The importance of culling in Johne's disease control
Introduction
Johne's disease is a chronic, progressive, and infectious intestinal disease caused by infection with Mycobacterium avium subsp. paratuberculosis (MAP). MAP spreads in herds primarily through fecal–oral transmission (Clarke, 1997), contaminated water and soils, in utero transmission, and infected milk and colostrum (Streeter et al., 1995). The typical symptoms of Johne's disease in cattle include reduced milk production, weight loss despite a normal appetite, and, in advanced cases, chronic diarrhea. There is no treatment available at present (National Research Council of the National Academies, 2003).
Johne's disease is globally widespread, and is currently among of the most important infectious disease of domestic ruminants in modern agricultural systems (NAHMS, 1997). Johne's disease is especially important to the dairy industry, due to the impact of the disease on milk production. It has been estimated to cost US dairy producers more than $200 million each year (Ott et al., 1999). Thus, MAP control is an important economic consideration for dairy producers.
In addition to the economic impact, a potential causative link between MAP and a chronic intestinal disease of humans, Crohn's disease, is being investigated (Collins, 1997, Feller et al., 2007, Mpofu et al., 2007, Behr and Kapur, 2008). As MAP is secreted in the milk of infected cattle and is highly prevalent in the US dairy industry, control of MAP transmission may be an important issue from a public health perspective.
Control of MAP spreading within and across herds is difficult, however, as Johne's disease has a long incubation period, and MAP can survive in the environment for a year or more (Whittington et al., 2004). In cattle, infection by MAP occurs at an early age ( year), and infected animals typically start to shed MAP several years after initial infection. When animals begin to shed MAP, generally as adults, the amount of the pathogen that is shed gradually increases. Clinical signs of Johne's disease only develop months to years after shedding of MAP starts (Whitlock et al., 2000).
To control the spread of MAP, test-based culling intervention is typically recommended. Current diagnostic tests, such as fecal culture test, fecal polymerase chain reaction (PCR) test, and enzyme-linked immunosorbent assay (ELISA) have high test sensitivities for detecting infectious animals shedding high levels of MAP, but relatively low test sensitivities for detecting infectious animals shedding low levels of MAP (Whitlock et al., 2000, Whitlock et al., 2007, Collins, 2005, Collins et al., 2006). A general practice on many dairy farms is to immediately cull animals shedding high levels of MAP, as they are considered to be a greater risk for spreading MAP. For animals shedding low levels of MAP, culling is generally delayed or not done at all; culling low shedding animals may be more costly to the herd than the infections they cause (Dorshorst et al., 2006).
In addition to culling of infectious animals, changes to management practices can reduce the transmission rates due to direct contacts between the susceptible and infectious animals. In the present study, we consider good and poor herd management as two extremes of a wide spectrum. Poor management signifies a failure to control the physical spread of MAP organisms between animal groups. This commonly includes using pooled colostrum for calves and poorly cleaned and/or shared calving pens. Good management involves cleaning calving pens between usage, housing calves separate from cows, and individual housing of calves (Nielsen and Toft, 2007). On real-life dairy farms, the implementation of these management procedures will vary and a wide range in good and poor managements may be observed. The goal of our research is to study the impact of culling strategies over the full range of observed management strategies.
The work presented here focuses on the efficacy of test-based culling strategies for MAP control in US dairy herds. First, we quantified the effect of MAP control by using the concept of the basic reproduction ratio . The expression of was derived from a mathematical model which we developed and published earlier for studying infection dynamics of MAP on US dairy herds (Mitchell et al., 2008). We applied a phase diagram approach to determine the effectiveness of test-and-cull strategies for good and poor herd managements (low- and high-transmission rates, respectively). Second, we derived the average detection time for infectious animals and described culling rates in terms of test characteristics such as test sensitivity, test turnaround time, and producer decisions such as testing interval and delayed culling. The effects of different test strategies and culling decisions for low shedding animals, based on the fecal culture test, were directly addressed. Third, we performed global uncertainty analysis (UA) and sensitivity analysis (SA) to understand the overall effect of all model parameters on . Fourth, we evaluated the impact of culling of only high shedding animals on MAP control by comparing three test methods (fecal culture, fecal PCR, and ELISA).
Section snippets
Model description
Mathematical models of Johne's disease have been developed to study infection dynamics on dairy farms (Collins and Morgan, 1992, Groenendaal and Galligan, 1999, Beyerbach et al., 2001, Groenendaal et al., 2003, Kudahl et al., 2007). We used the mathematical model proposed by Mitchell et al. (2008) for US dairy herds. In this model (Fig. 1), all animals are classified into six compartments according to their infection status: susceptible animals (), animals resistant to MAP infection due to
Phase diagrams of for farms with good and poor management
Phase diagrams of provide an overview to evaluate the effectiveness of culling strategies for MAP control (Anderson and May, 1999, Hethcote, 2000). In Figs. 3(a) and (b), the phase plane is defined as the culling plane formed by culling rates and . The critical curve divides the culling plane into two regions, disease free (), and endemic equilibrium (). The constraint culling rates and , for low and high shedders, respectively, were calculated by
Conclusions
The results given in this work, based on the simple mathematical model for US dairy herds, show that for farms with good management, culling of only high shedding animals is effective in controlling MAP transmission, but for farms with poor management, in addition to immediate culling of high shedding animals, culling of low shedding animals (based on the results of the fecal culture test), is necessary. Culling of low shedding animals is more efficient than that of high shedding animals to
Acknowledgments
This work was supported by USDA-CSREES/2004-35605-14243, Johne's Disease Integrated Program in Research, Education and Extension, and by USDA-ARS under the Regional Dairy Quality Management Alliance (RDQMA) Specific Cooperative Agreement. We thank Robert H. Whitlock, Cristina Lanzas, Renata Ivanek, Abani Pradhan, and Patrick Ayscue for helpful discussions and suggestions.
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2019, Acta TropicaCitation Excerpt :Moreover, it might potentially represent a threat to human health (Selim and Gaede, 2015). The carrier animals have ability to intermittent or later persistent shedding on MAP in feces start up 2 years before onset of clinical signs leading to contamination of environment (Mitchell et al., 2015), also MAP can be secreted in the milk (Lu et al., 2008). The susceptible animal are infected mainly via fecal-oral route, after long incubation period, animal showing chronic enteritis, severe emaciation and reduction in milk yield (Hussain et al., 2018).