A randomised vaccine field trial in Kenya demonstrates protection against wildebeest-associated malignant catarrhal fever in cattle

Ivermectin inhibits replication of the malignant catarrhal fever virus alcelaphine herpesvirus 1

Malignant catarrhal fever is a lymphoproliferative disease of cattle and other ungulates that is caused by genetically and antigenically related gamma herpesviruses of the genus Macavirus. Infection of the natural host species is efficient and asymptomatic but spread to susceptible hosts is often fatal with clinical signs including fever, depression, nasal and ocular discharge. There is no recognised treatment for MCF but a vaccine for one MCF virus, alcelaphine herpesvirus 1 (AlHV-1), has been described. In this paper we describe the inhibition of AlHV-1 replication and propagation by the anthelminthic drug ivermectin. Concentrations of 10 μM or greater led to significant reductions in both copy number and viable titre of virus tested in culture medium, with little replication detected at over 20 μM ivermectin. In the absence of alternative treatments, further testing of ivermectin as a candidate antiviral treatment for MCF may therefore be justified.

Wildebeest-Derived Malignant Catarrhal Fever: A Bovine Peripheral T Cell Lymphoma Caused by Cross-Species Transmission of Alcelaphine Gammaherpesvirus 1

Gammaherpesviruses (γHVs) include viruses that can induce lymphoproliferative diseases and tumors. These viruses can persist in the long term in the absence of any pathological manifestation in their natural host. Alcelaphine gammaherpesvirus 1 (AlHV-1) belongs to the genus Macavirus and asymptomatically infects its natural host, the wildebeest (Connochaetes spp.). However, when transmitted to several susceptible species belonging to the order Artiodactyla, AlHV-1 is responsible for the induction of a lethal lymphoproliferative disease, named wildebeest-derived malignant catarrhal fever (WD-MCF). Understanding the pathogenic mechanisms responsible for the induction of WD-MCF is important to better control the risks of transmission and disease development in susceptible species. The aim of this review is to synthesize the current knowledge on WD-MCF with a particular focus on the mechanisms by which AlHV-1 induces the disease. We discuss the potential mechanisms of pathogenesis from viral entry into the host to the maintenance of viral genomes in infected CD8⁺ T lymphocytes, and we present current hypotheses to explain how AlHV-1 infection induces a peripheral T cell lymphoma-like disease.

A Vaccine Targeting Ovine Herpesvirus 2 Glycoprotein B Protects against Sheep-Associated Malignant Catarrhal Fever

Malignant catarrhal fever (MCF) is a complex and often fatal disease of ungulates. Effective vaccines are needed to avoid MCF outbreaks and mitigate losses. This study aimed to evaluate a sheep-associated MCF (SA-MCF) vaccine candidate targeting ovine herpesvirus 2 (OvHV-2) glycoprotein B (gB). Rabbits were used as a laboratory animal model to test the safety, immunogenicity, and protective efficacy of a chimeric virus consisting of a recombinant, non-pathogenic strain of alcelaphine herpesvirus-1 encoding OvHV-2 ORF8 to express gB (AlHV-1^∆ORF73/OvHV-2-ORF8). Viral-vectored immunizations were performed by using the AlHV-1^∆ORF73/OvHV-2-ORF8 chimera alone or as a DNA prime (OvHV-2-ORF8)-virus boost regimen. The viral vector was inoculated by intravenous or intramuscular routes and the DNA was delivered by intradermal shots using a gene gun. The vaccine candidates were deemed safe as no clinical signs were observed following any of the immunizations. Anti-OvHV-2 gB antibodies with neutralizing activity were induced by all immunogens. At three weeks post-final immunization, all animals were challenged intranasally with a lethal dose of OvHV-2. MCF protection rates ranging from 66.7% to 71.4% were observed in vaccinated rabbits, while all mock-vaccinated animals developed the disease. The significant protective efficacy obtained with the vaccine platforms tested in this study encourages further trials in relevant livestock species, such as cattle and bison.

Development of a recombinant ELISA for ovine herpesvirus 2, suitable for use in sheep

The minor capsid protein of ovine herpesvirus 2, identified as a potential antigen for serological testing, was over-expressed and purified to allow its assessment in ELISA. The corresponding gene sequence (OvHV-2 orf65, Ov65) was modified to incorporate epitope tags and internal restriction enzyme sites in an E. coli codon-optimised version of the gene. This codon-optimised gene was then subject to internal deletions to identify regions of the protein that could be removed while maintaining protein solubility and antigenicity. It was found that a derivative with deletion of the conserved 5'-end of the gene (Ov65delB) expressed a polypeptide that was soluble when over-expressed in bacteria and was detected by OvHV-2 specific sera. Proteomic analysis of the affinity purified Ov65delB showed that it contained multiple predicted Ov65 tryptic peptides but also showed contamination by co-purifying E. coli proteins. An indirect ELISA, based on this affinity-purified OV65delB, was optimised for use with sheep and cattle samples and cut-off values were established based on known negative serum samples. Analysis of groups of samples that were either presumed infected (UK sheep) or tested OvHV-2 positive or negative by PCR (cattle MCF diagnostic samples) showed that the assay had 95 % sensitivity and 96 % specificity for sheep serum; and 80 % sensitivity and 95 % specificity for cattle serum. The lower sensitivity with cattle samples appeared to be due to a lack of serological response in some MCF-affected cattle. This recombinant antigen therefore shows promise as the basis of an inexpensive, simple and reliable test that can be used to detect OvHV-2-specific antibody responses in both MCF-affected animals and in OvHV-2 reservoir hosts.

Molecular detection and characterization of ovine herpesvirus-2 using heminested PCR in Pakistan

Background

Malignant catarrhal fever (MCF) is a highly fatal lymphoproliferative disease of cattle, deer, bison, water buffalo, and pigs caused by the gamma-herpesviruses alcelaphine herpesvirus-1 (AlHV-1) and ovine herpesvirus-2 (OvHV-2).

Objectives

This study aimed to determine the prevalence of OvHV-2 in sheep, goats, cattle, and buffalo in Rawalpindi and Islamabad, Pakistan, by applying molecular and phylogenetic methods.

Methods

Blood samples were aspirated from sheep (n = 54), goat (n = 50), cattle (n = 46) and buffalo (n= 50) at a slaughterhouse and several farms. The samples were subjected to heminested polymerase chain reaction (PCR), followed by sequencing and phylogenetic analysis of the OvHV-2 POL gene and the OvHV-2 ORF75 tegument protein gene.

Results

The highest percentage of MCF positive samples was in sheep (13%), whereas goat, cattle, and buffalo had lower positive percentages, 11%, 9%, and 6.5%, respectively. Four OvHV-2-positive PCR products obtained from sheep samples were sequenced. The sequences obtained were submitted to the NCBI GenBank database (MK852173 for the POL gene; MK840962, MK852171, and MK852172 for the ORF75 tegument protein gene). Phylogenetic analysis revealed a close similarity of study sequences with those of worldwide samples.

Conclusions

This study is the first cross-sectional study on the prevalence and molecular detection of OvHV-2 in apparently healthy cattle and buffalo that could be carrying OvHV-2 acquired from OvHV-2-positive sheep and goats. The results indicate that OvHV-2 is circulating in Pakistan. Further studies are needed to characterize OvHV-2 and elucidate further its prevalence.

Analysis of immune responses to attenuated alcelaphine herpesvirus 1 formulated with and without adjuvant

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MCF vaccine was tested with and without adjuvant and containing inactivated virus.

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Adjuvant was required for optimal virus neutralising antibody responses.

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Storage of AlHV-1 with Emulsigen adjuvant significantly reduced virus viability.

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Vaccination with adjuvant-inactivated AlHV-1 did not reduce antibody responses.

The experimental vaccine for bovine malignant catarrhal fever consists of viable attenuated alcelaphine herpesvirus 1 (AlHV-1) derived by extensive culture passage, combined with an oil-in-water adjuvant, delivered intramuscularly. This immunisation strategy was over 80% effective in previous experimental and field trials and protection appeared to be associated with induction of virus-neutralising antibodies. Whether the vaccine virus is required to be viable at the point of immunisation and whether adjuvant is required to induce the appropriate immune responses remains unclear. To address these issues two studies were performed, firstly to analyse immune responses in the presence and absence of adjuvant and secondly, to investigate immune responses to vaccines containing adjuvant plus viable or inactivated AlHV-1.

The first study showed that viable attenuated AlHV-1 in the absence of adjuvant induced virus-specific antibodies but the titres of virus-neutralising antibodies were significantly lower than those induced by vaccine containing viable virus and adjuvant, suggesting adjuvant was required for optimal responses. In contrast, the second study found that the vaccine containing inactivated (>99.9%) AlHV-1 induced similar levels of virus-neutralising antibody to the equivalent formulation containing viable AlHV-1.

Together these studies suggest that the MCF vaccine acts as an antigen depot for induction of immune responses, requiring adjuvant and a suitable antigen source, which need not be viable virus. These observations may help in directing the development of alternative MCF vaccine formulations for distribution in the absence of an extensive cold chain.

OvHV-2 Glycoprotein B Delivered by a Recombinant BoHV-4 Is Immunogenic and Induces Partial Protection against Sheep-Associated Malignant Catarrhal Fever in a Rabbit Model

An efficacious vaccine for sheep-associated malignant catarrhal fever (SA-MCF) is important for the livestock industry. Research towards SA-MCF vaccine development is hindered by the absence of culture systems to propagate the causative agent, ovine herpesvirus-2 (OvHV-2), which means its genome cannot be experimentally modified to generate an attenuated vaccine strain. Alternative approaches for vaccine development are needed to deliver OvHV-2 antigens. Bovine herpesvirus 4 (BoHV-4) has been evaluated as a vaccine vector for several viral antigens with promising results. In this study, we genetically engineered BoHV-4 to express OvHV-2 glycoprotein B (gB) and evaluated its efficacy as an SA-MCF vaccine using a rabbit model. The construction of a viable recombinant virus (BoHV-4-AΔTK-OvHV-2-gB) and confirmation of OvHV-2 gB expression were performed in vitro. The immunization of rabbits with BoHV-4-AΔTK-OvHV-2-gB elicited strong humoral responses to OvHV-2 gB, including neutralizing antibodies. Following intra-nasal challenge with a lethal dose of OvHV-2, 42.9% of the OvHV-2 gB vaccinated rabbits were protected against SA-MCF, while all rabbits in the mock-vaccinated group succumbed to SA-MCF. Overall, OvHV-2 gB delivered by the recombinant BoHV-4 was immunogenic and partly protective against SA-MCF in rabbits. These are promising results towards an SA-MCF vaccine; however, improvements are needed to increase protection rates.