Highly Pathogenic Avian Influenza (HPAI) H5N1 (clade 2.3.4.4b) has spillover into dairy cattle populations in the US. A year after the initial outbreak, many uncertainties remain about the virus's epidemiology. This study aims to examine cow-level factors such as days in milk (DIM), milk yield, parity, pregnancy, and days carrying calf (DCC) that may influence cow`s susceptibility to manifest clinical signs during a HPAI H5N1 outbreak using on-farm herd records. The aims were: (1) to describe the main characteristics of cows manifesting clinical signs, (2) to evaluate the relationship between pregnancy and the manifestation of clinical signs, and (3) to identify other risk factors associated with the manifestation of clinical signs in pregnant cows.
The study was performed in a commercial dairy farm (Colorado, US) that operates in two different locations moving cows between them based on the lactation stage. Due to data availability, the study included only animals from one location, focusing on mid-to-late lactation Holstein cows, most of which were pregnant and over 200 DIM. Cows were housed in 12 pens. Based on the distribution of measured cow-level factors on May 1, 2024, five pens were considered comparable, as they housed groups of cows with similar characteristics within the same facility type. These comparable pens were identified using ANOVA or Chi-Square Test of Independence, post-hoc tests, and network-based clustering.
The location experienced an H5N1 HPAI outbreak, peaking in late May 2024. The study population included all cows present in the herd on May 1, 2024, excluding those in the hospital pen (n = 3,281). Cows were classified as clinical cases if they had "FLU" recorded in their health records by August 31, 2024. The classification followed the outbreak response strategy, which involved treating only cows with clinical signs like reduced milk production, colostrum-like milk, severe dehydration, and anorexia while keeping records of treated animals.
To describe the characteristics of clinical cases (Aim 1), the proportion of cases across different strata was obtained using the 3,281 cows. T-test was used to compare mean differences in continuous variables (DIM, milk yield, and DCC) between cases and non-cases, while the Chi-Square Test of Independence was applied to assess differences in case status across categorical variables (parity, and pregnancy). To evaluate the relationship between clinical disease and pregnancy (Aim 2), a 1:1 matched cohort of pregnant and non-pregnant cows (n = 196), matched by parity, lactation stage, milk yield, and pen ID, was analyzed using conditional logistic regression, with case status as the outcome. For Aim 3, risk factors for clinical disease in pregnant cows were identified by analyzing all pregnant cows from comparable pens (n = 1,546) using mixed-effects logistic regression, with case status as the outcome and pen ID as the random effect.
Aim 1 showed an overall proportion of clinical cases of 14.0% (n = 458). Significant differences were found for pregnancy (6% in non-pregnant cows vs. 15% in pregnant cows; p < 0.001) and parity (9% in 1st parity cows, 16% in 2nd and 3rd parity cows, and 18% in cows with ≥4 parities; p < 0.001). Among all 12 pens, the proportion of cases ranged from 7% to 27%, and when restricted to comparable pens (n = 5), the variation remained (11% to 24%). For Aim 2, the matched OR, using non-pregnant cows as the reference group, was 4.9 (95% CI: 1.6–14.9). For Aim 3, the OR for multiparous cows vs. primiparous cows was 2.1 (95% CI: 1.5–2.8).
The findings suggest that cow-level factors (pregnancy and parity) and pen-level factors might contribute to HPAI H5N1 clinical disease. Expanding the number of herds studied is critical for a better understanding of these risk factors.
