A prospective cohort observational study investigating the association between the United Kingdom digital dermatitis genetic index and the disease prevalence in a population of Holstein heifers

The digital dermatitis index (DDI), a genetic index for digital dermatitis (DD) resistance has been previously associated with presence of DD in Holstein cows. The aim of this study was to further investigate this association in a population of Holstein heifers. Two cohorts of heifers from one farm were enrolled. For the first cohort, DD lesions from the hind feet were recorded with the animals standing, using at telescopic mirror once a month over a period of 3 mo. For the second cohort, DD lesions were recorded from the hind feet by picking them up in a foot-trimming chute once a month over a period of 4 to 6 mo. The M-stage scoring system was used, and 4 binary variables were created. The first binary variable created was grouping animals that never had any DD lesion (M0) and animals that had at least one lesion recorded throughout the study. Similar binary transformations were repeated for active (M1, M2, M4.1) lesions, chronic (M3, M4), and M2 lesions separately. Genomically estimated predicted transmitting abilities (gPTA) for the DDI were available for 519 heifers of the first cohort and 361 of the second. The DDI gPTA were standardized (rescaled to represent distance from the mean expressed in SD) and were also grouped into terciles (i.e., low-, medium- and high-genetic-merit groups) within each cohort. Univariable analysis was used to investigate the association between the 4 binary lesion-presence variables and the DDI gPTA for each cohort separately and multivariable logistic regression models were fitted for the second cohort. Survival (time to event) analysis was performed using the second-cohort dataset by fitting a parametric proportional hazard regression model using the Weibull distribution for interval censored data, with DDI and season at enrollment remaining as explanatory variables. There was no difference between low-, medium-, and high-genetic-merit heifers regarding the presence of any, active, or chronic lesions in the first cohort. However, for a one-unit increase in the DDI gPTA, the odds ratio (OR) of M2 lesion presence was 0.68 (95% CI: 0.50-0.92). Multivariable logistic regression models using data from the second cohort showed that for every one-unit increase in the DDI gPTA the OR for any, chronic, active, and M2 DD lesion presence were 0.64 (95% CI: 0.50-0.80), 0.60 (95% CI: 0.47-0.76), 0.55 (95% CI: 0.42-0.70), and 0.59 (95% CI: 0.41-0.84), respectively. The proportional hazard model showed that a one-unit increase in the DDI gPTA resulted in a hazard ratio of 0.70 (95% CI: 0.60-0.82). The model-adjusted median age at first DD event was 306 d (95% CI: 284-328) for heifers with a DDI of -1 compared with 377 d (95% CI: 348-411) for heifers with a DDI of +1. We show significant association between DDI gPTA and presence of DD lesions. Lower-genetic-merit animals developed a DD lesion at a younger age compared with their higher-merit counterparts. Genetic selection using the DDI could gradually reduce the disease prevalence during the rearing period of replacement animals.