Insights into cow-level risk factors for highly pathogenic avian influenza H5N1 clinical disease in lactating Holsteins from a single outbreak in Colorado: An observational study

-tests and chi-squared tests. The association between clinical disease and pregnancy was assessed using a 1:1 matched subset of pregnant and nonpregnant cows (n = 196), matched on parity, lactation stage, milk yield, and pen ID, and analyzed using conditional logistic regression. Other potential risk factors of clinical disease were evaluated in a subset of pregnant cows from comparable pens (n = 1,546) using mixed-effects logistic regression. The overall proportion of clinical cases reported was 14.0% (n = 458). Unadjusted analysis showed significant differences in pregnancy (6% in nonpregnant cows vs. 15% in pregnant cows) and parity (9% in first-parity cows, 16% in second- and third-parity cows, and 18% in cows with ≥4 parities). Across all 12 pens, the proportion of clinical cases ranged from 7% to 27%, and when restricted to comparable pens (n = 5), the variation was similar (11% to 24%). The matched odds ratio (OR), using nonpregnant cows as the reference group, was 4.9 (95% CI: 1.6-14.9), indicating higher odds of being a case among pregnant cows. Among pregnant cows, the OR for multiparous cows versus primiparous cows was 2.1 (95% CI: 1.5-2.8). The findings suggest that pregnancy and parity are 2 cow-level factors that contribute to HPAI H5N1 clinical disease. Expanding the number of herds studied is critical for a better understanding of these risk factors.