Analysis of correlated continuous repeated observations: modelling the effect of ketosis on milk yield in dairy cows

This study used mixed models analysis to demonstrate the advantages of a repeated measures technique for a continuous variable over a single measure technique. As an illustration, the loss of milk yield due to ketosis was studied in 2604 multiparous New York State Holstein cows belonging to eight herds, calving between 1991 and 1993. Two methods of analysis were presented: The first treated milk yield as a continuous, summary measure (projected 305-day milk yield); the second treated milk yield as repeated measurements (test-day milk yields). In the first example, with 305-day milk yield as the outcome, ketosis was treated as a binary covariate. Ketosis had no effect on the 305-day milk yield. In the second example, with monthly test-day milk yields as the outcome, four different covariance structures (simple, compound symmetry, autoregressive, and unstructured) to model the association among the repeated measurements were compared. With this approach, ketotic cows yielded significantly less milk per day both before and immediately after diagnosis than did non-ketotic cows. Based on the goodness-of-fit statistics, it was unclear whether an autoregressive or unstructured covariance structure was best. However, an autoregressive structure, in which the previous and current test-day milk yields are assumed to be correlated, was considered more suitable in this study; it is a simpler and more appropriate covariance structure for this particular problem than is an unstructured covariance structure. Nevertheless, with the test-day approach, any of these correlation structures could be used to estimate milk loss after disease. Based on these findings, it is recommended that a repeated measures approach, rather than a single measure approach, be used to study the short-term effect of disease on milk yield.     

Probability of and risk factors for introduction of infectious diseases into Dutch SPF dairy farms: a cohort study

A 2-year cohort study was conducted to investigate the probability of disease introduction into Dutch dairy farms. The farms were tested regularly for diseases and were visited biannually to collect management data. Ninety-five specific pathogen-free (SPF) dairy farms were selected from a database of bovine herpesvirus type 1 (BHV1)-free farms to study the probability of, and risk factors for, introduction of BHV1, bovine viral diarrhoea virus (BVDV), Salmonella enterica subsp. enterica serotype Dublin (S. dublin), and Leptospira interrogans serovar hardjo (L. hardjo).Although most of the 95 SPF farms had a low risk on introduction of infectious diseases, one disease was introduced into 12 farms and two diseases were introduced into one farm. Three farms experienced an outbreak of BHV1, one farm an outbreak of L. hardjo, two farms BVDV, six farms S. dublin, and one farm both BHV1 and S. dublin. The total incidence rate was 0.09 (0.06-0.12) per herd-year at risk. The results suggest that the "non-outbreak" farms were significantly more closed than the "outbreak" farms. Direct animal contacts with other cattle should be avoided and professional visitors should be instructed to wear protective clothing before handling cattle.     

畜产品安全中存在的问题与分析

阐述了我国畜产品安全现状，强调了其形势的严峻性和紧迫性，分析了当前存在的主要问题，并提出了改进措施和建议。     

Longitudinal data collection of Mycobacterium avium subspecies Paratuberculosis infections in dairy herds: the value of precise field data

Longitudinal infection data on Mycobacterium avium subspecies paratuberculosis (MAP) was collected on three dairy farms in Northeastern United States during approximately 10 years. Precise data on animal characteristics and animal location within farm were collected on these farms. Cows were followed over time with regard to MAP status during biannual fecal and serum sampling and quarterly serum sampling. Approximately 13 000 serum samples, 6500 fecal samples and 2000 tissue samples were collected during these years. Prevalence of positive samples was 1.4% for serological samples, 2.2% in fecal samples and 16.7% in tissue samples. Infection dynamics of MAP was studied and resulted in a number of potential changes in our understanding of MAP infection dynamics. First, a high prevalence of MAP infection was observed in these herds due to lifetime follow up of cows, including slaughter. Second, two distinctly different infection patterns were observed, so called non-progressors and progressors. Non-progressors were characterized by intermittent and low shedding of MAP bacteria and a virtual absence of a humoral immune response. Progressors were characterized by continuous and progressive shedding and a clearly detectable and progressive humoral immune response. Strain typing of MAP isolates on the three farms identified on two of three farms a dominant strain type, indicating that some strains are more successful in terms of transmission and infection progression. Continuous high quality longitudinal data collection turned out to be an essential tool in our understanding of pathobiology and epidemiology of MAP infections in dairy herds.     

Ecologic study of the risk factors for environmental mastitis in cows

An index was developed to measure the proportion of intramammary infections caused by environmental microorganisms on dairy farms. This environmental index can be interpreted as the probability that an intramammary infection was caused by an environmental pathogen, rather than by a contagious pathogen. Using the environmental index as the outcome variable, risk factors for environmental mastitis were studied on 10 dairy farms in New York. Turning the cows outside was associated with lower environmental index, and having cows drink from a stream increased the environmental index. Selective (rather than uniform) nonlactating cow intramammary treatment was related to a lower environmental index (apparently because the farms practicing selective nonlactating cow treatment suffered from epizootics of contagious mastitis).     

Cow-level evaluation of a kinetics ELISA with multiple cutoff values to detect fecal shedding of Mycobacterium avium subspecies paratuberculosis in New York State dairy cows

In control programs for Mycobacterium avium subsp. paratuberculosis (Map), the infection status of the cows in a herd is often obtained by testing (a sample of) the herd with an ELISA that may lack some sensitivity and specificity but that is fast and inexpensive. In New York State (NYS), an unabsorbed kinetics ELISA (KELA) has been used extensively for Map control. The objective of this study was to determine the relative sensitivity and specificity of the KELA for detection of fecal shedding of Map for the NYS dairy cow population, taking into account possible confounders such as different antigen batches and Map prevalence in a herd.

The data for the study consisted of all serum samples from NYS dairy cows with concurrent fecal culture results submitted to the NY Animal Health Diagnostic Laboratory (NYAHDL) between 1991 and 1996 (n = 10,562). The data represented cows with different levels of fecal shedding from herds with different within-herd Map prevalence, including herds that were whole herd fecal culture negative on repeated testing.

The cutoff values were based on the predictive value for fecal shedding obtained with a multiple logistic regression model that included variables for the three antigen batches and the Map prevalence in the herd. The KELA could not distinguish between non-shedders and low shedders (≤30 total colony forming units (TCFU)) and thus the predictive value of the KELA to detect moderate to heavy fecal shedders (>30 TCFU) was modeled. The three cutoff values of 65, 135 and 170 were based on low (<0.2), moderate (<0.80) and high (>0.95) probabilities for moderate to heavy fecal shedding. The sensitivity and specificity values relative to culture were 67% and 95.2%, 31% and 99.7%, and 11% and 99.9% for the three cutoff values, respectively. Cutoff values for the KELA decreased for herds with increasing within-herd Map prevalence. For the best positive predictive value of a KELA for moderate to heavy fecal shedding, the cutoff values should be determined based on the apparent within-herd prevalence in a herd.     

Forecasting clinical disease in pigs: comparing a naive and a Bayesian approach

In veterinary practice the clinician often evaluates and predicts herd health status over time according to clinical criteria. In this paper, we modeled three different clinical signs among pigs based on longitudinal clinical observations in 15 pig herds. We compared and discussed the outputs from two different approaches for making clinical forecasts in a herd: a naive approach using a simple time series model with previous disease observations as predictors and a Bayesian state space models approach, in which the time lag variable entered into the random component of the model. We used the Markov chain Monte Carlo technique to calculate posterior distributions of the forecasts. For the herd specific forecasts the results showed that there were only minor differences between the forecasts from the simple time series model and the median forecasts from the Bayesian model. However, the credibility intervals from the Bayesian model were wider than the forecasts from the simple model and, therefore the Bayesian model encompassed the variability in the forecasts better. Compared to the statistical model, the simple time series would be easier to implement in a practical setting. However, the latter lacks the inherent “generality” from the statistical model that allows the user to make statements about the distribution of the herds and to predict disease status based on the “average” correlation among the herds. The applicability of the Bayesian approach within a clinical decision-making framework was discussed, with special emphasis on the use of prior information and clinical forecasting.     

Variability of acetone in milk in a large low-production dairy herd: a longitudinal case study

The objective of this study was to relate acetone in milk with cow and management factors in one low producing dairy herd (5260 kg milk per 305-day lactation). Milk acetone was measured in regular monthly milk samples one to three times within 100 days of lactation in 4433 lactations (2639 cows, 7800 measurements) from one herd over a period of 32 months (1988-91). Associations between milk acetone and cow factors and surrogate measures of management were evaluated by variance components of multiple fixed effect models. Lactation stage, calendar month of study, production groups and milk yield were strong, and percentage milk fat and parity were weak predictors of milk acetone. There was a trend of increasing body weight loss from the first to the second month of lactation with increasing milk acetone level. A substantial increase in milk production in 1991 was accompanied by an almost twofold rise in milk acetone. It was concluded that environmental parameters had strong relationships with milk acetone even in this low-producing herd.     

Impact of imperfect Mycobacterium avium subsp. paratuberculosis vaccines in dairy herds: A mathematical modeling approach

The objective of this study was to investigate the potential impacts of imperfect Mycobacterium avium subsp. paratuberculosis (MAP) vaccines on the dynamics of MAP infection in US dairy herds using a mathematical modeling approach. Vaccine-based control programs have been implemented to reduce the prevalence of MAP infection in some dairy herds; however, MAP vaccines are imperfect. Vaccines can provide partial protection for susceptible calves, reduce the infectiousness of animals shedding MAP, lengthen the latent period of infected animals, slow the progression from low shedding to high shedding in infectious animals, and reduce clinical disease. To quantitatively study the impacts of imperfect MAP vaccines, we developed a deterministic multi-group vaccination model and performed global sensitivity analyses. Our results explain why MAP vaccination might have a beneficial, negligible, or detrimental effect in the reduction of prevalence and show that vaccines that are beneficial to individual animals may not be useful for a herd-level control plan. The study suggests that high efficacy vaccines that are aimed at reducing the susceptibility of the host are the most effective in controlling MAP transmission. This work indicates that MAP vaccination should be integrated into a comprehensive control program that includes test-and-cull intervention and improved calf rearing management.