75 years of bovine viral diarrhea virus: Current status and future applications of the use of directed antivirals

Bovine viral diarrhea virus (BVDV) was first reported 75 years ago and remains a source of major financial and production losses in the North American cattle industry. Currently, control methods in North America primarily center around biosecurity and vaccination programs; however, despite high levels of vaccination, the virus persists in the cattle herd due at least in part to the often-insidious nature of disease and the constant viremia and viral shedding of persistently infected animals which act as a reservoir for the virus. Continued development of targeted antivirals represents an additional tool for the prevention of BVDV-associated losses. Currently, in vivo studies of BVDV antivirals are relatively limited and have primarily been directed at the RNA-dependent RNA polymerase which represents the viral target with the highest potential for commercial development. Additional live animal studies have explored the potential of exogenous interferon treatment. Future research of commercial antivirals must focus on the establishment and validation of in vivo efficacy for compounds with demonstrated antiviral potential. The areas which provide the most viable economic justification for the research and development of antivirals drugs are the fed cattle sector, outbreak control, and wildlife or animals of high genetic value. With further development, targeted antivirals represent an additional tool for the management and control of BVDV in North American cattle herds.

Anti-Classical Swine Fever Virus Strategies

Classical swine fever (CSF), caused by CSF virus (CSFV), is a highly contagious swine disease with high morbidity and mortality, which has caused significant economic losses to the pig industry worldwide. Biosecurity measures and vaccination are the main methods for prevention and control of CSF since no specific drug is available for the effective treatment of CSF. Although a series of biosecurity and vaccination strategies have been developed to curb the outbreak events, it is still difficult to eliminate CSF in CSF-endemic and re-emerging areas. Thus, in addition to implementing enhanced biosecurity measures and exploring more effective CSF vaccines, other strategies are also needed for effectively controlling CSF. Currently, more and more research about anti-CSFV strategies was carried out by scientists, because of the great prospects and value of anti-CSFV strategies in the prevention and control of CSF. Additionally, studies on anti-CSFV strategies could be used as a reference for other viruses in the Flaviviridae family, such as hepatitis C virus, dengue virus, and Zika virus. In this review, we aim to summarize the research on anti-CSFV strategies. In detail, host proteins affecting CSFV replication, drug candidates with anti-CSFV effects, and RNA interference (RNAi) targeting CSFV viral genes were mentioned and the possible mechanisms related to anti-CSFV effects were also summarized.

Meeting report: 32nd International Conference on Antiviral Research

The 32nd International Conference on Antiviral Research (ICAR), sponsored by the International Society for Antiviral Research (ISAR), was held in Baltimore, Maryland, USA, on May 12-15, 2019. This report gives an overview of the conference on behalf of the Society. It provides a general review of the meeting and awardees, summarizing the presentations, and their main conclusions from the perspective of researchers active in many different areas of antiviral research and development. As in past years, ICAR promoted and showcased the most recent progress in antiviral research, and continued to foster collaborations and interactions in drug discovery and development. The 33rd ICAR will be held in Seattle, Washington, USA, March 30th-April 3rd, 2020.

African swine fever

The continuing spread of African swine fever (ASF) outside Africa in Europe, the Russian Federation, China and most recently to Mongolia and Vietnam, has heightened awareness of the threat posed by this devastating disease to the global pig industry and food security. In this review we summarise what we know about the African swine fever virus (ASFV), the disease it causes, how it spreads and the current global situation. We discuss current control methods in domestic and wild pigs and prospects for development of vaccines and other tools for control.

Low CD4/CD8 ratio in classical swine fever postnatal persistent infection generated at 3 weeks after birth

Classical swine fever virus (CSFV) is one of the most important pathogens affecting swine. After infection with a moderate virulence strain at 8 hours after birth, CSFV is able to induce viral persistence. These animals may appear clinically healthy or showed unspecific clinical signs despite the permanent viremia and high viral shedding, in absence of immune response to the virus. Given the role played by this infection in disease control, we aimed to evaluate the capacity of CSFV to induce postnatal persistent infection at 3 weeks after birth. Nine pigs were CSFV infected and sampled weekly during 6 weeks and viral, clinical, pathological and immunological tests were carried out. Also, the CD4/CD8 ratio was calculated with the purpose to relate this marker with the CSFV persistent infection. The IFN‐α response was detected mainly 1 week after infection, being similar in all the infected animals. However, 44.4% of animals were CSFV persistently infected, 33.3% died and 22.2% developed specific antibody response. Interestingly, in persistently infected pigs, the T‐CD8 population was increased, the T‐CD4 subset was decreased and lower CD4/CD8 ratios were detected. This is the first report of CSFV capacity to confer postnatal persistent infection in pigs infected at 3 weeks after birth, an age in which the weaning could be carried out in some swine production systems. This type of infected animals shed high amounts of virus and are difficult to evaluate from the clinical and anatomopathological point of view. Therefore, the detection of this type of infection and its elimination in endemic areas will be relevant for global CSF eradication. Finally, the low CD4/CD8 ratios found in persistently infected animals may be implicated in maintaining high CSFV replication during persistence and further studies will be performed to decipher the role of these cells in CSFV immunopathogenesis.

Epidemiology, diagnosis and control of classical swine fever: Recent developments and future challenges

Classical swine fever (CSF) represents a major health and trade problem for the pig industry. In endemic countries or those with a wild boar reservoir, CSF remains a priority for Veterinary Services. Surveillance as well as stamping out and/or vaccination are the principle tools of prevention and control, depending on the context. In the past decades, marker vaccines and accompanying diagnostic tests allowing the discrimination of infected from vaccinated animals have been developed. In the European Union, an E2 subunit and a chimeric live vaccine have been licensed and are available for the use in future disease outbreak scenarios. The implementation of commonly accepted and globally harmonized concepts could pave the way to replace the ethically questionable stamping out policy by a vaccination-to-live strategy and thereby avoid culling of a large number of healthy animals and save food resources. Although a number of vaccines and diagnostic tests are available worldwide, technological advancement in both domains is desirable. This work provides a summary of an analysis undertaken by the DISCONTOOLS group of experts on CSF. Details of the analysis can be downloaded from the web site at http://www.discontools.eu/.

3-(imidazo[1,2-a:5,4-b']dipyridin-2-yl)aniline inhibits pestivirus replication by targeting a hot spot drug binding pocket in the RNA-dependent RNA polymerase

The compound 3-(imidazo[1,2-a:5,4-b']dipyridin-2-yl)aniline (CF02334) was identified as a selective inhibitor of the cytopathic effect (CPE) caused by bovine viral diarrhea virus (BVDV) in a virus-cell-based assay. The EC50-values for inhibition of CPE, viral RNA synthesis and the production of infectious virus progeny were 13.0 ± 0.6 μM, 2.6 ± 0.9 μM and 17.8 ± 0.6 μM, respectively. CF02334 was found to be inactive in the hepatitis C subgenomic replicon system. CF02334-resistant BVDV was obtained and was found to carry the N264D mutation in the viral RNA-dependent RNA polymerase (RdRp). Molecular modeling revealed that N264D is located in a small cavity near the fingertip domain of the pestivirus polymerase. CF02334-resistant BVDV was proven to be cross-resistant to BPIP, AG110 and LZ37, inhibitors that have previously been described to target the same region of the BVDV RdRp. CF02334 did not inhibit the in vitro activity of recombinant BVDV RdRp, but did inhibit the activity of BVDV replication complexes. Taken together, these observations indicate that CF02334 likely interacts with the fingertip of the pestivirus RdRp at the same position as BPIP, AG110 and LZ37, which marks this region of the viral polymerase as a "hot spot" for inhibition of pestivirus replication.

Episodic adaptive diversification of classical swine fever virus RNA-dependent RNA polymerase NS5B

Classical swine fever virus (CSFV) is the pathogen that causes a highly infectious disease of pigs and has led to disastrous losses to pig farms and related industries. The RNA-dependent RNA polymerase (RdRp) NS5B is a central component of the replicase complex (RC) in some single-stranded RNA viruses, including CSFV. On the basis of genetic variation, the CSFV RdRps could be clearly divided into 2 major groups and a minor group, which is consistent with the phylogenetic relationships and virulence diversification of the CSFV isolates. However, the adaptive signature underlying such an evolutionary profile of the polymerase and the virus is still an interesting open question. We analyzed the evolutionary trajectory of the CSFV RdRps over different timescales to evaluate the potential adaptation. We found that adaptive selection has driven the diversification of the RdRps between, but not within, CSFV major groups. Further, the major adaptive divergence-related sites are located in the surfaces relevant to the interaction with other component(s) of RC and the entrance and exit of the template-binding channel. These results might shed some light on the nature of the RdRp in virulence diversification of CSFV groups.

Potential applications for antiviral therapy and prophylaxis in bovine medicine

Viral disease is one of the major causes of financial loss and animal suffering in today's cattle industry. Increases in global commerce and average herd size, urbanization, vertical integration within the industry and alterations in global climate patterns have allowed the spread of pathogenic viruses, or the introduction of new viral species, into regions previously free of such pathogens, creating the potential for widespread morbidity and mortality in naïve cattle populations. Despite this, no antiviral products are currently commercially licensed for use in bovine medicine, although significant progress has been made in the development of antivirals for use against bovine viral diarrhea virus (BVDV), foot and mouth disease virus (FMDV) and bovine herpesvirus (BHV). BVDV is extensively studied as a model virus for human antiviral studies. Consequently, many compounds with efficacy have been identified and a few have been successfully used to prevent infection in vivo although commercial development is still lacking. FMDV is also the subject of extensive antiviral testing due to the importance of outbreak containment for maintenance of export markets. Thirdly, BHV presents an attractive target for antiviral development due to its worldwide presence. Antiviral studies for other bovine viral pathogens are largely limited to preliminary studies. This review summarizes the current state of knowledge of antiviral compounds against several key bovine pathogens and the potential for commercial antiviral applications in the prevention and control of several selected bovine diseases.

Substituted 2,6-bis(benzimidazol-2-yl)pyridines: a novel chemical class of pestivirus inhibitors that targets a hot spot for inhibition of pestivirus replication in the RNA-dependent RNA polymerase

2,6-Bis(benzimidazol-2-yl)pyridine (BBP/CSFA-0) was identified in a CPE-based screening as a selective inhibitor of the in vitro bovine viral diarrhea virus (BVDV) replication. The EC50-values for the inhibition of BVDV-induced cytopathic (CPE) effect, viral RNA synthesis and the production of infectious virus were 0.3±0.1μM, 0.05±0.01μM and 0.3±0.04μM, respectively. Furthermore, BBP/CSFA-0 inhibits the in vitro replication of the classical swine fever virus (CSFV) with an EC50 of 0.33±0.25μM. BBP/CSFA-0 proved in vitro inactive against the hepatitis C virus, that belongs like BVDV and CSFV to the family of Flaviviridae. Modification of the substituents on the two 1H-benzimidazole groups of BBP resulted in analogues equipotent in anti-BVDV activity (EC50=0.7±0.1μM), devoid of cytotoxicity (S.I.=142). BBP resistant BVDV was selected for and was found to carry the I261M mutation in the viral RNA-dependent RNA polymerase (RdRp). Likewise, BBP-resistant CSFV was selected for; this variant carries either an I261N or a P262A mutation in NS5B. Molecular modeling revealed that I261 and P262 are located in a small cavity near the fingertip domain of the pestivirus polymerase. BBP-resistant BVDV and CSFV proved to be cross-resistant to earlier reported pestivirus inhibitors (BPIP, AG110 and LZ37) that are known to target the same region of the RdRp. BBP did not inhibit the in vitro activity of recombinant BVDV RdRp but inhibited the activity of BVDV replication complexes (RCs). BBP interacts likely with the fingertip of the pestivirus RdRp at the same position as BPIP, AG110 and LZ37. This indicates that this region is a "hot spot" for inhibition of pestivirus replication.

In vitro surrogate models to aid in the development of antivirals for the containment of foot-and-mouth disease outbreaks

Foot-and-mouth disease virus (FMDV) is a highly pathogenic member of the genus Aphthovirus (family Picornaviridae) that is only to be manipulated in high-containment facilities, thus complicating research on and discovery of antiviral strategies against the virus. Bovine rhinitis B virus (BRBV) and equine rhinitis A virus (ERAV), phylogenetically most closely related to FMDV, were explored as surrogates for FMDV in antiviral studies. Although no efficient cell culture system has been reported so far for BRBV, we demonstrate that infection of primary bovine kidney cells resulted in an extensive but rather poorly-reproducible induction of cytopathic effect (CPE). Madin-Darby bovine kidney cells on the other hand supported viral replication in the absence of CPE. Antiviral tests were developed for ERAV in Vero A cells employing a viral RNA-reduction assay and CPE-reduction assay; the latter having a Z' factor of 0.83±0.07. The BRBV and ERAV models were next used to assess the anti-aphthovirus activity of two broad-spectrum antiviral agents 2'-C-methylcytidine (2CMC) and ribavirin, as well as of the enterovirus-specific inhibitor enviroxime. The effects of the three compounds in the CPE-reduction (ERAV) and viral RNA-reduction assays (BRBV and ERAV) were comparable. Akin to 2CMC, compound A, a recently-discovered non-nucleoside pan-serotype FMDV inhibitor, also inhibited the replication of both BRBV and ERAV, whereas enviroxime was devoid of activity. The BRBV and ERAV surrogate models reported here can be manipulated in BSL-2 laboratories and may facilitate studies to unravel the mechanism of action of novel FMDV inhibitors.

The potential of antiviral agents to control classical swine fever: a modelling study

Classical swine fever (CSF) represents a continuous threat to pig populations that are free of disease without vaccination. When CSF virus is introduced, the minimal control strategy imposed by the EU is often insufficient to mitigate the epidemic. Additional measures such as preemptive culling encounter ethical objections, whereas emergency vaccination leads to prolonged export restrictions. Antiviral agents, however, provide instantaneous protection without inducing an antibody response. The use of antiviral agents to contain CSF epidemics is studied with a model describing within- and between-herd virus transmission. Epidemics are simulated in a densely populated livestock area in The Netherlands, with farms of varying sizes and pig types (finishers, piglets and sows). Our results show that vaccination and/or antiviral treatment in a 2 km radius around an infected herd is more effective than preemptive culling in a 1 km radius. However, the instantaneous but temporary protection provided by antiviral treatment is slightly less effective than the delayed but long-lasting protection offered by vaccination. Therefore, the most effective control strategy is to vaccinate animals when allowed (finishers and piglets) and to treat with antiviral agents when vaccination is prohibited (sows). As independent control measure, antiviral treatment in a 1 km radius presents an elevated risk of epidemics running out of control. A 2 km control radius largely eliminates this risk.

Intra-host variation structure of classical swine fever virus NS5B in relation to antiviral therapy

Classical swine fever (CSF) is one of most important diseases of the Suidea with severe social economic consequences in case of outbreaks. Antivirals have been demonstrated, in recent publications, to be an interesting alternative method of fighting the disease. However, classical swine fever virus is an RNA virus which presents a challenge as intra-host variation and the error prone RNA dependent RNA polymerase (RdRp) could lead to the emergence/selection of resistant variants hampering further treatment. Therefore, it was the purpose of this study to investigate the intra-host variation of the RdRp gene, targeted by antivirals, in respect to antiviral treatment. Using the non-unique nucleotide changes, a limited intra-host variation was found in the wild type virus with 2 silent and 2 non-synonymous sites. This number shifted significantly when an antiviral resistant variant was analyzed. In total 22nt changes were found resulting in 14 amino acid changes whereby each genome copy contained at least 2 amino-acid changes in the RdRp. Interestingly, the frequency of the mutations situated in close proximity to a region involved in antiviral resistance in CSFV and bovine viral diarrhea virus (BVDV) was elevated compared to the other mutations. None of the identified mutations in the resistant variant and which could potentially result in antiviral resistance was present in the wild type virus as a non-unique mutation. In view of the spectrum of mutations identified in the resistance associated region and that none of the resistance associated mutations reported for another strain of classical swine fever for the same antiviral were observed in the study, it can be suggested that multiple mutations confer resistance to some degree. Although the followed classical approach allowed the analysis the RdRp as a whole, the contribution of unique mutations to the intra-host variation could not be completely resolved. There was a significant difference in de number of unique mutations found between: 1/wild type virus and the antiviral resistant variant and 2/between both and the number to be expected from the error rate of the RT-PCR process. This indicates that the some of the unique mutations contributed to the intra-host variation and that the antiviral pressure also shifted this pattern. This is important as one of the non-synonymous mutations found in the resistant variant and which was located in the antiviral resistance associated region, was present in the wild type virus as a unique mutation. The findings presented in this study not only show the importance of intra-host variation analysis but also warrants further research certainly in view of the potential inclusion of antivirals in a control/eradication strategy.

Synthesis and antiviral activity of an imidazo[1,2-a]pyrrolo[2,3-c]pyridine series against the bovine viral diarrhea virus

A series of imidazo[1,2-a]pyrrolo[2,3-c]pyridines has been prepared and evaluated for their anti-BVDV activities in MDBK cells. From the synthesized analogues bearing modifications of the substituents at positions 2, 3, 7 and 8, compounds 10a, b, 16, 24, 25 and 26 exhibited significant anti-BVDV activities.

The reduction of CSFV transmission to untreated pigs by the pestivirus inhibitor BPIP: a proof of concept

5-[(4-Bromophenyl)methyl]-2-phenyl-5H-imidazo[4,5-c]pyridine (BPIP) is a representative molecule of a novel class of highly active in vitro inhibitors of the replication of Classical swine fever virus (CSFV). We recently demonstrated in a proof of concept study that the molecule has a marked effect on viral replication in CSFV-infected pigs. Here, the effect of antiviral treatment on virus transmission to untreated sentinel pigs was studied. Therefore, BPIP-treated pigs (n=4), intra-muscularly infected with CSFV, were placed into contact with untreated sentinel pigs (n=4). Efficient transmission of CSFV from four untreated seeder pigs to four untreated sentinels was observed. In contrast, only two out of four sentinel animals in contact with BPIP-treated seeder animals developed a short transient infection, of which one was likely the result of sentinel to sentinel transmission. A significant lower viral genome load was measured in tonsils of sentinels in contact with BPIP-treated seeder animals compared to the positive control group (p=0.015). Although no significant difference (p=0.126) in the time of onset of viraemia could be detected between the groups of contact animals, a tendency towards the reduction of virus transmission was observed. Since sentinel animals were left untreated in this exploratory trial, the study can be regarded as a worst case scenario and gives therefore an underestimation of the potential efficacy of the activity of BPIP on virus transmission.