Table 3.
Comparison of the prevalence of multi-drug resistance in isolates of Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni from treated and untreated animals.
| Antimicrobial treatment | Isolates resistant to at least 3 classes | Total isolates | Prevalence ratio* | p value |
|---|---|---|---|---|
| None | 9 | 194 | Referent | |
| 1 | 55 | 276 | 4.30 (2.18–8.48) | <0.0001 |
| 2 | 81 | 211 | 8.27 (4.27–16.02) | <0.0001 |
| 3+ | 49 | 103 | 10.25 (5.25–20.03) | <0.0001 |
| Unknown | 89 | 467 | 4.11 (2.11–7.99) | <0.0001 |
*
Numbers in parentheses are 95% confidence intervals. Prevalence ratio is the prevalence of a bacterial isolate from an animal in the given treatment group being classified as resistant to 3 or more of the 6 antimicrobial classes included in our study compared to the prevalence of a bacterial isolate from an untreated animal being classified as resistant to 3 or more of the 6 antimicrobial classes included in our study. The referent category for all ratios is a history of antibiotic treatments. Linear trend test (χ2 = 93, degrees of freedom = 1, p < 0.0001).