Factores de riesgo a nivel individual asociados con la IAAP en vacas Holstein en lactación: estudio de un brote
Alexandre Rico1, Noelia Silva-del-Rio1
(1)-University of California, Davis
OBJECTIVES: Highly Pathogenic Avian Influenza
(HPAI) H5N1 (clade 2.3.4.4b) has spillover into dairy cattle populations. Seven
months after the initial outbreak, there are still many unknowns about this
outbreak. This study aims to identify specific cow-level factors (i.e., days in
milk (DIM), milk yield, parity, pregnancy) that may increase the susceptibility
of cattle to H5N1 infection based on on-farm herd records.
METHODS: We enrolled 3,281 Holstein lactating cows from a
commercial operation with 12 pens. This herd housed mainly pregnant dairy cows
moved from a nearby sister facility. The study herd experienced the peak of the
outbreak during the last week of May. Cow information including DIM, milk
yield, parity, reproductive status, and pen allocation were obtained from herd
health records software using May 1st, 2024,
as the date. Among these, cases were defined as cows that had the item
“FLU” recorded in the herd health history using August 31st, 2024,
as the date. Comparable pens based on measured cow-level factors were
identified using cluster analysis. Univariate associations between cow-level
factors and the risk of being a case were explored using Chi-Square Test for
Independence and T-test. Pregnant and non-pregnant cows were compared using a
1:1 matched cohort based on parity, milk yield, lactation stage and pen
allocation. The study design was a nested case-control that was analyzed using
conditional logistic regression. The effect of the other cow-level factors was
estimated through a cohort-based case-control study, that only included
pregnant cows within comparable pens, using a mixed-effects logistic
regression, with pen as random variable. The significance level was 0.05.
RESULTS: The overall risk of being a case was 16.2%. In
the descriptive study (n=3281), DIM, milk yield, and days carrying calf for
pregnant cows did not show significant differences between cases and non-cases
(p > 0.05). In contrast, pregnancy, parity, and pen allocation were
significantly associated with the risk (p < 0.05). The risk across the 12
pens ranged from 7% to 27%, and there remained a significant difference among
comparable pens, ranging from 7 to 24%. In the second study, focused on the
effect of pregnancy (196 cows: 98 pregnant and 98 non-pregnant), the risk was
4% in non-pregnant cows and 18% in pregnant cows. The effect was significant
with an odds ratio (OR) of 4.9 (95% CI: 1.6, 14.9). The last study (n=1,546)
only identified parity as potential risk factor for being a case. The risk was
9% for first-parity cows and 18% for cows with more parities. The effect was
significant with OR of 2.1 (95% CI: 1.5, 2.8).
CONCLUSIONS: The three main findings observed in this
outbreak were as follows: pens housing similar cows showed different levels of
risk; among lactating cows, those that were pregnant had a higher likelihood of
being cases compared to those that were not pregnant; and, within the group of
lactating pregnant cows, 1st parity cows were less likely to be cases than cows
with multiple parities.
