Table 4. Immunoglobulin G anti-West Nile virus group-level prevalence in equines by group size, Camargue, France, 2000^a.

Size class	Total	Positive (%)	Prevalence Ratio	95% CI
1–2	1,006	149 (14.8%)	Ref.
3–6	294	73 (24.8%)	1.9	1.4-2.6
7–25	98	40 (40.8%)	4.0	2.6-6.1
>25	31	12 (38.7%)	3.6	1.7-7.6
Total	1,429	274 (19.2%)

^a Chi square = 55.68 (3 degrees of freedom), p<0.0001.