S. Wayne Martin

Evaluating the health status of herds based on tests applied to individuals

Abstract The effects of test sensitivity and specificity, and the impact of true prevalence of disease, on test results at the individual level are well known. When individual are tested to ascertain if an aggregate of animals (e.g. a herd) is affected by a condition of interest, the number of animals tested and the critical number of reactors used to decide the health status of the herd become very important in influencing the herd-level sensitivity and specificity. If the test specificity is less than 100%, as the number of animals tested increases, the probability of at least one false-positive animal increases—thus the herd specificity decreases. The herd sensitivity, herd negative predictive value and herd apparent prevalence increase directly with the number of animals tested, but the herd positive predictive value decreases. Herd sensitivity can be increased by using a test that is less than 100% specific. These features should be borne in mind when interpreting the natural history of disease, as well as when conducting disease surveys or disease-control campaigns based on surrogate tests.

Disease, production and culling in Holstein-Friesian cows III. Disease and production as determinants of disease

The effects of the level of milk production and previous disease occurrence (both within the same lactation and in the previous lactation) on the risk of disease occurrence in Holstein-Friesian cows were investigated. The data were from 2875 lactation records from 2008 cows in 32 commercial dairy herds within a 50 mile radius of Guelph, Ontario. Discriminant analysis was applied to data from a subset of 817 cows which completed 2 lactations during the study. This was done in order to simultaneously evaluate the effects of the level of milk production and disease occurrence in 1 lactation, on the risk of disease in a subsequent lactation. Many diseases tended to recur in the second lactation, but the level of milk production was not related significantly to the risk of any of the common disease conditions. A path model, with previous lactation production entered as an exogenous variable, was constructed in order to further evaluate the role of the level of milk production as a determinant of disease. Only milk fever was significantly associated with the level of milk production. Relationships amongst diseases within a lactation were identified by a modified case-control procedure and were used to construct several path models. Relationships amongst diseases in heifers, mature cows and all cows combined are discussed.

Disease, production and culling in Holstein-Friesian cows II. Age, season and sire effects

Effects of age, season and sire on the occurrence of common diseases of Holstein-Friesian cows were evaluated in a two and a half year study. The data were from 2875 lactation records from 2008 cows in 32 commercial dairy herds within a 50 mile radius of Guelph, Ontario. Age patterns for all disease conditions were determined using χ2 or quadratic regression analyses. Linear increases with age were observed for milk fever, mastitis requiring local therapy only, cystic ovaries and foot and leg problems. Quadratic relationships with age were observed for ketosis (clinical and subclinical), mastitis requiring systemic therapy, subclinical mastitis, dystocia, respiratory diseases and traumatic reticulo-peritonitis. No consistent age patterns were found for retained placentas, reproductive tract infections, abomasal displacements, teat injuries, digestive disorders and abortions. Seasonal patterns were evaluated using a formal statistical test for the seasonality of events. Significant peaks were found in the risk of ketosis and cystic ovaries during the winter, and in the risk of teat injuries in the late winter or early spring. Seasonal patterns, more complex than a simple unimodal pattern, were observed for mastitis requiring local therapy only, traumatic reticulo-peritonitis and abortions. Heritabilities were determined for 9 disease conditions. All heritabilities were low or 0 with the exception of clinical ketosis (h2 = 0.31).

Disease, production and culling in Holstein-Friesian cows: IV. Effects of disease on production

Abstract The effects of common clinical and two subclinical disease conditions (mastitis and ketosis) on milk production and reproductive performance in Holstein-Friesian cows were investigated in a two and a half year study. The data used were from 1315 lactation records and results from tests performed on 6241 composite milk samples. Path analyses were used to estimate the direct and indirect impact of disease on days to conception, milk production in a standard 305 day lactation and milk production per day. Indirect effects accounted for a large percentage of the total effect of some diseases on production. The path coefficients were quite sensitive to the units used to measure production, with deviation from the herd average being the preferable measure. These units controlled confounding due to herd level variables. Effects identified by the path analyses were substantially different from those determined on the basis of unconditional associations between diseases and production parameters. Subclinical mastitis and the reproductive diseases (most of which were subclinical in nature) were found to have the greatest detrimental impact on milk production per day. Several disease conditions had a beneficial effect on milk production and this was attributed to the therapy provided to cows affected with these conditions. All conditions related to reproductive performance had a detrimental effect on days to conception.

Disease, production and culling in Holstein-Friesian cows. I. The data

Abstract A two-and-a-half-year study was undertaken in 32 commercial dairy herds within a 50-mile radius of Guelph, Ontario, in order to provide a comprehensive evaluation of relationships amongst diseases, production parameters and risk of removal in Holstein-Friesian cows. Diseases consisted of common clinical diseases as well as two subclinical diseases (mastitis and ketosis). The data set consisted of a total of 2875 lactation records from 2008 cows. The means and standard deviations of some of the production parameters were: age 4.9 ± 2.5 year, breed class average for milk 140 ± 26 BCM units, milk production per day of life 18.0 ± 3.8 kg and days to conception 118 ± 60 days. Lactational incidence rates of the more common diseases were: reproductive tract infections 0.182, mastitis 0.168, cystic ovaries 0.104, subclinical ketosis 0.096, retained placenta 0.086 and ketosis 0.074. The average lactation mean log somatic cell count was 179 000 cells/ml. The times postpartum to the first diagnosis of all disease conditions are presented.

Associations of clinical findings, bacteriological and histological results of endometrial biopsy with reproductive performance of postpartum dairy cows

Abstract Ninety-seven cows from a commercial Holstein-Friesian dairy herd were studied. Cows were classified as having exhibited good or poor reproductive performance. Unconditional relationships were examined between prostaglandin administration, clinical and palpation findings and histological and bacteriological culture results of endometrial biopsies taken at Days 26 and 40 postpartum and the reproductive performance variable. Variables unconditionally associated with performance were then offered to logistic multiple-regression models to quantify the diagnostic efficacy of these findings. Logistic regression models were calculated for histological, bacteriological, and clinical findings (at Days 26 and 40) first separately and then in a combined model. Cows positive for Actinomyces pyogenes or anaerobic bacteria were more likely to be poor reproductive performers. Variables from Day 26 postpartum were more likely to be conditionally associated with reproductive performance than findings from Day 40. Ovarian findings from Day 26 were associated with subsequent reproductive performance only when the location of the structure relative to the previously gravid horn was considered. In the final combined model, increased inflammation in the stratum compactum and more areas of epithelium increased the likelihood of poor reproductive performance, and increased numbers of lymphocytic foci in the stratum compactum and larger follicles on the gravid-side ovary reduced the risk of poor reproductive performance. This model included information on 55 cows and achieved 84% correct predictions, with a sensitivity (for poor cows) of 92% and a specificity (good cows) of 77%.

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

Abstract 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 enrolment 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 2.2×105 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.

Disease, production and culling in Holstein-Friesian cows V. Survivorship

Abstract Effects of disease on the risk of removal in Holstein-Friesian cows were investigated in a two and a half year study. Records were available from 141 sold cows, 415 culled cows and 1452 comparison cows in 32 commercial dairy herds within a 50 mile radius of Guelph, Ontario. Removals were subdivided into culls (cows sent to slaughter) and sales for dairy purposes. Discriminant analysis was used to identify disease conditions associated with an increased risk of removal before the 150th day or after the 150th day of the lactation. Age and herd effects were controlled in all analyses and the effect of level of production was controlled in analyses of removals after Day 150. Subclinical mastitis was associated with increased risks of culling and of being sold for dairy purposes after Day 150. Mastitis, milk fever, feet and leg problems, teat injuries and respiratory disease were associated with an increased risk of culling early in the lactation. No disease conditions were significantly associated with the risk of being sold for dairy purposes early in the lactation. The survival experience of these cows was determined by the use of life table analyses, the median survival time being 5.6 years. Risk of removal was highest in cows aged 3.0–4.9 years and in cows over 7 years old. Survival was positively associated with level of milk production.

Geographical and temporal distribution of human giardiasis in Ontario, Canada.

BackgroundGiardia is the most frequently identified intestinal parasite in North America. Although information on geographical distribution of giardiasis is critical in identifying communities at high risk, little has been done in this area. Therefore, the objective of this study was to investigate the geographical and temporal distribution of human giardiasis in Ontario in order to identify possible high risk areas and seasons. Two spatial scales of analyses and two disease measures were used with a view to identifying the best of each in assessing geographical patterns of giardiasis in Ontario. Global Morans I and Moran Local Indicators of Spatial Associations were used to test for evidence of global and local spatial clustering, respectively.ResultsThere were seasonal patterns with summer peaks and a significant (P < 0.001) decreasing temporal trend. Significant (P < 0.05) global spatial clustering of high rates was observed at the Census Sub-division spatial scale but not at the Census Division scale. The Census Sub-division scale was a better scale of analyses but required spatial empirical Bayesian smoothing of the rates. A number of areas with significant local clustering of giardiasis rates were identified.ConclusionsThe study identified spatial and temporal patterns in giardiasis distribution. This information is important in guiding decisions on disease control strategies. The study also showed that there is benefit in performing spatial analyses at more than one spatial scale to assess geographical patterns in disease distribution and that smoothing of disease rates for mapping in small areas enhances visualization of spatial patterns.

Association between management practices and within-herd prevalence of Cryptosporidium parvum shedding on dairy farms in southern Ontario

Abstract To identify management practices associated with an increased within-herd prevalence of Cryptosporidium parvum shedding on dairy farms in southern Ontario, fecal samples were taken from 1089 calves aged 7–28 days, from 119 herds. Information on management practices was obtained by administering a questionnaire compiled using a modified Delphi technique. Data were analyzed using univariable and multivariable negative binomial regression. Overall, 30% of the calves in the study were shedding C. parvum oocysts, with at least one positive calf detected in 77% of herds. Within-herd prevalence ranged from 0 to 80%. Predictors significantly associated with an increased prevalence of shedding in multivariable modelling were the use of calf scour prophylaxis in cows (risk ratio [RR] 1.70, P <0.01) and calves (RR 1.38, P =0.02) and the feeding of milk replacer in the first week of life (RR 1.40, P =0.02). In contrast, the presence of concrete flooring in calf housing areas (RR 0.59, P <0.01) and the use of soap or detergent when washing calf feeding utensils (RR 0.61, P <0.01) appeared to be protective.