TABLE 4.

Factors associated (P ≤ 0.25) with each strain on univariate analysis

Strain	Factor	Category	No. of cases	Odds ratio	95% confidence interval	P value
A vs not A	Date of isolation	Sept./Oct.	4/43 (9.3%)	0.47	0.14, 1.53	0.04
		Novembera	13/72 (18%)	1
		December	16/65 (25%)	1.48	0.65, 3.38
		Jan./Feb.	1/23 (4.3%)	0.21	0.03, 1.49
	Farm	Farm O	4/22 (18%)	0.57	0.17, 1.91	0.03
		Farm W	4/30 (13%)	0.39	0.12, 1.29
		Farm Ma	18/64 (28%)	1
		Other	9/93 (9.7%)	0.27	0.11, 0.66
	Age	<30 days	1/11 (9.1%)	0.36	0.04, 3.27	0.21
		30–39 days	7/39 (18%)	0.79	0.26, 2.46
		40–49 days	12/53 (23%)	1.06	0.39, 2.92
		50–59 daysa	8/37 (22%)	1
		60–69 days	4/18 (22%)	1.04	0.27, 4.03
		70–89 days	1/15 (6.7%)	0.26	0.03, 2.28
		≥90 days	0/12 (0%)	0.001	<10−6, >106
	Date of birth	August	4/37 (11%)	0.31	0.09, 1.13	0.20
		Sept. 1–15	2/26 (7.7%)	0.21	0.04, 1.09
		Sept. 16–30	7/29 (24%)	0.81	0.26, 2.56
		Oct. 1–15a	9/32 (28%)	1
		Oct. 16–31	6/27 (22%)	0.73	0.22, 2.40
		Nov. 1–15	2/20 (10%)	0.28	0.05, 1.48
		After Nov. 15	4/14 (29%)	1.02	0.25, 4.11
B vs not B	Year of birth	1991	0/27 (0%)	0.0005	<10−6, >106	0.01
		1992	1/16 (6.3%)	0.96	0.09, 9.9
		1993	4/28 (14%)	2.39	0.49, 11.6
		1994a	3/46 (6.5%)	1
		1995	3/39 (7.7%)	1.19	0.23, 6.29
		1996	1/34 (2.9%)	0.43	0.04, 4.37
		1997	0/14 (0%)	0.0005	<10−6, >106
		1998	3/5 (60%)	21.5	2.53, 182
C vs not C	Year of birth	1991	0/27 (0%)	0.0002	<10−6, >106	0.007
		1992	4/16 (25%)	2.22	0.54, 9.2
		1993	7/28 (25%)	2.22	0.66, 7.46
		1994a	6/46 (13%)	1
		1995	2/39 (5.1%)	0.36	0.07, 1.90
		1996	4/34 (12%)	0.89	0.23, 3.43
		1997	0/14 (0%)	0.0002	<10−6, >106
		1998	0/5 (0%)	0.0002	<10−6, >106
	Date of birth	August	4/37 (11%)	0.53	0.13, 2.06	0.14
		Sept. 1–15	0/26 (0%)	0.0004	<10−6, >106
		Sept. 16–30	3/29 (10%)	0.50	0.11, 2.22
		Oct. 1–15a	6/32 (19%)	1
		Oct. 16–31	4/27 (15%)	0.75	0.19, 3.01
		Nov. 1–15	4/20 (20%)	1.08	0.26, 4.44
		After Nov. 15	1/14 (7.1%)	0.33	0.04, 3.07
	Site of infection	Lung	21/172 (12%)	2.36	0.53, 10.6	0.21
		Other	2/36 (5.6%)	1
D vs not D	Farm	Farm O	1/22 (4.5%)	0.29	0.03, 2.44	0.16
		Farm W	1/30 (3.3%)	0.21	0.02, 1.75
		Farm Ma	9/64 (14%)	1
		Other	5/93 (5.4%)	0.35	0.11, 1.09
	Date of isolation	Sept./Oct.	5/43 (12%)	1.45	0.41, 5.06	0.25
		Novembera	6/72 (8.3%)	1
		December	2/65 (3.1%)	0.35	0.07, 1.80
		Jan./Feb.	3/23 (13%)	1.65	0.38, 7.20
E vs not E	Farm	Farm O	0/22 (0%)	0.0005	<10−6, >106	0.05
		Farm W	2/30 (6.7%)	0.34	0.07, 1.66
		Farm Ma	11/64 (17%)	1
		Other	10/93 (11%)	0.58	0.23, 1.46
	Year of birth	1991	3/27 (11%)	0.51	0.13, 2.09	0.03
		1992	0/16 (0%)	0.0004	<10−6, >106
		1993	4/28 (14%)	0.69	0.19, 2.48
		1994a	9/46 (20%)	1
		1995	6/39 (15%)	0.75	0.24, 2.32
		1996	1/34 (2.9%)	0.13	0.02, 1.04
		1997	0/14 (0%)	0.0004	<10−6, >106
		1998	0/5 (0%)	0.0004	<10−6, >106
	Date of birth	August	3/37 (8.1%)	0.62	0.13, 2.99	0.17
		Sept. 1–15	4/26 (15%)	1.27	0.29, 5.67
		Sept. 16–30	0/29 (0%)	0.0007	<10−6, >106
		Oct. 1–15a	4/32 (13%)	1
		Oct. 16–31	3/27 (11%)	0.88	0.18, 4.30
		Nov. 1–15	4/20 (20%)	1.75	0.38, 7.97
		After Nov. 15	1/14 (7.1%)	0.54	0.05, 5.31
	Site of infection	Lung	20/172 (12%)	2.23	0.50, 10.0	0.25
		Other	2/36 (5.6%)	1
	Age	<30 days	0/11 (0%)	0.0004	<10−6, >106	0.09
		30–39 days	7/39 (18%)	2.48	0.59, 10.4
		40–49 days	7/53 (13%)	1.72	0.42, 7.16
		50–59 daysa	3/37 (8.1%)	1
		60–69 days	2/18 (11%)	1.42	0.22, 9.33
		70–89 days	0/15 (0%)	0.0004	<10−6, >106
		≥90 days	0/12 (0%)	0.0004	<10−6, >106

^aThis category was the reference category.