Establishment and characterisation of a porcine rubulavirus (LPMV) persistent infection in porcine kidney cells

Long-term RNA persistence of porcine rubulavirus (PorPV-LPMV) after an outbreak of a natural infection: the detection of viral mRNA in sentinel pigs suggests viral transmission

The persistence of porcine rubulavirus (PorPV-LPMV) in five pigs that had survived an outbreak of a natural infection was determined. After the resolution of the outbreak, each animal was housed in an isolation pen together with one sentinel pig. Approximately every 2 months thereafter one group of animals was euthanized and tissue samples taken for virological and serological analysis. Infectious virus was not isolated from any samples; antibodies to PorPV-LPMV were detected in convalescent pigs by virus neutralisation test and blocking ELISA but not in sentinel pigs. PorPV-LPMV mRNA of the nucleoprotein (NP) and phosphoprotein (P) genes was detected by a nested polymerase chain reaction (nPCR) in samples of trigeminal and optic nerves, cervical spinal cord, tonsils, salivary gland, lung and pancreas from convalescent pigs. mRNA was also detected in the midbrain, corpus callosum, or olfactory bulb in four out of five pigs by nRT-PCR, this result was confirmed by the sequencing of a 260bp PCR product of P gene region. The highest average viral copies/μg of total RNA occurred in the olfactory bulb and pancreas tissues of convalescent pigs and midbrain, tonsil and pancreas of sentinel pigs housed with the convalescent pigs. Satellitosis and gliosis of the midbrain, olfactory bulb, corpus callosum, medulla oblongata or choroid plexus were microscopically observed in four convalescent pigs. The control pig remained negative in all tests. The results indicate that PorPV-LPMV mRNA persists and induces a durable humoral immune response in pigs that have recovered from a natural infection. After a possible reactivation of the virus, it was transmitted to sentinel pigs in contact with the convalescent pigs.

Development of a real-time RT-PCR method for detection of porcine rubulavirus (PoRV-LPMV)

In order to provide a rapid and sensitive method for detection of the Porcine rubulavirus La Piedad-Michoacan-Mexico Virus (PoRV-LPMV), we have developed a specific real-time reverse transcriptase polymerase chain reaction assay. The detection of PoRV-LPMV, represents a diagnostic challenge due to the viral RNA being present in very small amounts in tissue samples. In this study, a TaqMan(®) real-time PCR assay was designed based on the phosphoprotein gene of PoRV-LPMV, to allow specific amplification and detection of viral RNA in clinical samples. Assay conditions for the primers and probe were optimized using infected PK15 cells and ten-fold serial dilutions of a plasmid containing the whole P-gene. The sensitivity of the developed TaqMan(®) assay was approximately 10 plasmid copies per reaction, and was shown to be 1000 fold better than a conventional nested RT-PCR. The performance of this real-time RT-PCR method enables studies of various aspects of PoRV-LPMV infection. Finally, the assay detects all current known variants of the virus.

Uptake of porcine rubulavirus (LPMV) by PK-15 cells

BACKGROUND

The porcine virus denominated La Piedad Michoacan Virus (LPMV) is a member of the family Paramyxoviridae and is the cause of a disease in pigs present only in Mexico. The disease is characterized by meningoencephalitis and respiratory distress in young pigs, epididymitis and orchitis in boars, and reproductive failure and abortion in sows.

METHODS

The cytopathology, morphology, and distribution of the hemagglutination neuraminidase (HN) and nucleoprotein (NP) proteins of LPMV were investigated following inoculation into PK-15 cells. The cytopathic effect was characterized by cytoplasmic vacuolation and the formation of syncytia and cytoplasmic inclusion bodies.

RESULTS

In immunofluorescence assays using a monoclonal antibody (MAb) against the HN protein at 5-60 min post-infection (early infection), a diffuse immunofluorescence was observed near the cell membrane and adjacent to the nuclear membrane. At 24 h post-infection (late infection), a dust-like immunofluorescence was observed throughout the cytoplasm. LPMV-infected cells incubated with the MAb against the NP protein showed punctate cytoplasmic fluorescence during the early stages of infection. At the late infection stage, these fluorescent particles became larger and were seen predominantly in the cytoplasm of syncytia. This pattern was also apparent by immunohistochemical labeling and immunogold electron microscopy. The latter technique revealed that HN protein was diffusely distributed throughout the cytoplasm. When using the MAb against the NP protein, nucleocapsid organization was the most prominent feature and resulted in the formation of cytoplasmic inclusion bodies visible by light and electron microscopy. Immunogold labeling of purified nucleocapsids was shown by electron microscopy. Virus particles and nucleocapsids were morphologically similar to members of the Paramyxoviridae family.

CONCLUSIONS

The morphologic characteristics of the virions and the distribution patterns of the HN and NP proteins in PK-15 infected cells indicate that the mechanisms of LPMV replication are generally similar to those of the members of the Paramyxoviridae family.