The reaction of pregnant ewes to inoculation with mucosal disease virus of bovine origin

Non-Bovine Species and the Risk to Effective Control of Bovine Viral Diarrhoea (BVD) in Cattle

Bovine viral diarrhoea virus (BVDV) is an economically important and highly prevalent virus of domestic cattle. Infections with BVDV may lead to both, reproductive and immunological effects that can result in widespread calf losses and increased susceptibility to diseases, such as mastitis and respiratory disease. While BVDV is generally considered to be host specific, it and other Pestivirus species, such as Border disease virus (BDV) in sheep, have been shown to be infecting species other than those from which they were originally isolated from. Recently BVDV was placed on the OIE’s list of notifiable disease and control and eradication programmes for BVDV have been developed throughout much of Europe, the United States, and the United Kingdom. While some countries, including Sweden and Ireland have successfully implemented eradication programmes, other countries such as New Zealand and Australia are still in the early stages of BVDV control. Despite effective control methods, incursions of BVDV into previously cleared herds still occur. While the cause of these incursions is often due to lapses in control methods, the ability of ruminant pestiviruses to infect species other than cattle poses the question as to whether non-bovine species could be impeding the success of BVDV eradication and control. As such, the aim of this review is to make mention of what is known about the cross-species transmission of BVDV, BDV and other pestiviruses between cattle and non-bovine ungulate species and draw conclusions as to the risk non-bovine species pose to the successful control and eradication of BVDV from cattle.

Investigation of AGID and two commercial ELISAs for the detection of Bovine viral diarrhea virus-specific antibodies in sheep serum

Effective control and the eventual eradication of Bovine viral diarrhea virus (BVDV) from cattle populations depend on the accurate identification of infected animals. Although typically a disease agent of cattle, BVDV is known to infect a wide variety of nonbovine species, including sheep. However, validation of serologic tests in these nonbovine species, particularly sheep, is lacking. We analyzed 99 sheep sera (57 samples from Pestivirus-naive sheep, and 42 samples from BVDV-inoculated sheep) in order to investigate 3 serologic tests: the agarose gel immunodiffusion (AGID) and 2 commercial enzyme-linked immunosorbent assays (ELISAs) for detection of BVDV antibodies. At the manufacturer's cutoff thresholds, the AGID performed with 95.2% diagnostic sensitivity; ELISA-A performed with sensitivity of 90.5% and ELISA-B with 69.1%. All 3 tests performed with 100% diagnostic specificity. Two-graph receiver operating characteristic analysis showed that performance characteristics were optimized, such that both diagnostic sensitivity and diagnostic specificity were >95% for both ELISAs, if the thresholds were altered to 34.9% inhibition for ELISA-A and 63.5 signal-to-noise ratio for ELISA-B.

Experimental infection of pregnant ewes with bovine viral diarrhea virus type-2 (BVDV-2): effects on the pregnancy and fetus

The reproduction effects of bovine viral diarrhea virus type-2 (BVDV-2) infection were investigated in ewes inoculated with a non-cytopathic BVDV-2 isolate at three stages of gestation. Virus inoculation was followed by a transient viremia, accompanied by a transient and mild hyperthermia and nasal discharge in a few animals. Some ewes were sacrificed at different time-points after virus inoculation to study the kinetics of fetal infection. Infectivity and viral antigens were detected in placentomes from day 7 to 36 post-inoculation (pi) and in fetal fluids and tissues between days 10 and 28 pi. Cardiac petechial hemorrhages and hemoperitoneum accompanied by a severe fibrinous ulcerative placentitis were observed in fetuses examined at days 21, 28 and 36 pi. Inoculation of ewes at days 55-60 of gestation resulted in a prolonged virus replication in placentomes and fetal tissues; ewes that were allowed to proceed with pregnancy had 77% of abortions or fetal and perinatal deaths. Seven stillbirths, unviable and viable lambs born to these ewes were virus-positive at birth. Infectious virus was repeatedly isolated from leukocytes of two lambs up to 2 and 6 months of age, indicating they were persistently infected. Ewes inoculated at days 65-70 of gestation had 66.6% of fetal and perinatal losses. Three viable lambs born to these ewes were healthy, BVDV antibody-positive and virus-negative. A transient viral replication in placentomes and in a few fetal tissues, followed by the rise of fetal neutralizing antibodies and virus clearance was the result of inoculating ewes at days 120-125 of gestation. Lambs born to these ewes were healthy, antibody-positive and virus-negative. These results demonstrate that the biology of BVDV-2 infection in pregnant sheep is essentially similar to that of BVDV-1 in pregnant cattle and sheep. These features make this species an attractive animal model for studying the pathogenesis of congenital BVDV-2 infection.

Detection of viral antigen in placenta and fetus of cattle acutely infected with bovine viral diarrhea virus

The reproductive organs and fetuses of seven Norwegian Red heifers were investigated for the presence of bovine viral diarrhea virus (BVDV) antigen during the time of initial transplacental transmission of the virus. The heifers were inoculated with a noncytopathogenic BVDV at day 85/86 of gestation and were slaughtered at day 7, 10, 14, 18, or 22 postinoculation (pi). Cryostat sections of uterus, ovaries, placentomes, intercotyledonary fetal membranes, and fetal organs were examined using immunohistochemical techniques. A double immunofluorescence technique was used to identify cells that showed staining with antibodies against the leukocyte common antigen CD45 or the intermediate filament vimentin and BVDV antigens. The earliest stage of infection at which BVDV antigen could be detected in the fetuses was 14 days pi. At this stage, BVDV antigen was detected in cells of mesenchymal origin in the lungs and in large cells that morphologically resembled immature megakaryocytes in the liver. In the intercotyledonary fetal membranes and in the placentomes, BVDV antigen was not detected until 18 and 22 days pi, respectively. BVDV antigen was not detected in maternal tissue from any of the heifers. The present results indicate that fetal infection with BVDV can take place without preceding or simultaneous high concentrations of BVDV in uterus or placenta of acutely infected heifers.

Distribution of viral antigen in uterus, placenta and foetus of cattle persistently infected with bovine virus diarrhoea virus

The tissue distribution and cellular localisation of bovine virus diarrhoea virus (BVDV) was investigated in the uterus, placentomes, intercotyledonary foetal membranes and foetal organs of three persistently infected (PI) pregnant heifers. The uterus and ovaries of a non-pregnant PI heifer were also included in the study. Cryostat sections were examined using immunohistochemical techniques and monoclonal antibodies against BVDV. A double immunofluorescence technique was used to identify BVDV positive cells that also showed staining for either the leukocyte common antigen CD45 or the cytoskeletal filament vimentin. BVDV antigen was detected in all the organs examined, and was present in both epithelial and non-epithelial cells. In all organs many of the virus-positive cells also showed reactivity for vimentin. In the foetal liver and spleen a small, scattered population of virus-positive cells showed reactivity for CD45. A few cells showed reactivity both for BVDV antigen and for CD45 in the placentomes and intercotyledonary foetal membranes. In contrast to earlier reports, only scattered cells in the foetal part of the placentomes, the cotyledons, showed reactivity for BVDV antigen. However, in the chorion of the intercotyledonary foetal membranes, a larger proportion of the trophoblast cells showed reactivity for BVDV, especially the binuclear trophoblast cells. In the uterus, pregnancy appeared to favour virus replication, as the section from the pregnant heifers showed much stronger staining and a higher proportion of viral antigen-positive cells than sections from the non-pregnant PI heifer.

Genetic identification of pestivirus strain Frijters as a border disease virus from pigs

A pig pestivirus isolate, the Frijters strain, was characterized by using reverse transcription-PCR (RT-PCR), followed by RFLP and direct sequencing of the amplicons. Restriction endonuclease enzyme AvaI and Bg/I digestion of 5' NC genomic region PCR products suggested that Frijters strain belongs to the border disease viruses (BDV). This finding was confirmed by nucleotide sequencing of the amplified part of the 5-NC genomic region, which revealed 94% and 95% nucleotide similarity between the Frijters strain and the BDV prototype strains Moredun cp and Moredun ncp, respectively. On the other hand, 55-77% nucleotide identity was observed between the Frijters strain and prototypes of BVDV and CSFV. The amino acid similarity in the Npro (p20) region was 89% between Frijters and the Moredun strains and 70-77% between Frijters and the BVDV of CSFV strains. It was concluded that the Frijters virus is a member of the BDV group of the Pestivirus genus. Considering that this virus circulates in the large swine populations of Europe, the spread of certain pestiviruses to more than one animal species should not be considered sporadic. Since the presence of BDV in swine herds might be a confusing factor during classical swine fever eradication programmes, highly specific methods are required for reliable identification of these isolates. RT-PCR, followed by RFLP and sequencing of the amplicons proved to be a reliable approach for the rapid detection and identification of pestiviruses.

Antigenically different pestivirus strains induce congenital infection in sheep: a model for bovine virus diarrhea virus vaccine efficacy studies

To study the efficacy and safety of bovine virus diarrhea virus (BVDV) vaccines there is need for a valid challenge model. We investigated whether sheep can be used in such a challenge model. We intranasally inoculated six groups (A-F) of seronegative sheep at day 49 of gestation with either of five antigenically different BVDV strains and one border disease virus strain. A seventh group (G) was housed for 10 days with a persistently infected calf and an eighth group (H) served as control. From each group half of the sheep were killed at 2 weeks, and half at 4 weeks after infection. For virus isolation five organs were collected from the sheep and seven from the fetuses. All sheep of groups A and H remained seronegative in the ELISA and in the serum neutralization test. At 2 and 4 weeks after infection virus was isolated from almost all fetal organs in six groups. In group A and in the control group no virus was isolated from the fetal organs. The virus distribution patterns in fetuses from sheep housed with the persistently infected calf or intranasally inoculated with the same strain were similar. We concluded that (i) antigenically different BVDV strains can induce congenital infection in sheep and that (ii) the consequences of a contact infection were similar to those after intranasal infection. In a second experiment we infected two groups of seronegative sheep with one of the strains used in the first experiment, before mating. A control group was left uninfected. The sheep were served and all sheep were challenged with antigenically homologous or heterologous BVDV at day 49 of pregnancy. Three weeks after challenge, sheep were killed and the procedure as in the first experiment was followed. None of the fetuses sheep were virus positive whereas all fetuses of the control sheep were virus positive. Hence, the immune response after BVDV infection protects fetuses against homologous and heterologous infection during pregnancy. Sheep may therefore be used in vaccination-challenge experiments to evaluate BVDV vaccine efficacy in preventing congenital infection.

Border disease in sheep

The current knowledge on border disease in sheep is reviewed. This is a congenital and teratogenic disorder induced by pestivirus. The history, etiology, epidemiology, clinical aspects, and pathologic lesions at postnatal and intrauterine infections (as well as in congenitally affected animals), pathogenesis, immunity, diagnosis, and control and prevention of the syndrome are discussed.

Variation in neuropathogenicity in sheep fetuses transplacentally infected with non-cytopathogenic and cytopathogenic biotypes of bovine-virus diarrhoea virus

Pregnant Merino ewes were inoculated intravenously between days 63 and 65 of gestation with a non-cytopathogenic (ncp) bovine-virus diarrhoea-virus (BVDV) isolate (experiment A). The histomorphological findings and the distribution of viral antigen, as revealed by immunohistochemistry in brains of fetuses from experiment A, were compared with those seen in fetal brains from a previous study (experiment B), in which pregnant ewes had been intravenously infected between days 65 and 68 of gestation with the cytopathogenic (cp) BVDV strain Indiana. The two viruses showed remarkable variations concerning their pathogenicity for the developing fetal brain. The cp BVDV had a much higher neuropathogenic potential than the ncp BVDV and induced severe intracranial malformations in most fetuses. In experiment A, exclusively relatively mild leucoencephalomalacic lesions occurred. Between fetuses of the two experiments, significant differences concerning the distribution of viral antigen and the inflammatory response were found. In the majority of fetal brains from experiment B examined at days 10, 14 and 21 post inoculation (p.i.), antigen-containing differentiated brain cells (neurons, astrocytes, oligodendrocytes) and undifferentiated cells in the periventricular germinal zones were seen throughout the different zones of the developing telencephalon and cerebellum. At 21 days p.i., a marked inflammatory response consisting of brain macrophages and other mononuclear cells occurred in the meninges and in the brain parenchyma of fetuses from experiment B. In brain sections of fetuses infected with ncp BVDV, in contrast to fetuses infected with cp BVDV, viral antigen was not detectable during the early stages (days 10 and 20) p.i., and histopathological lesions were not seen at this stage. At days 41 and 47 p.i., antigen-positive astrocytes and oligodendrocytes were found in the developing white matter of the telencephalon and cerebellum. Furthermore, antigen-containing neurons were seen in the developing cerebral cortex. Cellular infiltrations in fetal brains from experiment A were limited to the leucoencephalomalacic areas in the developing cerebral and cerebellar white matter and consisted exclusively of brain macrophages. Immunohistochemical staining in brain sections of fetuses from both experiments revealed that numerous perivascular cells contained viral antigen, whilst positive endothelial cells were exclusively found in fetuses from experiment A. From the findings of this study it was concluded that the cp BVDV stain used in experiment B has a marked tropism for the fetal brain and both its already differentiated and undifferentiated cell populations, and that the resulting brain lesions primarily are the consequence of a direct cytolysis of these cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Ruminant pestivirus infections in animals other than cattle and sheep

The current knowledge on infection with ruminant pestiviruses in animals other than cattle and sheep and reviewed. Natural infection is found to occur in goats, in captive and free-living ruminants, as well as in pigs. The viral agent, susceptible species, epidemiology, clinicopathologic lesions, pathogenesis, diagnosis, control, and consequences for control programs in domestic ruminants are discussed.

Virological and pathological findings in sheep fetuses following experimental infection of pregnant ewes with cytopathogenic-bovine-virus diarrhoea virus

Eighteen pregnant Merino ewes were inoculated intravenously between days 65 and 68 of gestation with the unpurified cytopathogenic (cp) bovine virus diarrhoea virus (BVDV) strain Indiana (experiment I). In experiment II, three ewes were inoculated with the same virus after two successive plaque isolations in order to compare its pathogenicity for the fetus with special regard to lesions in the fetal brain. In experiment I, fetal blood and tissue samples, allantoic fluids and placentomes were collected sequentially between 10 and 80 days post-inoculation (p.i.). BVDV was recovered from 6 of 19 fetuses examined during the first 3 weeks after inoculation. From fetuses sampled between 30 and 50 days p.i. virus was isolated from three cases only, and from 60 days p.i. onwards virus was no longer recovered. BVDV was longer detected in the allantoic fluid than in fetal tissues and continued to be present until 80 days post-inoculation. From tissue samples of two fetuses of experiment I, only non-cytopathogenic BVDV was isolated, whilst samples from seven fetuses contained the cp BVDV biotype as revealed by an immunoplaque assay. The cp biotype was also isolated from placentomes. In experiment II, virus was not isolated from any of the tissue samples of two living fetuses collected at 67 days post-inoculation. In both experiments, cp BVDV was recovered from allantoic fluid samples. In contrast to the developing fetal brain, other tissues or organs seemed to be less vulnerable to the cp BVDV strain Indiana. The partial purification of this virus strain did not affect its pathogenicity for the brains of the developing fetuses.

Genetic diversity and BVD virus

The various measures of genetic variation of BVD virus was reviewed with emphasis on the implications for future control of virus-induced disease and diagnosis. While experimental data does not support unique serotypes for BVDV, there is substantial antigenic variation among the isolates examined. This variation may permit fetal infections even in animals assumed to be well vaccinated. The genetic differences between cytopathic and noncytopathic strains of BVDV are expressed in infected cells by the production of a p80 protein by cytopathic strains. In addition, cellular gene inserts have been detected in cytopathic strains. Monoclonal antibodies have demonstrated a high degree of diversity with the pestivirus population. Grouping of BVDV isolates by monoclonal antibody analysis is suggestive at best. The use of nucleic acid probes as diagnostic reagents has been compromised by the nucleic acid sequence variation found in the BVDV isolates tested.

A survey of hairy shaker disease (border disease, hypomyelinogenesis congenita) in sheep

A survey by mail in 1979 of 995 sheep farmers in Victoria enquiring as to the occurrence of hairy shaker disease (HSD) in lambs on their farms resulted in a 48% response. A total of 17 farms reported the presence of HSD. One hundred and eleven ewes produced lambs with HSD representing a small proportion only of the ewes (0.8%) on these farms and an even smaller percentage of the total ewes (0.03%) in the survey. Abortions were not prevalent on the majority of farms reporting HSD. There was no evidence that HSD was more common in any particular area in Victoria. As was anticipated mucosal disease virus/bovine virus diarrhoea (MDV) antibody was often present in serums from affected sheep. It was concluded that at present, the hairy shaker form of border disease did not appear to be an economically important problem in Victoria.

Congenital infections with nonarbo togaviruses

The present review deals with the similarities and differences of selected aspects of prenatal pestivirus infections of domestic animals and congenital rubella of man. Hog cholera virus, bovine virus diarrhoea virus and border disease virus are antigenically closely related, but unrelated to rubella virus. The nonarbo togaviruses are capable of producing congenital infections resulting in a wide spectrum of abnormalities. The infected foetus can die in utero, in the neonatal period, or it may be born with teratogenic defects. In addition, apparently healthy progeny can be delivered that develop a late onset disease, months, or years after birth, or remain clinically normal for life. The ultimate outcome of a congenital infection is mainly determined by the stage of foetal development, at which infection occurs. Foetuses exposed to rubella virus raise an antibody response to the virus, whereas domestic animals frequently fail to respond immunologically to a congenital pestivirus infection. In congenital rubella the virus usually disappears from the host's body 1-2 years after birth. However, congenital pestivirus infections may be characterized by a lifelong and widespread persistence of virus in clinically healthy animals. Such animals are of significance in the epizootiology of bovine virus diarrhoea, border disease or hog cholera.

Pathology in the ovine foetus caused by an ovine pestivirus

Homogenised tissues or tissue culture supernatant fluid containing a noncytopathic pestivirus obtained from a lamb with a neurologic form of border disease, were inoculated into ewes at different stages of pregnancy. Foetal death occurred in 9 ewes of those inoculated between 19 and 47 days of pregnancy while 3 ewes did not lamb. Eight of the foetuses were aborted between 77 and 132 days of pregnancy; of these 6 were autolysed or mummified and one had arthrogryposis. The one full-term dead lamb had a hairy birth coat and lissencephalic micrencephaly. Foetal death occurred in only 7 of 14 ewes inoculated between 57 and 72 days of pregnancy. Four of these ewes aborted between 77 and 108 days of pregnancy and 3 gave birth to full-term, dead, hairy lambs. The remaining 7 ewes gave birth to live hairy lambs with severe inco-ordination. All lambs carried to term and aborted foetuses or lambs that could be examined had a range of intracranial malformations including focal leucomalacia, micrencephaly, hydranencephaly, porencephaly, lissencephaly and cerebellar hypoplasia. Some lambs also had skeletal abnormalities including arthrogryposis, scoliosis and brachygnathia inferior. The pestivirus isolate used in these trials produced more severe effects on the ovine foetus than previously observed in similar inoculation trials using pestivirus isolates from border disease lambs without nervous signs.

Properties of Border disease virus as studied in a sheep cell line

A fetal lamb muscle cell line has been isolated in which two strains of Border disease virus replicate well and induce a clear-cut cytopathic effect, thus providing a sensitive and practical assay system for both the virus and neutralizing antibodies. This system enabled us to study some characteristics of Border disease virus in comparison with two other agents responsible for hog cholera and bovine viral diarrhea. Our results indicate that the last virus and the Border disease agent are indistinguishable in vitro.