The development of oral lesions in lambs naturally infected with orf virus

Use of a local anaesthetic and antiseptic wound formulation for the treatment of lambs naturally infected with Orf virus

Contagious ecthyma (CE) is a worldwide highly contagious zoonotic viral skin disease of sheep and goats. Treatment for Orf virus (ORFV) infection usually involves topical and oral antibiotics. An anaesthetic and antiseptic topical gel (Multisolfen® or Tri-Solfen®; MS®, Medical Ethics, Australia) has been documented as an efficacious therapy for lesions from mucosal and epithelial viral infections in ruminants. The present study tested a new treatment protocol of MS® for CE therapy on-farm in 150 lambs naturally infected with ORFV. Lambs were divided into three cohorts of 50 lambs each (C, D and E). Cohort C was treated with MS® 3 times with an interval of 3 days between treatments, cohort D was treated daily with hypochlorous acid, whilst cohort E served as untreated controls. The lambs were examined clinically every two days, weight measured weekly, with whole blood and sterile swabs from ORFV lesions collected for haematological analysis and specific ORFV PCR. Cohort C presented fewer lambs displaying ORFV-associated lesions than other cohorts at different times of the experiment. Further, lesions treated with MS® were milder compared with other cohorts. However, following cessation of therapy, most of the lambs again developed ORFV-associated lesions. No differences between cohorts were observed in weight, haematological and PCR results. These findings suggest that topical treatment with MS® is effective for CE in field conditions, especially in the first stages of the clinical course, although treatment with MS® may need to be extended a minimum of 4 weeks.

Contagious ecthyma in Egypt: Clinical, virological and molecular explorations

Contagious Ecthyma (CE) is a highly contagious viral disease of sheep and goats with worldwide distribution. The present study aimed to provide a clinical description of contagious ecthyma in four sheep flocks and screen the possible genetic variation in the B2L gene of the detected isolates. Oral lesions were collected and inoculated into chorioallantoic membrane (CAM) of 11 days embryonated chicken eggs. Polymerase chain reaction and direct sequencing of the B2L gene was conducted. Infected sheep exhibited anorexia with a development of nodular lesions evolving in proliferative thick scabs around oral commissures. The inoculated CAM showed small-sized white pock lesions accompanied with thickening of CAM. The partial length of B2L gene (592 bp) was successfully amplified in samples collected from four flocks. The isolated strains belong to genotype I/I and I/II. Sequence and evolutionary analysis illustrate that B2L gene (ORF011) are highly conserved among Orf viruses isolated from different countries.

Identification of strain diversity and phylogenetic analysis based on two major essential proteins of Orf viruses isolated from several clinical cases reported in Malaysia

There is a little information on the characterization of Orf virus strains that are endemic in Malaysia. The relationship between the severity of disease and the molecular genetic profile of Orf virus strains has not been fully elucidated. This study documented the first confirmed report of contagious ecthyma causing by Orf virus in goats from a selected state of eastern peninsular Malaysia. The disease causes significant debilitation due to the inability of affected animals to suckle which brings a great economic loss to the farmers. A total of 504 animals were examined individually to recognize the affected animals with Orf lesion. Skin scrapping was used to collect the scab material from the infected animals. The presence of Orf virus was confirmed by combination of methods including virus isolation on vero cells, identification by Transmission Electron Microscopy (TEM) and molecular technique using PCR and Sanger sequencing. The results showed the successful isolation of four Orf virus strains with a typical cytopathic effects on the cultured vero cells line. The morphology was confirmed to be Orf virus with a distinctive ovoid and criss cross structure. The phylogenetic analysis revealed that these isolated strains were closely related to each other and to other previously isolated Malaysian orf viruses. In addition these Orf virus strains were closely related to Orf viruses from China and India. This study provides more valuable insight in terms of genotype of Orf virus circulating in Malaysia.

An association of Orf virus infection among sheep and goats with herd health programme in Terengganu state, eastern region of the peninsular Malaysia

BACKGROUND

Orf virus causes a scabby skin lesions which decreases productivity in small ruminants. The unknown status of this disease in the eastern region of Peninsular Malaysia warrants a study to determine sero-prevalence of orf with regards to farmers' compliance level towards the Herd Health Program (HHP) programme.

RESULTS

Out of 504 animals, 115 were positive for Orf-virus antibodies. An overall prevalence rate of 22.8% indicated a high prevalence of orf disease in this region. It was observed that 25.1% (92/367) of goats were positive and 16.8% (23/137) of sheep sero-converted for Orf virus antibody. Several factors were measured for their possible association with prevalence of Orf virus infection. The prevalence was higher in LY farm, JC breed, kid and female animals, and in the presence of disease lesion. Chi-square analysis showed a significant association of three risk factors which are species, age and sex of the animals (P < 0.05). Notwithstanding, all other variables showed no significant difference (P > 0.05). Farms surveyed usually practised intensive management system, keeping animals in the shade at all time, due to limited availability of suitable land as a free-range grazing area. An interview with small holder farmers revealed a lack of awareness of the main goals of herd health programme. An overall compliance level of 42.7% was observed for all HHP parameters. Among the 14 main components of HHP modules, animal identification had recorded highest compliance level (84.62%) while milking management recorded the least compliance (- 82.69%). That explained why there was a high sporadic prevalence of Orf infection in this region.

CONCLUSION

Good herd health supervision is a rehearsal target to prevent an outbreak and the spread of diseases thus reduces economic losses among farmers. Therefore, a good herd health programme should be in place, in order to prevent and control disease transmission as well as to improve herd immunity.

Phylodynamics of parapoxvirus genus in Mexico (2007-2011)

In this study we report for the first time the phylodynamics of the parapoxvirus (PPV) genus in Mexico. Based on the analysis by PCR of 124 epithelial samples collected between 2007 and 2011 from naturally infected goats, sheep and cows in Mexico, we found that different PPV were present in 21 out of the 24 states sampled during this study. Our phylogenetic analysis confirmed the presence of different PPV species in Mexico, and their phylogenetic relationship with other PPV circulating in the US and Canada. Furthermore, we describe the existence of two different ORFV phylogenetic groups that are clearly host associated (sheep or goat). Evidence of directional selection at five specific amino acid residues in the enveloped glycoprotein B2L might help to support this host predilection. Collectively, the results generated in this study highlight the importance of PPV genus in Mexico and open the possibility for future studies describing with more detail the importance of this genus in North America.

The whole genomic analysis of orf virus strain HN3/12 isolated from Henan province, central China

Background

The Orf virus (ORFV) is the causative agent of orf, a globally-occurring, acute, pustular, contagious disease affecting sheep, goats and humans with a worldwide distribution. Currently, the genomic analysis of four ORFV strains from the Fujian province in southern China and a NA1/11 strain isolated from the Jilin province in northeast China have been reported. However, little is known about the genomic information of ORFV strains from central China.

Results

From a recent outbreak in a sheep herd in the Henan province of central China, a novel ORFV strain (HN3/12) was isolated and cultured in ovine fetal turbinate (OFTu) cells. The strain was identified as HN3/12 and verified by PCR based on the DNA sequences of 011 and 059 genes. The whole genomic sequence of this isolate was determined by Next Generation Sequencing technology. To determine the genetic characteristics of the HN3/12 strain, phylogenetic analysis of the 011 and 059 genes and amino acid sequence alignment of the HN3/12 strain were performed and compared with reference parapoxvirus strains.

Conclusions

The HN3/12 genome is 136,643 bp in length, contains 63.67% G + C and encodes 132 putative genes. Phylogenetic analysis of the 011 and 059 nucleotide sequences showed that this viral strain was similar to the NA1/11 isolate. The homology analysis indicates that HN3/12 has 93% to 98% identity with published ORFV strains at amino acid level. When open reading frames (ORFs) were aligned among the HN3/12 and four Fujian ORFV strains, most of them have identities greater than 90% and only a few less than 60%. The availability of the whole genomic sequence of HN3/12 aids in our understanding of, and provides new insights into, the genetic diversity of ORFV.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-017-1178-1) contains supplementary material, which is available to authorized users.

Development of a lateral flow immunochromatographic assay for the rapid diagnosis of Orf virus infections

A rapid and simple lateral-flow immunochromatographic assay (LFIA) was developed for the specific detection of Orf virus (ORFV) using two distinct monoclonal antibodies (MAbs: 5A5 and 6F2) against the ORFV ORF011 protein. The MAb 5A5 was conjugated with colloidal gold, and the MAb 6F2 and goat anti-mouse IgG were sprayed onto a nitrocellulose membrane in strips at positions designated test (T) and control (C), respectively. The results showed that samples of ORFV complexed with colloidal gold-conjugated MAb 5A5, were captured by MAb 6F2 at the T line resulting in the appearance of a purple band. When samples did not contain ORFV or when they contained a quantity of ORFV below the detection limit of the test, only the C line was visible. The analysis of sensitivity of the test demonstrated that the lowest detected quantity of ORFV was 2.03×10(3.0) TCID50/ml. Storage at room temperature for 6 months did not result in the loss of performance of the LFIA test. Using loop-mediated isothermal amplification (LAMP) as a reference test, the relative specificity and sensitivity of the LFIA test were determined to be 100% and 92.1%, respectively. Based on these results, the LFIA test developed may be a suitable tool for rapid on-site testing for ORFV infection.

Isolation and Characterization of Monoclonal Antibodies Against a Virion Core Protein of Orf Virus Strain NA1/11 As Potential Diagnostic Tool for Orf Viruses

Orf is caused by the orf virus (ORFV) and is a non-systemic, widespread disease afflicting sheep, goats, wild ruminants, and humans. Recent outbreaks in sheep and goats in Jilin and other northern Chinese provinces raise concerns about orf control in China. Thirty-five hybridoma clones were constructed from splenocytes of BALB/c mice immunized with natural orf virus protein. These hybridomas were used to produce antibodies targeting ORFV proteins. Immunological characterization of these monoclonal antibodies (MAb) showed that the 5F2D8 hybridoma line produced MAb that can recognize the 100, 70, and 20 kDa bands from total viral lysate. This hybridoma was further characterized by immunoprecipitation and peptide sequencing. The results indicate that 5F2D8 specifically recognizes orf virus encoded protein ORFV086, a late expression virion core protein that plays important roles in progeny virus particle assembly, morphogenesis, and maturity. Further experiments demonstrate that this MAb did not react with other viral proteins of ORFV orthopoxviruses, but reacted strongly to different field isolates of orf viruses from China. Additionally, this anti-ORFV086 MAb possesses ORFV neutralizing capability. Sequence alignments and phylogenetic analysis determined that ORFV086 of NA1/11, clustered together with NZ2 and IA82, is highly conserved and has structural similarities with the Vaccinia virus core protein P4a. As such, this MAb has great potential as a diagnostic tool for orf viruses, in the further exploration of orf pathogenesis, and in disease control and prevention.

Orf virus infection in sheep or goats

Orf virus, a member of the genus Parapoxvirus, is the causative agent of contagious ecthyma ('Orf'). It is a pathogen with worldwide distribution, causing significant financial losses in livestock production. The disease mainly affects sheep and goats, but various other ruminants and mammals have been reported to be infected as well. It is also a zoonotic disease, affecting mainly people who come in direct or indirect contact with infected animals (e.g. farmers, veterinarians). The disease is usually benign and self-limiting, although in many cases, especially in young animals, it can be persistent and even fatal. Production losses caused by Orf virus are believed to be underestimated, as it is not a notifiable disease. This review of literature presents all latest information regarding the virus; considerations regarding treatment and prevention will be also discussed.

Diagnostic pathology in microbial diseases of sheep or goats

Post-mortem examination is a key step in the diagnostic process of infectious diseases in sheep and goats. Diagnostic pathology deals with identification and study of lesions, at the same time providing also significant clues regarding pathogenesis of the diseases. This article reviews the salient pathological findings associated with the most significant infectious diseases of sheep and goats present in countries where small ruminants are a relevant agricultural industry. Lesions are reviewed according to the different organ systems where they occur. Emphasis has been given in the description of the salient lesional patterns than can be identified in each organ and which can be of help in the differential diagnosis of the lesions caused by bacteria, viruses, fungi or prions. Finally, a review of the usefulness of ancillary tests that may be used on various tissue samples for performing an aetiological diagnosis, is included; the application of various techniques, from immunohistochemistry to molecular biology-based tests, is described.

Molecular genetic analysis of orf virus: a poxvirus that has adapted to skin

Orf virus is the type species of the Parapoxvirus genus of the family Poxviridae. It induces acute pustular skin lesions in sheep and goats and is transmissible to humans. The genome is G+C rich, 138 kbp and encodes 132 genes. It shares many essential genes with vaccinia virus that are required for survival but encodes a number of unique factors that allow it to replicate in the highly specific immune environment of skin. Phylogenetic analysis suggests that both viral interleukin-10 and vascular endothelial growth factor genes have been "captured" from their host during the evolution of the parapoxviruses. Genes such as a chemokine binding protein and a protein that binds granulocyte-macrophage colony-stimulating factor and interleukin-2 appear to have evolved from a common poxvirus ancestral gene while three parapoxvirus nuclear factor (NF)-κB signalling pathway inhibitors have no homology to other known NF-κB inhibitors. A homologue of an anaphase-promoting complex subunit that is believed to manipulate the cell cycle and enhance viral DNA synthesis appears to be a specific adaptation for viral-replication in keratinocytes. The review focuses on the unique genes of orf virus, discusses their evolutionary origins and their role in allowing viral-replication in the skin epidermis.

Pathogenesis in lambs and sequence analysis of putative virulence genes of Brazilian orf virus isolates

The parapoxvirus orf virus (ORFV) is the agent of contagious ecthyma, an ubiquitous mucocutaneous disease of sheep and goats that may present variable clinical presentations. We herein studied the pathogenesis of ORFV infection in lambs and analyzed three putative virulence genes of four Brazilian ORFV isolates. Lambs inoculated in the labial commissures with each ORFV isolate (n=4, viral titer 10(5.6) TCID50/ml) developed classical orf lesions, characterized by a progressive course of erythema/macules, vesicles, pustules and proliferative scabs. Lesions lasted an average of 22.9 days (18-26) and virus shedding was detected for approximately 24.6 days (18-30). Two isolates (SV269/11 and SV820/10) produced more severe, long-lasting lesions resulting in highest clinical scores. Lambs inoculated with isolate SV581/11 developed lesions markedly milder (lower clinical scores [p<0.05]) and more limited than the other groups. Virus shedding by SV581/11 group, however, lasted similarly or even longer than the other groups. Sequence analysis of three virulence genes (VEGF, VIR and IL-10v) revealed amino acid deletions and mutations in VEGF and IL-10v genes of SV581/11 and SV252/11, the isolate(s) producing milder lesions. Additionally, the VEGF gene of isolate SV581/11 presented the lowest amino acid identity with the other isolates and with ORFV standard strain OV-IA82. Thus, these results demonstrate that ORFV isolates may display differential virulence in lambs and these differences might be associated with genetic changes in putative virulence genes.

Heterogeneity among orf virus isolates from goats in Fujian Province, Southern China

Orf virus is a parapoxvirus that causes recurring contagious ecthyma or orf disease in goat, sheep and other wild and domestic ruminants. Infected animals show signs of pustular lesions on the mouth and muzzle and develop scabs over the lesions. Although the infection is usually cleared within 1–2 months, delayed growth and associated secondary infections could still impact the herds. Orf virus can also infect humans, causing lesions similar to the animals in pathological histology. Prior infection of orf virus apparently offers little protective immunity against future infections. Several gene products of orf virus have been identified as responsible for immunomodulatory functions. In our recent study of orf virus isolates from an area along the Minjiang River in northern Fujian Province, we found a high heterogeneity among isolates from 10 farms within a 120-kilometer distance. Only two isolates from locations within 1 km to each other had same viral genes. There is no correlation between the geographical distance between the corresponding collection sites and the phylogenetic distance in ORFV011 or ORV059 genes for any two isolates. This finding suggests that there are diverse populations of orf virus present in the environment. This may in part contribute to the phenomenon of recurring outbreaks and heighten the need for better surveillance.

The N terminus of orf virus-encoded protein 002 inhibits acetylation of NF-κB p65 by preventing Ser(276) phosphorylation

Orf virus-encoded protein 002 (ORFV002) inhibits NF-κB signaling pathway by decreasing the acetylation of NF-κB-p65 through interference of NF-κB p65′s association with NF-κB p300. However, the precise mechanism of how ORFV002 interferes with the NF-κB p65/p300 association is still unknown. Due to similarities of the amino acid sequences of ORFV002 and the adenovirus type 12 (Ad12) E1A protein (E1A-12), we hypothesized that the N-terminal 52 amino acids of ORFV002 might play an important role in this inhibition and constructed several in-frame fusions of ORFV002 to an enhanced green fluorescent protein (EGFP) reporter, including C-terminal and N-terminal deletion mutants of ORFV002. When the N-terminus of ORFV002 was absent, the localization of ORFV002 shifted mainly from the nucleus to the cytoplasm, and it's inhibition of NF-κB transactivation was lost. NF-κB p65 Lys³¹⁰ acetylation and Ser²⁷⁶ phosphorylation were detected in co-transfection experiments with NF-κB p65 and ORFV002 or its mutants with, or without, the N-terminal region. The results showed that the N-terminus of ORFV002 plays a crucial role in inhibiting both the acetylation and phosphorylation of NF-κB p65. Further investigation indicated that ORFV002 and its C-terminal deletion mutants interfered with NF-κB p65 (Ser²⁷⁶) phosphorylation induced by mitogen- and stress-activated protein kinase-1 (MSK1) and the interaction between NF-κB p65 and MSK1. Since phosphorylated NF-κB p65 recruits transcriptional co-activators such as p300 and CBP, we concluded that the N-terminus of ORFV002 inhibits acetylation of NF-κB p65 by blocking phosphorylation of NF-κB p65 at Ser²⁷⁶.

Orf in South Africa: endemic but neglected

A survey amongst sheep and goat producers and veterinarians was undertaken to collect epidemiological data on orf in South Africa. Previous epidemiological studies on the presence of the disease in the country have not been documented and this report is the first descriptive epidemiological study of orf in South Africa. A seven-month investigation, realised by direct and indirect interviews and field observation, enabled us to outline incidence and risk factors of this disease and to better understand how the local farmers in rural areas relate to it. The results may contribute to better management of the disease in rural areas. By means of molecular analyses the phylogenetic relationships between field isolates from different areas have been identified. The findings gave a first important contribution to the general assessment of the economic impact of orf virus infections and the extent of the risk to human health.

Isolation and phylogenetic analysis of orf virus from the sheep herd outbreak in northeast China

BACKGROUND

Orf is a zoonotic and epitheliotrophic contagious disease that mainly affects sheep, goats, wild ruminants, and humans with a worldwide distribution. To date, there is little information on the characterization of ORFV strains that are endemic in Mainland China. In addition, the relationship between the severity of disease and the molecular profile of ORFV strains has not been fully elucidated.

RESULTS

From the recent outbreak of a sheep herd in Nongan, northeast of China, the novel orf virus (ORFV) strain NA1/11 was successfully isolated. Western blot analysis indicated that the NA1/11 strain cross reacts with monoclonal antibody A3 and infected sheep ORFV antiserum. The purified virions revealed the typical ovoid shape when observed by atomic force microscopy. To determine the genetic characteristics of the NA1/11 strain, the sequences of ORFV011 (B2L), ORFV059 (F1L), ORFV109, ORFV110 and ORFv132 (VEGF) genes were amplified and compared with reference parapoxvirus strains. Non-metric multidimensional scaling (nMDS) was performed to analyze the nucleotide similarities between different ORFV strains.

CONCLUSIONS

Phylogenetic analysis based on ORFV 011 nucleotide sequences showed that the NA1/11strain was closely related to Xinjiang and Gansu strains. ORFV110 and ORFV132 genes are highly variable. The results revealed that precise phylogenetic analysis might provide evidence for genetic variation and movement of circulating ORFV strains in Northeast China. In addition, nMDS analysis showed that geographic isolation and animal host are likely major factors resulting in genetic differences between ORFV strains.

Identification and characterization of monoclonal antibodies against the ORFV059 protein encoded by Orf virus

Recent outbreaks of orf in China have been attributed to a novel strain of Orf virus (ORFV) designated ORFV-Jilin. Currently, monoclonal antibodies (Mabs) have not yet been developed against this specific pathogen even though such entities could have potential applications regarding the diagnosis and characterization of ORFV-Jilin. Therefore, the current study was undertaken to generate Mab against the immunodominant ORFV059 protein of this virus. For this purpose, the ORFV-Jilin ORFV059 protein was expressed in Escherichia coli and subsequently used as an antigen to immunize mice and for the initial screening of hybridomas prepared from the mice for their ability to produce anti-ORFV059 protein Mabs via an indirect ELISA. Ten, positive hybridomas were identified in this manner and verified based on the ability of their released Mab to react specifically with both naturally and artificially expressed ORFV059 protein in Western blots. The two hybridomas with the greatest propensity to secrete Mab were subcloned three times before being introduced intraperitoneally into mice. Afterwards, both Mab were separately purified from the mice's ascetic fluids and found to successfully recognize the ORFV-Jilin ORFV059 protein in a variety of immunological assays. Thus, the widespread utility of these Mab as a diagnostic core reagent should prove invaluable for further investigations regarding the mechanisms of orf pathogenesis and the control of this disease. In this regard, it should be noted that Mab A3 was used to confirm the predicted late expression of the ORFV-Jilin ORFV059 protein during virus replication.

Zoonotic Poxviruses Associated with Companion Animals

Simple Summary

Contemporary enthusiasm for the ownership of exotic animals and hobby livestock has created an opportunity for the movement of poxviruses—such as monkeypox, cowpox, and orf—outside their traditional geographic range bringing them into contact with atypical animal hosts and groups of people not normally considered at risk. It is important that pet owners and practitioners of human and animal medicine develop a heightened awareness for poxvirus infections and understand the risks that can be associated with companion animals and livestock. This article reviews the epidemiology and clinical features of zoonotic poxviruses that are most likely to affect companion animals.

Abstract

Understanding the zoonotic risk posed by poxviruses in companion animals is important for protecting both human and animal health. The outbreak of monkeypox in the United States, as well as current reports of cowpox in Europe, point to the fact that companion animals are increasingly serving as sources of poxvirus transmission to people. In addition, the trend among hobbyists to keep livestock (such as goats) in urban and semi-urban areas has contributed to increased parapoxvirus exposures among people not traditionally considered at high risk. Despite the historic notoriety of poxviruses and the diseases they cause, poxvirus infections are often missed. Delays in diagnosing poxvirus-associated infections in companion animals can lead to inadvertent human exposures. Delays in confirming human infections can result in inappropriate treatment or prolonged recovery. Early recognition of poxvirus-associated infections and application of appropriate preventive measures can reduce the spread of virus between companion animals and their owners. This review will discuss the epidemiology and clinical features associated with the zoonotic poxvirus infections most commonly associated with companion animals.

Virological and clinico-pathological features of orf virus infection in experimentally infected rabbits and mice

Many aspects of the biology of orf virus (ORFV) infection remain poorly understood and attempts to establish animal models have yielded conflicting and non-reproducible results. We herein describe the characterization of ORFV infection and disease in rabbits and mice. A protocol of intradermal inoculation was employed to inoculate 10(8.5)TCID₅₀/mL of ORFV strain IA-82 in the skin of ears, of the back and labial commissures. All inoculated rabbits presented a clinical course characterized by erythema, macules, papules/vesicles or pustules that eventually dried originating scabs. Local signs started around days 3 and 4 post-inoculation (pi) and lasted 3-10 days. Virus was recovered from lesions between days 2 and 14pi. Histological examination of lesions revealed focal proliferative dermatitis with ballooning degeneration and eosinophilic intracytoplasmic inclusion bodies in keratinocytes, histological hallmarks of contagious ecthyma in sheep. A similar, albeit milder clinical course occurred in 5/10 inoculated mice; virus was recovered from lesions from three animals. Inoculated lambs - used as controls - developed severe lesions of contagious ecthyma. VN tests performed at day 28pi failed to detect neutralizing antibodies in all inoculated animals. In contrast, convalescent rabbit sera were positive by ELISA at dilutions from 100 to 400. These results show that rabbits are susceptible to ORFV infection and thus may be used to study selected aspects of ORFV biology.

In vitro antiviral activity of dehydroepiandrosterone and its synthetic derivatives against vesicular stomatitis virus

In this work the antiviral activity of 20 dehydroepiandrosterone (DHEA) analogs with different substituents at positions C-3, C-15, C-16 and C-17 were evaluated against vesicular stomatitis virus (VSV) in Vero cell cultures. The selectivity indexes (SI) obtained with DHEA and epiandrosterone (EA) were 50 and 72.6, respectively. The work showed that the compounds 21-norpregna-5,17(20)-dien-3beta,16alpha-diyl-diacetate, 17,17-ethylendioxyandrostan-5,15-dien-3beta-ol and 3beta-hydroxypregn-17(20)-en-16-one had higher SI values than ribavirin, which was used as a reference drug. The antiviral mode of action of DHEA was also investigated against VSV replication in Vero cells, and time of addition experiments showed that DHEA mainly affected a late event in the virus growth cycle. Analysis of RNA and protein synthesis indicated that DHEA adversely affected positive strand RNA synthesis and viral mature particle formation.