Factors associated with spread of pseudorabies virus among breeding swine in quarantined herds.

Knowledge of the factors that place susceptible gilts at highest risk of pseudorabies virus (PRV) infection in a quarantined herd is crucial to reduce spread of PRV within the herd. Cohorts of PRV seronegative gilts were monitored in 17 herds that were endemically infected with PRV to determine the location of breeding females at the time of infection with PRV and identify herd characteristics and management and housing factors that may influence spread of PRV in the breeding section of swine herds endemically infected with PRV. Blood samples were collected every 1 to 2 months for an average of 13.6 months. In addition, blood was collected from a representative sample of finishing pigs (greater than or equal to 20 weeks old) 3 times per year to determine their serologic PRV status. Incidence rates and relative risks of PRV infection were estimated for 4 areas of the breeding section: gestation barn, gilt pool, farrowing room, and breeding area. Overall, 28, 11, 8, and 2 females became infected with PRV in each of these areas, respectively. The greater number of females infected in the gestation barns, compared with the number of females infected in other locations, is probably a consequence of being at risk for a longer period rather than of a higher incidence rate. Herd size, common housing for gilts in the gilt pool and sows, and serologic pattern of PRV infection in finishing pigs were associated with the detection of spread of PRV in the breeding section of the 17 herds.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Spread of pseudorabies virus among breeding swine in quarantined herds.

In theory, pseudorabies virus (PRV) may be eliminated from any size of breeding herd by phased test and removal if replacement gilts are not infected with PRV, culling decisions are partially based on PRV status, and the cull rate is higher than the incidence rate of PRV. Annual cull rates are commonly at least 50%, but little information exists on the incidence of PRV within enzootically infected swine herds. The purpose of this study was to develop a method by which spread of PRV could be detected among breeding swine within enzootically infected herds and to determine the incidence of PRV infection in these herds. Data were collected from 17 herds that were quarantined for PRV and ranged in size from 120 to 1,100 sows. At each herd, within the first 5 days of introduction, a group of approximately 30 replacement gilts was identified, vaccinated with a glycoprotein X-deleted PRV vaccine, and blood sample was collected. The owner of 1 herd had a nonvaccinated breeding herd and elected to leave incoming gilts nonvaccinated. After vaccination, blood samples were collected every 1 to 2 months for an average of 13.6 months. Serum samples from vaccinated gilts were tested for antiglycoprotein X antibodies by a specific differential ELISA. Samples from nonvaccinated gilts were evaluated by serum neutralization test. Product-limit method was used to estimate the probability of not becoming infected with PRV. Spread was detected in 7 of 8 herds that had more than 400 sows and in 2 of 9 herds that had less than 400 sows.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prevalence of pseudorabies virus infection and associated infections in six large swine herds in Illinois.

Sera were collected from 6 large farrow-to-finish swine herds infected with pseudorabies virus (PRV) in Illinois. All herds were participating in the Large Herd Cleanup Study, a USDA-initiated project to evaluate the feasibility of eradicating pseudorabies from large farms (greater than 400 sows) by use of a combination of vaccination and management changes. Herd size ranged between 425 and 1,500 breeding females. Between April and July 1990, sera for measurement of PRV antibodies were obtained from 113 to 156 sows and 112 to 162 finishing pigs (body weight greater than 70 kg)/herd. Duplicate sera from 30 sows and 30 market-weight pigs/herd were obtained for measurement of serum antibodies to the following associated organisms: swine influenza virus, transmissible gastroenteritis virus, encephalomyocarditis virus, Actinobacillus pleuropneumoniae, Eperythrozoon suis, and 6 serovars of Leptospira interrogans. Prevalence of PRV antibodies attributable to field virus infection ranged between 53.8 and 100% for sows and between 0.7 and 97.3% for finishing pigs, as determined by the appropriate differential test for the vaccine being used on each farm. In only 1 herd, PRV seroprevalence was increased with higher sow parity. For associated infections, the risk of seropositivity attributable to PRV was not significant (for most infections) on all farms and varied among farms. Thus, pseudorabies did not appear, in general, to increase susceptibility to infection with other disease agents.     

Factors associated with time under quarantine for swine herds in the voluntary phase of the Illinois pseudorabies eradication program

The association of herd characteristics and intervention strategies with time under quarantine was evaluated for 163 farrow-to-finish swine herds enrolled in the voluntary phase (1986–1987) of the pseudorabies virus (PRV) eradication program in the state of Illinois (USA). Vaccination was the intervention strategy used most widely (69% of herds), particularly in larger herds. Depopulation was used primarily when PRV seroprevalence was high, and test-and-removal when seroprevalence was low. Approximately 50% of the herds were released from quarantine within 3 years of developing a herd clean-up plan.

Multiple regression analysis using the Cox proportional hazards model indicated the following. Vaccination had a strong association with a longer time until release from quarantine (P<0.001). This is attributed to the lack of a vaccine differential test during this time, which made diagnosis of natural infection more difficult. Offspring segragation was associated with a longer time under quarantine (all herds: P=0.05; non-vaccinated herds: P=0.004). Delay in implementation of a herd clean-up plan was also associated with longer time under quarantine (all herds: P=0.012; non-vaccinated herds: P<0.001). Herds with higher seroprevalence at the time of agreement to a herd plan required a longer time under quarantine (all herds: P<0.001). This result was apparent for non-vaccinated herds (P=0.001), and thus is not merely a consequence of vaccination. Herds in areas with a high geographic density of quarantined herds required a longer time before release from quarantine (all herds: P=0.003), although this trend was not apparent for non-vaccinated herds (P=0.39). After taking PRV seroprevalence into account, there was no apparent association of time under quarantine with sow herd size (all herds: P=0.057; non-vaccinated herds: P=0.81) or confinement housing (all herds: P=0.19; non-vaccinated herds: P=0.91).     

Epizootic of pseudorabies among ten swine herds.

An epizootic of pseudorabies in 10 swine herds located near Greensburg in Decatur County, Indiana was investigated. Records of Decatur County swine herds previously quarantined because of pseudorabies were obtained. For the 1988 epizootic, clinical findings were considered, location of each of the 10 swine herds in relation to the other herds was determined, and potential fomites were considered. Meteorologic data prior to, during, and after the epizootic were obtained, correlated, and analyzed. A source-receptor model, based on wind direction and speed data, was used to determine whether weather conditions in the Greensburg area enhanced the potential for aerosol spread of pseudorabies virus between herds. On the basis of lack of other modes of spread of pseudorabies and on meteorologic and source-receptor data, aerosol transmission of pseudorabies virus between the 10 herds was probable.     

Factors affecting the geographic distribution of pseudorabies (Aujeszky's disease) virus infection among swine herds in Illinois

Data on the geographic distribution of swine herds tested for pseudorabies virus (PRV) in the state of Illinois (USA) were analyzed to determine whether the prevalence of PRV-infected herds was clustered geographically at the county level. Second-order analysis of spatial dependence indicated there was a spatial clustering of counties of high PRV prevalence rates and that this clustering was greater than the observed clustering of counties with a large number of swine herds. The clustering of county PRV prevalence rates was most apparent within a radius of 120 km (on the average, approximately two couties away). The association of county PRV prevalence rates with average herd size, geographic density of swine herds in the country and regional (within 120 km) density of PRV-infected herds was analyzed using multiple linear regression. The primary factor affecting county PRV prevalence rates was the regional PRV density, which interacted with the other model variables. For counties with a low regional density of PRV infection, county PRV prevalence rates charged little with a change in county herd density or average herd size. In contrast, for counties with a high regional density of PRV infection, PRV prevalence within a county increased with increasing average herd size and increasing geographic density of swine herds in the county. The results of the current and previous studies implicate an important role for the geographic proximity of infected herds in the spread of PRV among swine herds.     

Embryo transfer for conserving valuable genetic material from swine herds with pseudorabies

Embryo transfer was used to conserve genetic material from 2 swine herds seropositive for pseudorabies virus (PRV). Embryos (n = 805) were recovered from 38 PRV-seropositive Duroc sows in Iowa and, after 4 to 10 hours' culture and shipment to Illinois, were transferred to 34 recipients from a herd seronegative for PRV. All recipients remained seronegative for PRV, and 22 of the recipients farrowed 208 pigs (189 alive) that also were seronegative for PRV. There was no evidence of PRV in the embryo recovery medium or in the uterine and oviductal cells recovered with the embryos. Transfer of morulae resulted in higher (P less than 0.02) farrowing rates than did transfer of 4- to 8-cell embryos, but litter size was not affected.     

Vaccination of swine with thymidine kinase-deficient mutants of pseudorabies virus

A mutant of pseudorabies virus (PRV) deficient in thymidine kinase (TK-) activity was isolated and characterized. The mutant grew well in cell culture and did not revert to the thymidine kinase-positive phenotype. The PRV-TK- was not virulent when inoculated intranasally into 3-to 4-week-old pigs and could not be reactivated from the ganglia of these pigs by explantation and cocultivation with susceptible cells several weeks after virus inoculation. Pigs that had been exposed to PRV-TK- were immune to challenge exposure with a virulent strain of PRV. Furthermore, the challenge virus was not recovered from the ganglia of most of these pigs, indicating that colonization of the ganglia by a super-infecting virulent PRV strain was considerably reduced by vaccination.     

猪伪狂犬病病毒的分离与鉴定

猪伪狂犬病是由疱疹病毒科的猪疱疹病毒1型病毒引起的一种急性传染病，多数动物均可感染，猪是本病的主要宿主和带毒者。本病主要以成年妊娠母猪流产、死产、木乃伊胎、新生仔猪急性死亡及3周龄以内的仔猪表现神经症状为主要特征。近年来我国也相继有本病发生的报道。1997年10月黑龙江省牡丹江地区某养猪场从外地引进种母猪，回来后直接放入后备猪舍与其他后备母猪混养，5日后同舍母猪出现厌食、发热、精神沉郁，有的呕吐、咳嗽。     

Serologic status of pseudorabies virus in growing-finishing pigs in quarantined herds

It has been reported that pseudorabies virus (PRV) stops spreading within growing-finishing sections of a large percentage of infected farrow-to-finish herds. This study was designed to follow the PRV status of growing-finishing pigs in a sample of infected herds. Fifteen infected herds were selected, of which 11 had seropositive finishing pigs and 4 had seronegative finishing pigs. These herds were visited quarterly for one year, and a cross section of growing-finishing pigs was tested for the presence of anti-PRV antibodies. The 4 herds that initially were seronegative remained seronegative, whereas of the 11 herds initially seropositive, 4 remained seropositive, 4 became seronegative, and 3 became temporarily seronegative before becoming seropositive again. Three characteristics serologic profiles were observed: one indicating continued viral spread; one indicating no spread for at least the preceding 3 months; and one indicating that PRV spread had recently ceased in this section of the herd. Results of our study indicated that periodic monitoring of a cross section of the growing-finishing pigs for their PRV serologic status was valuable for determining whether PRV was actively spreading in this section of the herd.     

Identification of pseudorabies virus-infected swine herds by evaluating the serostatus of boars or finishing pigs

Data were collected from 39 Minnesota swine farms quarantined for pseudorabies virus (PRV) infection. Each herd was serologically evaluated for antibodies to PRV in the sows, boars, and finishing pigs. To identify PRV-seropositive swine herds, the Kappa statistic was used to estimate the effectiveness of evaluating the PRV serostatus of boars or of finishing pigs. Using the serostatus of all herd boars, the sensitivity (with 95% confidence interval) of identifying PRV-infected herds was 58 +/- 22%, and the specificity was 100 +/- 0%; Kappa statistic was 0.55. Using the serostatus of a representative sample of finishing pigs, the sensitivity of identifying PRV-infected herds (with 95% confidence interval) was 63 +/- 22%, and specificity was 87 +/- 23%; Kappa statistic was 0.40. The PRV serostatus of herd boars or of a representative sample of finishing pigs did not accurately reflect the PRV serostatus of the herd.     

Latent infection and subsequent reactivation of pseudorabies virus in swine exposed to pseudorabies virus while nursing immune dams

The ability of pseudorabies virus (PRV) to infect and establish latency in pigs with passively acquired (maternal) antibody for PRV was tested by exposing such pigs to the virus and subsequently attempting to reactivate latent virus by administering large doses of dexamethasone. Pigs of each of 4 litters that had nursed gilts with relatively high (512, gilts 1 and 2), moderate (32, gilt 3), and no (less than 2, gilt 4) serum titers of virus-neutralizing (VN) antibodies for PRV were allotted to 3 treatment groups (A, B, C) when they were 2 weeks old. Group-A pigs were separated from littermates and dam and thereafter kept in isolation; group-B pigs were experimentally exposed oronasally to PRV and 1 hour later returned to their dam; group-C pigs were kept with their dam and potentially exposed to PRV by contact with littermates of group B. Sera obtained from pigs at selected intervals until they were 17 weeks old were tested for VN activity and for precipitating activity for radiolabeled viral proteins. All group-A pigs remained clinically normal throughout the experiment. Depending on the initial amount of passively acquired antibody, little or no serum VN or precipitating activity remained by the time these pigs were 17 weeks old. Group-B and -C pigs, with relatively high amounts of passively acquired antibody when exposed to PRV, also remained clinically normal. However, most became latently infected as subsequently evidenced by either dexamethasone-induced or noninduced virus reactivation. Noninduced reactivation may have been initiated by weaning the pigs when they were about 8 weeks old.(ABSTRACT TRUNCATED AT 250 WORDS)     

猪伪狂犬病病毒的分离鉴定与防制

从福清海口某猪场病猪淋巴结分离到1株病毒。该病毒接种Balb/c小鼠出现神经症状,死亡率为100%;接种Vero细胞出现拉网病变;PRV阳性血清能特异性中和该分离毒。以上结果证实该病毒为猪伪狂犬病毒。确诊后采取隔离、消毒及注射疫苗等措施,取得了一定的效果。     

Shedding patterns in swine of virulent and attenuated pseudorabies virus

Shedding patterns of 2 virulent (P-2208 and KC-152-D) and 1 attenuated (BUK) strains of pseudorabies virus (PRV) were determined in groups of intranasally inoculated feeder pigs. Clinical signs observed following inoculation with the P-2208 and KC-152-D strains included increase in rectal temperatures up to 42.2 C, anorexia, severe respiratory disturbance, and fatal CNS signs in 2 cases. Clinical signs in pigs inoculated with 7.2 X 10(7) median tissue culture infective dose (TCID50) of the BUK strain were limited to depression and a rise in rectal temperatures to 40.5 C for 3 to 4 days. Evaluation of the efficacy of the virus isolation method used showed that the presence on swabs of only 12.5 TCID50 of the P-2208 strain or 8.4 TCID50 of the BUK strain resulted in a 50% chance of virus recovery. Intranasal inoculations with 500 TCID50 of the P-2208 or KC-152-D strain did not result in synchronous infection of the whole group. Intranasal inoculations with 5,000 TCID50 of the KC-152-D strain or 50,000 TCID50 of the P-2208 strain resulted in continuous virus shedding in all pigs between postinoculation days (PID) 4 and 13 (KC-152-D strain) or 14 (P-2208 strain). Some of the pigs in these 2 groups further shed the P-2208 or KC-152-D strain in a continuous or discontinuous pattern up to PID 19 (P-2208 strain) or 20 (KC-152-D strain). The time of onset or the level of virus neutralizing antibody production in individual pigs was not found to have an influence on their shedding patterns.(ABSTRACT TRUNCATED AT 250 WORDS)