# 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

## TL;DR

This study found that pregnant cows and those with multiple lactations are more likely to show clinical signs of HPAI H5N1 in dairy herds.

## Contribution

The study identifies pregnancy and parity as significant risk factors for HPAI H5N1 clinical disease in lactating dairy cows.

## Key findings

- Pregnant cows had 4.9 times higher odds of clinical disease compared to nonpregnant cows.
- Cows with multiple lactations had 2.1 times higher odds of clinical disease than first-lactation cows.
- Clinical disease incidence varied widely across pens with similar cow characteristics.

## Abstract

Summary: This observational study examined cow-level risk factors associated with clinical highly pathogenic avian influenza (HPAI) H5N1 in lactating Holsteins during a May 2024 outbreak on a dairy farm in Colorado. Using herd records, the study aimed to characterize clinical cases, evaluate the association between pregnancy and clinical disease, and identify risk factors specific to pregnant cows. Overall, 14.0% of the herd developed clinical disease, with incidence varying across pens (ranging from 7% to 27%), even among those housing cows with similar characteristics. Pregnant cows were 4.9 times more likely to develop clinical disease compared with nonpregnant cows, whereas cows with multiple lactations had 2.1 times greater odds compared with first-lactation cows. These results suggest that pregnancy status, parity, and pen-level factors may play an important role in determining susceptibility to clinical disease during an HPAI outbreak in dairy cattle.

Summary: This observational study examined cow-level risk factors associated with clinical highly pathogenic avian influenza (HPAI) H5N1 in lactating Holsteins during a May 2024 outbreak on a dairy farm in Colorado. Using herd records, the study aimed to characterize clinical cases, evaluate the association between pregnancy and clinical disease, and identify risk factors specific to pregnant cows. Overall, 14.0% of the herd developed clinical disease, with incidence varying across pens (ranging from 7% to 27%), even among those housing cows with similar characteristics. Pregnant cows were 4.9 times more likely to develop clinical disease compared with nonpregnant cows, whereas cows with multiple lactations had 2.1 times greater odds compared with first-lactation cows. These results suggest that pregnancy status, parity, and pen-level factors may play an important role in determining susceptibility to clinical disease during an HPAI outbreak in dairy cattle.

•Pregnant cows had 4.9 times higher odds of clinical disease than nonpregnant cows.•Multiparous cows had 2.1 times higher odds of clinical disease than first-parity cows.•Clinical disease varied widely across pens housing cows with similar characteristics.

Pregnant cows had 4.9 times higher odds of clinical disease than nonpregnant cows.

Multiparous cows had 2.1 times higher odds of clinical disease than first-parity cows.

Clinical disease varied widely across pens housing cows with similar characteristics.

Highly pathogenic avian influenza (HPAI) H5N1 (clade 2.3.4.4b) has spilled over into dairy cattle populations in the United States. A year after the initial outbreak, many uncertainties remain about the virus' epidemiology. The objective of this observational study was to examine cow-level factors that may influence cow's susceptibility to manifest clinical signs during an HPAI H5N1 outbreak using on-farm herd records. The study was conducted on a commercial dairy farm in Colorado that housed only mid-to-late lactation cows and experienced the peak of its outbreak in late May 2024. Herd records were used to identify clinical cases. The study population comprised 3,281 cows that were housed across 7 freestalls and 5 open-lot pens. Pens were classified as comparable if they housed cows with similar individual-level characteristics, as determined using a network-based clustering approach based on ANOVA and chi-squared tests. Univariate associations between case status and potential risk factors were evaluated in the full study population using t-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.