Preben Willeberg

Risk factors for infection of sow herds with porcine reproductive and respiratory syndrome (PRRS) virus

In 1992, the porcine reproductive and respiratory syndrome virus (PRRSV) of European type (PRRSV-EU) was introduced in Denmark. By 1996, the virus had spread to approximately 25% of the Danish herds. In January 1996, a modified-live vaccine based on the American type of the virus (PRRSV-US) was used in replacement boars for Danish artificial insemination (AI) centres and from July 1996, the vaccine was used in PRRSV-EU infected herds for prevention of disease. Soon after vaccine introduction, PRRSV non-infected herds experienced outbreaks of disease due to infection with PRRSV-US. In this study, we investigated the risk factors (biosecurity level, animals, exposure from PRRSV-US-infected neighbour herds, semen, herd size, pig density and herd density) for infection with PRRSV-US in a cohort of 1071 sow herds; we used a nested case-control study. The retrospective observation period lasted from June 1996 (when they all were non-infected) to October 1997. Seventy-three non-vaccinated, closed sow herds became infected with the vaccine strain during this period. Each case herd was matched with two control herds from the cohort (controls had not been infected at the time of infection in the case herds). The data were analysed using a Cox-regression model. The hazard of infection increased significantly with exposure from PRRSV-US-infected neighbouring herds, purchase of animals from herds incubating PRRSV-US infection, increasing herd size and purchase of semen from boars at PRRSV-US-infected AI centres. The results are consistent with the modified-live vaccine strain spread to other herds by trade with animals and semen and by neighbour (area) transmission. We suggest that virus spread by aerosols was a frequent mode of transmission.

Prevalence of subclinical Salmonella enterica infection in Danish finishing pig herds.

Our aim was to determine the Salmonella enterica prevalence in 96 randomly selected Danish pig herds, based on serological examination of blood samples and bacteriological examination of faecal samples (collected simultaneously from the same pens). For comparison, 39 high-seroprevalence herds were included in the study. The representativeness of the selected herds was assessed, based on descriptive statistics of herd size and type. Totals of 1330 pen samples and 6814 blood samples were examined.The results from the meat-juice screening in the Danish S. enterica Control Programme were available for 3372 meat-juice samples from 91 of the 96 randomly selected herds and 1195 meat-juice samples from 37 of the 39 high-seroprevalence herds. Of the 96 randomly selected herds, 23 herds had no positive pen samples (out of 10), no positive blood samples (out of 50) and no positive meat-juice samples (out of approximately 30-40 samples in 6 months). Ten herds had one or more positive meat-juice samples but were otherwise negative. S. Typhimurium was isolated from 30 of the 39 high-seroprevalence herds. Our conclusions were: (1) The within-herd seroprevalence among the 96 randomly selected Danish pig herds was low (average within-herd seroprevalence=2%, maximum=32%). (2) Among the 39 high-seroprevalence herds (recently assigned level 3 in the S. enterica Control Programme), S. enterica was isolated from 77% of the herds when 10 pen samples were examined bacteriologically. (3) Seropositivity tended to be related to the presence of S. Typhimurium.

Empirical and theoretical evidence for herd size as a risk factor for swine diseases

Abstract Herd size is frequently studied as a risk factor for swine diseases, yet the biological rationale for a reported association with herd size (whether positive or negative) is rarely adequately discussed in published epidemiological studies. Biologically plausible reasons for a positive association between herd size and disease include a greater risk of introduction of pathogens from outside the herd, greater risk of transmission of pathogens within and among herds when the herd is large, and effects of management and environmental factors that are related to herd size. However, compared with owners of small herds, owners of large herds might more frequently adopt management and housing practices that mitigate this theoretically increased risk. We used studies of pleuritis, pneumonia and pseudorabies to describe the epidemiological issues involved in evaluations of the relationship between management factors, herd size and disease. In future studies, we recommend that (i) herd size be measured in a way that best characterizes the true population at risk; (ii) studies that evaluate management-related risk factors should account for herd size wherever possible; (iii) population-based studies of the interrelationships among management factors and between management factors, herd size, herd density and pig density be done; (iv) likely biological reasons for any herd-size effect be postulated; and (v) the distribution of herd sizes in the source population and the study sample be described.

A longitudinal study of serological patterns of respiratory infections in nine infected Danish swine herds.

Sixteen litters of seven pigs from each of nine Danish farrow-to-finish herds were followed to investigate the serological patterns caused by natural infection with Mycoplasma hyopneumoniae, Pasteurella multocida toxin and Actinobacillus pleuropneumoniae serotypes 2, 5-7, 12. In seven of the herds, pigs were followed as two separate cohorts started 4 weeks apart, and in two herds only one cohort was followed.A total of 999 pigs were included in the study. The pigs were blood sampled at weaning and subsequently every fourth week until slaughter. All pigs were examined for antibodies against M. hyopneumoniae (enzyme-linked immunosorbent assay), P. multocida toxin (enzyme-linked immunosorbent assay) and A. pleuropneumoniae serotypes 2, 5-7, 12 (complement-fixation tests). The most-common pattern (28%) of seroconversion was that of pigs first seroconverting to A. pleuropneumoniae serotype 2, followed by seroconversion to M. hyopneumoniae. Each herd had a dominant serotype of A. pleuropneumoniae to which most pigs seroconverted. Seroconversion to the respiratory pathogens occurred mainly in the growing-to-finishing units (8-24 weeks). The risk of seroconversion to the P. multocida toxin was very low (<20%) and occurred late.None, four and seven herds tested seropositive to PRRS and to swine influenza virus subtypes H3N2 and H1N1, respectively, when testing 10 pigs per herd (selected randomly among the study pigs) at the age of 20 weeks.

The need for built-in validation of surveillance data so that changes in diagnostic performance of post-mortem meat inspection can be detected

Time trends in animal-disease surveillance often are evaluated on the basis of crude estimates of apparent prevalence. In addition to possible changes in the true prevalence of the condition, changes in apparent prevalence over time might reflect changes in sensitivity and/or specificity of the diagnostic classification used. To illustrate this, comparative post-mortem meat inspection data from four Danish slaughter plants sampled in 1993-1994 and 1997-1998 were used to obtain latent-class model estimates of the sensitivity and specificity of traditional and extended post-mortem meat inspection of visceral and parietal chronic pleuritis (CP), respectively.True prevalence of CP was estimated for each study period and slaughter plant by latent-class models. Estimated sensitivities of traditional post-mortem meat (TPM) inspection ranged from 28.8 to 61.4% (1993-1994) and 39.2 to 87.3% (1997-1998). An increase in sensitivity with time was seen for all slaughter plants. Estimated sensitivities of extended post-mortem meat (EPM) inspection ranged from 85.7 to 94.8% (1993-1994) and 73.8 to 93.0% (1997-1998). All estimated specificities were >93.3%. The possible association of the estimated true prevalence of CP with time (1993-1994 versus 1997-1998) was investigated with a logistic-regression model with random effects. A slight, but non-significant decrease in the odds of CP from 1994 to 1998 was found (odds ratio=0.9). In this and similar situations, one should consider conducting ongoing double-classification of samples of units followed by statistical estimation of true prevalences, sensitivities and specificities, so that decisions can be based on such estimates rather than on crude apparent prevalences.

Ad hoc method for the assessment on listing and categorisation of animal diseases within the framework of the Animal Health Law

Abstract The European Commission has requested EFSA to assess animal diseases according to the criteria as laid down in Articles 5, 7, 8 and Annex IV for the purpose of categorisation of diseases in accordance with Article 9 of the Regulation (EU) No 2016/429 (Animal Health Law). This scientific opinion addresses the ad hoc method developed for assessing any animal disease for the listing and categorisation of diseases within the Animal Health Law (AHL) framework. The assessment of individual diseases is addressed in distinct scientific opinions that are published separately. The assessment of Articles 5, 8 and 9 criteria is performed on the basis of the information collected according to Article 7 criteria. For that purpose, Article 7 criteria were structured into parameters and the information was collected at parameter level. The resulting fact sheets on the profile and impact of each disease were compiled by disease scientists. A mapping was developed to identify which parameters from Article 7 were needed to inform each Article 5, 8 and 9 criterion. Specifically, for Articles 5 and 9 criteria, a categorical assessment was performed, by applying an expert judgement procedure, based on the mapped information. The judgement was performed by EFSA Panel experts on Animal Health and Welfare in two rounds, individual and collective judgement. The output of the expert judgement on the criteria of Articles 5 and 9 for each disease is composed by the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported.

The cox regression model applied to risk factor analysis of infections in the breeding and multiplying herds in the Danish SPF system

Abstract The Cox regression model is discussed with emphasis on model examination and selection. The model is applied in a multivariate risk factor analysis of Mycoplasma hyopneumoniae (MH) infections in the breeding and multiplying herds in the Danish specific pathogen-free (SPF) pig production system. The analysis showed that the pattern of infections was in agreement with the theory that both airborne transmission and spread through trade in subclinically infected animals are major causes of MH infections.

Infectious and rearing-system related risk factors for chronic pleuritis in slaughter pigs

Chronic pleuritis (CP) in Danish pigs for slaughter is by far the most frequent finding at the routine post-mortem meat inspection. An initial investigation published in 1990 demonstrated infectious and management-related risk factors. Serological testing for additional infectious agents, as well as the need to consider the effect of disease clustering at the herd level, required a re-analysis of the data. Our re-analysis used a representative sample of 4,800 pigs originating from 623 Danish herds. Each pig was examined for the presence of CP and progressive atrophic rhinitis (PAR). The gender of the pig, the weight of the carcass, and the herd of origin were also recorded. Individual blood samples were examined for seropositivity for Actinobacillus pleuropneumoniae (AP) serotypes 2, 6, 7, 12, Haemophilus parasuis, Mycoplasma hyopneumoniae (MYC) and swine influenza (SI). Herd-level information retrieved through a questionnaire included health status, production type, herd size (i.e. pigs per year) and vaccination procedures. Associations between CP and infectious, individual and herd-related factors were investigated by logistic regression with random effects. Among pigs from herds with conventional health status, seropositivity for AP serotypes 2 and 6, and MYC had odds ratios (ORs) of CP of 9.0, 1.6 and 1.8, respectively. Neither seropositivity for AP serotype 7 nor SI were associated with CP by themselves, but interacted: OR of CP of 5.3 (1.8) when present at the same time among pigs exhibiting (not exhibiting) PAR. An association of PAR with CP was found, and PAR interacted with AP serotype 7: OR=10.0 (4.3) when both factors were present among pigs exposed (non-exposed) to SI. The OR (0.97) for an increase of carcass weight by 1 kg was negligible. In pigs from specific pathogen-free (SPF) herds, seropositivity for MYC and herd size were associated with CP. Moreover, for a herd size of 1,000 pigs, CP was associated with exposure to MYC by an OR of 3.3 (decreasing to 1.9 when the herd size was increased by 1,000). Farrow-to-finish as opposed to finishing herd had an OR of CP of 3.2. In conventional herds, seropositivity for AP serotype 2 and MYC were associated with 51% and 29% of the occurrence of CP. In SPF herds, farrow-to-finish as opposed to finishing herds was associated with 47% of the occurrence of CP. Seropositivity for MYC was associated with 33% (39%) of the occurrence of CP in herds with a size > (< or =) 1,500 pigs.

Estimation of sensitivity, specificity and predictive values of two serologic tests for the detection of antibodies against Actinobacillus pleuropneumoniae serotype 2 in the absence of a reference test (gold standard)

Latent-class models were used to determine the sensitivity, specificity and predictive values of a polyclonal blocking enzyme-linked immunosorbent assay (ELISA) and a modified complement-fixation test (CFT) when there was no reference test. The tests were used for detection of antibodies against Actinobacillus pleuropneumoniae serotype 2 in a survey of respiratory diseases in Danish finishing pigs. The estimates were obtained by maximum-likelihood and also by a Bayesian method (implemented with Gibbs sampling). Possible dependence of diagnostic errors was investigated by comparing models where independence was assumed to models allowing for conditional dependence, given the true disease status. No strong evidence of conditional dependence in either test sensitivity or specificity was found. Assuming independence, maximum-likelihood estimates and 95% confidence intervals of the sensitivity and specificity of the ELISA were 100% and 92.8% (90.1-95.5%) and the corresponding values of the CFT were 90.6% (85.8-95.4%) and 98.6% (98.0-99.3%), respectively. Bayesian estimates and posterior 95% credible intervals of the sensitivity and specificity of the ELISA were 99.7% (98.7-100%) and 92.7% (89.9-95.3%) and of the CFT were 90.6% (86.0-95.3%) and 98.7% (98.0-99.3%). The sensitivity and specificity of a combined test, where the CFT is subsequently applied to the pig sera that test positive in the ELISA, were estimated at 90.2% (85.6-95.0%) and 99.9% (99.8-100%), respectively. The cost of the combined test was less than the cost of the use of the CFT alone, at prevalences <54%. Prevalences and predictive values and their 95% limits were estimated in six sub-samples of data. The estimates of sensitivity and specificity obtained in the present investigation generally validate those reported from other sources.

Bovine somatotropin and clinical mastitis: epidemiological assessment of the welfare risk

Abstract Published data on clinical mastitis in BST trials are re-evaluated applying general epidemiological methods. Results of pooled analyses are used to estimate relative risks and population attributable fractions. The estimates suggest, that BST treatment is associated with 15–45% excess incidence of clinical mastitis, which is probably due to an indirect causal effect through increased milk yield. The increased risk is of concern to the welfare of future populations of dairy cows. It will limit the acceptability and usage of BST under common management conditions, and it should be taken into account in evaluating BST preparations for marketing authorization.