|Van Kessel, Jo Ann|
Submitted to: Journal of Theoretical Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2008
Publication Date: 5/14/2008
Citation: Lu, Z., Mitchell, R.M., Smith, R.L., Van Kessel, J.S., Chapagain, P.P., Schukken, Y.H., Grohn, Y.T. 2008. The Importance of Culling in Johne’s Disease Control. Journal of Theoretical Biology. 254(2008):135-146. Interpretive Summary: Johne’s disease is a chronic, progressive, infectious intestinal disease of cattle and is caused by infection with the bacterium, Mycobacterium avium subsp. paratuberculosis (MAP). A large number of US dairy herds are infected with MAP and the dairy industry incurs large economic losses as a result of Johne’s disease. MAP infection is characterized by a very long incubation period and identification of animals that are infected is very difficult. The recommendation is usually to remove from the herd (cull) cows that have been identified as shedding large numbers of MAP in their feces. In this study, we used a modeling approach to quantify the effects of MAP control in US dairy herds that were well or poorly managed with respect to Johne’s disease. We also studied the impact of varying test intervals (time between testing the animals for MAP) and the type of test used for MAP detection. Our study shows that, in the case of good herd management, culling of only the animals shedding high levels of MAP is effective in controlling MAP transmission. However, culling of animals shedding low numbers of MAP is also necessary when herd management is poor. When the interval between animal testing is long, culling of infectious animals is not generally effective to control MAP. The results of this study will allow producers to understand the potential for various management strategies to control Johne’s disease in their dairy herd.
Technical Abstract: Johne’s disease is caused by Mycobacterium avium subsp. paratuberculosis (MAP) infection and results in economic losses in dairy industry. To control MAP transmission in herds, test-based culling has been recommended and immediate culling of high shedding animals is typically implemented. In this study, we quantified the effects of MAP control in US dairy herds, using the basic reproduction ratio R0. The effectiveness of culling strategies was evaluated for good and poor herd management (low- and high-transmission rates, respectively) by a phase diagram approach. To establish a quantitative relationship between culling rates and test properties, we defined the average detection times for low and high shedding animals. The effects of various culling strategies and test characteristics, such as test sensitivity, test turnaround time, and testing interval, were analyzed. To understand the overall effect of model parameters on R0, we performed global uncertainty and sensitivity analyses. We also evaluated the effectiveness of culling only high shedding animals by comparing three test methods (fecal culture, fecal PCR, and ELISA). Our study shows that, in the case of good herd management, culling of only high shedding animals is effective in controlling MAP transmission. However, in the case of poor management, in addition to immediate culling of high shedding animals, culling of low shedding animals (based on the fecal culture test) is necessary. Culling of low shedding animals may be delayed 6 - 12 months, however, if a shorter testing interval is applied. This study suggests that if farmers prefer culling only high shedding animals, faster MAP detection tests (such as the fecal PCR and ELISA) of higher sensitivity should be applied with high testing frequency, particularly on farms with poor management. Culling of infectious animals with a longer testing interval is not effective in general to control MAP.