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).