A comparison of differential diagnostic tests to detect antibodies to pseudorabies glycoproteins gX, gI, and gIII in naturally infected feral pigs

A poly(dimethylsiloxane)-based solid-phase microchip platform for dual detection of Pseudorabies virus gD and gE antibodies

Pseudorabies caused by pseudorabies virus (PRV) infection is still a major disease affecting the pig industry; its eradication depends on effective vaccination and antibody (Ab) detection. For a more rapid and accurate PRV detection method that is suitable for clinical application, here, we established a poly(dimethylsiloxane)-based (efficient removal of non-specific binding) solid-phase protein chip platform (blocking ELISA) for dual detection of PRV gD and gE Abs. The purified gD and gE proteins expressed in baculovirus were coated into the highly hydrophobic nanomembrane by an automatic spotter, and the gray values measured by a scanner were used for the S/N (sample/negative) value calculation (gD and gE Abs standard, positive: S/N value ≤0.6; negative: S/N value >0.7; suspicious: 0.6 < S/N ≤ 0.7). The method showed an equal sensitivity in the gD Ab test of immunized pig serum samples compared to the neutralization test and higher sensitivity in the gE Ab test compared to the commercial gE Ab detection kit. In the clinical evaluation, we found an agreement of 100% (122/122) in the gD Ab detection compared to the neutralization test and an agreement of 97.5% (119/122) in the gE Ab detection compared to the commercial PRV gE Ab detection kit. In summary, the protein chip platform for dual detection of PRV gD and gE Abs showed high sensitivity and specificity, which is suitable for PRV immune efficacy evaluation and epidemic monitoring.

A high-temperature passaging attenuated Pseudorabies vaccine protects piglets completely against emerging PRV variant

Emerging variant of pseudorabies virus (PRV) have evaded the antiviral immunity of commercially available PRV vaccine and have led to PRV outbreaks in Chinese pig farms. Here, we attenuated a PRV variant strain by serial passages in vitro and evaluate the protective efficacy of the attenuated strain as a vaccine candidate. The virulent PRV variant strain JS-2012 was continuously passaged in Vero cells at 40°C and attenuated rapidly. After 90 passages in Vero cells, the passaged virus lost its ability to cause death in 2-week-old piglets. The 120th passage virus was avirulent in the sucking piglets. An attenuated strain, JS-2012-F120 derived from the 120th passage virus by three rounds of plaque cloning grew better than its parent strain JS-2012 in Vero cells and showed notably different cytopathic effects and plaque morphology from JS-2012. PCR combined with sequence analysis showed that JS-2012-F120 contained a 2307-bp deletion covering nucleotide 487 of gE gene to 531 of US2 gene. After inoculation with JS-2012-F120, young piglets were completely protected from challenge with the classical and emerging virulent PRVs. Moreover, the piglets did not develop specific gE antibodies. Thus, JS-2012-F120 appears to be a promising marker vaccine to control PRV variant circulating in Chinese pig farms, and the high-temperature passaging in vitro was an efficient method to attenuated alphaherpesvirus.

A live, attenuated pseudorabies virus strain JS-2012 deleted for gE/gI protects against both classical and emerging strains

Emerging pseudorabies virus (PRV) variant have led to pseudorabies outbreaks in Chinese pig farms. The commercially available PRV vaccine provides poor protection against the PRV variant. In this study, a gE/gI deleted PRV strain JS-2012-△gE/gI was generated from a PRV variant strain using homologous DNA recombination. Compared to the parental strain JS-2012, JS-2012-△gE/gI grew slowly and showed small plaque morphology on Vero cells. The safety and immunological efficacy of JS-2012-△gE/gI was evaluated as a vaccine candidate. JS-2012-△gE/gI was avirulent to suckling piglets, but was able to provide full protection for young piglets against challenge with both the classical virulent PRV and the emerging PRV variant. After sows were vaccinated with the gE/gI-deleted strain, their suckling offspring were resistant to an otherwise lethal challenge with the classical and the variant PRVs. Piglets inoculated with JS-2012-△gE/gI did not develop PRV-specific gE-ELISA antibodies. Thus, JS-2012-△gE/gI appears to be a promising marker vaccine candidate to control PRV variant circulating in pig farms in China.

Variation of Aujeszky's disease viruses in wild swine in USA

In the United States of America, Aujeszky's disease (pseudorabies) has been eradicated from all domestic swine. Some re-emergence of infection occurred as vaccine use diminished. Sporadic outbreaks have also occurred because of the reservoir of infection in feral swine that have spread across the southern two-thirds of the country and Hawaii. In order to be able to understand the origins of re-emerging virus, sequence analysis of variable genes in pseudorabies virus (PRV) has been used to differentiate strains. Most PRV from feral swine can be distinguished from virus circulating in domestic pigs during the national epizootic. However, several feral swine isolates of PRV from south central states are closely related or identical in sequence to strains from domestic pigs. Extensive study by PCR for the presence of virus in the oral cavity of feral pigs disclosed that the viral DNA is distributed widely in tonsils salivary glands, taste buds and even mucosa in the vicinity of tusks. Clearly the virus in feral swine has multiple mechanisms of transmission to insure persistent infection and the threat of re-emergence in domestic swine continues.

Antibodies against pseudorabies virus in feral swine in southeast Brazil

Serum samples collected from 358 wild boars (Sus scrofa) in breeding farms in São Paulo, southeast Brazil, from 1998 to 2001, were tested for antibodies against pseudorabies virus (PRV) by means of serum neutralization (SN) and enzyme-linked immunobsorbent assay (ELISA). Seropositive animals were detected in three of seven herds analyzed. Overall seroprevalence as assessed by SN was 30.7%, ranging from 25.2% to 100% for the herds that presented seropositive animals. Indirect ELISA detected lower seroprevalence (19.3%). Sensitivity and specificity of ELISA were equal to 57.3% and 97.6%, respectively. Agreement was equal to 85.2% (P<0.0001). These results showed that PRV infections occurred in farmed feral swine in southeast Brazil, and affect pseudorabies eradication program.

Molecular biology of pseudorabies virus: impact on neurovirology and veterinary medicine

Pseudorabies virus (PRV) is a herpesvirus of swine, a member of the Alphaherpesvirinae subfamily, and the etiological agent of Aujeszky's disease. This review describes the contributions of PRV research to herpesvirus biology, neurobiology, and viral pathogenesis by focusing on (i) the molecular biology of PRV, (ii) model systems to study PRV pathogenesis and neurovirulence, (iii) PRV transsynaptic tracing of neuronal circuits, and (iv) veterinary aspects of pseudorabies disease. The structure of the enveloped infectious particle, the content of the viral DNA genome, and a step-by-step overview of the viral replication cycle are presented. PRV infection is initiated by binding to cellular receptors to allow penetration into the cell. After reaching the nucleus, the viral genome directs a regulated gene expression cascade that culminates with viral DNA replication and production of new virion constituents. Finally, progeny virions self-assemble and exit the host cells. Animal models and neuronal culture systems developed for the study of PRV pathogenesis and neurovirulence are discussed. PRV serves asa self-perpetuating transsynaptic tracer of neuronal circuitry, and we detail the original studies of PRV circuitry mapping, the biology underlying this application, and the development of the next generation of tracer viruses. The basic veterinary aspects of pseudorabies management and disease in swine are discussed. PRV infection progresses from acute infection of the respiratory epithelium to latent infection in the peripheral nervous system. Sporadic reactivation from latency can transmit PRV to new hosts. The successful management of PRV disease has relied on vaccination, prevention, and testing.

Survival and immunization of raccoons after exposure to pseudorabies (Aujeszky's disease) virus gene-deleted vaccines

In a controlled experiment, 16 wild-trapped raccoons were exposed to 1 of 2 genetically modified live pseudorabies virus (PRV) vaccines used in swine. One vaccine had genes deleted for thymidine kinase (TK(-)) and glycoprotein G (gG(-)); the other had an additional deletion for glycoprotein E (gE(-)). These vaccines were administered orally and intranasally at four dose levels: 10(3), 10(4), 10(5), and 10(6) TCID(50). The 21 days survival rate was 37.5% for the gG(-)TK(-) vaccine; all of the survivors developed antibodies to PRV. All animals receiving the gG(-)gE(-)TK(-) vaccine survived; 75% (all except the lowest dose) developed anti-PRV antibodies. Survivors were challenged intranasally with a 3.2x10(3) TCID(50) dose of the virulent wildtype PRV Shope strain. Two of the remaining three gG(-)TK(-) vaccinated raccoons survived the challenge; for the gG(-)gE(-)TK(-) vaccine, the survival rate was 50% (4/8). The raccoons with higher vaccine-induced antibody titers were more likely to survive the challenge with the virulent PRV; there was a 100% mortality rate for raccoons lacking detectable anti-PRV antibodies. This experiment indicates that exposure of raccoons to modified live gene-deleted PRV vaccines may result in an immune response, and that this immunity provides some protection against exposure to virulent virus.

Effect of maternally acquired Aujeszky's disease (pseudorabies) virus-specific antibody in pigs on establishment of latency and seroconversion to differential glycoproteins after low dose challenge

This study investigated whether (1) passively immune pigs could become latently infected after challenge with low doses of wild type pseudorabies virus (PRV) and (2) if seroconversion to PRV could be consistently detected using two commercially available differential diagnostic ELISAs. Three litters of piglets with passively acquired PRV serum neutralizing (SN) antibody (geometric mean titers 47.03 to 95.10) were challenged at 6 to 12 days of age with 236 to 500 TCID50 of Shope strain virus; pigs were vaccinated at 11 weeks of age with a commercially available genetically engineered vaccine (TK- gE- gG- Iowa S62 strain PRV). Vaccination was intended to reduce the risk of reactivation of latent infection resulting in spread of virulent PRV infection to previously uninfected pigs during the experiment. Vaccination at this age also approximated common field practices in infected herds. After 15 weeks, all challenged pigs were seropositive on the PRV glycoprotein (g or gp) E differential ELISA but were seronegative on the gG differential ELISA. All three challenge groups had pigs that were latently infected as evidenced by the detection of PRV DNA by polymerase chain reaction (PCR) assay of their trigeminal ganglia (TG). There was a significant inverse relationship observed for age at challenge and the proportion of PCR positive pigs in the group 15 weeks postchallenge (p = 0.0004). This trend was independent of the passively acquired PRV SN antibody titers at challenge. In this study, passively acquired antibody did not provide protection against establishment of latent infection in piglets after exposure to low doses of virulent PRV. These latent infections were detected serologically by only one of two available differential diagnostic ELISA.

Mechanisms of transmission of Aujeszky's disease virus originating from feral swine in the USA

To understand the possible mechanisms of transmission of Aujeszky's disease virus (pseudorabies or PRV) from a feral pig reservoir, intranasal infections were initiated in domestic pigs and in pigs from a herd derived from captured feral pigs. Virus strains originating from feral pigs and from domestic pigs were compared. Similar shedding patterns were obtained in both feral-derived and domestic pigs, however, virus strains from feral pigs were markedly attenuated. Virus could be isolated after acute infection from nasal secretions, tonsils and occasionally from genital organs. In studies of transmission of PRV by cannibalism, either latently infected or acutely infected tissue was fed to both domestic and feral-derived pigs. In two similar experiments, latently infected tissue did not transmit virus, but tissues from acutely infected pigs did transmit infection. Cannibalism was observed typically in both types of pigs older than 6 weeks of age. It was concluded that transmission of PRV originating from feral pigs can occur by several mechanisms including the respiratory route and by cannibalism of pigs that die of acute infection. Transmission of PRV from feral swine may, however, result in sub-clinical infection.

The role of biotechnologically engineered vaccines and diagnostics in pseudorabies (Aujeszky's disease) eradication strategies

Modern-day biotechnology has an almost unlimited number of possibilities for reducing the impact of hereditary and infectious diseases. To date one of its most visible and rewarding applications for veterinary medicine has been in the genetic engineering of vaccines and diagnostics to assist in the eventual eradication of pseudorabies (PR, Aujeszky's disease). In the following review we summarize some of the most pertinent issues relative to PR eradication and point out the present and potential role of biotechnology in achieving our goal.

Evaluation of serological tests for the detection of pseudorabies gE antibodies during early infection

Two enzyme-linked immunosorbent assays (ELISAs) and a particle concentration fluorescence immunoassay (PCFIA) were compared for their ability to detect antibodies against pseudorabies virus (Aujeszky's disease virus) glycoprotein E (gE) in the early stages of infection in pigs previously vaccinated with gE-deleted pseudorabies vaccines. Seventy pigs were included in the study. Five groups of 6 pigs each were vaccinated with one of 5 different pseudorabies virus (PRV) gE-deleted vaccines, and subsequently infected intranasally with 10(5.6) TCID50 of the Iowa 4892 pneumotropic strain of PRV. This entire procedure was repeated using 10(4.6) TCID50 of the Rice strain of PRV. Five unvaccinated control pigs were also challenged with each virus strain. Three control pigs died before seroconverting, leaving 67 pigs for comparison. Blood samples were drawn from experimentally inoculated pigs on the day of vaccination, the day of challenge, and on 4-10, 14, and 21 days postchallenge (DPC). Serology test sensitivity estimates and comparisons among tests were made for each sampling day. Results of this study demonstrated differences among the tests in the time from inoculation to initial antibody detection, and the time to detect 50% and 75% of the infected pigs. The average time until first detection of pigs as seropositive for gE antibodies by PCFIA was 7.5 DPC. The blocking ELISA detected pigs as seropositive an average of 8.8 DPC, and the indirect ELISA first detected gE antibodies by 9.3 DPC. Fifty percent of the pigs were detected as seropositive by days 7, 8, and 9 for the PCFIA, blocking ELISA, and indirect ELISA, respectively. Similarly, 75% of the pigs were detected as seropositive by days 8, 9, and 10 for the PCFIA, blocking ELISA, and indirect ELISA, respectively. All pigs were detected as seropositive by 14 DPC for all 3 tests.

Circumvention of maternal antibody interference by immunization of newborn pigs with glycoprotein gIII-deleted marker vaccine

Maternal antibodies interfere with the immunization of swine by modified live-virus pseudorabies virus (PRV) vaccines. To test the hypothesis that a PRV vaccine attenuated by deletions in the thymidine kinase (TK) and gIII genes might reduce interference by maternal antibodies, pigs with moderate to low levels of colostral PRV antibodies were immunized with the TK- gIII-OMNIMARK-PRV vaccine. Vaccinates and non-vaccinates were challenged intranasally with virulent PRV at 7 weeks of age. In support of the hypothesis, it was found that central nervous system (CNS) and/or respiratory disease developed in six out of 10 controls with a fatal outcome in one, while two out of 13 vaccinates showed only very mild and transient CNS or respiratory disease signs with no fatalities. All vaccinates gained weight while non-vaccinates initally lost weight. At post-challenge day (PCD) 11, vaccinates showed 4.5 lb/pig greater weight gain than non-vaccinates. Virus neutralization (VN) analyses before and after challenge showed that vaccinates had been primed immunologically. In another experiment, newborn pigs from a pseudorabies disease-quarantined herd with high VN antibody titres were vaccinated, respectively, with the gIII-TK-OMNIMARK-PRV vaccine, a TK-gI-gX-vaccine, or no vaccine and challenged with virulent PRV at 14 weeks of age when VN titres were < 1:2. By PCD 9, the TK-gIII-group had outgained the TK-gI-gX- and the control groups, respectively, by 6.0 and 3.2 lb per pig.

Evaluation of serological pseudorabies tests for the detection of antibodies during early infection

Six enzyme-linked immunosorbent assays, a latex agglutination test, and the standard microtitration serum virus neutralization test were compared for their ability to detect antibodies against pseudorabies virus (PRV) during the early stages of infection. Thirty-five pigs were infected intranasally with 10(5)-10(7) TCID50 of either the Iowa 4892 pneumotropic or the Becker strain of PRV. Blood samples were drawn from experimentally inoculated animals on days 4-10, 14, and 21 postchallenge. Test sensitivity estimates and comparisons among tests were made for each sampling day over the 21-day monitoring period. Results of this study demonstrated differences among tests in 1) the time from inoculation to initial antibody detection, 2) the time to detect > or = 95% of the infected pigs, and 3) the time from initial antibody detection to determination of > or = 95% as positive. By day 10 postchallenge, no statistically significant difference in diagnostic sensitivity was observed among the 8 tests compared in the study.

Gene specific assay to differentiate strains of pseudorabies virus

The need for compatibility between Pseudorabies vaccination and disease eradication measures has caused the production and release of diverse Pseudorabies virus (PRV) vaccine strains with altered genetic makeups due to the deletion of specific genes. These genes code for antigens used as differential serologic markers. By use of polymerase chain reaction (PCR), it is possible to determine, in a rapid and sensitive way, if a given PRV strain has a "wildtype" genotype, or if instead it carries a deletion for a specific gene. A sequence of 217 bp was selected as an amplification target within the gene of the essential glycoprotein 50 (gp50). Another sequence of 173 bp was selected in the joint area of glycoprotein 63 (gp63) and glycoprotein I (gI) genes. Under optimal amplification conditions, the simultaneous use of both PCR tests allowed us to differentiate specifically gI negative strains from several other wild type PRV strains, utilizing cell culture-propagated virus, acutely and latently infected neural tissue of mice and pigs as source of DNA targets. This kind of test will be useful for the rapid identification of PRV strains detected in tissues from individual animals, especially in cases of single reactors occurring in vaccinated herds. At the same time, the gene-defined PCR test will be useful for the evaluation of vaccines in their ability to prevent latency, by permitting unequivocal differentiation between vaccine and challenge virus strains.

Evaluation of the sensitivity and specificity of two diagnostic tests for antibodies to pseudorabies virus glycoprotein X

The diagnostic performance of 2 enzyme-linked immunosorbent assays (gX-T, gX-H) for antibodies to pseudorabies virus (PRV) glycoprotein X (gX) were evaluated using 311 serum samples from a nonvaccinated quarantined herd. When the standardized virus neutralization (VN) test, which uses the Shope strain (VN Shope), was used as the comparative diagnostic standard, the gX-T test had a 7% false-negative rate and a 52% false-positive rate, and the gX-H test had a 19% false-negative rate and a 19% false-positive rate. When the VN test with a Bartha recombinant strain (VN Bartha gIIIKa) was used as the diagnostic standard, the gX-T test had a 9% false-negative rate and a 26% false-positive rate, and the gX-H test had a 24% false-negative rate and a 11% false-positive rate. Thus, the gX-T test was more sensitive and the gX-H test was more specific. Additional diagnostic tests on 79 serum samples from a noninfected herd did not produce false positives for the gX-H test, but there was an 8% false-positive rate for the gX-T test. Previous studies from our laboratory have demonstrated that VN Bartha gIIIKa has higher sensitivity than VN Shope, without losing specificity, and thus is a better comparative diagnostic standard. When adding a suspect range to the gX-T test, using the same criteria as the suspect range for the gX-H test, the false-positive rate of the gX-T test was reduced to 5% when evaluated versus VN Bartha gIIIKa in the infected herd and to 1% for the PRV-negative herd.(ABSTRACT TRUNCATED AT 250 WORDS)