Factors associated with the seroprevalence of pseudorabies virus in breeding swine from quarantined herds.

Strategies for the elimination of pseudorabies virus (PRV) from swine herds include test and removal, offspring segregation, and depopulation/repopulation. The prevalence of PRV in a herd is a major factor in selection of the most appropriate strategy. The purpose of the study reported here was to describe the prevalence of PRV in adult swine in PRV quarantined herds in Minnesota, and to determine herd factors associated with the seroprevalence. Questionnaires describing the health history of the herd, management practices, and design of the swine facilities were obtained from the owners of 142 quarantined herds. Blood was collected from 29 finishing pigs over the age of 4 months, up to 29 adult females, and all herd boars. Factors considered to be significant in a bivariate analysis were combined in a stepwise multiple logistic regression analysis. The prevalence of PRV-seropositive adults in each herd was bimodally distributed among the 142 herds. In 42 (30%) of the herds, none of the females tested was seropositive, which represented the lower mode. At least 90% of the adults tested were seropositive in 30 (21%) of the herds and represented the higher mode. The odds of the breeding swine of a given herd having a PRV seroprevalence of greater than or equal to 20% as compared with having a seroprevalence of less than 20% was 1.654 times higher per 50 adults in the herd, 13.550 times higher if the finishing pigs were seropositive, 2.378 times higher if sows were housed inside during gestation, and 1.481 times lower per number of years since the imposition of quarantine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Factors affecting the risk of infection with pseudorabies virus in Illinois swine herds

A case-control study of pseudorabies virus (PRV) infection in Illinois swine herds was conducted to identity risk factors associated with PRV infection. Factors identified as being associated with increased risk of PRV infection included percentage of herd in total confinement (adjusted OR (aOR)=19.7, 95% confidence interval (CI): 3.3–117.2) and having two or more PRV positive herds in the township (aOR=3.2, 95% CI: 1.0–10.2). A protective factor identified in the study included using one's own vehicle to transport pigs to market rather than hiring truckers (aOR=0.2, 95% CI: 0.07–0.6). A protective factor for producers who used their own vehicle for transporting pigs was cleaning the truck after off-site trips (aOR=0.09, 95% CI: 0.03–0.2). Management factors which can be most easily altered by producers who wish to prevent PRV infection in their herd include purchasing one's own vehicle for transport of pigs, and cleaning out this vehicle carefully after off-site visits. Total confinement herds and herds in areas where PRV is endemic appear to be at higher risk of becoming infected with PRV, and managers should be especially aware of herd security measures.     

Herd factors affecting the selection and success of intervention strategies in the program for eradication of pseudorabies (Aujeszky's disease) virus from Illinois swine farms

The program for eradication of pseudorabies virus (PrV) from swine herds in Illinois was evaluated with respect to compliance with Livestock Conservation Institute (LCI) guidelines for selection of intervention strategies and for the effectiveness of these interventions under different herd conditions. The sample consisted of 395 swine operations quarantined between 1988 and 1994. These herds were followed until the end of 1996. The association of herd characteristics (number of sows, sow PrV seroprevalence, type of housing, number of PrV-seropositive farms within 1.5 mi) at the time of quarantine with the producer's selection of an initial intervention strategy (vaccination, offspring segregation, test-and-removal, depopulation-and-repopulation) was analyzed using logistic multiple regression. The interaction of herd characteristics with intervention strategies to affect the duration of quarantine was analyzed using multivariable Cox regression.

Factors favoring selection of vaccination were increased herd size, higher sow PrV seroprevalence, and more PrV-seropositive farms within 1.5 mi. Offspring segregation was preferred when sow PrV seroprevalence was higher, and test-and-removal was preferred when seroprevalence was lower. Depopulation-and-repopulation was more likely in outdoor operations. Except for depopulation-and-repopulation, selection of an intervention strategy was in accordance with LCI guidelines.

Vaccination and offspring segregation were associated with longer times under quarantine, and test-and-removal and depopulation-and-repopulation with shorter times. Test-and-removal was more effective in reducing the duration of quarantine when sow PrV seroprevalence was low. Vaccination increased the duration of quarantine less when sow PrV seroprevalence was high. Vaccination increased the duration of quarantine more when there were one or more PrV-seropositive farms within 1.5 mi than when there were no PrV-seropositive farms within 1.5 mi. It is apparent that herd characteristics affect the duration of quarantine and therefore need to be taken into account in the selection of a PrV-intervention strategy.     

Estimated BVDV-prevalence, -contact and -vaccine use in dairy herds in Northern Portugal

A study to evaluate BVDV-prevalence, recent -contact and -vaccine use in dairy herds in the "Entre Douro e Minho" (EDM) region in North Portugal was carried out in 124 dairy herds in 2003. Herds were visited to ascertain BVDV-vaccine use and to collect a bulk tank milk (BTM) sample and serum from 1268 cattle to analyse BVDV-antibodies using an NS2-3 ELISA. Fifty-three percent of farmers used inactivated BVDV-vaccines whilst the remaining farmers were not presently using BVDV-vaccines. BMT-antibody results included 35% positives, 25% negative and 39% inconclusive, and were similar in vaccinated and non-vaccinated herds (p>0.05) and allowed estimating a 10% BVDV herd-prevalence from prior knowledge of the relationship between BMT-antibody results and probability of PI cattle in the herd. Overall individual seroprevalence was 27% and was 23% in non-vaccinated and 36% in vaccinated cattle (p<0.05). Contact of the herd with BVDV was assessed according to seroprevalence in young and adult cattle in the herd and it was estimated that 35% of herds were infected or had recent contact with BVDV, 40% were not infected and did not have recent contact with BVDV and the BVDV-infection and -contact status of remaining herds was undetermined. The results from this study indicate BVDV is endemic and BVDV-vaccines are widespread in the dairy-cattle population in EDM region in Portugal.     

Aujeszky’s disease and the effects of infection on Japanese swine herd productivity: a cross-sectional study

Pseudorabies virus (PRV) is endemic in some regions of Japan. We investigated the effects of PRV infection status on herd productivity. Serum samples were obtained from 48 swine herds in Japan. Within each herd, three serum samples were obtained from growing pigs at four different ages, as well as from sows in low and high parity groups. Sera were tested for antibodies against wild-type PRV via competitive ELISA. Herds were classified into PRV positive and negative groups based on serological results. Herds infected with PRV exhibited postweaning mortalities (6.84%) that were significantly (P=0.0018) higher than those in unaffected herds (4.73%). Because of the reduced productivity in PRV positive herds, the current PRV eradication program must be strengthened.     

Association between soil type and paratuberculosis in cattle herds

OBJECTIVE: To determine the association between soil type and paratuberculosis in cattle. SAMPLE POPULATION: Soil samples and test results for paratuberculosis in 92 Indiana cattle herds. PROCEDURE: Testing records from herds in which > or = 20 cattle were tested for paratuberculosis by use of an ELISA between 1998 and 2002 were identified. Soil type was characterized on the basis of herd location. Clusters of herds with seroprevalence greater than the median seroprevalence were identified. Association between clusters and soil types was estimated by logistic regression, adjusted for herd type (dairy or beef). RESULTS: A spatial cluster of greater than the median seroprevalence was identified in northeast Indiana. Soils with low silt content were associated (odds ratio [OR], 7.2; 95% confidence interval [CI], 2.1 to 24.5) with this cluster. Adjusting for herd type did not substantially alter this association (OR, 6.7). Herds located in areas with sandy loam (OR, 6.2; 95% CI, 1.4 to 27.4) and loam (OR, 3.6; 95% CI, 1.0 to 13.2) soils were also more likely to be within the cluster of greater than the median seroprevalence. Herds located in areas of silt loam soils were less likely (OR, 0.2; 95% CI, 0.1 to 0.7) to be included in this cluster. CONCLUSIONS AND CLINICAL RELEVANCE: Spatial distribution of herds with greater than the median seroprevalence of paratuberculosis was associated with soil characteristics. Survival of Mycobacterium avium subsp paratuberculosis may be enhanced by silt or sand content in loamy soils. These results may be used to modify paratuberculosis control programs.     

Prevalence of herds with young sows seropositive to pseudorabies (Aujeszky's disease) in northern Belgium

In Belgium, pseudorabies in swine has been the subject of a mandatory eradication programme since 1993. From December 1995 to February 1996, a survey was conducted in the five provinces of northern Belgium to estimate the provincial pseudorabies virus (PRV) herd seroprevalence. Seven hundred and twenty randomly selected herds were included in this survey. To detect recently infected animals, only young sows were sampled. The results show that 44% of these herds had an important number of PRV-seropositive young sows. The highest herd seroprevalence was observed in West Flanders (68%), followed by Antwerp (60%), East Flanders (43%), Limburg (18%), and Flemish Brabant (8%). Assuming a diagnostic test sensitivity and specificity of 95% and 99%, respectively, and a true PRV within-herd prevalence of 43%, the overall true PRV herd prevalence was estimated to be 35%. A logistic multiple-regression revealed that the presence of finishing pigs was associated with a two-fold increase in odds of a herd being seropositive (odds ratio (OR)=2.07, 95% confidence interval (CI)=1.31–3.26); a breeding herd size ≥70 sows was associated with a four-fold increase in odds of a herd being seropositive (OR=4.09, 95% CI=2.18–7.67); a pig density in the municipality of ≥455 pigs/km² was associated with a 10-fold increase in odds of a herd being seropositive (OR=9.68, 95% CI=5.17–18.12). No association was detected between the PRV herd seroprevalence and purchase policy of breeding pigs (purchased gilts, or use of homebred gilts only).     

Bovine viral diarrhoea virus in beef cattle herds of Yucatan, Mexico: seroprevalence and risk factors

A survey of bovine viral diarrhoea virus (BVDV) infection was carried out from June 2001 to July 2002 in a non-vaccinated beef cattle population from the livestock region of Yucatan, Mexico, to assess seroprevalence and identify risk factors related to seroprevalence. The aim was also to estimate the intra-herd correlation (r_e) and design effect (D) of BVDV seropositivity. Cattle were selected by a two-stage cluster sampling. Blood samples were collected from 560 animals originating from 40 herds. Sera were tested for antibodies against BVDV using an indirect ELISA test. The sensitivity and specificity of the test was 97.9 and 99.7%, respectively. Risk factors regarding the herd and each animal sampled were recorded through a personal interview at the time of blood sampling. Twenty-four of the 40 herds had at least one seropositive animal. The animal true seroprevalence was estimated as 14%. The marginal logistic regression model used to describe the data found a significant (p < 0.05) association of herd size–cow-origin interaction. The interaction was due to a higher risk of seropositivity in the category of herds with ≤100 animals and purchased cows (OR = 1) as compared to herds with ≤100 animals and cows born in the farm (OR = 0.23). Seropositivity between cows purchased and cows born in the farm was similar for herd sizes of 101–196 and >196 animals. The r_e and D values were 0.17 ± 0.05 and 3.16 ± 0.57, respectively.     

Pseudorabies virus is transmitted among vaccinated conventional pigs, but not among vaccinated SPF pigs

Whereas the reproduction ratio (R) of pseudorabies virus (PRV) in vaccinated specific pathogen free (SPF) pigs without maternally derived antibodies under experimental conditions has repeatedly been shown to be significantly below 1, R in vaccinated conventional pigs in the field with maternally derived antibodies was significantly above 1. To exclude the difference in husbandry conditions as a cause for this discrepancy, we quantified and compared the transmission of PRV in both groups under identical experimental conditions. Whereas none of the SPF sentinel pigs became infected (R=0, significantly<1), all conventional sentinel pigs did become infected (R=2.5, significantly>1). Moreover, only one SPF pigs shed virus in saliva, the mean cumulative titre being almost a 100-fold less than in conventional pigs (17 pigs, P=0.003). In addition, the mean proliferation of peripheral blood lymphocytes in response to PRV antigens was significantly higher in SPF pigs than in conventional pigs at all points studied (P<0.0001). Moreover, the virus-neutralising antibody titre after vaccination was significantly higher in SPF pigs than in conventional pigs. We conclude that the discrepancy in transmission between vaccinated SPF pigs and vaccinated conventional pigs cannot be attributed to the experimental conditions.     

The new classification system for slaughter-pig herds in the Danish Salmonella surveillance-and-control program

The Danish surveillance-and-control program for Salmonella in slaughter pigs was introduced in 1995. The key element of the program is a quick and correct identification of herds with high seroprevalence. After 5 years, the classification scheme was evaluated--and a revision was made. Data from two Salmonella screenings including a total of 1902 slaughter pig herds were used. For each herd, information was available on Salmonella status based on both microbiology and serology. Based on analyses of these data, suitable changes in the scheme were identified and their effect estimated by use of data from the Danish Salmonella Database including all herds in 2000. The classification scheme has been adjusted on the following points. (1) The sampling has been simplified into 60, 75, or 100 samples per herd per year depending on herd size. This means more-precise estimates for the seroprevalence among smaller herds. (2) Herds with an annual kill or=index 40, and the limit between Levels 2 and 3 to >or=index 70. If the Danish swine producers are interested, a Level 0 may be introduced (consisting of seronegative herds as an indication of a negligible Salmonella prevalence). The classification scheme was introduced in August 2001.     

Factors associated with circulation of pseudorabies virus within swine herds

Data were collected from 104 Minnesota swine farms quarantined for pseudorabies virus (PRV) infection. Each herd was serologically evaluated for the presence of antibodies to PRV in finishing pigs. Herd management practices, swine housing design, and disease profiles were described for each farm. Multiple logistic regression analysis was used to determine which factors were associated with circulation of PRV in the finishing pigs of farrow-to-finish farms. Sixty-seven (64%) of the herds had no serologic evidence of PRV circulation in the finishing section, whereas 37 herds (36%) contained at least one PRV seropositive finishing pig. The odds of a given finishing herd being seropositive for PRV were 2.85 times higher if the finishing pigs were housed in confinement (P = 0.01), 2 times higher if Actinobacillus (Haemophilus) pleuropneumoniae was a clinical problem in the herd (P = 0.03), 1.36 times less for each year that passed since the herd quarantine was issued (P = 0.01), 1.74 times higher if clinical signs of PRV were reported (P = 0.04), and 1.52 times higher if animal protein was included in at least one of the rations (P = 0.08).     

Suboptimal reproductive performance of dairy cattle kept in smallholder herds in a rural highland area of northern Tanzania

The objectives of the present study were to assess the reproductive performance and cumulative incidence risk of reproductive disorders and to compare the success of artificial insemination (AI) to natural service (by handmating; NS) in dairy cattle kept in smallholder herds under a zero-grazing system in a rural highland area of Tanzania. Data on occurrence of all normal and abnormal reproductive events were collected for 215 adult animals belonging to 74 households.

The median and range of the intervals: intercalving, calving to first service, and calving to pregnancy were 477 (335–860), 154 (38–486) and 206 (61–567) days, respectively. Breed and parity did not affect the reproductive parameters (P>0.05). However, cows in the milked group had a shorter median calving interval than those in the suckled group (P<0.001). The overall percentage pregnant and the percentage pregnant to first service were higher in the NS than in the AI group (49 vs. 32%; P=0.007) and (67 vs. 25%; P<0.001), respectively. The median numbers of services per pregnancy were not different between the AI (3) and NS (2) groups (P=0.17). The cumulative incidence risk of abortion, dystocia, prolapse, retained fetal membranes, mastitis, milk fever and cyclic non-breeders were 16.0, 1.7, 2.5, 4.2, 5.0, 1.7, and 6.1%, respectively. Hoof overgrowth (4.6%) and hoof deviation (4.6%) were the most-frequent digital problems. We concluded that reproductive parameters and cumulative incidence risk of abortion show suboptimal reproductive performance in rural-based, zero-grazed smallholder dairy herds in Tanzania particularly those using AI.     

THE EFFECT OF CALFHOOD VACCINATION WITH STRAIN 19 ON THE SEROLOGICAL DIAGNOSIS AND ERADICATION OF BOVINE BRUCELLOSIS

Data from 42,224 cattle from 694 herds collected during the brucellosis eradication campaign were used to examine the effects of calfhood strain 19 vaccination. The prevalence of infection in vaccinated herds was 1.8% compared with 9.1% in non-vaccinated herds (p< 0.005). The mean titre in the former group was lower (p< 0.001). Vaccinated herds required 3.3 herd tests to achieve a provisionally free status compared with 4.8 in non-vaccinated herds (0.001 < p < 0.005). Vaccination did not significantly reduce the number of herd tests in herds with less than 100 breeding females. In tests after the initial herd test only 0.5% reactors were found in vaccinated herds compared with 6.9% in non-vaccinated herds (p< 0.005). There were 0.9% false positive to the Rose Bengal plate test in non-vaccinated and 2.1% in vaccinated animals (p< 0.005) in non-infected herds. In infected herds this percentage was 3.0% and 4.2% respectively by (p< 0.05). In the non-infected herds there were 0.04% false positives to the complement fixation test out of 10,506 non-vaccinated cattle tested and 0.2% out of 24,734 vaccinated cattle.     

A seroepidemiological study of bovine pestivirus in Queensland beef and dairy herds conducted in 1994/95

OBJECTIVE: To describe the distribution and prevalence of cattle herds with detectable antibody to bovine pestivirus in Queensland in 1994/95. MATERIALS AND METHODS: The study used 7,838 serum samples collected from 250 herds in Queensland, as part of a structured animal health surveillance program conducted in 1994 and 1995. Samples were collected from female cattle bred on the property. In each herd, 10 to 20 heifers less than two years of age and 10 to 15 older cows were sampled giving a 95% probability of detecting one or more seropositive animals if the seroprevalence was approximately 10% or greater. Sera were analysed for antibodies to bovine pestivirus using a virus neutralisation test. RESULTS: Total cattle numbers in sampled herds varied from 62 to 24,600 head, while total area of properties sampled varied from 50 to 395,400 hectares. Eleven percent of herds contained no seropositive animals among those sampled, and in 38% of herds, all sampled cattle aged one to two years of age were seronegative. There was a trend for larger herds to have one or more animals seropositive for bovine pestivirus (chi-squared for Linear trend = 3.656, p = 0.056). Herds with more than 500 head of cattle were significantly more likely than herds with less than 500 head to contain one or more seropositive animals in any age group (prevalence ratio = 1.12; 95% confidence interval 1.01 - 1.23; p = 0.026). Age specific seroprevalence increased from around 10% in heifers, to between 75% and 85% in cows aged 10 years. The average annual incidence risk for bovine pestivirus infection varied from 0.12 to 0.24 seroconversions per cattle year at risk, and did not vary with age. The overall crude seroprevalence adjusted for herd size was 45%. There was a wide range of seroprevalence recorded for each level of stocking intensity. CONCLUSIONS: This survey provides valuable baseline data on bovine pestivirus infection in Queensland cattle herds.     

A seroepidemiological survey of Leptospira bratislava infections in Danish sow herds

A total of 796 sows and gilts from 30 Danish sow herds were examined three times at intervals of 6 weeks for serum antibodies to Leptospira bratislava by the microscopic agglutination technique (MAT) test. The prevalence of seroreactors with positive titer values, 1:100, at the three successive tests were 2.7%, 2.5% and 2.9%; 4.5% of the animals were positive in at least one of the three tests, and 2.2% showed a greater than two-fold rise in titer between two consecutive samplings. Of the 30 herds, 21 (70%) had ever-positive within-herd prevalences in sows and gilts of 4–13%. The risk of a herd having one or more positive sow was positively associated with a herd size of > 141 sows, and distinct regional differences in the prevalence of positive herds were observed. The reproductive performance of the 21 herds with seroreactions was poorer than the performance of the nine herds without positive reactions concerning the variables: ‘days from weaning to last service’ (2.7 days more, P = 0.07), ‘percentage of sows returning to heat’ (4.0 percentage units more, P = 0.03), ‘services per farrowing’ (0.04 more, P = 0.04), ‘farrowing percentage’ (4.3 percentage units lower, P = 0.06), and ‘stillborn pigs per farrowing’ (0.16 more, P = 0.02). No association between the MAT serological status of the herd and the incidence of medical treatments of sows and gilts could be found. A high prevalence and low cumulative incidence of seroreactors was demonstrated in first-parity gilts, followed by a low prevalence and cumulative incidence from parity 2 to 3, and a high prevalence and cumulative incidence at the fifth parity.     

Seroprevalence of and risk factors for infectious bovine rhinotracheitis in beef cattle herds of Yucatan,Mexico

The main objective of this cross-sectional study was to estimate the seroprevalence of infectious bovine rhinotracheitis (IBR) in a population of non-vaccinated beef cattle in the livestock region of Yucatan, Mexico and to determine potential risk factors related to the seroprevalence. Also, we estimated the intraherd correlation (r_e) and design effect (D) of IBR seropositivity. Cattle were selected by two-stage cluster sampling. Blood samples were collected from 564 animals from 35 herds. Sera were tested for antibodies against IBR using the serum-neutralisation test. Information regarding the herd and each animal sampled were recorded through a personal interview with the farmer or farm manager. The data were analysed using fixed-effects logistic multiple regression. Thirty-four of the 35 herds had at least one seropositive animal. The animal true seroprevalence was 54.4%. Animals in large herds or in production had higher odds of seropositivity than those in small herds or growing. The r_e and D were 0.17 and 3.62, respectively.     

A retrospective study into characteristics associated with the seroprevalence of pseudorabies virus-infected breeding pigs in vaccinated herds in the southern Netherlands

Vaccination programs to eradicate pseudorabies virus (PRV) are being considered in several countries. Knowledge of factors that influence PRV transmission within vaccinated breeding herds may contribute to the success of these programs. A multivariate analysis of variance of the PRV-seroprevalence in sows in 209 herds (average seroprevalence 67.0% per herd) in the southern Netherlands revealed the following risk indicators: (1) presence of finishing pigs; (2) production type (producers of finishing piglets had a higher seroprevalence than producers of breeding stock); (3) vaccination of the sows during nursing (in comparison with vaccinating all sows simultaneously at 5 month intervals, or vaccination during the second half of gestation); (4) pig density in the municipality where the herd was located (seroprevalence increased with higher pig density); (5) herd size less than 100 sows; (6) average within-herd parity (seroprevalence increased with higher withinherd parity); (7) replacement pigs raised on the premises; (8) vaccine strain administered to the sows. Purchase policy (breeding pigs purchased between 10 weeks and 7 months of age, or use of home-bred gilts only) did not significantly contribute to the multivariate model.     

Effect of husbandry methods on seropositivity to African swine fever virus in Sardinian swine herds

Multiple logistic regression was used on serological data collected in the context of the Sardinian African swine fever (ASF) eradication program from pig farms in the province of Nuoro, Sardinia. The monthly percentage of ASFV-positive herds decreased significantly from October 1994 through March 1996 (P < 0.001). The farm-level risk of seropositivity to African swine fever virus (ASFV) was higher in free-range farms than in partial-confinement farms (odds ratios (OR) varied between 4.9 in October 1994, and 5.7 in March 1996, P < 0.001). The risk of infection for total-confinement farms was one-fifth of the risk for partial-confinement farms in October 1994 (OR = 0.2, P < 0.001), whereas in March 1996, the estimated OR was 0.57 and not significant (upper confidence limit = 1.1). The maintenance of ASFV in Sardinia was primarily associated with free-range pig farms. The natural logarithm of the number of pigs tested per visit in a farm was positively associated with the risk of herd seropositivity (OR = 2.6, P < 0.001).     

Use of an enzyme-linked immunosorbent assay for detection of infection with pseudorabies virus on a herd basis.

The use of an ELISA that can differentiate between swine infected with pseudorabies virus (PRV) and swine vaccinated with a specific PRV vaccine was evaluated on an individual and herd basis, and a system for interpreting ELISA results on a herd basis was developed. In 17 herds, recently introduced replacement gilts, seronegative for PRV, were vaccinated with a thymidine kinase- and glycoprotein X (gpX)-deleted vaccine. After vaccination, blood samples were collected from these gilts approximately every 1 to 2 months for up to 19 months. Serum samples were analyzed for antibodies to gpX antigen, using a commercially available ELISA kit according to the manufacturer's protocol. Herd status was determined as positive, suspect, or negative, according to the serum sample:negative control (S:N) values of the samples collected from the herd. From the 17 herds, 130 evaluations were performed. On 49 (38%) of the 130 herd evaluations, 1 or more gilts had suspect test results. Additional testing was required in 19 (39%) of these 49 herd evaluations to determine the PRV infection status of the herd. Status of herds having gilts with suspect results and no positive results was usually negative after retesting. Herds having gilts with positive results were unlikely to have negative status after retesting.     

Estimating sample sizes for a two‐stage sampling survey of seroprevalence of pseudorabies virus (PRV)‐infected swine at a regional level in the Netherlands

Summary

In the European Union, vaccination campaigns against Pseudorabies virus (PRV) in swine have been started to eradicate PRV. Specific sampling designs are needed to monitor PRV seroprevalence at a regional level. This paper demonstrates how sampling theory can be applied to design a disease seroprevalence survey, using PRV as an example. In the spring of 1994, the four regions in the Netherlands covered by the regional Animal Health Services were monitored with respect to PRV seroprevalence. Per region, blood samples from approximately 1400 herds, with two animals per herd, were collected. The sampling design accounted for stratification by fattening pig and sow population within each region. The regional PRV seroprevalence of swine in the Southern region was the highest (24.9%), closely followed by the PRV seroprevalence of swine in the Eastern region (20.5%). These regions have the highest density of swine in the Netherlands. The PRV seroprevalence in the Western and Central region (11.7%) was about half of the seroprevalence in the Southern and Eastern regions; the lowest regional PRV seroprevalence was observed in the Northern region (3.5%). The Northern part also has the lowest pig density. The PRV seroprevalence was approximately two times higher in sows than in fattening pigs.