Natural infection of cattle with an atypical `HoBi'-like pestivirus – Implications for BVD control and for the safety of biological products

During a study on Bovine Viral Diarrhoea (BVD) epidemiology in Thailand, a pestivirus was detected in serum from a calf. Comparative nucleotide sequence analysis showed that this virus was closely related to a recently described atypical pestivirus (D32/00_'HoBi') that was first isolated from a batch of foetal calf serum collected in Brazil. The results from virus neutralisation tests performed on sera collected from cattle in the herd of the infected calf, showed that these cattle had markedly higher antibody titres against the atypical pestivirus 'HoBi' than against Bovine Viral Diarrhoea Virus types 1 and 2, or Border Disease Virus. The results also supported, consequently, the results from the molecular analysis, and demonstrated that a 'HoBi'-like pestivirus had been introduced to, and was now circulating in the herd. This study is the first to report a natural infection in cattle with a virus related to this atypical pestivirus, and it suggests that this group of pestiviruses may already be spread in cattle populations. The findings have implications for BVD control and for the biosafety of vaccines and other biological products produced with foetal calf serum. Consequently, these atypical pestiviruses should be included in serological assays, and any diagnostic assay aimed at detection of pestiviruses in biological products or animals should be tested for its ability to detect them.

[Seroepidemiological survey of sheep in Carinthia for the dissemination of ruminant pestiviruses]

In this study serological investigations were performed to determine the prevalence of pestiviral infections in sheep in one Federal State of Austria, namely Carinthia. 1527 blood samples from sheep in 147 flocks were collected and tested by Enzyme-linked immunosorbent assay and virus-neutralisation tests for antibodies to ruminant pestiviruses. The estimated flock prevalence was 47.6%, the individual prevalence 16.3%. Significant geographical variations in the flock as well in the individual prevalence were found. The highest prevalence in sheep and in sheep flocks was established in the region Spittal/Drau with 25.9% and 69.7%.The individual and the flock prevalence was significantly higher on farms where cattle or sheep from other farms were present than on farms with no cattle (p < 0.017). All Enzyme-linked immunosorbent assay positive sera were tested for Bovine viral diarrhea virus-1 (strain NADL), Bovine viral diarrhea virus-2 (strain 125) and for Border disease virus (strain MOREDUN) by virus neutralisation tests. Seventy out of 249 positive samples revealed the highest titres (> or = two-fold) to Bovine viral diarrhea virus-1 and 25 to Border disease virus. The remaining positive samples did not show clear results because of cross reactions.

Development of a real time RT-PCR to detect and type ovine pestiviruses

A real time one-step RT-PCR was designed to detect and type border disease virus (BDV), bovine viral diarrhea virus (BVDV) type 1 and BVDV type 2 in ovine samples. The real time RT-PCR was shown to behave in a linear manner and had limits of detection of 100-1000 copies of viral RNA as judged by in vitro transcribed RNA. The real time RT-PCR was validated on 50 clinical samples from UK flocks and was more sensitive than a virus isolation and a classical nested RT-PCR (nRT-PCR). The results of real time RT-PCR virus typing agreed completely with sequencing. The majority of ovine isolates were BDV; a small proportion were BVDV type 1. BVDV type 2 was not detected in any sample. This test appears reliable and can be used for the typing of ovine pestiviruses in the UK.

[Detection and species-specific differentiation of pestiviruses using real-time RT-PCR]

An important prerequisite for an efficient eradication of pestiviruses like bovine viral diarrhea virus or classical swine fever virus are sensitive and specific detection methods. Beside antigen detection with antigen capture ELISAs and virus isolation using cell culture, the detection of virus genomes by reverse transcriptase polymerase chain reaction (RT-PCR) becomes more and more important. By using real-time RT-PCR, the disadvantages of conventional PCR methods concerning the risk of contamination and missing specificity controls are minimized. In the following, the validation and application of different real-time RT-PCR systems for the detection of pestiviruses are presented. In addition, a new "panpesti" probe was tested, and the ability to differentiate between the major species of pestiviruses with known PCR protocols was determined using the Light Cycler PCR machine.

[Heterogeneity of ruminant pestiviruses: academic interest or important basis for the development of vaccines and diagnostics?]

Pestiviruses cause economically important diseases of farm animals. Members of the Pestiviruses are bovine viral diarrhea virus 1 (BVDV-1), BVDV-2, classical swine fever virus (CSFV) and border disease virus (BDV). Phylogenetic analyses based on the entire nucleic acid sequence encoding the Npro allow a statistically significant segregation of established species and of subgroups within the species. BVDV-1 strains isolated in Germany can be associated with at least five different subgroups. In contrast all BVDV-2 isolates detected in Germany so far are closely related, belonging to one subgroup. A group of virus isolates from sheep and zoo animals is clearly different from established pestivirus species and can be designated as BDV-2. Antigenetic relatedness of pestiviruses was studied using defined virus isolates and antisera in cross-neutralization assays. Six antigenic groups were distinguished corresponding to the genetic clusters BVDV-1, BVDV-2, CSFV, BDV-1, BDV-2 and Giraffe-1. A significant antigenic difference was also observed between members of subgroups 1a and 1b of BVDV-1. Studies on the genetic and antigenic heterogeneity of pestiviruses are important for the development of new vaccines, diagnostic tests and for eradication programs.

Leukopenia and thrombocytopenia in pigs after infection with bovine viral diarrhoea virus-2 (BVDV-2)

The aim of this study was to investigate whether bovine viral diarrhoea virus-2 (BVDV-2) is pathogenic for pigs, which organs become infected and whether or to which extent the virus is excreted into the environment. Ten pigs were observed for clinical reactions after infection with a BVDV-2 strain, that has been shown to be pathogenic in calves under experimental conditions. Samples were taken to monitor thrombocyte and leukocyte counts as well as antibody development. Post mortem examinations were performed at 7, 11 and 27 days after infection. Tissue samples were collected for virus isolation, histological and immunohistological examination. All ten pigs became infected and BVDV could be re-isolated from the lymphocytes, the plasma and different lymphatic organs. The infection passed clinically inapparent, apart from a slight increase in body temperature in some animals. Some animals developed a slight leukopenia and/or thrombocytopenia. There were no macroscopic or histological lesions observed that could specifically be related to the inoculation of BVDV-2. With respect to all parameters studied, the infection and the consequences thereof were clearly less pronounced in pigs as compared to cattle, the natural host. Our results indicate, that pigs infected with BVDV-2 might develop antibodies that cross-react in tests for antibodies against classical swine fever virus.

Enzyme-linked immunosorbent assay using a virus type-specific peptide based on a subdomain of envelope protein E(rns) for serologic diagnosis of pestivirus infections in swine

Peptides deduced from the C-terminal end (residues 191 to 227) of pestivirus envelope protein E(rns) were used to develop enzyme-linked immunosorbent assays (ELISAs) to measure specifically antibodies against different types of pestiviruses. The choice of the peptide was based on the modular structure of the E(rns) protein, and the peptide was selected for its probable independent folding and good exposure, which would make it a good candidate for an antigenic peptide to be used in a diagnostic test. A solid-phase peptide ELISA which was cross-reactive for several types of pestivirus antibodies and which can be used for the general detection of pestivirus antibodies was developed. To identify type-specific pestivirus antibodies, a liquid-phase peptide ELISA, with a labeled, specific classical swine fever virus (CSFV) peptide and an unlabeled bovine viral diarrhea virus peptide to block cross-reactivity, was developed. Specificity and sensitivity of the liquid-phase peptide ELISA for CSFV were 98 and 100%, respectively. Because the peptide is a fragment of the E(rns) protein, it can be used to differentiate between infected and vaccinated animals when a vaccine based on the E2 protein, which is another pestivirus envelope protein, is used.

Genetic diversity of pestiviruses: identification of novel groups and implications for classification

The complete Npro coding sequences were determined for 16 pestiviruses isolated from cattle, pig, and several wild ruminant species including reindeer, bison, deer, and bongo. Phylogenetic analysis enabled the segregation of pestiviruses into the established species bovine viral diarrhea virus-1 (BVDV-1), BVDV-2, border disease virus (BDV), and classical swine fever virus (CSFV). For BVDV-1 five distinct subgroups were identified, while BVDV-2, BDV, and CSFV were each subdivided into two subgroups. The virus isolates from bongo and deer as well as one porcine virus isolate belong to BVDV-1. Interestingly, the isolates from reindeer and bison are distinct from the established pestivirus species. The Npro sequences from these two viruses are more similar to BDV than to the other pestivirus species. Calculation of the pairwise evolutionary distances allowed a clear separation of the categories species, subgroup, and isolate only when the reindeer/bison viruses were considered as members of an additional pestivirus species. Furthermore, the entire E2 coding sequences of a representative set of virus isolates covering all recognized species and subgroups were studied. Segregation of pestiviruses based on the E2 region was identical with that obtained with the N(pro) sequences.

Immunohistochemical diagnosis of pestivirus infection associated with bovine and ovine abortion and perinatal death

OBJECTIVE

To establish a reliable, rapid, economical method for detection of pestivirus infection in bovine and ovine fetuses and to examine participation of these viruses in abortions and neonatal mortality.

ANIMALS

213 bovine and 31 ovine fetuses, as well as 36 newborn calves and 25 lambs, which had died within 3 days after birth, were tested for bovine viral diarrhea virus (BVDV) and border disease virus by use of different methods.

PROCEDURE

Detection of BVDV in fetuses was performed by immunohistochemical methods, using a panel of monoclonal antibodies against pestivirus antigens on cryostat and paraffin sections and by virus isolation in cell culture; in some instances, an antigencapture ELISA was performed. Results of the various methods were compared.

RESULTS

Sensitivity of BVDV detection by immunohistochemical methods and virus isolation in cell culture was equal; however, it decreased in association with autolysis. In autolytic fetuses, use of formalin-fixed, paraffin-embedded brain sections was the most favorable method. Antigen detection by ELISA was less sensitive.

CONCLUSIONS

Immunohistochemical analysis of cryostat sections of brain, skin, thyroid gland, abomasum, and placenta is a rapid, sensitive method for detecting pestiviruses in fetuses. In the presence of advanced autolysis, this method used on formalin-fixed, paraffin-embedded brain sections is recommended over the other described methods.

Antigenic differences among pestiviruses

Monoclonal antibody typing of pestiviruses, supplemented with genetic sequence comparisons, has shown that at least four antigenic clusters can be identified within the genus. One cluster represents "classic" bovine viral diarrhea strains, which also have been isolated occasionally from sheep and pigs, whereas a second group corresponds to classic swine fever (hog cholera) strains. A distinct group of "true" border disease isolates is found in sheep and has been isolated occasionally from swine. The fourth group is antigenically quite distinct form the others and has been isolated from sheep, pigs, and cattle. It includes recent American isolates from cases of acute severe hemorrhagic disease. The implications of these findings for diagnosis and control are discussed.

[Epidemiological characterization of a pestivirus isolate from a viremic pig out of a mixed pig-cattle herd]

A pestivirus isolated from a healthy pig out of a mixed pig-cattle holding was identified by use of monoclonal antibodies as a porcine virus related or identical to bovine viral diarrhoea (BVD) virus. About 7% of the pigs of this herd showed neutralizing antibodies (nab) against BVD and Border Disease (BD) virus and against the homologous porcine nonclassical swine fever (CSF) pestivirus isolate 10421/Han94. The nab titres against this virus were clearly higher than against CSF virus strain Alfort/187. Amongst the cattle kept in the farm no BVD viraemic animal was detected. About 13% of them showed nab to BVD virus with higher titres against the BVD virus strain NADL than against the porcine pestivirus virus isolate 10421/Han94. There was no of a BVD virus transmission from the cattle to the pigs within this herd. The problem of misinterpretation of non-CSF virus isolation from pigs is discussed. The necessity of identifying pestiviruses of porcine origin by use of species-specific monoclonal antibodies is stressed in order to prevent erroneous declarations of CSF outbreaks.

Ruminant pestiviruses

The ruminant pestiviruses, bovine virus diarrhoea virus (BVDV) and border disease virus (BDV) are highly successful and important pathogens which infect ruminant species worldwide. Although the serological relationships among ruminant pestiviruses require further clarification, there is growing evidence for two antigenic groups, one of which predominates in cattle and one in sheep. The success of pestiviruses stems from the ability of the non-cytopathic (NCP) biotype of the virus to cross the placenta and establish a persistent infection (PI) in the developing foetus. This biotype should be regarded as the 'normal' biotype with the cytopathic (CP) biotype being an abnormal virus that is usually isolated only from PI animals dying from mucosal disease. Recent molecular evidence points to CP viruses arising from their NCP counterparts by recombination events that include the insertion of host RNA and/or the duplication of viral RNA sequences. However, the biological mechanism through which CP viruses kill cells remains unknown. Virtually all CP and NCP viruses cause only mild, transient clinical symptoms in healthy adult animals and stimulate a protective immune response. Despite the urgent requirement for a safe, effective vaccine, there is still no commercial vaccine that has been shown to immunize dams so that foetal infection is prevented. In the absence of an effective vaccine, reliable diagnostic techniques are essential to implement effective control measures. There is now a range of monoclonal antibody-based enzyme-linked immunosorbent assays for identifying PI or convalescent animals. These tests are specific, rapid, sensitive and reliable but may themselves become redundant as they are superceded by ever-increasing molecular biology-based techniques.

[Field infection with BVD virus in swine: epidemiology and diagnosis]

In a pig breeding herd in Lower Saxony infertility of breeding sows had been repeatedly observed. Growth retardation and post mortem findings in two piglets gave clinical indication to swine fever/hog cholera. A virus was isolated and typed by monoclonal antibodies as pestivirus not identical with hog cholera virus (HCV). In neutralization tests applying the field isolate, HCV and bovine viral diarrhea (BVD) virus the sera breeding sows and weaner pigs yielded high neutralizing antibody titres against the pestivirus field isolate but low titres against HCV. Specific antibodies against HCV were ruled out by a complex trapping blocking (CTB) ELISA. Intranasal inoculation of a weaner pig with spleen homogenate led to a short-termed viraemia without clinical signs but seroconversion with high antibody titres against the homologous pestivirus. In an in-contact pig no virus was detected and no antibody demonstrated within a period of 18 weeks.