Sheep persistently infected with Border disease readily transmit virus to calves seronegative to BVD virus

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.

Eradication of Bovine Viral Diarrhoea (BVD) in Cattle in Switzerland: Lessons Taught by the Complex Biology of the Virus

Bovine viral diarrhoea virus (BVDV) and related ruminant pestiviruses occur worldwide and cause considerable economic losses in livestock and severely impair animal welfare. Switzerland started a national mandatory control programme in 2008 aiming to eradicate BVD from the Swiss cattle population. The peculiar biology of pestiviruses with the birth of persistently infected (PI) animals upon in utero infection in addition to transient infection of naïve animals requires vertical and horizontal transmission to be taken into account. Initially, every animal was tested for PI within the first year, followed by testing for the presence of virus in all newborn calves for the next four years. Prevalence of calves being born PI thus diminished substantially from around 1.4% to <0.02%, which enabled broad testing for the virus to be abandoned and switching to economically more favourable serological surveillance with vaccination being prohibited. By the end of 2020, more than 99.5% of all cattle farms in Switzerland were free of BVDV but eliminating the last remaining PI animals turned out to be a tougher nut to crack. In this review, we describe the Swiss BVD eradication scheme and the hurdles that were encountered and still remain during the implementation of the programme. The main challenge is to rapidly identify the source of infection in case of a positive result during antibody surveillance, and to efficiently protect the cattle population from re-infection, particularly in light of the endemic presence of the related pestivirus border disease virus (BDV) in sheep. As a consequence of these measures, complete eradication will (hopefully) soon be achieved, and the final step will then be the continuous documentation of freedom of disease.

Global Distribution and Genetic Heterogeneity of Border Disease Virus

Border disease virus (BDV) belongs to the genus Pestivirus of the family Flaviviridae. Interspecies transmission of BDV between sheep, cattle, and pigs occurs regularly, sometimes making diagnosis a challenge. BDV can yield substantial economic losses, including prenatal and postnatal infections in lambs, which are the primary source of infection and maintenance of the virus in the population. Since BDV is antigenically and genetically related to bovine viral diarrhea virus (BVDV), it might pose a significant risk to cattle, influencing BVDV eradication campaigns. Similarly, the presence of BDV in swine herds due to pestivirus spillover between small ruminants and pigs might cause uncertainty in classical swine fever virus (CSFV) diagnostics. Therefore, knowledge of BDV epidemiology in different geographical regions will help prevent its spread and optimize control measures. Previous epidemiological studies have shown that various BDV genotypes are predominant in different countries. This review provides an overview of the spread of BDV world-wide in different host species.

Genomic characterization of pestiviruses isolated from bovine, ovine and caprine foetuses in Turkey: A potentially new genotype of Pestivirus I species

This study was carried out to investigate the frequency and genetic diversity of pestiviruses in abortion cases in cattle and small ruminants in Turkey. During January 2012 and December 2017, a total of 2029 aborted foetuses (553 bovine foetuses, 1,388 sheep foetuses and 88 goat foetuses) were collected from different regions of Turkey. Real-time RT-PCR (RRT-PCR) assays were used to detect pestiviral RNA in aborted foetuses. To confirm the cause of abortion, pestivirus-positive foetuses were also examined for the presence of Brucella spp., Campylobacter spp., Chlamydophila abortus (C. abortus), akabane virus, bluetongue virus and Schmallenberg virus by molecular detection methods. Pestiviral RNA was detected in 61 (11%) of the 553 bovine foetuses, 124 (8.9%) of the 1,388 sheep foetuses and 3 (3.4%) of the 88 goat foetuses. Furthermore, C. abortus DNA was detected in 3 pestivirus-positive sheep foetuses, whereas other infectious agents were not detected in pestivirus-positive foetuses. Genetic characterization of the pestivirus RRT-PCR positive samples was conducted by sequencing 5' untranslated (5' UTR) and non-structural autoprotease (N^pro ) genomic regions. A total of 68 sequences were obtained, and phylogenetic analyses revealed that all sequences belonged to BVDV-1, including 1b (8/68), 1f (2/68), 1l (4/68), 1r (10/68), Aydin-like pestivirus (20/68) and one unknown genotype (24/68). The 5' UTR and N^pro sequences of this unknown genotype differed from pestiviruses previously described, providing evidence for the presence of an emerging genotype within the species Pestivirus I, tentatively named as 'Konya-like' pestivirus. 'Konya-like' pestivirus was the dominant genotype in sheep foetuses, whereas Aydin-like pestivirus was found to be the predominant genotype in bovine foetuses. To the best my knowledge, this is the first report of Aydin-like pestivirus infection in cattle. The information provided in this study contributes to the understanding the dissemination and evolution of pestiviruses and could be beneficial for developing more effective vaccines.

Risk factors associated with the within-farm transmission of bovine viral diarrhea virus and the incidence of persistently infected cattle on dairy farms from Ibaraki prefecture of Japan

For understanding the factors affecting bovine viral diarrhea virus (BVDV) transmission, this study investigated the distribution of BVDV and the epidemiological features of persistently infected (PI) cattle in Ibaraki Prefecture of Japan, and identified farm-level risk factors associated with BVDV infection, with a focus on within-farm transmission and PI animal detection. Among all 377 dairy farms, forty-four PI cattle were identified on 22 farms. Thirty-eight and six PI cattle were born on their current farms or purchased, respectively. Twenty-six PI cattle were born from pregnancies on their current farms, seven from pregnancies in summer pastures, and eight from pregnancies on other farms. The within-farm seroprevalence on farms with PI animals was significantly higher than that on farms without PI cattle. Of 333 farms holding homebred cattle without movement records, antibody-positivity in homebred cattle was observed on 194 farms; these cattle were likely infected by within-farm transmission. Herd size, summer pasturing, and BVDV infection status of the nearest dairy farm were risk factors associated with within-farm transmission. Likewise, herd size, summer pasturing, and the proportion of purchased cattle were related to PI animal occurrence. This study shows the risk of within-farm transmission and occurrence of PI animals after the introduction of BVDV via purchasing and summer pasturing, and illustrates the significant role of PI cattle in circulating BVDV. More effective measures for screening BVDV infection and PI animals, including intensive tests targeting moved cattle and newborn calves, and bulk milk surveillance, are required to control the spread of BVDV in Japan.

Serological evidence for exposure to bovine viral diarrhoea virus in sheep co-grazed with beef cattle in New Zealand

Aims: To determine whether sheep that co-grazed with cattle that were suspected to be positive for bovine viral diarrhoea (BVD) virus had serological evidence of exposure to the virus.Methods: Eighteen commercial farms that routinely co-grazed cattle and sheep in the same paddocks were recruited through purposive sampling. The recruiting veterinarians identified nine farms with cattle herds that were known or highly suspected to be positive for BVD and nine farms that were considered to be free of BVD. Blood samples were taken from 15 ewes aged 1 year on each farm and samples were submitted to a commercial diagnostic laboratory to test for antibodies against pestiviruses using an ELISA. All samples that were positive were then tested using a virus neutralisation test (VNT)for antibodies against BVD virus.Results: Of the 270 blood samples, 17 were positive for pestivirus antibodies by ELISA and these originated from two farms that were known or suspected to have BVD virus-positive cattle. None of the samples from the nine flocks co-grazed with cattle herds that were known or suspected to be BVD virus-negative were positive for pestivirus antibodies. Within the two positive farms, 2/15 samples from the first farm and 15/15 samples from the second farm were antibody-positive. When the 17 positive blood samples were submitted for VNT, all 15 samples from the second farm tested positive for BVD virus antibodies with the highest titre being 1:512.Conclusions and clinical relevance: In this small sample of New Zealand sheep and beef farms with suspected BVD infection in cattle, there was evidence of pestivirus exposure in co-grazed sheep. Although we were unable to confirm the origin of the exposure in these sheep, these findings highlight that farmers who are trying to eradicate BVD from their cattle should be mindful that the infection may also be circulating in sheep, and both populations should be considered a possible risk to each other for generating transient and persistent infections. Further work is needed to estimate the true prevalence of New Zealand sheep flocks that are affected by BVD and the associated economic impacts.

Pestivirus Infections in Semi-Domesticated Eurasian Tundra Reindeer (Rangifer tarandus tarandus): A Retrospective Cross-Sectional Serological Study in Finnmark County, Norway

Members of the Pestivirus genus (family Flaviviridae) cause severe and economically important diseases in livestock. Serological studies have revealed the presence of pestiviruses in different cervid species, including wild and semi-domesticated Eurasian tundra reindeer. In this retrospective study, serum samples collected between 2006 and 2008 from 3339 semi-domesticated Eurasian reindeer from Finnmark County, Norway, were tested for anti-pestivirus antibodies using an enzyme linked immunosorbent assay (ELISA) and a subset of these by virus neutralization test (VNT). A seroprevalence of 12.5% was found, varying from 0% to 45% among different herding districts, and 20% in western Finnmark, as compared to 1.7% in eastern Finnmark. Seroprevalence increased with age. Pestivirus-specific RNA was not detected in any of the 225 serum samples tested by real-time RT-PCR. Based on VNT results, using a panel of one bovine viral diarrhea virus (BVDV) strain and two border disease virus (BDV) strains, the virus is most likely a reindeer-specific pestivirus closely related to BDV. A characterization of the causative virus and its pathogenic impact on reindeer populations, as well as its potential to infect other domestic and wild ruminants, should be further investigated.

Pathogen spread and globalization: The case of Pestivirus heterogeneity in southern Italy

Pestiviruses are responsible for widespread diseases affecting cattle, pigs and other ruminants, presenting a wide range of clinical manifestations, with significant impact on animal production. Given the recent various reports of a relatively high number of new strains and atypical genomic variants, in the present study, ninety-seven genomic sequences from southern Italy have been evaluated applying the palindromic nucleotide substitutions method, based on 5'-UTR secondary structure alignment and computing genetic distance among strains in the internal ribosome entry site. Sequence analysis revealed a highly heterogeneous virus population, indicating the introduction of virus variants of Bovine viral diarrhea virus and Border disease virus species from foreign countries. The application of different analytical procedures was useful to avoid interpretation difficulties. Circulation of heterogeneous virus populations showed the need for more accurate epidemiological investigations and stringent veterinary controls to protect animal health and welfare.

Border disease in cattle

Within the family Flaviviridae, viruses within the genus Pestivirus, such as Border disease virus (BDV) of sheep, can cause great economic losses in farm animals. Originally, the taxonomic classification of pestiviruses was based on the host species they were isolated from, but today, it is known that many pestiviruses exhibit a broad species tropism. This review provides an overview of BDV infection in cattle. The clinical, hematological and pathological-anatomical findings in bovines that were transiently or persistently infected with BDV largely resemble those in cattle infected with the closely related pestivirus bovine viral diarrhoea virus (BVDV). Accordingly, the diagnosis of BDV infection can be challenging, as it must be differentiated from various pestiviruses in cattle. The latter is very relevant in countries with control programs to eradicate BVDV in Bovidae, as in most circumstances, pestivirus infections in sheep, which act as reservoir for BDV, are not included in the eradication scheme. Interspecies transmission of BDV between sheep and cattle occurs regularly, but BDV in cattle appears to be of minor general importance. Nevertheless, BDV outbreaks at farm or local level can be very costly.

Seroprevalence of border disease virus and other pestiviruses in sheep in Algeria and associated risk factors

BACKGROUND

Border disease virus (BDV) is a pestivirus responsible for significant economic losses in sheep industry. The present study was conducted between 2015 and 2016 to determine the flock seroprevalence of the disease in Algeria and to identify associated risk factors. 56 flocks from nine departments were visited and 689 blood samples were collected from adult sheep between 6 and 24 months of age (n = 576) and from lambs younger than 6 months (n = 113). All samples were tested by RT-PCR as well as by Ag-ELISA, to detect Persistently Infected (PI) animals. Serum samples from adults were tested by Ab-ELISA (Enzyme Linked Immuno-Sorbent Assay), to detect specific antibodies against pestivirus and 197 of them were further characterized by VNT (virus neutralization test) for the detection of neutralizing antibodies specific for BDV and for Bovine virus diarrhea virus (BVDV-1 and BVDV-2).

RESULTS

No PI animals were found among the 689 sheep tested. 144/197 sera were positive in VNT for BDV, and 2 sera were strongly positive BVDV-2. Fifty-five flocks (98%) had at least one seropositive animal and the apparent within-flock seroprevalence was estimated to be 60.17% (95% C.I.: 52.96-66.96). The true seroprevalence based on estimated sensitivity and specificity of the Ab-ELISA was 68.20% (95% C.I.; 60.2-76.3). Several risk factors were identified as linked to BDV such as climate, landscape, flock management and presence of other ruminant species in the farm.

CONCLUSION

These high seroprevalence rates suggest that BDV is widespread and is probably endemic all over the country. Further studies are needed to detect and isolate the virus strains circulating in the country and understand the distribution and impact of pestiviruses in the Algerian livestock.

Insemination with border disease virus-infected semen results in seroconversion in cows but not persistent infection in fetuses

BACKGROUND

This study examined various health variables in cows after artificial insemination with Border disease virus (BDV)-infected semen and the occurrence of persistent infection in ensuing fetuses. Five cows were inseminated (day 0) with BDV-infected semen as well as with semen from a fertile Eringer bull. One cow, inseminated with virus-free semen only, served as a control. Clinical examination, assessment of eating and rumination activities, measurement of intraruminal temperature and leukocyte count were used to monitor the health of the cows. Blood samples were collected at regular intervals for the detection of viral RNA and antibodies against BDV, and the cows were slaughtered on day 56. The uteri, placentae and fetuses were examined macroscopically, histologically, immunohistochemically and by means of molecular methods for the presence of pestiviruses.

RESULTS

The demeanour, eating and rumination activities and intraruminal temperature were not affected by insemination with BDV-infected semen, whereas the total leukocyte and lymphocyte counts dropped transiently and were significantly lower on day 6 than on day 0. Seroconversion occurred by day 28 in the five infected cows but not in the control cow. The uteri, placentae and fetuses had no macroscopic or histological lesions, and immunohistochemical examination and RT-PCR were negative for pestiviruses.

CONCLUSIONS

The findings showed that cows inseminated with BDV-infected semen seroconverted and fetuses thus produced were not persistently infected. Transmission of BDV to cattle through infected semen, therefore, seems to be of minor importance.

Control of Bovine Viral Diarrhea

Bovine viral diarrhea (BVD) is one of the most important infectious diseases of cattle with respect to animal health and economic impact. Its stealthy nature, prolonged transient infections, and the presence of persistently infected (PI) animals as efficient reservoirs were responsible for its ubiquitous presence in cattle populations worldwide. Whereas it was initially thought that the infection was impossible to control, effective systematic control strategies have emerged over the last 25 years. The common denominators of all successful control programs were systematic control, removal of PI animals, movement controls for infected herds, strict biosecurity, and surveillance. Scandinavian countries, Austria, and Switzerland successfully implemented these control programs without using vaccination. Vaccination as an optional and additional control tool was used by e.g., Germany, Belgium, Ireland, and Scotland. The economic benefits of BVD control programs had been assessed in different studies.

Factors associated with the bovine viral diarrhoea (BVD) status in cattle herds in Northwest Germany

In Germany, all calves are tested for the presence of bovine viral diarrhoea/mucosal disease virus (BVDV) virus since January 1, 2011. The basis for this compulsory investigation is the BVDV Federal Regulation (BVDVV), which demands testing of calves before the age of six months and according to the new regulation of June 2016 within four weeks or before entering another stock. In 2012, a questionnaire was sent to 7250 Lower Saxony cattle farmers to identify potential factors associated with the presence of BVDV. Completed questionnaires were received from 2542 farms for further analysis. For BVD status determination of these farms, the diagnostic results of 425,911 ear notch samples of calves as part of the BVD eradication period from June 2010 to December 2013 were used. For the analysis of the completed questionnaires, a univariable analysis was performed by the chi-square or Wilcoxon test for each variable studied. In addition, a multivariable logistic model was performed. Four potential risk factors remained after a backward selection in the final logistic regression model: the dairy production compared to the suckling and other types of production, the herd size, the purchase of animals and the location in western region in comparison with the central and eastern regions. In summary, according to the results of this study, the farm with the highest probability of a BVDV infection in Lower Saxony is a large dairy farm that purchases cattle and is located in a cattle-dense region. When the complete eradication of the virus will be achieved, the results of the present study may help to conduct a risk-oriented monitoring programme.

Border Disease Virus Infection of Bovine Placentas

Subsequent to a previous study of border disease virus (BDV) horizontal transmission from a persistently BDV-infected calf to 6 seronegative pregnant heifers, the heifers were slaughtered 60 days after exposure to the infected calf, and their fetuses and placentas were examined. Immunohistochemical examination of fetal organs and placenta showed positive labeling of moderate intensity for pestivirus antigen in 3 of 6 heifers. BDV infection in these 3 animals was confirmed by the detection of BDV RNA in different organs using reverse transcription quantitative polymerase chain reaction. In the placenta, the positive cells were visualized mostly on the fetal side. In those 3 heifers that harbored an infected fetus, the placental tissue in the placentome region showed a moderate to severe mononuclear and fibrosing placentitis and, in severe cases, necrotic areas. The inflammatory population was composed predominantly of T and B cells, a substantial number of macrophages, and, to a lesser extent, plasma cells. This is a novel report of placentitis in persistently BDV-infected fetuses from pregnant heifers that became acutely infected by cohousing with a calf persistently infected with BDV, which extends previous reports on bovine viral diarrhea virus-infected and BDV-infected cattle and sheep, respectively.

Complete Genome Sequences of Three Border Disease Virus Strains of the Same Subgenotype, BDSwiss, Isolated from Sheep, Cattle, and Pigs in Switzerland

We report here the complete genome sequences of three border disease virus (BDV) strains of the same subgenotype isolated in Switzerland from a sheep, a cow, and a pig, respectively. This is the first report of full-length sequences of a tentatively new subgenotype isolated from three different species of cloven-hoofed farm animals.

Detection of border disease virus in Mexican cattle

The genus Pestivirus within Flaviviridae is comprised of four recognized species, namely, bovine viral diarrhoea virus 1 (BVDV-1), bovine viral diarrhoea virus 2 (BVDV-2), border disease virus (BDV) and classical swine fever virus (CSFV). BDV, while primarily infecting sheep and goats, has also been reported in cattle and wild animals. Infections of sheep and goats result in economic loss due to abortions and the birth of persistently infected animals that have poor production and reduced life expectancy. In this study, we report the detection of BDV in cattle serum collected as part of pestivirus surveillance programme from six regions of Mexico, where a 67.1% of BVDV seroprevalence was calculated previously. Phylogenetic analyses based on comparison of the 5'UTR region typed the Mexican strains as BDV-1. Border disease (BD) is listed as an exotic disease in Mexico, and the origin of BDV found in these cattle is unclear. This is the first identification of BDV in Mexican cattle.

Influence of border disease virus (BDV) on serological surveillance within the bovine virus diarrhea (BVD) eradication program in Switzerland

BACKGROUND

In 2008, a program to eradicate bovine virus diarrhea (BVD) in cattle in Switzerland was initiated. After targeted elimination of persistently infected animals that represent the main virus reservoir, the absence of BVD is surveilled serologically since 2012. In view of steadily decreasing pestivirus seroprevalence in the cattle population, the susceptibility for (re-) infection by border disease (BD) virus mainly from small ruminants increases. Due to serological cross-reactivity of pestiviruses, serological surveillance of BVD by ELISA does not distinguish between BVD and BD virus as source of infection.

RESULTS

In this work the cross-serum neutralisation test (SNT) procedure was adapted to the epidemiological situation in Switzerland by the use of three pestiviruses, i.e., strains representing the subgenotype BVDV-1a, BVDV-1h and BDSwiss-a, for adequate differentiation between BVDV and BDV. Thereby the BDV-seroprevalence in seropositive cattle in Switzerland was determined for the first time. Out of 1,555 seropositive blood samples taken from cattle in the frame of the surveillance program, a total of 104 samples (6.7%) reacted with significantly higher titers against BDV than BVDV. These samples originated from 65 farms and encompassed 15 different cantons with the highest BDV-seroprevalence found in Central Switzerland. On the base of epidemiological information collected by questionnaire in case- and control farms, common housing of cattle and sheep was identified as the most significant risk factor for BDV infection in cattle by logistic regression.

CONCLUSION

This indicates that pestiviruses from sheep should be considered as a source of infection of domestic cattle and might well impede serological BVD surveillance.

Genetic diversity of ruminant Pestivirus strains collected in Northern Ireland between 1999 and 2011 and the role of live ruminant imports

BACKGROUND

The genus pestivirus within the family Flaviviridae includes bovine viral diarrhoea virus (BVDV) types 1 and 2, border disease virus (BDV) and classical swine fever virus. The two recognised genotypes of BVDV are divided into subtypes based on phylogenetic analysis, namely a-p for BVDV-1 and a-c for BVDV-2.

METHODS

Three studies were conducted to investigate the phylogenetic diversity of pestiviruses present in Northern Ireland. Firstly, pestiviruses in 152 serum samples that had previously tested positive for BVDV between 1999 and 2008 were genotyped with a RT-PCR assay. Secondly, the genetic heterogeneity of pestiviruses from 91 serum samples collected between 2008 and 2011 was investigated by phylogenetic analysis of a 288 base pair portion of the 5' untranslated region (UTR). Finally, blood samples from 839 bovine and 4,437 ovine animals imported in 2010 and 2011 were tested for pestiviral RNA. Analysis of animal movement data alongside the phylogenetic analysis of the strains was carried out to identify any links between isolates and animal movement.

RESULTS

No BVDV-2 strains were detected. All of the 152 samples in the first study were genotyped as BVDV-1. Phylogenetic analysis indicated that the predominant subtype circulating was BVDV-1a (86 samples out of 91). The remaining five samples clustered close to reference strains in subtype BVDV-1b. Out of the imported animals, 18 bovine samples tested positive and 8 inconclusive (Ct ≥36), while all ovine samples were negative. Eight sequences were obtained and were defined as BVDV-1b. Analysis of movement data between herds failed to find links between herds where BVDV-1b was detected.

CONCLUSION

Given that only BVDV-1a was detected in samples collected between 1968 and 1999, this study suggests that at least one new subtype has been introduced to Northern Ireland between 1999 and 2011 and highlights the potential for importation of cattle to introduce new strains.

Absence of circulation of Pestivirus between wild and domestic ruminants in southern Spain

Ruminant pestiviruses (family Flaviviridae) affect both wild and domestic ruminants worldwide, causing reproductive disorders and severe economic losses. Wild (n=1442) and domestic (n=373) ruminants from southern Spain were tested for the presence of antibodies to pestiviruses. Seropositivity was detected by both ELISA and virus neutralisation test in 1/892 (0.1 per cent) red deer, 29/125 (23.2 per cent) cattle and 17/157 (10.8 per cent) sheep. Pestivirus-specific antibodies to bovine viral diarrhoea virus (BVDV) and border disease virus (BDV) genotypes 1, 4 and 5 were detected. Significantly higher antibody titres to BDV-4 compared with BVDV-NADL were found in one cattle farm. Results indicate that wild ruminants may act as spillover hosts from domestic ruminants, and therefore they do not represent a risk for domestic ruminants in southern Spain. The wide distribution of pestiviruses among sheep and cattle suggests that transmission between these species may occur.

Transmission of border disease virus from a persistently infected calf to seronegative heifers in early pregnancy

Background

This study describes the transmission of border disease virus (BDV) from a persistently infected calf to seronegative heifers in early pregnancy, resulting in persistently infected fetuses. On day 50 of pregnancy (= day 0 of the infection phase), six heifers were co-housed in a free stall with a bull calf persistently infected with BDV (pi BVD) for 60 days. The heifers underwent daily clinical examination, and blood samples were collected regularly for detection of pestiviral RNA and anti-pestivirus antibodies. After day 60 (= day 110 of pregnancy), the heifers were slaughtered, and the fetuses and placentae underwent post-mortem and immunohistochemical examination and RT-PCR for viral RNA detection.

Results

Three heifers had mild viraemia from day 8 to day 14, and by day 40 all heifers had pestivirus antibodies identified as anti-BDV antibodies in the serum neutralisation test. The placenta of the three viraemic heifers had histological evidence of inflammation, and fetal organs from these heifers were positive for pestivirus antigen by immunohistochemical examination and for BD viral RNA by RT-PCR and sequencing. Thus, co-housing of heifers in early pregnancy with a pi-BDV calf led to seroconversion in all heifers and persistent fetal infection in three.

Conclusions

Considering that pi-BDV cattle can infect other cattle and lead to persistent infection of the fetus in pregnant cows, BDV should not be ignored in the context of the mandatory BVDV eradication and monitoring program. This strongly suggests that BDV should be taken into account in BVD eradication and control programs.

Experimental infection of rabbits with bovine viral diarrhoea virus by a natural route of exposure

Bovine viral diarrhoea virus (BVDV) is an important pathogen of cattle that can naturally infect a wide range of even-toed ungulates. Non-bovine hosts may represent reservoirs for the virus that have the potential to hamper BVDV eradication programs usually focused on cattle. Rabbits are very abundant in countries such as the United Kingdom or Australia and are often living on or near livestock pastures. Earlier reports indicated that rabbits can propagate BVDV upon intravenous exposure and that natural infection of rabbits with BVDV may occur but experimental proof of infection of rabbits by a natural route is lacking. Therefore, New Zealand White rabbits were exposed to a Scottish BVDV field strain intravenously, oro-nasally and by contaminating their hay with virus. None of the animals showed any clinical signs. However, the lymphoid organs from animals sacrificed at day five after exposure showed histological changes typical of transient infection with pestivirus. Most organ samples and some buffy coat samples were virus positive at day five but saliva samples remained negative. Development of antibodies was observed in all intravenously challenged animals, in all of the nebulised group and in four of six animals exposed to contaminated hay. To our knowledge this is the first report of BVDV propagation in a species other than ruminants or pigs after exposure to the virus by a natural route. However, to assess the role of rabbits as a potential reservoir for BVDV it remains to be determined whether persistent infection caused by intra-uterine infection is possible and whether BVDV is circulating in wild rabbit populations.