In vivo and in vitro reactivation of latent pseudorabies virus in pigs born to vaccinated sows

Latency of pseudorabies virus (PRV) was established in 8 of 9 pigs born to 2 vaccinated sows. Pigs had high, low, or no maternal antibody titers at the time of the initial inoculation. At postinoculation months 3 to 4, latent PRV could be reactivated in vivo by the administration of large doses of corticosteroids. In most pigs, the stress-simulating treatment resulted in recrudescence of virus shedding after lag periods of 4 to 11 days. In 3 pigs, virus shedding was without clinical signs of disease, whereas clinical signs that developed in 4 pigs appeared to be due to the corticosteroid treatment, rather than to the reactivation of PRV. Pigs with a log10 neutralizing antibody titer of less than or equal to 2.55 at the onset of corticosteroid treatment had a booster response. Reactivated PRV spread to sentinel pigs housed with the inoculated pigs. Reactivation of PRV was also demonstrated in vitro. Explant cultures of trigeminal ganglia from pigs killed between postinoculation months 4 to 5 produced infectious virus. Restriction endonuclease analysis indicated that the reactivated PRV was indistinguishable from virus isolated shortly after the primary infection. Seemingly, pigs with maternal antibodies can become latently infected and therefore may be regarded as potential sources of dissemination of PRV.     

Attenuated properties of thymidine kinase-negative deletion mutant of pseudorabies virus

A thymidine kinase (TK)-negative (TK-) deletion mutant of the Bucharest (BUK) strain of pseudorabies virus (PRV) was isolated. The mutant, designated as PRV (BUK d13), did not revert to TK-positive (TK+), even when propagated in medium that selected for TK+ viruses. The mutant also replicated equally well at 39.1 C and 34.5 C, and was easily distinguished from other PRV strains by molecular hybridization experiments, restriction nuclease fingerprints, and plaque autoradiography or other assays for the TK phenotype. The PRV (BUK d13) had greatly reduced virulence for mice and rabbits, compared with parental TK+ strains, PRV (BUK-5) and PRV (BUK-5A-R1), and provided mice with solid protection against the TK+ BUK and Aujeszky strains of PRV. Experiments were done in 5- to 6-week-old pigs to assess the safety and efficacy of PRV (BUK d13) in the natural host. In one experiment, pigs were vaccinated IM with 7.5 X 10(8) plaque-forming units of TK- PRV (BUK d13), and were then challenge exposed intranasally (IN) with 4.3 X 10(8) TCID50 of virulent PRV [Indiana-Funkhauser (IND-F)]. Vaccinated pigs did not have clinical signs of illness after vaccination or after challenge exposure. One nonvaccinated control pig died on postchallenge day 4; a 2nd nonvaccinated control pig became moribund, but eventually recovered. Pigs developed virus-neutralizing antibodies after vaccination, and had a secondary immunologic response after challenge exposure; however, PRV was not isolated from the tonsils or trigeminal ganglia of vaccinated pigs at postchallenge exposure day 11.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vaccine genotype and route of administration affect pseudorabies field virus latency load after challenge

The influence of vaccine genotype and route of administration on the efficacy of pseudorabies virus (PRV) vaccines against virulent PRV challenge was evaluated in a controlled experiment using five genotypically distinct modified live vaccines (MLVs) for PRV. Several of these MLVs share deletions in specific genes, however, each has its deletion in a different locus within that gene. Pigs were vaccinated with each vaccine, either via the intramuscular or intranasal route, and subsequently challenged with a highly virulent PRV field strain. During a 2-week period following challenge with virulent PRV, each of the vaccine strains used in this study was evaluated for its effectiveness in the reduction of clinical signs, prevention of growth retardation and virulent virus shedding. One month after challenge, tissues were collected and analyzed for virulent PRV latency load by a recently developed method for the electrochemiluminescent quantitation of latent herpesvirus DNA in animal tissues after PCR amplification. It was determined that all vaccination protocols provided protection against clinical signs resulting from field virus challenge and reduced both field virus shedding and latency load after field virus challenge. Our results indicated that vaccine efficacy was significantly influenced by the modified live vaccine strain and route of administration. Compared to unvaccinated pigs, vaccination reduced field virus latency load in trigeminal ganglia, but significant differences were found between vaccines and routes of administration. We conclude that vaccine genotype plays a role in the effectiveness of PRV MLVs.     

High- and low-challenge exposure doses used to compare intranasal and intramuscular administration of pseudorabies virus vaccine in passively immune pigs.

We compared 3 modified-live pseudorabies virus (PRV) vaccine strains, administered by the intranasal (IN) or IM routes to 4- to 6-week-old pigs, to determine the effect of high- and low-challenge doses in these vaccinated pigs. At the time of vaccination, all pigs had passively acquired antibodies to PRV. Four experiments were conducted. Four weeks after vaccination, pigs were challenge-exposed IN with virulent virus strain Iowa S62. In experiments 1 and 2, a high challenge exposure dose (10(5.3) TCID50) was used, whereas in experiments 3 and 4, a lower challenge exposure dose (10(2.8) TCID50) was used. This low dose was believed to better simulate field conditions. After challenge exposure, pigs were evaluated for clinical signs of disease, weight gain, serologic response, and viral shedding. When vaccinated pigs were challenge-exposed with a high dose of PRV, the duration of viral shedding was significantly (P less than 0.05) lower, and body weight gain was greater in vaccinated pigs, compared with nonvaccinated challenge-exposed pigs. Pigs vaccinated IN shed PRV for fewer days than pigs vaccinated IM, but this difference was not significant. When vaccinated pigs were challenge-exposed with a low dose, significantly (P less than 0.05) fewer pigs vaccinated IN (51%) shed PRV, compared with pigs vaccinated IM (77%), or nonvaccinated pigs (94%). Additionally, the duration of viral shedding was significantly (P less than 0.05) shorter in pigs vaccinated IN, compared with pigs vaccinated IM or nonvaccinated pigs. The high challenge exposure dose of PRV may have overwhelmed the local immune response and diminished the advantages of the IN route of vaccination.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pseudorabies virus latency and reactivation in vaccinated swine

Latency and reactivation of pseudorabies virus in swine was studied. Thirty-one pigs were assigned to 5 groups and were given 1 of 4 vaccines; 10 remained unvaccinated controls. All pigs were then challenge exposed with a sublethal dose of virulent pseudorabies virus. One hundred one days after challenge exposure, all pigs were treated with dexamethasone to reactivate the virus. Virus-positive tonsil and nasal mucus isolates were recovered from 29 of the 31 pigs over a 12-day period. Frequency and duration of virus-positivity were significantly (P less than 0.05) and consistently lower among vaccinated pigs than among the unvaccinated controls. It was concluded that vaccination before challenge exposure had little or no effect on the rate of establishment of virus latency, but that vaccination reduced shedding after subsequent reactivation of the virus.     

A study of the ability of a TK-negative and gI/gE-negative pseudorabies virus (PRV) mutant inoculated by different routes to protect pigs against PRV infection

The capacity of a TK-negative (TK-) and gI/gE-negative (gI/gE-) pseudorabies virus (PRV) mutant to protect pigs against Aujeszky's disease carried out by experimental infection with a virulent PRV strain, was tested. There were three groups, each of four susceptible pigs which were inoculated twice by two different schedules. Group 1 received the modified virus by the intradermal (first inoculation)-intramuscular (second inoculation) routes; group 2 was treated by the intranasal (first inoculation)-intramuscular (second inoculation) routes. The third group was left untreated as the control. All of the pigs were challenged intranasally with a virulent PRV strain and they were subsequently injected with dexamethasone. Two pigs in each group were necropsied on days 5 and 15 after dexamethasone inoculation. The challenge exposure resulted in mild clinical signs, increase in growth and a shorter period of virus shedding in vaccinated pigs, whereas the control group showed severe signs of Aujeszky's disease. No difference in the titre of the virulent virus which was excreted by pigs of all three groups, was observed and all animals seroconverted. Both the mutant strain and the wild-type virus established a latent infection although only the latter was reactivated and shed. Slight lesions were observed in target tissues of the vaccinated animals and no significant differences were detected between the two inoculation schedules.     

Effect of various vaccination procedures on shedding, latency, and reactivation of attenuated and virulent pseudorabies virus in swine.

Various procedures of vaccination for pseudorabies were compared for their effects on shedding, latency, and reactivation of attenuated and virulent pseudorabies virus. The study included 6 groups: group 1 (10 swine neither vaccinated nor challenge-exposed), group 2 (20 swine not vaccinated, but challenge-exposed), and groups 3 through 6 (10 swine/group, all vaccinated and challenge-exposed). Swine were vaccinated with killed virus IM (group 3) or intranasally (group 4), or with live virus IM (group 5) or intranasally (group 6). The chronologic order of treatments was as follows: vaccination (week 0), challenge of immunity by oronasal exposure to virulent virus (week 4), biopsy of tonsillar tissue (week 12), treatment with dexamethasone in an attempt to reactivate latent virus (week 15), and necropsy (week 21). Vaccination IM with killed or live virus and vaccination intranasally with live virus mitigated clinical signs and markedly reduced the magnitude and duration of virus shedding after challenge exposure. Abatement of signs and shedding was most pronounced for swine that had been vaccinated intranasally with live virus. All swine, except 4 from group 2 and 1 from group 4, survived challenge exposure. Only vaccination intranasally with live virus was effective in reducing the magnitude and duration of virus shedding after virus reactivation. Vaccination intranasally with killed virus was without measurable effect on immunity. Of the 55 swine that survived challenge exposure, 54 were shown subsequently to have latent infections by use of dexamethasone-induced virus reactivation, and 53 were shown to have latent infections by use of polymerase chain reaction (PCR) with trigeminal ganglia specimens collected at necropsy. Fewer swine were identified by PCR as having latent infections when other tissues were examined; 20 were identified by testing specimens of olfactory bulbs, 4 by testing tonsil specimens collected at necropsy, and 4 by testing tonsillar biopsy specimens. Eighteen of the 20 specimens of olfactory bulbs and 3 of the 4 tonsil specimens collected at necropsy in which virus was detected by PCR were from swine without detectable virus-neutralizing antibody at the time of challenge exposure. One pig that had been vaccinated intranasally with live virus shed vaccine virus from the nose and virulent virus from the pharynx concurrently after dexamethasone treatment. Evaluation of both viral populations for unique strain characteristics failed to provide evidence of virus recombination.     

Studies on the ability of a 98-kilodalton pseudorabies virus diagnostic antigen to detect latent infections induced by low-dose exposure to the virus

The effect of low-dose challenge of immunity with pseudorabies virus (PRV) on subunit-vaccinated pigs was studied in 2 experiments. In the first experiment, we studied the effect of challenge dose on the antibody response to an early excreted 98-kilodalton PRV-glycoprotein that was used as a diagnostic antigen in the ELISA. In the second experiment, we studied the effect of low doses of virus on the establishment of latent infections in subunit-vaccinated pigs. The relationship of virus exposure dose and vaccine dose to the response of pigs to diagnostic antigen was studied in 18 pigs. Two groups of 3 pigs were vaccinated with a total of 200 micrograms of a lectin-derived PRV subunit vaccine over a 5-week period. Two groups of 3 pigs were similarly vaccinated with a total of 100 micrograms. Two groups of 3 pigs served as nonvaccinated controls. One group of pigs from each of the preceding categories was intranasally exposed to 10(6.0) and 10(2.7) plaque-forming units (PFU) of virus. Antibody to diagnostic antigen was detected by the ELISA and radioimmunoprecipitation 3 to 7 days earlier in pigs exposed to 10(6.0) PFU, demonstrating that the size of the virus challenge dose affects the antibody response to diagnostic antigen. The establishment of latent infections by low PRV doses and the ability to detect these infections was studied in 10 subunit-vaccinated pigs. Each pig was intranasally exposed to 10(2.3) PFU of virus (day 0).(ABSTRACT TRUNCATED AT 250 WORDS)     

Detection of pseudorabies virus infection in subunit-vaccinated and nonvaccinated pigs using a nucleocapsid-based enzyme-linked immunosorbent assay.

The potential of a pseudorabies virus (PRV) nucleocapsid protein (NC)-based enzyme-linked immunosorbent assay (ELISA) as a screening assay for PRV infection in subunit-vaccinated and nonvaccinated pigs was studied. The NC-ELISA compared favorably to a commercial ELISA for detecting PRV infection in nonvaccinated pigs. Virus-specific antibody was first detected by the NC-ELISA between days 14 and 21 in 5 pigs challenged intranasally with 10(4) PFU of virus. Antibody continued to be detected in these pigs through day 42, when the experiment was terminated. The NC-ELISA also detected antibody in 23 of 24 pigs from PRV-infected herds. In contrast, the commercial ELISA detected antibody 1 week earlier than the NC-ELISA in experimentally infected pigs but failed to detect antibody in 3 naturally exposed pigs that were identified by the NC-ELISA. Infection in these animals was confirmed by radioimmunoprecipitation analysis. The potential usefulness of the NC-ELISA for detecting infection in vaccinated pigs was also evaluated. The nucleocapsid-specific antibody responses of 10 PRV envelope glycoprotein subunit-vaccinated pigs were monitored prior to and following nasal exposure to a low dose (10(2.3) PFU) of PRV. Sera were collected periodically for 113 days after infection. Nucleocapsid-specific antibody responses measured by the NC-ELISA remained below the positive threshold before challenge but increased dramatically following virus exposure. Maximum ELISA responses were obtained on day 32 postchallenge (p.c.). Mean ELISA responses decreased thereafter but remained well above the positive threshold on day 113 p.c. PRV nucleocapsid protein can be used effectively as antigen in the ELISA for detecting PRV infection in both nonvaccinated and subunit-vaccinated pigs.     

伪狂犬病病毒弱毒株LY株的分离鉴定

从辽阳某猪场的10日龄仔猪中分离到1株病毒,经纯化后测得其毒价为107.29TCID50/mL.细胞中和试验表明,该病毒能被猪伪狂犬病病毒标准阳性血清所中和.电镜下可见到典型的疱疹病毒粒子,具有囊膜及外周纤突.所分离的病毒对氯仿、胰蛋白酶、乙醚敏感,在pH5.0～9.0下稳定,56℃ 30 min可以灭活.应用特异性引物,通过PCR能扩增出伪狂犬病病毒1 240 bp的gD基因.分离病毒对3日龄乳鼠有一定的致病力,但对家兔、3～5日龄仔猪及妊娠母猪都有很高的安全性.用不同剂量的病毒培养液肌肉注射于3～5日龄仔猪,14 d后用105.7TCID50伪狂犬病病毒强毒攻击,所有试验仔猪均可得到有效保护.用分离毒免疫母猪,其后代可获高滴度的母源抗体,15日龄的仔猪能抵抗105.7TCID50强毒的攻击.试验的结果初步说明,所分离的病毒为伪狂犬病病毒(命名为PRV LY株),并可能是一株弱毒株,而且具有很好的免疫保护作用.     

Interaction of a combined feline viral rhinotracheitis-feline calicivirus vaccine and the FVR carrier state.

The effect of field feline viral rhinotracheitis (FVR) virus challenge on cats previously vaccinated with a combined FVR/feline calicivirus intramuscular vaccine was studied in relation to the development of an FVR carrier state. There was no virus shedding of either of the two vaccine viruses following vaccination. Treatment with corticosteroid 60 days after vaccination and before challenge with FVR virus did not induce virus re-excretion in vaccinates or controls; neither did similar treatment induce shedding 63 days after challenge of both vaccinates and controls with virulent field virus. After a further 55 days however, FVR virus shedding was elicited in one of four previously vaccinated and challenged cats compared with two of four unvaccinated and challenged controls. Two sentinel cats remained virologically and serologically free of FVR throughout. The vaccine was shown to be effective in controlling the disease; 12 weeks after initial vaccination no clinical signs were seen in three of four cats following intranasal challenge with 10(5)CCID50 of virulent field FVR virus, and a mild transient unilateral ocular and nasal discharge was seen in the remaining cat for one day only. Severe clinical signs of approximately 10 days' duration were seen in all four unvaccinated challenged controls. The virological and serological responses of the cats were also recorded.     

Efficacy of a pseudorabies virus vaccine based on deletion mutant strain 783 that does not express thymidine kinase and glycoprotein I.

The vaccine efficacy of a genetically engineered deletion mutant strain of pseudorabies virus, strain 783, was compared with that of the conventionally attenuated Bartha strain. Strain 783 has deletions in the genes coding for glycoprotein I and thymidine kinase. In experiment 1, which had a 3-month interval between vaccination and challenge exposure, strain 783 protected pigs significantly (P less than 0.05) better against virulent virus challenge exposure than did the Bartha strain. The growth of pigs vaccinated with strain 783 was not arrested, whereas that of pigs vaccinated with the Bartha strain was arrested for 7 days. Of 8 pigs given strain 783, 4 were fully protected against challenge exposure; none of the pigs given strain Bartha was fully protected. In experiment 2, which had a 3-week interval between vaccination and challenge exposure, the growth of pigs vaccinated with strain 783 was arrested for 3.5 days, whereas that of pigs vaccinated with the Bartha strain was arrested for 6 days. In experiment 3, pigs with moderate titer of maternal antibodies were vaccinated twice IM or once intranasally with either strain 783 or Bartha and were challenge-exposed 3 months after vaccination. Pigs given strain 783 twice IM were significantly (P less than 0.05) better protected than were the other pigs. They had growth arrest of only 6 days, compared with 9 days for pigs of other groups, and shed less virus after challenge exposure. Results of this study indicate that the vaccine based on the deletion mutant strain 783 is more efficacious than is the Bartha strain of pseudorabies virus.     

Evaluation in swine of a subunit vaccine against pseudorabies

A subunit vaccine against pseudorabies virus (PRV) was prepared by treating a mixture of pelleted virions and infected cells with the nonionic detergent Nonidet P-40 and emulsifying the extracted proteins incomplete Freund's adjuvant. Three 7-week-old pigs without antibodies against PRV were given 2 IM doses of this vaccine 3 weeks apart. Thirty days after the 2nd vaccination, 10(6) median tissue culture infective doses (TCID50) of a virulent strain of PRV were administered intranasally. Tonsillar and nasal swabs were collected daily between 2 and 10 days after challenge exposure. The pigs vaccinated with the subunit vaccine were not found to shed virulent PRV. Two groups of five 7-week-old pigs vaccinated with commercially available vaccines, either live-modified or inactivated virus, and subsequently exposed to 10(6) TCID50 of virulent PRV, shed virulent virus for up to 8 days. The subunit vaccine induced significantly higher virus-neutralizing antibody titers than either the live-modified or inactivated virus vaccine.     

A Study of the Ability of a TK‐Negative and gI/gE‐Negative Pseudorabies Virus (PRV) Mutant Inoculated by Different Routes to Protect Pigs Against PRV Infection

The capacity of a TK‐negative (TK ^‐) and gI/gE‐negative (gI/gE ^‐) pseudorabies virus (PRV) mutant to protect pigs against Aujeszky's disease carried out by experimental infection with a virulent PRV strain, was tested. There were three groups, each of four susceptible pigs which were inoculated twice by two different schedules. Group 1 received the modified virus by the intradermal (first inoculation)‐intramuscular (second inoculation) routes; group 2 was treated by the intranasal (first inoculation)‐intramuscular (second inoculation) routes. The third group was left untreated as the control. All of the pigs were challenged intranasally with a virulent PRV strain and they were subsequently injected with dexamethasone. Two pigs in each group were necropsied on days 5 and 15 after dexamethasone inoculation. The challenge exposure resulted in mild clinical signs, increase in growth and a shorter period of virus shedding in vaccinated pigs, whereas the control group showed severe signs of Aujeszky's disease. No difference in the titre of the virulent virus which was excreted by pigs of all three groups, was observed and all animals seroconverted. Both the mutant strain and the wild‐type virus established a latent infection although only the latter was reactivated and shed. Slight lesions were observed in target tissues of the vaccinated animals and no significant differences were detected between the two inoculation schedules.     

The response of pigs inoculated with a thymidine kinase-negative (TK-) pseudorabies virus to challenge infection with virulent virus

12 Large-White-Landrace piglets were subdivided in four groups of 3 and housed in separate units. The piglets of three groups were inoculated with the 86/27V 6C2 thymidine kinase negative (TK-) mutant of pseudorabies virus (PRV), by different routes. A second inoculation with the same mutant was given to the pigs 21 days later. The animals of a fourth group were left as uninoculated controls. 21 days following the second inoculation with the TK- mutant all pigs were challenge infected with the virulent PRV. On post challenge day (PCD) 30 all pigs were killed and samples for virus detection and histology were taken from several organs. The inoculated TK- mutant of PRV did not induce any ill effects in the pigs except a transient febrile reaction in some animals. Virus was recovered from nasal swabbings from one pig 2 days after the first inoculation of the mutant. After challenge exposure with virulent PRV, the TK- mutant-inoculated pigs were apparently protected, whereas the control pigs all were severely affected and recovered very slowly over 3 weeks. Virus was isolated from the nasal swabbings from the TK- mutant-inoculated pigs on PCDs 2 and 4, whereas the nasal swabbings from the control piglets were all positive for virus from PCD 2 through PCD 10. DNA analysis of the virus recovered showed a pattern identical to that of the virulent PRV. Histologic lesions were found in the respiratory and the central nervous systems, however, the lesions in the TK- mutant-inoculated pigs were much milder compared to those registered for the control pigs. Virus was not isolated from any of the tissue samples that were tested, but viral DNA with sequences typical of PRV genome was detected by PCR in all samples of trigeminal ganglia from either the TK- mutant-inoculated pigs or from the controls.     

A vaccine strain of pseudorabies virus with deletions in the thymidine kinase and glycoprotein X genes

A pseudorabies virus (PRV) mutant with deletions in genes for glycoprotein X (gX) and thymidine kinase, designated delta GX delta TK, was constructed and evaluated as a vaccine for protecting swine against PRV-induced mortality. Doses greater than or equal to 10(3) plaque-forming units (PFU) of this strain given to mice provided protection from challenge exposure with virulent PRV. Sera tested from mice inoculated with delta GX delta TK had high titers of neutralizing antibody to PRV, but reactivity in the same sera was not significantly different from that in sera from noninoculated mice (controls) when sera from both groups were evaluated by use of an ELISA with gX antigen produced in Escherichia coli. Compared with noninoculated pigs (controls), those given delta GX delta TK (greater than or equal to 10(2) PFU) were protected completely from lethal challenge exposure, without experiencing adverse effects on weight gain and with reduction of shedding of virulent challenge virus. Serotest results indicated that, although inoculated pigs responded with strong neutralizing antibody titers, the response of delta GX delta TK-inoculated pigs to gX, as determined by ELISA before challenge exposure, was not significantly greater than the ELISA values obtained from control pigs. The ELISA values from a group of pigs inoculated with a commercially available vaccine were significantly (P less than 0.05) higher than those of control pigs. The experimental vaccine, delta GX delta TK, was avirulent for mice, swine, and sheep, but was mildly virulent for calves (mortality, 1 of 12) and more virulent for dogs (mortality, 3 of 6) and cats (mortality, 2 of 6).(ABSTRACT TRUNCATED AT 250 WORDS)     

Specificity of the indirect enzyme-linked immunosorbent assay for detection of pseudorabies virus antibodies in pigs exposed to bovine herpesvirus-1

Six 5-week-old pigs were inoculated intranasally (IN) with 10(7.6) TCID50 of bovine herpesvirus-1 (BHV-1). Three of the pigs also were inoculated IV with a similar dose of BHV-1. Clinical responses were not observed in these 6 pigs before oronasal challenge exposure with 10(7.8) TCID50 of virulent pseudorabies virus (PRV) at postinoculation day 42. Two pigs inoculated IN with BHV-1 and challenge exposed with PRV remained healthy, whereas the remaining 4 pigs developed severe clinical signs of pseudorabies and were moribund at postinoculation day 50 (8 days after challenge exposure). Anti-BHV-1 antibodies were demonstrable by ELISA in all 6 pigs and by serum neutralization (SN) in 5 pigs before challenge exposure with PRV. Anti-PRV antibody was not detected by ELISA or SN before challenge exposure to PRV. After challenge exposure to PRV, pigs with humoral antibody to BHV-1 responded anamnestically, and anti-PRV antibody activity was demonstrable by ELISA and SN in the 2 surviving pigs.     

Sensitive glycoprotein gIII blocking ELISA to distinguish between pseudorabies (Aujeszky's disease)-infected and vaccinated pigs.

A blocking enzyme-linked immunosorbent assay (ELISA) test has been developed to distinguish pseudorabies virus (PRV) (Aujeszky's disease virus) -infected pigs from those immunized with a glycoprotein g92 (gIII) deletion mutant, PRV (dlg92dltk) [OMNIMARK-PRV]. This blocking ELISA test utilizes an anti-PRV gIII monoclonal antibody (mAbgIII)-horseradish peroxidase (HRPO) conjugate, TMB for color development and a cloned PRVg92 (gIII) antigen to coat wells of microtiter test plates. Undiluted sera are used to block the binding of the mAbgIII-HRPO conjugate to the antigen. The gIII blocking ELISA is specific and has a sensitivity comparable to screening ELISA and latex agglutination tests. PRV-negative sera and sera from pigs vaccinated once, twice, or four times with the gIII-negative vaccine all showed negative S/N values of greater than 0.70 (S/N defined as the optical density at 630 nm of test sera/optical density at 630 nm of negative control sera). Sera from PRV-infected herds, sera from pigs experimentally infected with virulent PRV, and sera from pigs vaccinated with modified-live or inactivated gIII+ vaccines were positive for gIII antibodies (S/N less than 0.7). Sera from pigs experimentally infected with 200 PFU virulent PRV seroconverted to gIII+ antibodies 7-10 days postinfection. Sera from pigs vaccinated with gpX- and gI- vaccines seroconverted to gIII+ antibodies 7-8 days after vaccination. The gIII antibodies persisted after gIII+ vaccinated for at least 376 days postvaccination. Sera from pigs protected by vaccination with PRV (dlg92dltk) and then challenge exposed to virulent PRV at 21 days postvaccination showed gIII+ antibodies by 14 days postchallenge. The specificity and sensitivity of the gIII blocking ELISA assay was further demonstrated on the United States Department of Agriculture-National Veterinary Services Laboratory (USDA-NVSL) sera from the 1988 PRV check set and the 1989 gIII PRV check set by comparing the gIII blocking ELISA assay with virus neutralization, screening/verification ELISA and latex agglutination assays.     

Successful pseudorabies vaccination in maternally immune piglets using recombinant vaccinia virus vaccines

Three gilts were vaccinated with a NYVAC vaccinia recombinant expressing glycoprotein gD of pseudorabies virus (PRV) (NYVAC/gD). After farrowing, the piglets were allowed to nurse normally to obtain colostral immunity and then were divided into four groups, receiving NYVAC/gD, a NYVAC recombinant expressing glycoprotein gB of PRV (NYVAC/gB), an inactivated PRV vaccine (iPRV), or no vaccine. The piglets were vaccinated twice, three weeks apart beginning at approximately two weeks of age and later challenged with virulent PRV oronasally. Piglets that received NYVAC/gB or iPRV were the best protected based on lack of mortality, lower temperature responses, decreased weight loss and decreased viral shedding after challenge. These results indicate effective strategies for stimulating active immune response while still under the protection of maternal immunity.     

Antibody response of pigs to inactivated monovalent and bivalent vaccines for porcine parvovirus and pseudorabies virus

Groups of pigs vaccinated with an inactivated bivalent vaccine containing porcine parvovirus (PPV) and pseudorabies virus (PRV) developed geometric mean titers (GMT) of humoral antibody for each of the viruses as high or slightly higher than those of other groups of pigs that were vaccinated with inactivated monovalent vaccines containing one or the other of the same viruses. An increase in GMT after challenge exposure of vaccinated pigs to live virus indicated that vaccination did not prevent virus replication. However, an indication that replication was less extensive in vaccinated pigs was provided by the following. Although neither vaccinated nor nonvaccinated (control) pigs had clinical signs after exposure to the live PPV, the effect of vaccination was evident by the fact that GMT were higher in nonvaccinated pigs after exposure than they were in vaccinated pigs. Conversely, all pigs exposed to live PRV had clinical signs, but these signs varied between mild-to-moderate and transient for vaccinated pigs to severe and fatal for nonvaccinated pigs.