Pathogenicity and persistence of pleural effusion disease virus isolates in rabbits.

Nine isolates of pleural effusion disease agent or virus (PEDV) from treponema-infected rabbits in various countries were examined for pathogenicity and persistence in rabbits. The isolates showed a wide range of pathogenicity and were categorized into three groups according to the severity of the acute infection. Group 1 comprised isolates causing more than 50% mortality, group 2 isolates causing mortality below 50%, while group 3 comprised isolates causing almost subclinical infections. The range between group 1 and group 3 was similar to that observed with virulent and avirulent progeny of the original PEDV isolate. Infection by each of the nine isolates resulted in a chronic low level viraemia which persisted for up to 2 years or more. Viral progeny of pathogenic isolates obtained in serum after the 2nd month of infection failed to induce clinical disease on rabbit inoculation. The chronic, subclinical infection was associated with a moderate, continued increase in serum IgG, but circulating immune complexes could not be demonstrated. Two years after infection slight histopathological changes were present in lymph nodes, spleen, liver, heart and lung. Evidence of immune complex disease could not be demonstrated.

Pathogenetic observations on pleural effusion disease in rabbits.

A pathogenetic study of pleural effusion disease (PED) in rabbits was made, using the virulent PED agent or virus (PEDV) and an avirulent derivate of this isolate. Independent of infective dose within the range examined, the virulent isolate caused fatal clinical disease, whereas the avirulent isolate caused subclinical infection. The two isolates differed in rapidity of initial spread of infection and in the maximum virus titres in serum, but they both resulted in a similar low level persisting viraemia. Circulating virulent virus gradually became avirulent during the viraemia. Avirulent infection induced protective immunity to virulent challenge during the first week after primary infection, but full clinical protection was not established until after the fourth week. The findings, corrobated with other closely comparable observations, suggest that the emergence of PED as an intercurrent mortality problem during rabbit passage of pathogenic Treponema pallidum is the result of a specific selective pressure on a benign passenger virus. The expression of virulence of PEDV appears to be dependent on length of interval between passages.

Pleural effusion disease in rabbits. Properties of the aetiological agent.

The size and heat sensitivity of Pleural effusion disease (PED) agent or virus (PEDV) propagated in rabbits were examined. The infectious particles were estimated to be between 25 and 50 nm by filtration. Residual infectivity of infectious serum was 0.1 per cent after heating at 56 degrees C for 4 hours. PEDV and the Stockholm agent appeared identical concerning pathogenic and immunogenic properties by infection experiments and protection tests in rabbits. Two of the three PEDV isolates were less pathogenic but appeared immunogenically identical to PEDV. The third isolate, obtained from the laboratory, which several years previously had supplied material for demonstration of the Stockholm agent, differed from PEDV in pathogenic and immunogenic properties. Serological examinations of paired rabbit sera did not indicate any antigenic relationship between PEDV and representative members of the two mammalian coronavirus antigenic groups. It is concluded that the aetiological agent of PED is a virus not belonging to the coronaviridae.

Interferon in rabbit sera after inoculation with Treponema pallidum suspensions contaminated with PED virus.

Paired rabbits sera were examined for the presence of interferon and pathogenicity for rabbits. The sera were obtained before and 48 hours after inoculation with Treponema pallidum suspensions of rabbit origin in 12 selected laboratories. Classical interferon, detectable in dilutions from 1/9 to 1/81, were found in 27 out of 39 postinoculation sera from which the pleural effusion disease (PED) virus was isolated. Serum interferon was not detectable in dilutions greater than or equal to 1/9 in 16 virus-negative postinoculation sera or in any of the 55 preinoculation sera. Interferon was found more often in sera from which highly virulent strains of PED virus were isolated than in sera from which strains of low virulence were isolated. The serum interferon assay provides useful presumptive evidence of contamination of rabbit-passaged treponemes with PED virus, but the assay is least useful when PED virus is present subclinically.

Pleural effusion disease in rabbits. Observations on viraemia, immunity and transmissibility.

Baby rabbits surviving infection with pleural effusion disease virus (PEDV) developed viraemia persisting for at least six months. Only the infectious serum samples collected during the first 2 months of disease could transfer the typical PED. Six months after neonatal infection, virus concentration in serum was 102 to 104 rabbit-infectious doses per ml, the level of IgG appeared elevated, and serum rendered non-infectious by ether-treatment had a protective effect in passive immunisation experiments. No evidence of glomerulonephritis or deposits of immunoglobulins could be demonstrated in the kidneys. During the nursing period PEDV was transmitted from infected baby rabbits to two out of four dams, but not to control litter-mates. After the nursing period control rabbits, caged together with the viraemic rabbits for 60 to 150 days, remained free from PEDV infection.

Pleural effusion disease agent as passenger of Treponema pallidum suspensions from rabbits. Survey of laboratories.

Material from rabbits used for the propagation of Treponema pallidum in 12 selected laboratories was examined for a viral passenger agent of the treponemal suspensions. An agent which causes clinical or subclinical pleural effusion disease (PED) in rabbits and which is serologically identical or related to the PED agent isolated in Copenhagen was found as a contaminant in treponemal suspensions in laboratories in Europe, the USA, and Japan. The experience gained in the Scandinavian treponematoses laboratories suggests that strains of T pallidum contaminated with the PED agent can be purified by passage through hamsters.

Pleural effusion disease in rabbits. Interferon in body fluids and tissues after experimental infection.

The distribution of interferon in body fluids and tissues was studied in 18 rabbits infected experimentally with the agent of pleural effusion disease (PED). Circulating interferon of the classical type was demonstrable 12 h after inoculation, and a maximum response was attained 2-3 days later. Circulating interferon disappeared between 6 and 8 days after inoculation. Interferon titres of serum were closely correlated with the early phase of febrile response and probably also with the initial growth phase of the PED agent. The interferon titres of pleural fluid exceeded by far the titres of other body fluids and tissues. No interferon could be demonstrated in brain, liver and urine.

Pleural effusion disease in rabbits. Histopathological observations.

Pleural effusion disease (PED) is a generalized infection of laboratory rabbits caused by an unidentified agent, believed to be a virus. The histopathological response of 17 rabbits infected experimentally with this agent was studied. The light microscopical changes were minimal and the most consistent findings were alterations of the lymphoid tissue. Fatal infections were characterized by a uniform reduction of the splenic white pulp, focal degenerative changes of the thymus and lymph nodes and probably slight proliferative changes of the kidney glomeruli. In surviving animals there were transient myocardial and hepatic lesions and, after clinical recovery, proliferative changes in spleen, lymph nodes, interstitial lung tissue and probably kidney glomeruli. The results do not permit any conclusions to be drawn regarding the aetiology or the pathogenesis of PED infection.

Uveitis in rabbits with pleural effusion disease. Clinical and histopathological observations.

Pleural effusion disease (PED) is a generalized infection of laboratory rabbits caused by a virus-like agent. The disease was first described in the late sixties as a mortality problem among rabbits used for the propagation of Nichols' pathogenic Treponema pallidum in Scandinavian laboratories using the T. pallidum immobilization (TPI) test for the serological diagnosis of syphilis. The iridocyclitis, described as a manifestation of PED, has been studied in detail in rabbits experimentally infected with the PED agent. All rabbits surviving the acute phase of infection developed a non-pyogenic, non-granulomatous anterior uveitis during the "viraemic" stage of infection. The ocular signs of disease culminated between days 3 and 6 and disappeared within 2 to 3 weeks. No recurrence of uveitis was observed during a 6-months observation period, nor by subsequent subcutaneous re-inoculation of the PED agent. Histologically, the uveal reaction was mild and regressed almost completely within 4 weeks. Discrete choroidal inflammatory foci occurred in some of the re-inoculated rabbits, but without changes in the anterior eye segment. The uveitis of pleural effusion disease in rabbits seems to be caused directly by the virus-like agent. It might possibly serve as a simple and reproducible model in further uveitis research.

Pleural effusion disease in rabbits, clinical and post mortem observations.

In Denmark and probably in other countries as well, the infectious agent causing intercurrent death of rabbits by passages of Nichols pathogenic Treponema pallidum has been studied in rabbits in the absence of T. pallidum. This agent can be propagated in rabbits at intervals of 2 to 30 days and, depending on the interval between passages and the number of passages, the mortality may vary from zero to almost 70 per cent. Based on the post mortem findings in fatal cases, the name pleural effusion disease is suggested for this rabbit infection. Iridocyclitis, haematological and biochemical changes are signs of the disease not described previously. The source of the infectious agent is discussed.