Anne Braad Kudahl

Effect of paratuberculosis on slaughter weight and slaughter value of dairy cows

The effect of infection with Mycobacterium avium ssp. paratuberculosis (MAP) on slaughter weight and slaughter value of dairy cows was evaluated. Two data sets were analyzed: 1) recordings from 1,031 cows from herds in a pilot study to control MAP infections, and 2) recordings from 36,455 cows from herds participating in the Danish MAP control program. The effect of stage of MAP infection on carcass weight and slaughter value was assessed by ANOVA. Infection stage was diagnosed by repeated milk antibody ELISA in both data sets. Furthermore, repeated fecal culture was recorded in data set 1 and occurrence of enteritis or enteric edema found at slaughter was recorded in data sets 1 and 2. Compared with presumably unaffected cows with at least 2 ELISA negative tests, slaughter weight and value were reduced by 10 and 17%, respectively, in cows with positive ELISA at slaughter. If the cow was also positive using fecal culture, slaughter weight and value were reduced up to 15 and 31%, respectively. The slaughter weight and value were reduced an additional 20 and 31%, respectively, for cases with recorded enteritis or edema. Thereby, summarized weight losses of up to 31% and slaughter value losses up to 48% occurred. Cows with negative fecal cultures had reduced slaughter results only if they were ELISA-positive in the last 2 tests. Losses of both slaughter weight and slaughter value caused by MAP were more severe than previously estimated. These losses could be predicted by repeated milk ELISA tests with or without confirmation with fecal culture.

Gross margin losses due to Salmonella Dublin infection in Danish dairy cattle herds estimated by simulation modelling

Salmonella Dublin affects production and animal health in cattle herds. The objective of this study was to quantify the gross margin (GM) losses following introduction and spread of S. Dublin within dairy herds. The GM losses were estimated using an age-structured stochastic, mechanistic and dynamic simulation model. The model incorporated six age groups (neonatal, pre-weaned calves, weaned calves, growing heifers, breeding heifers and cows) and five infection stages (susceptible, acutely infected, carrier, super shedder and resistant). The effects of introducing one S. Dublin infectious heifer were estimated through 1000 simulation iterations for 12 scenarios. These 12 scenarios were combinations of three herd sizes (85, 200 and 400 cows) and four management levels (very good, good, poor and very poor). Input parameters for effects of S. Dublin on production and animal health were based on literature and calibrations to mimic real life observations. Mean annual GMs per cow stall were compared between herds experiencing within-herd spread of S. Dublin and non-infected reference herds over a 10-year period. The estimated GM losses were largest in the first year after infection, and increased with poorer management and herd size, e.g. average annual GM losses were estimated to 49 euros per stall for the first year after infection, and to 8 euros per stall annually averaged over the 10 years after herd infection for a 200 cow stall herd with very good management. In contrast, a 200 cow stall herd with very poor management lost on average 326 euros per stall during the first year, and 188 euros per stall annually averaged over the 10-year period following introduction of infection. The GM losses arose from both direct losses such as reduced milk yield, dead animals, treatment costs and abortions as well as indirect losses such as reduced income from sold heifers and calves, and lower milk yield of replacement animals. Through sensitivity analyses it was found that the assumptions about milk yield losses for cows in the resistant or carrier stages had the greatest influence on the estimated GM losses. This was more influential in the poorer management scenarios due to increased number of infected cows. The results can be used to inform dairy farmers of the benefits of preventing introduction and controlling spread of S. Dublin. Furthermore, they can be used in cost-benefit analyses of control actions for S. Dublin both at herd and sector level.

Age-structured dynamic, stochastic and mechanistic simulation model of Salmonella Dublin infection within dairy herds

In the demand for a decision support tool to guide farmers wanting to control Salmonella Dublin (S. Dublin) in Danish dairy herds, we developed an age-structured stochastic, mechanistic and dynamic simulation model of S. Dublin in dairy herds, which incorporated six age groups (neonatal, preweaned calves, weaned calves, growing heifers, breeding heifers and cows) and five infection states (susceptible, acutely infected, carrier, super shedder and resistant). The model simulated population and infection dynamics over a period of 10 years in weekly time steps as: 1) population sizes of each of the six age-groups; 2) S. Dublin incidence and number of animals in each infection state; and 3) S. Dublin related morbidity and mortality in the acutely infected animals. The effects of introducing one infectious heifer on the risk of spread of S. Dublin within the herd and on the duration of infection were estimated through 1000 simulation iterations for 48 scenarios. The scenarios covered all combinations of three herd sizes (70, 200 and 400 cows), four hygiene levels indicating infectious contact parameters, and four herd susceptibility levels indicating different susceptibility parameters for the individual animals in each of the six age groups in the herd. The hygiene level was highly influential on the probability that the infection spread within the herd, duration of infection and epidemic size. The herd susceptibility level was also influential, but not likely to provide sufficient prevention and control of infection on its own. Herd size did not affect the probability of infection spread upon exposure, but the larger the herd the more important were management and housing practices that improve hygiene and reduce susceptibility to shorten durations of infection in the herd and to increase the probability of extinction. In general, disease and mortality patterns followed epidemic waves in the herds. However, an interesting pattern was seen for acute infections and abortions in adult cattle after the first 2 years of infection in herds with poor hygiene and high susceptibility. Repeated infections in young stock lead to a high proportion of resistant adult cattle, which lead to a dampening effect on acute infections in adults and associated abortions. Sensitivity analyses of 24 alternative scenarios showed that a super shedder state was not essential to mimic the infection dynamics and persistence patterns known from field studies, but a persistent carrier state was required in the model to mimic real life S. Dublin infections.

Effect of management on prevention of Salmonella Dublin exposure of calves during a one-year control programme in 84 Danish dairy herds

Studies reporting on how to control Salmonella in cattle herds have mainly been theoretical simulation models or case reports describing control of clinical salmonellosis outbreaks. The objective of this observational study was to investigate which management routines were associated with successful control of Salmonella Dublin in calves in dairy herds with previous signs of endemic infection. A total of 86 bulk-tank milk Salmonella Dublin antibody-positive bovine dairy herds were enrolled in the study in September 2008 and were all encouraged to control spread of the infection. One year later it was assessed if they were successful. The criterion for successful control was defined as the 10 youngest calves above three months of age testing Salmonella Dublin antibody-negative, indicating that exposure to Salmonella of these calves from birth until close to the day of testing had been successfully prevented. Management routines were registered through telephone interviews based on a questionnaire resulting in 45 variables for analysis. By the end of the study, a total of 84 herds had completed the interviews and had serum samples collected from calves. Data were analysed using two statistical methods: logistic regression analysis and discriminant analysis. Both analyses identified that increased probability of successful control was strongly associated with avoiding purchase of cattle from test-positive herds. Additionally, ensuring good calving area management, separating calf pens by solid walls rather than bars and not introducing biosecurity routines between the barn sections (e.g. boot wash, change of clothing) were associated with increased probability of successful control in the logistic analysis. The latter may seem illogical, but may be explained by successful herds already having good hygienic routines in place and therefore not having introduced new routines between barn sections in the study period. The discriminant analysis furthermore identified successful control to be associated with preventing cows from calving before being moved to the designated calving pen, by only letting one person be responsible for colostrum management and by not feeding poorer quality colostrum to bull calves than to heifer calves. The results are useful for dairy cattle producers and veterinary authorities to substantiate advice on management practices that are likely to lead to successful control of Salmonella Dublin.

Evaluation of milk yield losses associated with Salmonella antibodies in bulk tank milk in bovine dairy herds

The effect of Salmonella on milk production is not well established in cattle. The objective of this study was to investigate whether introduction of Salmonella into dairy cattle herds was associated with reduced milk yield and determine the duration of any such effect. Longitudinal data from 2005 through 2009 were used, with data from 12 mo before until 18 mo after the estimated date of infection. Twenty-eight case herds were selected based on an increase in the level of Salmonella-specific antibodies in bulk-tank milk from <10 corrected optical density percentage (ODC%) to ≥70 ODC% between 2 consecutive three-monthly measurements in the Danish Salmonella surveillance program. All selected case herds were conventional Danish Holstein herds. Control herds (n=40) were selected randomly from Danish Holstein herds with Salmonella antibody levels consistently <10 ODC%. A date of herd infection was randomly allocated to the control herds. Hierarchical mixed effect models with the outcome test-day yield of energy-corrected milk (ECM)/cow were used to investigate daily milk yield before and after the estimated herd infection date for cows in parities 1, 2, and 3+. Control herds were used to evaluate whether the effects in the case herds could be reproduced in herds without Salmonella infection. Herd size, days in milk, somatic cell count, season, and year were included in the models. Yield in first-parity cows was reduced by a mean of 1.4 kg (95% confidence interval: 0.5 to 2.3) of ECM/cow per day from 7 to 15 mo after the estimated herd infection date, compared with that of first-parity cows in the same herds in the 12 mo before the estimated herd infection date. Yield for parity 3+ cows was reduced by a mean of 3.0 kg (95% confidence interval: 1.3 to 4.8) of ECM/cow per day from 7 to 15 mo after herd infection compared with that of parity 3+ cows in the 12 mo before the estimated herd infection. We observed minor differences in yield in second-parity cows before and after herd infection and observed no difference between cows in control herds before and after the simulated infection date. Milk yield decreased significantly in affected herds and the reduction was detectable several months after the increase in bulk tank milk Salmonella antibodies. It took more than 1 yr for milk yield to return to preinfection levels.

Strategies for time of culling in control of paratuberculosis in dairy herds

Effect of time for culling cows infected with Mycobacterium avium ssp. paratuberculosis on prevalence and profitability was identified through simulations. Seven test-and-cull strategies with different culling criteria and no attempts to close infection routes were compared with strategies with (1) no control and (2) closure of infection routes and no culling. The effects on true prevalence and gross margin were evaluated in a herd with typical reproduction management (heat detection rate of 38%). This was repeated in a herd with poor reproduction management (heat detection rate of 28%), because poor reproduction leads to lack of replacement animals, which was hypothesized to affect the economic effects of culling. Effects of varying prices of milk, replacement heifers, and hourly wages were also evaluated. The simulated results predicted that immediate culling after the first positive antibody ELISA test would be the most effective culling strategy to reduce prevalence. However, closing transmission routes was even more effective in reducing the prevalence. In the first 3 to 6 yr, all test-and-cull strategies reduced gross margin by US

Sole haemorrhages in Danish bull calves: Prevalence and risk factors

5 to 55/stall per year. These losses were fully compensated by increased gross margin in yr 6 to 19. In the short run (7 yr with typical reproduction and 10 yr with poor reproduction), it was most profitable to cull test-positive cows when their milk yield decreased below 85% of that expected according to their parity and lactation stage, especially in herds with poor reproduction management. However, this strategy only stabilized the prevalence and did not reduce it. In the long term (>7 yr from implementation of a strategy), it was most profitable to cull cows immediately or as soon as possible after testing positive the first time. Varying milk prices did not affect the ranking between the different culling strategies. Increased market price (20%) of replacement heifers made all culling strategies less profitable and made culling based on a milk yield criterion the most profitable culling strategy for a longer period (11 to 13 yr). A 20% reduction in heifer price made immediate culling after a positive test the most profitable strategy overall in herds with typical reproduction, and after 9 yr in herd with poor reproduction. To conclude, the ideal culling strategy depends on the aim of intervention, the time horizon, and the reproductive capabilities combined with prices of replacement animals.

A stochastic model simulating paratuberculosis in a dairy herd

To evaluate the prevalence of sole haemorrhages among Danish dairy breed bull calves and associated risk factors, 730 calves were hoof trimmed shortly before slaughter and the presence of sole haemorrhages was recorded. Associations between the outcomes sole haemorrhages and severe sole haemorrhages, and the explanatory variables daily weight gain, liver abscesses and flooring, were analysed. Of the 730 calves, 545 (74.7%) had sole haemorrhages and 119 (16.3%) had severe sole haemorrhages. Calves housed on slatted concrete floors with cubicles had higher odds of sole haemorrhages (odds ratio, OR, 2.69, 95% confidence interval, CI, 1.81-4.00; P<0.001) and severe sole haemorrhages (OR 2.07, 95% CI 1.36-3.14; P<0.001) than calves housed on deep litter straw. The odds of severe sole haemorrhages increased with increasing daily weight gain (OR 1.28, 95% CI 1.07-1.54 for an increase of 100g/day; P=0.008).