Population dynamics of swine influenza virus in farrow-to-finish and specialised finishing herds in the Netherlands.

Influenza virus infections with subtypes H1N1, H3N2 and H1N2 are very common in domestic pigs in Europe. Data on possible differences of population dynamics in finishing pigs in farrow-to-finish herds and in specialised finishing herds are, however, scarce. The presence of sows and weaned piglets on the same premises may, however, affect the exposure of finishing pigs to influenza viruses. In a longitudinal study on 14 farrow-to-finish herds and 15 finishing herds, groups of pigs were followed by repeatedly testing the same animals for antibodies against all three influenza virus subtypes (H1N1, H3N2 and H1N2). At the end of the finishing period, the seroprevalences in farrow-to-finish and specialised finishing herds were 44.3% and 62.0%, respectively for H1N1, 6.6% and 19.3%, respectively for H3N2, and 57.2% and 25.6%, respectively for H1N2. For all three subtypes, the incidence of influenza virus infections was highest at the beginning of the finishing period in farrow-to-finish herds, while the incidence of influenza virus infections was highest at the end of the finishing period in finishing herds. Respiratory disease, probably related to the influenza infections, was observed in five of these herds only, but also occurred at the beginning of the finishing period in farrow-to-finish herds and at the end of the finishing period in finishing herds. The observed differences of population dynamics of influenza virus may affect choice and timing of intervention measures.

Effect of maternally derived antibodies on the clinical signs and immune response in pigs after primary and secondary infection with an influenza H1N1 virus.

The aim of this study was to determine the role of maternally derived antibodies (MDA) against an influenza H1N1 virus in the clinical protection of piglets and especially their effect on the development of the active immunity after an infection with a homologous influenza H1N1 virus. Twenty piglets with MDA and 10 piglets without MDA were housed together and inoculated twice with influenza H1N1 virus, at 7 and 15 weeks of age. Nine piglets without MDA were added to these groups at 12 weeks of age to be inoculated at 15 weeks of age only. Clinical signs, body temperature, growth performance, virus excretion, antibody responses, and influenza-specific T-cell response were monitored. It was shown that MDA protect piglets against the clinical consequences of a primary influenza infection, but that this protection is not complete. A short but significant rise in body temperature was observed and growth seemed to be inhibited due to the infection. Piglets with MDA shed virus for a longer period after an infection than piglets without MDA. Piglets with and without MDA were protected against the clinical consequences of a secondary infection. However, both after primary and secondary infection significant differences in immune responses were observed that indicated that pigs with MDA developed a weaker immunity than pigs without MDA. Furthermore, overall growth performances from weaning to slaughter show a trend in favour of pigs without maternal antibodies, compared to pigs with maternal antibodies, mainly caused by a significant better performance in the second half of the finishing period. The results of this study provide us insight in the role of MDA in clinical protection and their influence on active immunity after an influenza virus infection of pigs. Furthermore, it leads us to the discussion about the profitability of massive sow herd vaccinations in an attempt to increase MDA levels in piglets, taking into account the overall performance of these piglets and the possible effects on antigenic drift.

Estimating the incidence of influenza-virus infections in Dutch weaned piglets using blood samples from a cross-sectional study.

A cross-sectional study was carried out on 32 Dutch breeding herds to estimate the incidence of influenza-virus infections in piglets before the start of the finishing period, at the age of approximately 10 weeks. Longitudinal studies on two herds (8 and 10 litters, respectively) were done to obtain an average decay function for maternal antibodies.Each participating farm in the cross-sectional study was visited twice within 5 months; each time, blood samples were taken randomly from one compartment (a separate room with separate air flow) of 4-5-week-old piglets and one compartment of 8-9-week-old piglets. These blood samples (a total of 2598; 16-23 per compartment, depending on its size) were tested in a haemagglutination inhibition test for antibodies against influenza-virus subtypes H1 and H3. Samples from 8-9-week-old piglets from the first sampling period (n=660) were also tested in an IgM ELISA. For each individual herd and each influenza-virus subtype separately, the decay function derived from the longitudinal studies was used to calculate an expected seroprevalence in 8-9-week-old piglets, which was then compared to the observed seroprevalence. Depending on subtype and sampling period, between 10 and 15 of the 32 herds were suspected of virus circulation during the weaning period because the observed seroprevalence was significantly higher than the expected seroprevalence (P<0.05). In the first sampling period the IgM ELISA confirmed six of these outbreaks. However, due to the small window of detection of the IgM ELISA (compared to the length of the weaning period), it will always underestimate the number of infections. Infections in the first half of the weaning period will no longer be detectable because IgM antibodies have already disappeared. In individual pigs, an incidence of 16-17% was estimated for each subtype over a 4-week period between the age of 4-5 and 8-9 weeks. For each influenza subtype, 80% of the piglets will enter the finishing facilities without antibodies or with decaying maternal antibodies. These piglets may be susceptible to an infection with influenza virus.

A longitudinal study of Salmonella enterica infections in high-and low-seroprevalence finishing swine herds in The Netherlands.

The purpose of this investigation was to study the incidence and course of Salmonella infections in finishing pig herds in order to asses the stability of a given Salmonella herd status. Five low- and 7 high-seroprevalence herds were followed for seven sampling rounds. Each round, blood and faecal samples were tested in an indirect ELISA and by bacteriological culturing, respectively. In high-seroprevalence herds a positive Salmonella status was an indication of a long-term problem and the status was relatively stable over time. The herds experiencing clinical salmonellosis were not necessarily the herds with the highest seroprevalence. It is possible to deliver sero-negative finishers to the slaughterhouse, even though these pigs were seropositive as growers. In three out of five low-prevalence herds, major infection incidents occurred, indicating that changes in the Salmonella status should be anticipated. Low-prevalence herds can remain negative over a longer period of time as a result feeding a complete liquid feed containing fermented by-products.

Administration of acidified drinking water to finishing pigs in order to prevent Salmonella infections.

The aim of the study was to test whether acidified drinking water, with two millilitres of an acid mixture per litre, was able to reduce the number of Salmonella infections in finishing pig herds. In each compartment, half of the pens were supplied with acidified water and the other pens served as negative control. In three herds the required dose was not applied to the pigs as a result of various practical problems. In another herd, all pigs remained seronegative throughout the study. Analysis of the remaining three herds showed a large and significant treatment effect in one herd (P<0.001). As a result of the small number of observations and the overall lower seroprevalence in the control groups, the other two herds only showed a statistical trend to a treatment effect (0.10

Salmonella seroprevalence at the population and herd level in pigs in The Netherlands.

The aim of this study was to provide baseline data on the population and herd Salmonella seroprevalence in sows and finishers. For the population estimates in 1996 and 1999 and the herd prevalences for sows and gilts, blood samples from swine vesicular disease (SVD) and pseudorabies monitoring programmes were used and tested in an indirect Salmonella enzyme-linked immunosorbent assay (ELISA). The herd prevalence for finishers was determined using blood samples collected at two slaughterhouses. The population prevalence for finishers in 1996 and 1999 was 23.7 and 24.5%, respectively, and for sows 40.5 and 60.4%, respectively. The prevalence in free range (FR) finishers was significantly higher (44.6%) than in intensively housed finishers in 1999, identifying a hazard group for possible extra pork and pork product contamination. Of 406 finishing herds, 9% were completely seronegative for Salmonella (cut-off OD%>10). Of these 406 finishing herds, 69.7% had Salmonella-status I (low prevalence), 21.7% status II (moderate prevalence) and 8.6% status III (high prevalence) (cut-off OD%>40). In 46 multiplying sow herds, 20 breeding sow herds and 20 matching replacement gilt herds, the average herd prevalences were 54, 44.4 and 19.3%, respectively. Two gilt herds were completely seronegative. The prevalence in the gilt herds was never higher than in the matching breeding sow herds. Agreement on methodology and calibration of ELISA tests would make these results comparable between countries and is a prerequisite for a co-ordinated and integrated program to reduce Salmonella in pork in the European Union.

Herd level husbandry factors associated with the serological Salmonella prevalence in finishing pig herds in The Netherlands.

A national program to reduce Salmonella in pork and pork products should include monitoring and intervention at farm level. To develop an adequate intervention strategy at farm level, risk factors for Salmonella infections in finishing pigs have to be determined. In this study, blood samples were collected randomly at two slaughterhouses from slaughter pigs. Samples were tested by the Dutch Salmonella ELISA, based on the O-antigens 1, 4, 5, 6, 7 and 12, using a cut-off of OD%=10. This ELISA has been calibrated against the Danish ELISA to give comparable results. Workers from herds from which at least forty blood samples had been collected, were asked to participate in a questionnaire. In total, 353 questionnaires were obtained and analysed. Significant risk factors associated with the proportion of seropositive samples were identified by multiple linear logistic regression. The feeding of a complete liquid feed containing fermented by-products and the omission of disinfection after pressure washing a compartment as part of an all-in/all-out procedure, were both associated with a lower Salmonella seroprevalence. A small to moderate herd size (<800 finishing pigs), a previous diagnosis of clinical Salmonella infection in the herd, the use of tylosin as an antimicrobial growth promoter in finishing feed, or herds which had more than 16% of the livers of their pigs condemned at the slaughterhouse as a result of white spots were associated with a higher Salmonella seroprevalence. Hypothetical intervention strategies based on these risk factors can be studied for their effect on the Salmonella seroprevalence and practical applicability in field studies.

Experimental quantification of the transmission of Sarcoptes scabiei var. suis among finishing pigs.

In this study, the rate of S. scabiei var. suis transmission among finishing pigs was quantified in a contact transmission experiment. Forty piglets originating from a mange free farrow-to-finish herd were randomly allocated to three groups and one S. scabiei var. suis infested finishing pig was subsequently added to each of these groups. After 35 days, the three seeder pigs were removed from the groups and the remaining 40 pigs were re-allocated to five pens. Ear scrapings, to be examined for mites, were collected from each pig on days 1, 14, 28, 42, 56, and 84 of the experiment. Blood samples, to be tested for antibodies against S. scabiei, were collected from each pig on days 0, 14, 28, 42, 56, 70, 84 and 112 after the introduction of the seeder pigs. From the results of the ear scrapings and the blood samples the number of susceptible (not infested) and infested pigs was derived at the time of each sample collection and the number of new infestations in the intervals between the sample collections. From these data the infestation rate parameter beta (average number of new infestations per infested pig per day) was estimated by use of a Generalised Linear Model (GLM) and accordingly, beta was estimated at 0.056 (95% CI: 0.037-0.085) infestations per infested pig per day.Next, by use of beta, the transmission of S. scabiei was simulated in a population of 100 finishing pigs for 100 days after the introduction of a single infested pig. For this purpose, 500 simulations were done. The 90% confidence interval of the number of infested pigs at day 100 ranged from 12 to 88 (median: 63). It was concluded that transmission of S. scabiei among finishing pigs is slow. Due to the presumed lower contact rate between sows as compared to finishing pigs, it is anticipated that transmission of S. scabiei among sows will even be slower than among finishers These findings are of particular interest for the development of surveillance programmes for S. scabiei free herds.

Production performance and pruritic behaviour of pigs naturally infected by Sarcoptes scabiei var. suis in a contact transmission experiment.

Reports on the effects of mange on the production performance of pigs are conflicting. So far, studies have used experimental infections, by depositing encrusted lesions from chronically infected pigs into the ears of experimental pigs. However, this is a poor representation of what happens under natural field conditions. The purpose of our study was to quantify the effects of sarcoptic mange on production performance and pruritus in pigs that were infected by contact with S. scabiei var. suis-infected pigs. A total of 80 piglets were matched by sex and weight and randomly divided between experimental and control compartments. In the experimental compartment, each of three naturally S. scabiei var. suis-infested pigs were randomly allocated to three pens with 13 susceptible pigs each. From day 0 to 35, the growth performance of pigs in the experimental compartment was significantly (P=0.04) worse (35 g/d) than of pigs in the control compartment. From day 35 to 112, there was a statistical trend (P=0.10) that the growth performance of pigs in the experimental compartment was lower (50 g/d) than that of pigs in the control compartment. For the complete fattening period (0-112 or more days), the growth performance of pigs in the experimental compartment was significantly (P=0.05) worse (41 g/d) than that of pigs in the control compartment. Mean feed conversion ratio (kg feed per kg gain) was 2% higher in the experimental compartment compared with the control compartment. Pigs in the experimental compartment had a nine times (95% CI: 2 - 44) higher chance of showing pruritic behaviour than pigs in the control compartment.

[Effect of the Stellamune Mycoplasma vaccine on growth, energy conversion, death, and medication use in fattening pigs on a pig farm chronically infected with Mycoplasma hyopneumoniae]. / Het effect van het Stellamune Mycoplasmavaccin op groei, energiewaardeconversie, sterfte en medicijngebruik van vleesvarkens op een chronisch met Mycoplasma hyopneumoniae besmet varkensbedrijf.

The effect of Stellamune Mycoplasma vaccine, administered to piglets aged 2-15 days and then 13-15 days later, on daily weight gain, energy conversion, and use of medication was examined in fattening pigs on a chronically Mycoplasma hyopneumoniae infected pig farm. Half of the piglets were vaccinated and the other half acted as controls. In the study design, half of the pens in the fattening unit were allocated to vaccinated pigs; the other half to non-vaccinated pigs, pen was the experimental unit. In the fattening pens sows and castrated boars were separated. The study consisted of a total of 37 pens with vaccinated, and 37 pens with non-vaccinated pigs in 12 different compartments within the pig herd. In the finishing period, mean growth performance and mean energy conversion (EV/kg) of vaccinated animals was 65 grams/day higher and 0.07 EV/kg lower than in control pigs. Furthermore, the incidence of individual curative medication against respiratory problems was more than 4 times higher in control pigs than in vaccinated pigs. There was a tendency for a higher number of group medications against respiratory problems in control pigs than in vaccinated pigs. It is concluded that, in this herd, vaccination against M. hyopneumoniae was successful from an economic point of view.

Aujeszky's disease virus eradication campaign successfully heading for last stage in The Netherlands.

The Aujeszky's disease virus (ADV) eradication campaign in The Netherlands is based on compulsory vaccination and certification of ADV-free herds and consists of three stages: reduction of transmission of ADV, tracing and elimination of remaining sources of ADV, and, in the last stage prohibition of vaccination. This paper summarizes the progress made in the first two stages of the eradication. Between 1994-1999, ADV seroprevalence decreased sharply in all four regions in The Netherlands, both in the sow and the finishing pig population. In the sow population, the mean ADV prevalence in the northern, eastern and western regions decreased to less than 0.5% in the first quarter of 1998, and to approximately 1% in the southern region. For the finishing pig population, the mean ADV prevalence decreased to less than 0.6% in all four regions in the first quarter of 1998. The proportion of sow herds that were certified ADV-free increased steadily from approximately 40% in the last quarter of 1996 to 96% in the first quarter of 1999. The proportion of finishing pig herds that were certified ADV-free showed the same spectacular increase as in the sow population: from approximately 14% in the last quarter of 1996 to 90% in the first quarter of 1999. It is anticipated that the last stage of the ADV eradication campaign, stopping vaccination, will be achieved very soon. Monitoring wild boars for ADV between 1994 and 1999 indicated that ADV is uncommon within the wild boar population. Therefore, it seems that ADV infection in the wild boar population is not an important reservoir in The Netherlands.

Validation of ELISAs for the detection of antibodies to Sarcoptes scabiei in pigs.

An Enzyme-linked ImmunoSorbent Assay (ELISA) was developed for the detection of antibodies to Sarcoptes scabiei. This 'Animal Health Service'-ELISA (AHS-ELISA) was compared with a commercial test (Checkit(R) Sarcoptest) using experimental and field sera. The experimental study was a contact infestation experiment. Eighty piglets were randomly divided between the experimental and control group. After introduction of three Sarcoptes scabiei var. suis infested pigs in the experimental group, both groups were monitored by determining scratching indices, taking ear scrapings and blood samples in Weeks 0, 2, 4, 6, 8, 12 and 16. Four pigs in the control group were immunised with either Dermatophagoides pteronyssinus (Dp) antigens (n=2), or Acarus siro (As) antigens (n=2). In the control group all (non-immunised) pigs were negative in all tests. In the experimental group only slightly elevated scratching indices were observed, with a maximum in Week 8. After 2 weeks for the first time an ear scraping was positive (2.5%). In Week 8 the highest number of positive ear scrapings were found (25.0%). Positive results in the Sarcoptest were first obtained in Week 12 (10.5% positive), while eventually 29.0% of the finishing pigs were positive after 16 weeks. The AHS-ELISA first detected a serological response after 6 weeks (5. 0% positives), increasing until after 16 weeks a large proportion (74.2%) of the finishing pigs were seropositive, making the AHS-ELISA the most sensitive test. In the AHS-ELISA one As-immunised pig remained seronegative, but the other hyper-immunised pigs crossreacted. In the Sarcoptest, only Dp-immunised pigs had elevated Optical Densities (OD's) albeit below the cut-off level. Although hyper-immunisation is not a representation of field conditions, it cannot be excluded that the AHS-ELISA is not 100% specific.Field samples were taken from 20 sows in 30 herds, classified as mange-free, suspect, or infested. On a herd level there was high agreement among the ELISAs. Both serological tests were suitable to distinguish mange-free herds from infested herds. In one infested herd the decline of maternal antibody in piglets was studied by sampling 40 piglets from 20 different litters. The lowest average OD using the AHS-ELISA was found at 5 weeks of age, followed by a significant increase at 7 weeks. The average OD with the Sarcoptest was at a minimum level at 3 weeks, but no increase was found later. For screening of herds, interference of maternal antibodies is avoided by sampling at an age of 7 weeks or older.

Salmonella infections in finishing pigs in The Netherlands: bacteriological herd prevalence, serogroup and antibiotic resistance of isolates and risk factors for infection.

Salmonellae are wide spread in man and animals world wide and are of increasing significance as causative agents of foodborne diseases in man. The European Union, national authorities and the pig industry are therefore more and more interested in the Salmonella status of the pig population. The aim of this study was to estimate the bacteriological prevalence of Salmonella in finishing pig herds, the serogroup and the resistance to antibiotics of the isolated Salmonellae and a preliminary risk analysis of factors associated with infection. For this, 317 finishing pig herds were randomly selected from a database containing 1500 herds in the southern part of the Netherlands. In each herd 24 samples of fresh faeces were collected from two compartments with pigs close to market weight. Per compartment 12 samples of faeces were pooled into one pooled sample. Pooled samples were cultured in duplicate. Salmonella spp. were recovered from 71 out of 306 herds (23%) in which two compartments could be sampled. A total of 108 isolated Salmonella's were serotyped: 71 serogroup B, 3 serogroup C1, 6 serogroup C2, 22 serogroup D1, and 6 isolates neither serogroup B, C or D1. Of a total of 115 Salmonella isolates tested, none were resistant to colistin, enrofloxacin, flumequin or gentamicin. Automated liquid feeding of by-products, and membership of an Integrated Quality Control (IQC) production group were associated with a decreased risk of infection, while use of trough feeding was associated with an increased risk of infection. It is necessary to test these presumed risk factors in intervention studies to evaluate their potency to reduce the Salmonella prevalence in finishing pigs and thereby reduce the risk of Salmonellosis in people consuming pork.

Survey of infectious agents involved in acute respiratory disease in finishing pigs.

Outbreaks of respiratory disease constitute a major health problem in herds of finishing pigs and their aetiology often remains unclear. In this study, 16 outbreaks of respiratory disease with acute clinical signs in finishing pigs were investigated to determine which infectious agents were involved. From each herd four diseased and two clinically healthy pigs were examined pathologically and for the presence of viruses, bacteria and mycoplasmas. In addition, paired blood samples from 10 groupmates of the diseased pigs were tested for antibodies against commonly known causal agents of respiratory disease. A clear diagnosis was possible in 12 of the 16 outbreaks. Seven were due to an infection with influenza virus and five were due to an infection with Actinobacillus pleuropneumoniae. A combination of influenza virus and A pleuropneumoniae may have caused one other outbreak, but no clear cause could be established for the other three outbreaks.

Effect of hammer mill screen size and addition of fibre or S-methylmethionine-sulphonium chloride to the diet on the occurrence of oesophagogastric lesions in fattening pigs.

Four groups of about 86 pigs from a common source were fed a grower diet from 25 kg to 45 kg liveweight, and then from 45 to 107 kg liveweight they were offered one of four diets ad libitum: A) normal commercial feed, ground through a 3 mm screen (the control diet), B) the same diet ground through a 6 mm screen, C) the control diet to which lucerne meal was added before the diet was ground to increase its crude fibre content, and D) the control diet to which was added 400 ppm S-methylmethionine-sulphonium chloride (MMSC). All the diets were pelleted. Approximately 21 per cent of the animals fed the control diet had severe oesophagogastric erosions and/or ulcers after slaughter. The addition of 400 ppm MMSC decreased (P = 0.066) the proportion of severe oesophagogastric erosions and/or ulcers by about 50 per cent compared with the control diet. The diet with the higher crude fibre content (but finely ground) did not have a significant effect on the proportion of severe oesophagogastric erosions and/or ulcers. There was a tendency for the pigs fed the diet ground through a 6 mm screen instead of a 3 mm screen, to have fewer severe oesophagogastric erosions and/or ulcers. However, there were only small differences between the particle size distribution obtained from the wet sieve analysis of the two diets. As a result, the observed tendency for a decrease in the proportion of severe oesophagogastric erosions and/or ulcers in pigs fed the diet milled through the larger screen size was of questionable significance.(ABSTRACT TRUNCATED AT 250 WORDS)

An observational study into herd-level risk indicators of return to oestrus more than five days after insemination in sow herds.

In a study population of 90 sow herds in the southern Netherlands, the occurrence of sows returning to oestrus more than five days after insemination (SRO) was investigated. In these herds information with regard to the herd system, herd size, and breeding management was recorded. The occurrence of SRO was recorded with the on-farm computer management system CBK-plus. The incidence of SRO was defined as the average number of gilts or sows returning to oestrus more than five days after insemination per 100 first inseminations. The average incidence of SRO in the study population was 18.3 per 100 first inseminations. Multivariate analysis resulted in a model with a significant effect of herd operation (herds with only sows had a lower incidence of SRO than herds with sows and finishing pigs), breeding strategy (herds with two or more inseminations per oestrus in a sow had a lower incidence of SRO than herds with one insemination per oestrus) and percentage of sows artificially inseminated (a higher percentage of artificial insemination was associated with a higher incidence of SRO).

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

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.

Prevalence and development of antibodies neutralizing the haemolysin and cytotoxin of Actinobacillus pleuropneumoniae in three infected pig herds.

Sero-epidemiological studies were carried out in pigs aged 1 to 24 weeks in three herds in which Actinobacillus pleuropneumoniae was endemic. The sera were tested in the complement-fixation test and for their ability to neutralize the haemolytic and cytotoxic activities of the A. pleuropneumoniae serotypes isolated from the herds. Almost all (98%) sera from 1-week-old piglets neutralized the haemolytic and cytotoxic activities but only 21% fixed complement. At the end of the finishing period, most pigs (82%) had sera that neutralized haemolytic and cytotoxic activities and only 22% fixed complement. In longitudinal studies the neutralization titres decreased during the first 12-13 weeks of age. Thereafter, 75% of the pigs had increased titres in the haemolysin- and cytotoxin-neutralization tests and only 5% of the pigs had increased titres in the complement-fixation test. In none of these pigs were clinical signs of pleuropneumonia seen. Thus in these endemically infected herds the prevalence of complement-fixing antibodies was low, whereas the prevalence of neutralizing antibodies was high. The fact that serum-neutralizing titres are low in 12-week-old pigs might be the reason that pigs of this age are the most vulnerable to the disease.

Reduction of the prevalence of pseudorabies virus-infected breeding pigs by use of intensive regional vaccination.

The influence of an intensive vaccination program on prevalence of pseudorabies virus (PRV)-infected breeding pigs was examined in a region where the disease was enzootic. In a 2-year period, significantly (P < 0.005) greater decrease in the prevalence of PRV-infected breeding pigs was observed in herds participating in the regional vaccination program and in herds outside the trial region that intensified PRV vaccination during the study than in herds that applied routine PRV control measures. In the regional vaccination program, introduction of breeding stock from outside the area was significantly (P < 0.05) associated with higher prevalence of PRV-infected pigs at the end of the study. These results indicate that transmission of PRV can be markedly reduced by use of an accurately applied intensive vaccination program.