Calf-level risk factors for neonatal diarrhea and shedding of Cryptosporidium parvum in Ontario dairy calves.

This work was conducted to investigate calf-level factors that influence the risk of neonatal diarrhea and shedding of Cryptosporidium parvum oocysts in calves, on dairy farms in Ontario with histories of calf diarrhea or cryptosporidiosis. Fecal samples were collected weekly for 4 weeks from each of 1045 calves under 30 days of age on 11 dairy farms in south-western Ontario during the summer of 2003 and the winter of 2004. A questionnaire designed to gather information on calf-level management factors was administered on farm for each calf in the study. Samples were examined for C. parvum oocysts by microscopy, and a subset of specimens was also tested for enterotoxigenic Escherichia coli, Salmonella, bovine rotavirus and bovine coronavirus. The consistency of each sample was scored and recorded at the time of collection in order to assess the presence or absence of diarrhea. In addition, a blood sample was taken from each calf upon enrollment in the study, for assessment of maternal antibody transfer and for polymerase chain reaction testing for persistent bovine viral diarrhea virus infection. Using the GLLAMM function in Stata 9.0, multilevel regression techniques were employed to investigate associations between management practices and the risk of C. parvum shedding or diarrhea. C. parvum oocysts were detected in the feces of 78% of the 919 calves from which all four fecal samples had been collected. Furthermore, 73% of the 846 calves for which all four fecal consistency scores had been recorded were diarrheic at the time of collection of at least one sample. Significant predictors of the calf-level risk of C. parvum shedding included the use of calf diarrhea prophylaxis in pregnant cows, and the type of maternity facilities in which the calves were born. Factors associated with an increased risk of diarrhea were leaving the calf with the dam for more than an hour after birth, and the birth of a calf in the summer as opposed to winter. Calves shedding C. parvum oocysts had 5.3 (95% CI 4.4, 6.4) times the odds of diarrhea than non-shedding calves, controlling for other factors included in the final multivariable model. Furthermore, infected calves shedding more than 2.2 x 10(5) oocysts per gram of feces were more likely to scour than infected calves shedding lower numbers of oocysts (OR= 6.1, 95% CI 4.8, 7.8). The odds of diarrhea in calves shedding oocysts that had been allowed to remain with their dams for more than an hour were higher than the odds of diarrhea in shedding calves that had been separated from their dams within an hour after birth.

Comparison of two atmospheric-dispersion models to assess farm-site exposure to sour-gas processing-plant emissions.

We describe two approaches for exposure assessment that we used in a large-scale retrospective cattle study conducted in Alberta, Canada. Sulfur dioxide (SO(2)) was the surrogate measure of exposure to a complex mixture of combusted sour-gas emissions. Monthly air pollution dispersion modeling (1985-1994) (based on individual industrial source processing-plant engineering specifications, emission volumes, and meteorologic information) provided exposure isopleths of sulfur dioxide concentration from each of 231 sour-gas processing-plants across the province. In contrast, a simpler measure of proximity to source(s) of varying emission rates was applied in a geographical information system based on simplified pollution decay at increasing distances from each point source. Province-wide (663,000 km(2)) surface analysis (by exposure-level classification) produced a contingency coefficient of 0.68 between the two exposure estimates. Annual exposure estimates at the 1382 dairy and 5726 beef cow-calf farms studied were highly correlated over the 10-years period (r(spearman)=0.82 and 0.83, respectively), while monthly exposure estimates were somewhat less correlated (r(spearman)=0.80 and 0.82, respectively) for the two exposure assessment methods. Crude exposure estimates from each method were similar in both direction and magnitude.

A Bayesian approach to estimating the performance of a bovine virus diarrhoea virus (BVDV) antibody ELISA bulk-tank milk test.

We investigated the operational performance (sensitivity and specificity) of a bovine virus diarrhoea virus (BVDV) antibody ELISA bulk-tank milk test for predicting the herd BVDV antibody status in young stock (as a relatively precise indicator of active BVDV infection). The study was based on results from the annual screenings under the Norwegian bovine virus diarrhoea (BVD) control and eradication program, lasting from 1993 to 1997. Empirical information from these annual screenings was the basis for prior assumptions about the true prevalence of young-stock-positive herds. Assumptions about prior distributions for sensitivity and specificity were based on the literature. Improved posterior test performance estimates were achieved applying a Bayesian approach using Gibbs sampling simulation. The simulations were run separately for each year, and yielded median values for sensitivity of 87% at the cut-off used in the BVD program. The posterior distributions were wide indicating much uncertainty in these estimates. The specificity estimates ranged from 79 to 92% and had narrower posterior estimates. The estimates differed by year. When running the same simulation procedures at a lower cut-off --after altering the sensitivity and specificity priors--the median sensitivity estimates increased to about 95%; the median specificity ranged from 71 to 83%. Due to low prevalence, the Bayesian method lacked power to assess the test sensitivity. A technically simpler descriptive graphing procedure (based on empirical information) provided equally useful insight into the bulk-tank milk test performance.

Group-level factors associated with chemotherapeutic treatment regimens in land-based trout farms in Ontario, Canada.

The lowest level at which fish farmers ordinarily make management decisions is the individual holding unit. To identify factors associated with chemotherapeutic treatment initiation at the holding-unit level, we created a unit of measurement called the "farm-tank-lot" (FTL), which allowed the movements and mixing of groups of fish to be followed during an entire production cycle. Each FTL was comprised of fish with a common history housed in a specific holding unit. Our 21-month prospective observational study (conducted on 14 land-based trout farms in Ontario, Canada) showed that the FTL was a biologically meaningful unit of concern and a feasible unit of measurement on land-based trout farms.Multivariable logistic and Poisson regressions revealed that fish size and growth rate both were associated negatively with the probability and frequency of treatment. FTLs that existed for longer periods of time were more likely to be treated and treated more often. There was a significant farm effect. Future field studies of disease in cultured fish should incorporate these factors into their design and analysis.

A hierarchical trend model for bovine virus diarrhoea virus (BVDV) sero-conversion in Norwegian dairy herds from 1993 through 1997.

The Norwegian bovine virus diarrhoea (BVD) control-and-eradication program performed annual testing of the national cattle population since its first screening in January 1993. A bulk-tank milk ELISA antibody test was used for the initial screening of dairy herds. Based on the annual bulk-tank milk-test result, a binary variable denoting herd sero-conversion (a surrogate measure for incidence) was created. The count of herds with sero-conversion in each community was regressed on year and the initial herd-sero-prevalence for each community in a Poisson hierarchical trend model - modelling the risk of sero-conversion. By using this modelling approach, estimates of trend specific for each hierarchical level of organisation were included in the trend model (community, veterinary district and county) could be estimated. The main BVD trend showed a steadily declining sero-conversion risk. The communities in the highest herd-sero-prevalence quartile in 1993 continued to have the highest sero-conversion risk throughout the study period--decreasing from an average predicted sero-conversion risk in 1993 of 0.12 (95% CI; 0.10, 0.13) to 0.02 (0.007, 0.04) in 1997. There was an expressed variation in the level of sero-conversion for all the three hierarchical levels, but the trend only varied at the lowest level (community).