Table 1.
Results of stochastic modeling to simulate outbreaks of bovine tuberculosis in herds of various sizes and replacement rates following introduction of 1 latently infected animal.
| Replacement rate (1/y) | Herd size | No test-based culling | Slaughterhouse detection-based testing and culling | Constant testing test-based culling | ||||
|---|---|---|---|---|---|---|---|---|
| Probability of fadeout | Time to fadeout (mo) | Probability of fadeout | Time to fadeout (mo) | Probability of fadeout before detection | Time to detection of M bovis (mo) | Probability of fadeout | ||
| 0.33 | Small | 0.66 (0.63–0.72) | 16 (1–116) | 0.99 (0.98–0.998) | 24 (1–119) | 0.88 (0.87–0.91) | 29 (1–119) | 0.999 (0.995–1) |
| Medium | 0.62 (0.57–0.67) | 15 (1–118) | 0.99 (0.98–0.998) | 23 (1–119) | 0.58 (0.53–0.63) | 27 (1–117) | 0.99999 (0.999–1) | |
| Large | 0.53 (0.48–0.58) | 12 (1–110) | 0.99 (0.98–0.999) | 27 (1–116) | 0.58 (0.53–0.63) | 30 (1–112) | 0.9999 (0.999–1) | |
| 0.17 | Small | 0.43 (0.38–0.48) | 20 (1–115) | 0.96 (0.95–0.98) | 39 (1–119) | 0.77 (0.72–0.81) | 43 (1–120) | 0.999 (0.994–1) |
| Medium | 0.43 (0.37–0.48) | 19 (1–111) | 0.97 (0.95–0.99) | 37 (1–119) | 0.75 (0.70–0.79) | 42 (1–120) | 0.999 (0.994–1) | |
| Large | 0.43 (0.37–0.47) | 20 (1–117) | 0.97 (0.95–0.98) | 38 (1–119) | 0.74 (0.70–0.79) | 42 (1–120) | 0.999 (0.993–1) | |
Small, medium and large herds comprised 40, 276, and 3,000 cattle, respectively. The time of introduction of the infected animal was considered day 0. The model was run for a 10-year period; variability was assessed by performing 1,000 simulations of 1,000 herds each for each of the 6 scenarios. In the base model, no test-based culling was performed. Under the current slaughterhouse detection-based testing and culling strategy, whole-herd tests were performed after detection of M bovis infection in an animal via postmortem inspection. Under the constant testing strategy, whole-herd tests were performed annually unless infection was detected. For both testing strategies, test-based culling was implemented at 3 month intervals after detection according to USDA recommendations. Fadeout refers clearance of M bovis infection from the herd. Probabilities of fadeout are presented as mean (range) data; time to detection and time to fadeout are presented as median (range).