovine tuberculosis (bTB), modeling has been used to provide estimates of (i) within- and between-herd transmission rates (Alvarez et al, 2012, Barlow et al, 1997, Griffin et al, 2000, Kao et al, 1997, Kean et al, 1999, Perez et al, 2002a), (ii) the duration of the period between infection and detection/shedding of the pathogen (commonly referred to as ‘latency’) (Conlan et al, 2012, Fischer et al, 2005, Kao et al, 1997, Perez et al, 2002a, Smith et al, 2013a), (iii) bTB diagnostic technique reliability (sensitivity and specificity) (Conlan et al, 2012, Fischer et al, 2005, Smith et al, 2013a), (iv) disease dynamics in wildlife and contribution of wildlife reservoirs in disease incidence and/or persistence in livestock (Anderson et al, 2013, Barlow, 2000, Delahay et al, 2013, Graham et al, 2013, Kean et al, 1999), and (v) the effectiveness of alternative control measures in cattle and wildlife reservoirs (Fischer et al, 2005, Hardstaff et al, 2013, Kao, Roberts, 1999, Perez et al, 2002b, Smith et al, 2013b). Here, we offer a comprehensive review of models assessing bTB within-herd transmission, including a simplified description of the methodologies and assumptions adopted to formulate the models, a summary of the values used to parameterize or obtained from the modeling exercises, and a discussion on the most important conclusions that may be extracted from existing knowledge. We emphasize the review of studies published since the last review of the subject was published more than 10 years ago (Goodchild and Clifton-Hadley, 2001).