Visna Virus Infection Research Articles

A chronic indurative mastitis in sheep, associated with maedi/visna virus infection.

A possible association of a chronic indurative mastitis with mvv (maedi visna virus) infection in sheep was investigated. Sheep of four flocks (group A), in which insufficient lamb growth apparently associated with insufficient milk production and probably related to udder induration, was a serious problem, were clinically examined in mid-lactation. The results were compared with four mvv-free flocks (group B) without such complaints. The incidence of udder induration in group A (n = 263) was about eight times higher than in group B (n = 206): 63.1 versus 8.0%. The clinical picture differed essentially between the groups. In group A the udder abnormalities were of a diffuse and indurative nature, involving both udder halves, while in group B the udder lesions were mainly nodular and often limited to one udder half. Bacteriological examination revealed a difference in infection rate of the udders (6.8% in group A versus 14.1% in group B). A substantial difference was observed in a comparison of the bacteriological infection rate of the clinically abnormal udders (5.4% in group A, versus 47.0% in group B). Serological examination with an indirect ELISA revealed 81% seropositive sheep in group A, versus 0% in group B. Twelve sheep of group A and five of group B, called for reason of mastitis, were selected for pathological examination. The gross and microscopic lesions showed a pronounced difference between both groups. In group A a diffuse interstitial mastitis with slight to moderate fibrosis and a pronounced lymphoid hyperplasia was observed. In group B the mastitis had a nodular character, with a chronic galactophoritis, extensive fibrosis, and in some cases chronic abscesses.(ABSTRACT TRUNCATED AT 250 WORDS)

The emergence of antigenic variants is a rare event in long-term visna virus infection in vivo.

Six sheep persistently infected with visna virus were studied for 4 1/2 to 5 3/4 years until they became ill. Virus was isolated at intervals from peripheral blood leukocytes and cerebrospinal fluid (CSF), and at the time of sacrifice from various parts of the brain and the lungs. Both brain and lungs showed lesions typical of advanced visna/maedi. All the sheep formed antibodies in sera and CSF. Virus isolates from each sheep were tested in neutralization tests against sera and CSF collected from the same animal. In one sheep all isolates were found to be identical to the inoculated virus by this test. In each of the other sheep an antigenic variant emerged from 1 to 3 years after inoculation and remained in circulation even after the formation of autologous antibodies. In one case a variant was isolated from the lungs, whereas in all cases the virus isolated from the brain was identical to the inoculated virus. The results show that antigenic variants are rare in visna and do not seem to have a role in the pathogenesis of the disease.

Wild-type temperature-sensitive and -resistant visna viruses: isolation and biological comparison.

The plaques formed by wild-type (WT) populations of strain K485 visna (V-K485) virus, strain K796 visna (V-K796) virus, and of progressive pneumonia virus in sheep choroid plexus (SCP) cell cultures are heterogeneous in size. Plaque purification procedures showed that this heterogeneity was due to the presence of two biologically different viruses, large plaque-forming (Lpf) virus and small plaque-forming (Spf) virus. The V-K796 Lpf virus, the V-K796 Spf virus, and the V-K796 WT virus are antigenically similar to each other. Although the V-K796 Spf virus is less cytopathic than the V-K796 Lpf virus, both viruses have a 14- to 16-hour latent period in SCP cells and have similar rates of synthesis for the initial 24 to 30 hours after infection. The V-K796 Spf virus is considered a temperature-sensitive visna virus since, unlike the V-K796 Lpf virus, it does not produce plaques at 41 degrees C and it is more thermosensitive than the V-K796 Lpf variant. Population analyses of two strains of wild-type visna virus and one strain of progressive pneumonia virus demonstrated that these populations contain variants with the phenotypes of V-K796 Spf virus and V-K796 Lpf virus, that the Spf virus was not selected against in sheep, and that cultivation of the wild type visna viruses at 37 degrees C selected against the temperature-sensitive variants. The existence of the temperature-sensitive Spf viruses in these wild-type virus populations suggests that the Spf variants maintain the persistent visna virus infection in sheep and delay the development of the host's cell-mediated immune response to the viral proteins.

Gene expression in visna virus infection in sheep.

Visna is a slow degenerative disease of the central nervous system (CNS) of sheep caused by a nontumorigenic retrovirus. During the course of this disease, visna virus establishes a persistent infection of the CNS, lung and haematopoietic system, despite a specific humoral and cellular immune response. We have studied visna virus life cycle at the single-cell level in choroid plexus of experimentally infected animals, using a very sensitive and quantitative in situ hybridization assay. We report here that although proviral DNA is synthesized in significant amounts, its expression is blocked at the transcriptional level. This restriction of proviral DNA transcription offers an explanation for the slowness of the disease and the persistence of the infection.

Early Immune Responses in Visna, a Slow Viral Disease of Sheep

The initial phase of the long incubation period of visna virus infection of sheep was studied by intracerebral inoculation of the 1514 strain of visna virus into 12 BorderLeicester sheep. Cell-free virus was recoverable only during the first two weeks after inoculation. Thereafter, virus could be recovered by cocultivation of cerebrospinal fluid (CSF) cells or peripheral blood leukocytes (PBL) or by explantation of tissues. Virus-specific cell-mediated immunity, measured by stimulation of [3H]thymidine incorporation into PBL and CSF cells, appeared within one week, peaked by two weeks, and then fell to control values by six weeks after inoculation. Histopathologic changes consisted of an initial acute meningoencephalitis changing to a more chronic inflammatory process with the development of dense perivascular infiltration and formation of germinal centers. During the chronic phase virus-neutralizing antibody first appeared in the serum and then in the CSF. We conclude that the initial immune responses of sheep to infection with visna virus are similar to those seen in acute viral infections but fail to eliminate the virus, which eventually results in the slowly evolving neurologic disease visna.

Ovine cells: Their long-term cultivation and susceptibility to visna virus

Sheep choroid plexus (SCP) cells have been subcultured more than 120 times and have undergone over 300 cell generations. These fibroblastic-appearing SCP II-B cells contain ovine-specific antigens, have an absolute plating efficiency of 23 to 28% and are as susceptible to visna virus infection and virus-induced cytopathology as their low passage level counterparts. Cultures of low, relatively high and high passage level SCP cells produced equivalent amounts of visna virus at similar rates when infected with equal amounts of visna virus. The passage level of the SCP II-B cells, their elapsed number of cell generations, their possession of ovine-specific antigens and their full susceptibility to visna virus allow these cells to be considered an established line of sheep cells.

Infectivity of visna virus DNA

Abstract Deoxyribonucleic acid isolated from cells infected with the RNA containing slow virus visna confers on uninfected cells the capacity to synthesize new virus. The cytopathic agent isolated in these experiments has the biological and biochemical properties of visna virus, and the active material is DNA by several criteria, the most important of which is the loss of infectivity with prior treatment of the DNA with deoxyribonuclease. The efficiency of infection with DNA is markedly enhanced by presenting DNA to the cell as a calcium DNA phosphate precipitate. Because of the lytic nature of visna virus infection, the infectivity of visna DNA can be assayed quantitatively by plaque formation. Our major findings with this infectivity assay include the following: (1) Duplex DNA is infectious while denatured DNA is not. (2) At least one complete copy of proviral DNA is integrated in the host genome. (3) The minimal infective size of duplex DNA corresponds to a transcript of one subunit of viral RNA. (4) However, the concentration dependence of infectivity is two hit over the size range of 10–30 million molecular weight. These findings suggest that the unique genetic information of the viral genome is distributed over more than one subunit and that the proviral DNA transcripts are unlinked in the cell. The usefulness of the DNA infectivity assay and the implications of the visna provirus for persistent viral infections are discussed.

Long-term Visna virus infection of sheep choroid plexus cells: initiation and preliminary characterization of the carrier cultures.

Two sheep choroid plexus cell cultures were maintained and propagated for 413 days since being infected with strain K796 visna virus. The majority of the cells in these cultures contained visna virus-specific antigen on days 93 and 105 after infection. Reverse transcriptase-like activity similar to that present in visna virus preparations was obtained from these cultures when very little plaque-forming virus was being synthesized. The persistently infected cultures are resistant to the cytopathic effect which occurs in uninfected cultures upon exposure to visna virus. Persistently infected cells require more time than uninfected cells to become confluent. Less than 0.02 percent of the persistently infected sheep choroid plexus cells form macroscopic colonies within 14 days, whereas 20 to 30 percent of the cells from uninfected cultures form macroscopic colonies within this time.

Visna virus infection of American lambs.

Random-bred fetal and 4-week-old American lambs, inoculated intracerebrally with visna virus, developed a persistent infection in the brain and sometimes in the lung. The pathologic changes present in these lambs were similar to the early lesions of visna in Icelandic sheep, thus providing a possible model for the study of virus-induced demyelinating disease.