The required sample size in vaccination-challenge experiments with infectious bronchitis virus, a meta-analysis.

For statistical, animal welfare and financial reasons the choice of the number of chickens per group in experiments is important. This estimation, together with the number of tracheal organ cultures (TOCs) that need to be examined from each chicken in order to assess protection, should be based on the difference in level of protection that one would like to be able to detect (effect size), the expected variability of the results between and within the chickens, the desired confidence level and the power of the study. To obtain data that would facilitate this estimation, a meta-analysis was performed on the data from 18 infectious bronchitis virus (IBV) vaccination-challenge experiments performed at the Dutch Animal Health Service Deventer, the Netherlands (GD) in order to determine and quantify the source of variation in the mean level of protection of different groups. For the calculations, 137 groups of chickens were subdivided into 10 clusters based on age (young or adult), vaccination (none, homologous or heterologous), challenge (IBV or mock infected) and location of vaccination (isolator at GD or in the field). The results were used to estimate the required number of chickens per group for the different clusters using 2, 5 or 10 TOCs per chicken to be able to detect effect sizes of 6.25%, 12.5%, 25% and 50% between groups of chickens with 95% confidence (P<0.05) and 80% power. The number of chickens that was required for the mentioned effect sizes varied greatly from 2 to 650. This meta-analysis provided data that allow research workers to estimate the number of chickens that should be included in each group in order to obtain reliable results based on particular combinations of infectious bronchitis vaccination and challenge strains as defined by the presented clusters.

Efficacy of infectious bronchitis virus vaccinations in the field: association between the alpha-IBV IgM response, protection and vaccine application parameters.

Infectious bronchitis virus (IBV) is, in spite of vaccination, still a major cause of respiratory problems in broilers and of poor egg production in breeders and layers in many parts of the world. A possible cause of the insufficient protection induced by vaccination is an inadequate application of the vaccine. This paper reports the results of two field studies. In the first, the results of the alpha-IBV IgM enzyme-linked immunosorbent assay (ELISA) on post-vaccination sera were compared with the efficacy of the IBV vaccination against homologous challenge of the same broilers. The results showed that groups with at least 50% positive sera in the IgM ELISA at 10 days post vaccination had a high level of protection against challenge. Most groups of broilers with a low level of IgM ELISA positives had a low or moderate level of protection against challenge. In a second field study, the IgM response to IBV vaccination was compared with detailed information of the vaccination process of 360 spray-vaccinated flocks of about 2-week-old broilers, layer pullets, broiler breeders and broiler grandparents. The information included parameters such as flock size, type of chicken, housing, age of the chicken, application route, vaccine, dose, water quantity and temperature, ventilation and light management, combination with other vaccines and temperature of the house. The aim was to identify factors that might be associated positively or negatively with the IgM response and thereby with the expected level of protection against homologous challenge under field conditions. Significant associations were detected between the level of IgM response and factors regarding type of bird, flock size, housing type, ventilation management, light management, age/interval of vaccination, interval between vaccination and blood sampling, and temperature of the water that was used to reconstitute the vaccine. This knowledge can be useful to improve the average efficacy of IBV vaccination in the field.

A study on the severity and relevance of porcine circovirus type 2 infections in Dutch fattening pigs with respiratory diseases.

This study was set up to get more insights in the severity and relevance of porcine circovirus type 2 (PCV2) infections in Dutch fattening farms in an endemic PCV2-situation with no clinical signs of post-weaning multisystemic wasting syndrome (PMWS). In part A of the study, in total 29 commercial fattening farms with varying percentages of pneumonia and pleurisy at slaughter were examined. Blood samples were collected at random by cross-sectional sampling; 10 in the age of 10-12 weeks, 10 at the age of 16 weeks and 10 blood samples at the end of the finishing period (20-22 weeks of age). Serum samples were examined for the presence of PCV2 IgM and IgG antibodies and for antibodies against other porcine lung pathogens. In part B, 8 "high" and 8 "low" herds were selected. The 8 "high" herds were defined as herds having high percentages of lung lesions (pneumonia) at slaughter, and the 8 "low" herds had low percentages of pneumonia at slaughter. For both the "high" and "low" herds, 3 pigs showing signs of respiratory distress were selected for necropsy (n=48). Lung tissue samples were examined post-mortem for macroscopic and histopathological lesions, and for the presence of bacteria and viruses. The results of part A showed that, pigs at 16 weeks of age with IgM antibodies against PCV2 had a lower probability of having pleuritis at slaughter (OR 0.34, P<0.000). Pigs in the age category of 20-22 weeks, and with IgM antibodies against PCV2, also had a lower probability of having pneumonia at slaughter (OR 0.29, P=0.032). In part B lobus apicalis pneumonia, PCV2 in macroscopically unaffected lungs, Pasteurella multocida, Mycoplasma hyopneumoniae, and swine influenza viruses were all found significantly more often in "high" than in "low" pigs at autopsy. High PCV2 DNA loads (>10(4) PCV2 DNA copies/mg) were found in lungs of 14 (58%) "high", and in 7 (29%) of the "low" pigs (P=0.13). In 11 of the 19 affected lungs from "high" pigs, high PCV2 DNA loads were found in combination with one or more other lung pathogens, while this was found only in 5 of the 17 affected lungs from "low" pigs (P=0.02). This study confirms the hypothesis that PCV2 plays a role in pneumonia and pleurisy in 10-24 weeks old fattening pigs, not only in herds with a high prevalence of PMWS, but also in herds with no clinical signs of PMWS.

[Udder health: the influence of BVD-free status]. / Uiergezondheid: de invloed van de BVD-vrije status.

The effect of BVD v-free certification of BVD v-infected dairy herds on udder health was determined, by comparing parameters of udder health in 319 cases and 629 controls. Cases were dairy herds that originally had at least one BVD V antigen-positive animal but which subsequently became BVD v-free and maintained this certified status for at least 2 years. Controls had an unknown BVD V status. Each case was matched with two controls by region and herd size. The three udder health parameters were bulk-milk somatic cell count (scc), the proportion of cows with a high scc, and the proportion of cows with a high scc after previously having a normal scc (these cows were considered to have a new intramammary infection). Udder health was better in the cases in the 2 years preceding the BVD V-free period than in the controls. BVD V-free status was not associated with bulk-milk scc or the proportion of cows with a high scc but was associated with occurrence of a new intramammary infection:fewer case cows had a new intramammary infection than control cows (difference 0.6%; P < 0.05).

Prevalence of VTEC O157 in dairy and veal herds and risk factors for veal herds.

The aim of this study was to determine the herd prevalence of veal and dairy herds and to identify risk factors for VTEC O157 positive veal herds. The study was based on monitoring data from November 1996 through July 2005 of 1051 dairy herds and 930 veal herds. The herd level prevalence (95% CI) was 8.0% (6.4-9.6) for dairy herds and 12.6% (10.5-14.7) for veal herds. Within the population of veal herds, a prevalence of 39.8% (33.9-45.6) was found for pink veal herds (n = 269) and 1.5% (0.7-2.8) for white veal herds (n = 661). Multivariable logistic regression showed that the type of veal (pink vs. white; OR = 21.6; 95% CI: 10.4-45.0), ventilation (mechanical vs. natural; OR = 0.4; 95% CI: 0.2-0.8), time between arrival in the herd and sampling (3-5 months vs. 0-2 months: OR = 2.33; 95% CI: 1.1-5.1, > or = 6 months vs. 0-2 months: OR = 4.11; CI: 1.9-8.9), other feed than the 7 most common (yes vs. no; OR = 2.1; 95% CI: 1.2-3.7) and at least one dog present in the stable (yes vs. no; OR = 2.6; 95% CI: 1.5-4.6) were significantly (P < 0.05) associated with the presence of VTEC O157. The large difference in the VTEC O157 prevalences for pink veal and white veal production might have been caused by a very different management of these type of herds. However, this could not be studied with the data collected.

The effect of becoming BVDV-free on fertility and udder health in Dutch dairy herds.

The aim of the study was to investigate the effect of BVDV-free certification of dairy herds on fertility and udder health. Cases were defined as dairy herds that had at least one BVDV-antigen positive animal, subsequently gained the BVDV-free status by participating in the BVDV-control programme of the Animal Health Service (AHS) and maintained this status for at least 2 years. Controls had an unknown status for BVDV and two controls were matched to one case by region and herd size. Data concerning fertility and milk production of all herds were provided by The Dutch Royal Cattle Syndicate (NRS). After validation, data of 79,607 cows of 392 case herds and 124,831 cows of 730 control herds were analysed on ten fertility and three udder health parameters. For the analyses all observations were aggregated at herd level. To account for the matching, differences for fertility parameters were calculated between each of the two pairs of case-control within a matching code. The analyses were performed with these differences as dependent variables. Mixed models and GEE models were used for the statistical analyses of fertility and udder health. Case herds had a significantly lower abortion rate in the BVDV-free period than controls herds (10.3% versus 11.6%, P<0.01) while there was no significant difference for the other fertility parameters. There was no effect on mastitis prevalence or bulk-milk SCC but the mastitis incidence significantly decreased for case herds in the BVDV-free period (cases 0.6 % lower than controls, P<0.05). In our study the effect of getting the BVDV-free status may have been underestimated for several reasons like an unknown status for control herds, not knowing when an acute infection occurred in case herds and not knowing the management for both cases and controls. Interestingly, both significant variables, being abortions and mastitis incidence, are parameters that are more difficult to influence by the farmer than the other parameters (e.g. calving interval).