The contribution of previous lameness events and body condition score to the occurrence of lameness in dairy herds: a study of two herds

It has been demonstrated that low body condition and previous occurrence of lameness increase the risk of future lameness in dairy cows. To date the population attributable fraction (PAF), which provides an estimate of the contribution that a risk factor makes towards the total number of disease events in a population, has not been explored for lameness using longitudinal data. Estimation of PAF helps to identify control measures that could lead to the largest improvements on-farm. The aim of this study was to use longitudinal data to evaluate the proportion of lameness that could be avoided in two separate herds (two populations), through i) reduced recurrence of previous lameness events ii) and moving body conditions score (BCS) into optimal ranges. Data were obtained from two UK dairy herds; Herd A, a 200-cow herd with 8 years of data from a total of 724 cows where lameness events were based on weekly locomotion scores (LS; 1 to 5 scale) and Herd B, a 600-cow herd with data recorded over 44 months from a total of 1,040 cows where treatment of clinical cases was used to identify lameness events. The PAF for categories of BCS were estimated using a closed equation appropriate for multiple exposure categories. Simulation models were used to explore theoretical scenarios to reflect changes in BCS and recurrence of previous lameness events in each herd. For Herd A, 21.5% of the total risk periods (cow-weeks) contained a lameness event (LS 3, 4 or 5), 96% of which were repeat events and 19% were recorded with BCS < 2 (3-weeks previously; 0 to 5 scale). When lameness events were based on two consecutive weeks of LS 4 or 5, 4% of risk periods were recorded as lame, of which 89.5% were repeat events. For Herd B, 16.3% of the total risk periods (consecutive 30-days) contained a lameness event (72.6% were repeat events) and 20% were recorded with BCS ≤ 2 (0 to 120 days previously). The median PAF for all previous lameness was between 79 and 83% in the two herds. Between 9 and 21% of lameness events could be attributed to previous lameness occurring > 16 weeks before a risk period. The median PAF estimated for changes in BCS were in the region of 4 to 11%, depending on severity of lameness. Repeated bouts of lameness made a very large contribution to the total number of lameness events. This could either be because certain cows are initially susceptible and remain susceptible, due to the increased risk associated with previous lameness events, or due to interactions with environmental factors. This area requires further research.