Biomedical subjects

G van Schaik

Publications and source records attributed to G van Schaik.

12 recordsLinked to original sources

PubMed2003

Prevalence estimates for paratuberculosis adjusted for test variability using Bayesian analysis.

The ELISA tests that are available to detect an infection with Mycobacterium avium subsp. paratuberculosis (MAP) have a limited validity expressed as the sensitivity (Se) and specificity (Sp). In many studies, the Se and Sp of the tests are treated as constants and this will result in an underestimation of the variability of the true prevalence (TP). Bayesian inference provided a natural framework for using information on the test variability (i.e., the uncertainty) in the estimates of test Se and Sp when estimating the TP. Data from two prevalence studies for MAP using an ELISA in several regions in two locations were available for the analyses. In location 1, all cattle of at least 3 years of age were sampled in approximately 90 randomly sampled herds in each of the four regions of the country. In location 2, in 30 randomly sampled herds in each of three regions, approximately 30 randomly selected cows were sampled. Information about the unknown test Se and Sp and MAP prevalence was incorporated into a Bayesian model by joint prior probability distributions. Posterior estimates were obtained by combining the actual likelihood with the prior distributions using Bayes' formula. The corrected cow-level TP (proportion of infected cows in a herd) was low, 5.8 and 3.6% in locations 1 and 2, respectively. Certain regions within a location differed significantly in herd-level TP (proportion of infected herds). The herd-level TP was 54.3% in location 1 (95% credible interval (CI) 46.1, 63.3%) and 32.9% in location 2 (95% CI: 14.4, 73.3%). The variation in the herd-level TP estimate for location 2 was more than three times as large as the variation in location 1 mainly because of the relatively small number of investigated herds in location 2. In future prevalence studies for MAP, sample size calculations should be based on a very low cow-level prevalence. Approximately 50 and 90% of the herds in the current study had an estimated cow-level TP below 4 and 10%, respectively.

Animals↗

PubMed2003

Pooled fecal culture sampling for Mycobacterium avium subsp. paratuberculosis at different herd sizes and prevalence.

A stochastic spreadsheet model was developed to obtain estimates of the costs of whole herd testing on dairy farms for Mycobacterium avium subsp. paratuberculosis (Map) with pooled fecal samples. The optimal pool size was investigated for 2 scenarios, prevalence (a low-prevalence herd [< or = 5%] and a high-prevalence herd [> 5%]) and for different herd sizes (100-, 250-, 500- and 1,000-cow herds). All adult animals in the herd were sampled, and the samples of the individuals were divided into equal sized pools. When a pool tested positive, the manure samples of the animals in the pool were tested individually. The individual samples from a negative pool were assumed negative and not tested individually. Distributions were used to model the uncertainty about the sensitivity of the fecal culture at farm level and Map prevalence. The model randomly allocated a disease status to the cows (not shedding, low Map shedder, moderate Map shedder, and heavy Map shedder) on the basis of the expected prevalence in the herd. Pooling was not efficient in 100-cow and 250-cow herds with low prevalence because the probability to detect a map infection in these herds became poor (53% and 88%) when samples were pooled. When samples were pooled in larger herds, the probability to detect at least 1 (moderate to heavy) shedder was > 90%. The cost reduction as a result of pooling varied from 43% in a 100-cow herd with a high prevalence to 71% in a 1,000-cow herd with a low prevalence. The optimal pool size increased with increasing herd size and varied from 3 for a 500-cow herd with a low prevalence to 5 for a 1,000-cow herd with a high prevalence.

Animals↗

PubMed2002

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.

Animal Husbandry↗

PubMed2002

Trends in somatic cell counts, bacterial counts, and antibiotic residue violations in New York State during 1999-2000.

Milk quality data on a month-by-month basis from March 1999 to December 2000 were studied from five of the largest milk plants operating in New York State. The analyses focused on bulk tank somatic cell count (SCC), bacterial counts in the form of plate loop count (PLC), and antibiotic residue violations in the pool of milk of New York State, their mutual relation, and the influence of farm size. The average SCC was 363,000 cells/ml, the average PLC was 24,400 bacteria/ml, and the average number of antibiotic residue violations in the pool of milk was 3.9 per 1000 producers. Each month between 72 and 88% of the milk pool had SCC levels in compliance with the European Union (EU) requirements (SCC < 400,000 cells/ml). The findings in this study suggest that larger farms had lower SCC and PLC but more antibiotic violations. However, the larger farms contribute most to the SCC and PLC of the total pool of milk. Farms with high SCC also had higher PLC and more antibiotic violations. Measurable improvements in overall quality of the pool of milk in New York state would most likely occur by targeting incentives, education, and training programs for any farms with very high SCC and for larger farms with SCC between 400,000 and 750,000 cells/ml.

Animal Husbandry↗

PubMed2001

An economic model for on-farm decision support of management to prevent infectious disease introduction into dairy farms.

A more-closed farming system can be a good starting point for eradication of infectious diseases from within a herd. The economic implications of a more-closed farming system will not always be obvious to farmers. The management decisions are related to different parts of the farm and are farm-specific. To support these decisions, a model was developed of the economic consequences of a more-closed system (a simple static and deterministic design was used). The risk factors in the model were based solely on bovine herpesvirus type 1 (BHV1) but losses due to introduction of BVDV, L. hardjo, and S. dublin were added to the model. The model was verified and partly validated and a sensitivity analysis was done. The cost to one 55-cow dairy farm that refrained from purchasing cattle, provided protective clothing to professional visitors and a temporary employee, and built and maintained a double fence around 6 ha of land to prevent over-the-fence contacts was Dfl. 4495 over 5 years. The probability of disease introduction was decreased by 74%. The prevented losses for disease introduction amounted to Dfl. 7033 over 5 years (net benefits of Dfl. 2538 over 5 years).A more-closed system would be still beneficial when a sanitary barrier was used instead of just protective clothing, when the probability of introduction of infectious diseases was decreased, and when odds ratios in the model were replaced by more-conservative relative risks. The benefits became negative when a farm had to build and maintain a double fence around 12 ha instead of 6 ha, when the probability of introduction of all diseases was decreased by 50%, and when the estimations were based solely on BHV1.

Animal Husbandry↗

PubMed2001

[Risk and economics of disease introduction to dairy farms].

A more closed farming system will enhance the success of disease eradication programmes, because the introduction or re-introduction of infectious diseases is less likely. The objective of the study was to obtain input for the development of an on-farm decision support model to calculate the economic consequences of a more closed farming system. The input was based on bovine herpesvirus 1 (BHV1), since there were numerous data on this disease, but a more closed farming system will prevent introduction of other diseases as well (i.e. bovine virus diarrhoea virus (BVDV), L. hardjo, and S. dublin). Direct animal contacts, such as purchase of cattle, participation in cattle shows, and cattle that escape and mingle with other cattle, were found to be important risk factors for the introduction of BHV1. Furthermore, the use of protective farm clothing was found to be an important preventive factor. The effect of an IBR outbreak at an IBR-free farm on milk production caused limited losses of on average 0.9 kg per cow per day during 9 weeks, but the variability was high (95% CI 0-2 kg). Nine percent of Dutch IBR-free dairy farms that were also at risk for BVDV, L. hardjo or S. dublin had one introduction per year of one of these four diseases. All these results were incorporated in the economic model. Management measures to reduce the probability of introduction of BHV1, the costs of these measures, and the risk reduction after these measures were obtained from other sources. The calculations showed that the implementation of a more closed system will be profitable for most farms. The profitability will increase when a farm is at risk for more diseases, but will decrease when farms are limited in their facilities to rear replacement heifers or when a large proportion of pasture adjoins pasture of other cattle farms.

Animal Husbandry↗

PubMed2001

Risk factors for introduction of BHV1 into BHV1-free Dutch dairy farms: a case-control study.

In May 1998, a compulsory eradication programme for BHV1 started in the Netherlands. In December 1999 approximately 24% of Dutch dairy farms were certified BHV1-free (Animal Health Service (AHS)). Ninety-three certified BHV1-free dairy farms participated in a cohort study that investigated the probability of introduction of infectious diseases. The probability of introduction of BHV1 was determined from March 1997 until April 1999. Ninety of these farms remained BHV1-free and could be used as control farms. From January 1997 until March 1998, BHV1 was introduced into 41 BHV1-free dairy farms in the Netherlands (case farms). Management data were collected for both cases and controls and were complete for 37 case farms and 82 control farms. For small data sets and for data in which both low and high frequencies were expected in the contingency tables, the asymptotic methods were unreliable. Our data set clearly resembled such a data set; the risk factors were rare events because the BHV1-free farms were closed farms on which few direct animal contacts occurred. Therefore, an exact stratified modelling approach was most suitable for the data. The study showed that dairy farms should prevent cattle from escaping or mingling with other cattle and that professional visitors should always wear protective farm clothing.

Animal Husbandry↗

PubMed1999

Modeling the effect of an outbreak of bovine herpesvirus type 1 on herd-level milk production of Dutch dairy farms.

One of the impacts of disease is its effect on milk production. In the present study the effect of an outbreak of bovine herpesvirus type 1 on milk production at the herd level of certified bovine herpesvirus type 1-free dairy farms was modeled. The objective was to study several linear models to quantify the effects of a bovine herpesvirus type 1 outbreak on milk production accounting for the repeated measurements and incorporating our assumptions about the most likely duration of effects of this virus. Because milk production is measured at regular intervals, the data consisted of repeated measurements at the herd and cow levels. A marginal model, a subject-specific random-effect model, and a transition model were developed. The effect of a bovine herpesvirus type 1 outbreak was statistically significant in the random-effect model, and this model fitted the investigated farms best. However, a transition model might be a better model for generalizing the results to the whole population of Dutch dairy farms. The effect of a bovine herpesvirus type 1 outbreak on milk production derived from the random-effect model amounted to, on average, a loss of 0.92 kg of milk per cow per day during a period of 9 wk. The milk production loss varied from almost none to 2 kg of milk per cow per day. This reduction resulted in an average loss of Dfl 372 (Dfl1 = $US $0.50) with lower and upper confidence limits of, respectively, Dfl 12 and Dfl 730 per bovine herpesvirus type 1 outbreak.

Animals↗

PubMed1998

Adaptive conjoint analysis to determine perceived risk factors of farmers, veterinarians and AI technicians for introduction of BHV1 to dairy farms.

A study was carried out to determine the possibility of a more-closed farming system for (Dutch) dairy farms. The objective of the study was to provide effective and economically profitable management advice for improving the animal-health status of farms. Management measures will only be successfully applied if supported by farmers and their advisors (such as veterinarians). Therefore, the perception of farmers and advisors of the importance of various risk factors for the introduction of diseases to a farm was determined by using bovine herpes virus type 1 (BHV1) as an example. As part of the study, an evening-long workshop was organized and run thrice. In total, 49 farmers, veterinarians and AI technicians participated in these workshops. The computerized questionnaire technique was based on adaptive conjoint analysis (ACA). ACA has the advantage that participants can work with a large number of risk factors in a relatively short period of time. Another advantage of ACA (compared with standard questionnaires) is that the answers from each participant can be checked with regard to consistency with respect to the importance assigned to them. Data from participants with inconsistent responses can be excluded from further analyses. The results of the ACA interview were compared with the risk factors reported in the literature as being associated with BHV1 status (e.g. purchase of cattle, participation in cattle shows) and with farmers' actual management to prevent the introduction of diseases. The workshop participants were all operating in the dairy sector and they seemed well aware of the risk of direct animal contacts for the introduction of BHV1. Farmers thought visitors to be more risky than did AI technicians and (especially) veterinarians. Farmers who purchased cattle or participated in cattle shows were of the opinion that the risks of direct animal contacts were more important than did farmers who were not involved in those practices. Farmers whose farms were BHV1-positive (and participated in cattle shows more often) thought the risk of participation smaller than did farmers with BHV1-negative farms.

Animal Husbandry↗

PubMed1998

[Introduction of BHV1 on dairy farms. Risk assessment by cattle farmers and veterinarians].

A study is being carried out at Wageningen Agricultural University together with, among others, the Animal Health Service to determine the possibilities and economic consequences of a more closed farming system for (Dutch) dairy farms. Three identical workshops, held in the evening, were organized as part of the study. The opinion of farmers and their veterinarians on the importance of risk factors for the introduction of diseases on a farm was determined, using Bovine Herpes Virus type 1 (BHV1) as an example. In total, 27 farmers and 13 veterinarians participated in the workshops and completed a computerized questionnaire that was based on Adapted Conjoint Analysis (ACA). The results of the farmers and veterinarians were compared. Both farmers and veterinarians seemed well aware of the risk of direct animal contacts for introduction of BHV1. Farmers thought visitors to be of more risk than veterinarians. By making use of information obtained from the ACA workshops, it will be possible to improve the advice given to different groups in the dairy sector.

Animals↗

PubMed1998

Risk factors for existence of Bovine Herpes Virus 1 antibodies on nonvaccinating Dutch dairy farms.

A more closed farming system may prevent introduction of infectious diseases on dairy farms and can be a good starting point for control of these diseases. Data were available on the presence of Bovine Herpes Virus 1 (BHV1) antibodies in bulk milk and/or blood samples of Dutch dairy farms. Furthermore, information about the possible risk factors for introduction of infectious diseases was collected on 214 of these dairy farms. Data of 107 farms which had been never vaccinated against BHV1 remained for the analysis. A positive BHV1 status on these 107 farms could only be caused by introduction of BHV1. Risk factors for introduction of BHV1 on the farms were quantified using logistic regression. BHV1-positive farms purchased cattle and participated in cattle shows more often compared with BHV1-negative farms. A BHV1-positive farm also had more (professional) visitors in the barn who used farm clothing less often. The BHV1-positive farms were found to be situated closer to other cattle farms compared with the BHV1-negative farms.

Analysis of Variance↗

PubMedDate not supplied

Cost-benefit analysis of vaccination against paratuberculosis in dairy cattle.

Paratuberculosis is an infectious and incurable disease which causes considerable economic losses in dairy cattle, due mainly to premature disposal and losses of milk production. In 1984 the Animal Health Service North-Netherlands started a vaccination trial in which young calves were vaccinated once, to test whether vaccination reduced the production losses and whether the overall costs of vaccination were outweighed by the benefits. Vaccination against paratuberculosis reduced the number of clinically infected animals by almost 90 per cent. It also reduced the numbers of subclinically infected animals and animals with a positive histological and/or bacteriological test result. Although vaccination did not prevent losses in milk production, it reduced the infection pressure and the clinical signs of the disease. Partial budgeting showed that vaccination against paratuberculosis was highly profitable. The costs of vaccination were US$15 per cow and the benefits (total returns minus costs) were US$142 per cow.

Animals↗