|Van Kessel, Jo Ann|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2010
Publication Date: 7/24/2010
Publication URL: http://hdl.handle.net/10113/46263
Citation: Smith, R.L., Strawderman, R.L., Schukken, Y.H., Wells, S.J., Pradhan, A.K., Espejo, L.A., Whitlock, R.H., Van Kessel, J.S., Smith, J.M., Wolfgang, D.R., Grohn, Y.T. 2010. The effect of Jonhe's Disease status on reproduction and culling in dairy cattle. Journal of Dairy Science. 93:3513-3524. 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. Data from 6 commercial US dairy herds over a 3-7 year period were analyzed to determine the impact of Mycobacterium avium subsp. paratuberculosis (MAP) on time to culling (removal of cow from the herd) and time to calving (interval between births). Animals positive for MAP were culled at higher rates than negative animals, and cows shedding high quantities of MAP had longer calving intervals than other cows. Higher culling rates and longer calving intervals are not desirable from the dairy producers’ perspective and can have a negative economic impact. These results demonstrate two aspects of the economic impact of MAP that should be considered when determining optimal MAP control measures.
Technical Abstract: Among the costs attributed to Mycobacterium avium subsp. paratuberculosis (MAP) infection in dairy cattle, the impacts on reproduction and culling are the least documented. In order to properly estimate the cost of MAP infections in a dairy herd, the rates of calving and culling were calculated for cows in each stage of MAP infection relative to uninfected cows. Two well-defined databases, each consisting of culling and reproduction data from 3 commercial dairy herds, were used for analysis, consisting of 2,856 cows with 6,380 calvings and 1,696 cullings. Every cow in each study herd was tested regularly for MAP, and herds were followed for between 4 and 7 years. Infection status (non-shedding vs. low- or high-shedding) was defined as a time-dependent variable for all cows with at least one positive test result. A Cox regression model, stratified on herd and controlling for the time-dependent infection variable, was used to analyze time to culling. Non-shedding animals were significantly less likely to be culled in comparison with animals in the low- and high-shedding categories. Time to calving was analyzed using a proportional rates model, an analog to the Andersen-Gill regression model suitable for recurrent event data, stratifying on herd and weighted to adjust for the dependent censoring caused by the culling effects described above. High-shedding animals had lower calving rates in comparison with non-shedding animals. There was no discernible difference in calving interval length between non-shedding and low-shedding animals. These results can be incorporated into economic models for the cost of MAP in dairy herds.