THE EFFECT OF BOVINE EPHEMERAL FEVER VIRUS ON THE BOVINE FOETUS

Six pregnant heifers that were experimentally infected with bovine ephemeral fever virus produced normal calves. Foetuses of 8 heifers that were immune to bovine ephemeral fever were inoculated with bovine ephemeral fever virus. One was aborted after 36 days, but the cause of abortion could not be determined. One was sacrificed after 28 days. The foetus and its membranes were normal and neither virus nor antibody was demonstrated. The other 6 heifers produced normal calves. One calf, 501, inoculated after 160 days gestation, contained high levels of neutralising antibody in serum before ingestion of colostrum. The others, inoculated at gestational ages of 52 days to 157 days showed no evidence of neutralising antibody in blood samples collected at birth.     

THE INFECTION OF MERINO SHEEP WITH BOVINE EPHEMERAL FEVER VIRUS

Four Merino sheep inoculated intravenously with bovine blood containing ephemeral fever virus showed no clinical signs of ephemeral fever. Two sheep showed a mild haematological response and developed a neutralising antibody which closely paralled that of a steer inoculated at the same time. Leucocytes harvested from these 2 sheep on days 3 and 4 after inoculation with virus reproduced ephemeral fever when inoculated into susceptible steers whilst those harvested on days 1, 2 and 5 did not. Even though this indicates that EFV can multiply in some sheep when they are inoculated intravenously, it cannot be inferred that natural infection occurs.     

THE SUSCEPTIBILITY OF YOUNG AND NEWBORN CALVES TO BOVINE EPHEMERAL FEVER VIRUS

The pathogenicity of a field strain, 417, of bovine ephemeral fever (BEF) virus for newborn and young calves was investigated. Three colostrum-deprived newborn calves inoculated intravenously developed severe clinical disease and viraemia, and produced long-lasting neutralising antibody. The incubation period in these animals was 10 and 11 days, compared with 5 to 7 days for older calves. Two newborn calves which received colostrum from immune dams and 2 which received colostrum from non-immune dams failed to respond clinically to intravenous inoculation with strain 417. The neutralising antibody response of these calves was of short duration. Four calves, 7 to 8 weeks old and lacking detectable neutralising antibody to BEF virus, or having low levels of antibody, did not develop clinical disease when inoculated intravenously. Four calves 12 to 14 weeks of age and free of detectable neutralising antibody to BEF virus developed clinical disease when inoculated with strain 417.     

SOME ASPECTS OF THE EPIZOOTIOLOGY OF BOVINE EPHEMERAL FEVER IN AUSTRALIA

Serum samples collected from cattle in Western Australia, Northern Territory, Queensland and New South Wales in 1966 had neutralising antibodies to ephemeral fever virus, although the last major epizootic of ephemeral fever was in 1955-56. The incidence of antibodies ranged from 1.5% in Western Australia to 29.0% in Queens- land, and 57.6% of serums assayed were of low titre (2 to 5). Antibodies were not found in serums collected from cattle in Victoria, South Australia, southern Western Australia or Norfolk Island. After the 1967-68 epizootic the pro-porton of cattle with antibody ranged from 3.1% to 47.6% in herds with antibody in Victoria to 81.8% to 91.7% in herds in Queensland, and 58.2% of serums assayed had antibody titres greater than 45. Cattle with low levels of antibody in 1966 had high levels after the 1967-68 epizootic, although it is not known what pro-portion showed clinical signs of ephemeral fever during the epizootic. Serum samples collected in 1966, and which contained low levels of antibody, were fractionated by gel filtration and the neutralising activity was confined to the 7S globulin fraction. In one cow experimentally infected with ephemeral fever virus, the neutralising activity at 15 days after inoculation was confined to the 19S globulin fraction, in both the 19S and 7S fractions at 22 days but was almost totally confined to the 7S fraction by day 36. The significance of the results is discussed, and it is suggested that ephemeral fever virus remains enzootic in areas of Australia between major epizootics, but the infecting virus may be of low pathogenicity and immunogenicity for cattle, resulting mainly in subclinical infections.     

THE SUSCEPTIBILITY OF CALVES TO INFECTION WITH A STRAIN OF BOVINE EPHEMERAL FEVER VIRUS INOCULATED INTRACEREBRALLY

Three newborn calves were inoculated intracerebrally with bovine ephemeral fever virus strain 525. The 2 that lacked detectable neutralising antibody to bovine ephemeral fever vaccine developed fatal encephalitis after 4 and 7 days respectively. The third calf which had a low level of maternal antibody remained healthy and developed antibody that became undetectable after 6 months. Bovine ephemeral fever virus strain 525 was reisolated from the brains of both dead calves by intracerebral inoculation of suckling mice. Homogenates that were prepared from the brains of the calves failed to produce disease or to induce antibody formation in susceptible calves when inoculated intravenously. Strain 525 of BEF virus has been shown to possess a degree of neurovirulence for laboratory animals that has not been reported for other strains (Tzipori and Spradbrow 1974). Although this strain is unable to produce viraemia in calves after I/V inoculation, the present study shows that strain 525 can multiply in the brain tissues of calves and cause death after I/C inoculation.     

STUDIES ON THE PATHOGENESIS OF BOVINE EPHEMERAL FEVER

A study of the pathogenesis of bovine ephemeral fever confirmed that the major clinical signs were fever lasting no more than 2 days, with increased respiratory rate, dyspnoea and some degree of lameness. Haematological observations revealed a neutrophilia with a left shift and a lymphopaenia at the time of peak clinical reaction. The net result was a slight leucopaenia on the day after this reaction. The most prominent pathological changes involved the lungs and synovial joints. Pulmonary emphysema and alveolar collapse with bronchiolitis, degenerative changes in synovial membranes and increased synovial fluid were observed. Specific fluorescence indicating the presence of BEF viral antigen could be detected at the time of peak clinical response in individual cells in the lungs, spleen and lymph nodes as well as neutrophils. Before and after the peak fever some fluorescence was seen in cells which appeared to be reticular cells in the lymph nodes. Viral isolation in mice could be made from blood, lungs, spleen and lymph nodes over a period of no more than 3 days. It is postulated that viral growth takes place mainly in the reticuloendothelial cells in the lungs, spleen and lymph nodes and not in vascular endothelium or lymphoid cells.     

表达猪瘟病毒E2蛋白的重组腺病毒对猪的免疫效力评价

为了进一步验证含有猪瘟病毒E2基因的重组腺病毒（rAdV—E2）在猪体上的免疫效力,将rAdV—E2按108TCID50／头接种猪2次,同时用野生型腺病毒wtAdV作为阴性对照,当抗体上升到一定程度后用致死剂量的猪瘟强毒石门株进行攻击。结果表明,rAdV—E2免疫组（n=5）所有免疫猪在加强免疫后均产生了猪瘟特异性中和抗体,并于加强免疫后3w达到峰值,攻毒后所有猪只抗体迅速升高,除了一头猪短期体温升高外,未出现任何其它临床症状;而野生型腺病毒wtAdV免疫组（n=5）猪只在攻毒前一直没有检出特异性抗体,攻毒后全部出现典型的猪瘟临床症状和严重的病毒血症,剖检时可见典型猪瘟病理变化。这表明构建的猪瘟病毒E2基因重组腺病毒rAdV—E2免疫猪后产生了很好的免疫效果,有望成为具有开发价值的活载体疫苗。