Research Project: Development of a Vaccine and Improved Diagnostics for Malignant Catarrhal Fever

Location: Animal Disease Research

2023 Annual Report

Objectives
Malignant catarrhal fever (MCF) is an often-fatal viral disease that affects primarily ruminants. The disease is caused by gamma herpesviruses in the MCF group of the Macavirus genus. These viruses are carried asymptomatically by certain animal species that serves as virus reservoirs but can cause disease when transmitted to other non-adapted, susceptible species. Ovine herpesvirus-2 (OvHV-2), transmitted by sheep, is globally distributed and the most frequent cause of MCF worldwide. In North America, sheep-associated MCF is a leading cause of death in American bison, which are highly susceptible to the disease. Overall, MCF results in significant economic impact to agriculture not only due to the loss of animals but also because it imposes restrictions to multispecies operations. With no treatment or vaccines available, currently the only way to control the disease is separating carrier and susceptible animals. Therefore, availability of an effective vaccine to MCF and reliable diagnostic assays are top priorities for the agricultural industry. Using rabbits as a laboratory animal model, we have recently demonstrated that protection to MCF can be achieved when using recombinant viral vectors carrying OvHV-2 genes as potential vaccines. In this project we propose to test the safety, efficacy, and duration of protection of selected vaccine candidates in relevant livestock species. We also plan to improve disease control by developing a point-of-care serological assay for detection of MCF virus reservoirs. Objective 1: Assess the safety and efficacy of new MCF vaccine platforms and delivery systems. Sub-objective 1.A: Evaluate the safety of immunogen formulations to deliver OvHV-2 gB as an SA-MCF vaccine in cattle and bison. Sub-objective 1.B: Determine the efficacy of a vaccine targeting OvHV-2 gB in preventing SA-MCF in bison. Sub-objective 1.C: Assess the risk of SA-MCF vaccinated bison to develop MCF upon exposure to OvHV-2 infected sheep. Objective 2: Improve MCF diagnosis by developing new optimized diagnostic assays.

Approach
This project focuses on two important aspects of malignant catarrhal fever (MCF) research: Objective 1 is development of a safe and effective sheep-associated (SA)-MCF vaccine for clinically susceptible species and Objective 2 is improvement of diagnostic capabilities for detection of MCF virus reservoirs. In Sub-objective 1.A, we will test a recombinant viral vectored vaccine expressing ovine herpesvirus-2 (OvHV-2) glycoprotein B (gB) in cattle and bison to determine its safety and immunogenicity. Vaccination will be done with the viral vector in adjuvant alone or following prime immunization with DNA encoding OvHV-2 gB. Experimental animals will be monitored for potential adverse effects from immunizations and development of immune responses to the OvHV-2 antigen. Our hypothesis is that immunization with the vaccine formulations and regimes tested does not cause adverse reactions in cattle or bison. In Sub-objective 1.B, we will determine the efficacy of a vaccine in preventing SA-MCF in bison. The hypothesis for this sub-objective is that immunization with a vaccine targeting OvHV-2 gB protects bison from SA-MCF following challenge with a lethal dose of OvHV-2. Experimental animals will be challenged at different times post-vaccination to evaluate the duration of protection induced by vaccination. In Sub-objective 1.C, we will assess the risk of SA-MCF vaccinated bison to develop MCF upon exposure to OvHV-2 infected sheep. This will be a field trial to test the hypothesis that bison immunized with the SA-MCF vaccine have reduced risk of developing MCF than non-vaccinated animals when exposed to OvHV-2 infected sheep. Vaccinated and non-vaccinated control bison will be exposed to a flock of OvHV-2 infected sheep maintained at close proximity for at least 3 months. Development of disease in both groups will be used to calculate protection rates and risks. In Objective 2 we will improve MCF diagnosis by developing a point-of-care serological assay for detection of MCFV antibodies. The goal is to validate and optimize a lateral flow immunoassay using recombinant OvHV-2 gB coupled to quantum dots as antigen. This is expected to result in a rapid, simple, sensitive, and inexpensive method to identify infected animals that can serve as viral reservoirs.

Progress Report
This report documents fiscal year (FY) 2023 progress for project 2090-32000-045-000D, titled, “Development of a Vaccine and Improved Diagnostics for Malignant Catarrhal Fever”. In support of Sub-Objective 1A, ARS researchers in Pullman, Washington, conducted experiments to evaluate the safety and immunogenicity of a sheep-associated malignant catarrhal fever (SA-MCF) vaccine candidate, AlHV-1/OvHV-2-gB, in both cattle and bison. This vaccine candidate is based on a mutated alcelaphine herpesvirus-1 (AlHV-1) that expresses the ovine herpesvirus-2 (OvHV-2) envelope glycoprotein B (gB) as the vaccine target. Promising results regarding the safety and efficacy of this vaccine candidate in rabbits, used as a laboratory animal model, prompted its evaluation in MCF natural hosts. In the past year, ARS researchers conducted two vaccine trials, one in cattle and one in bison, using groups of immunized (AlHV-1/OvHV-2-gB in adjuvant) and control (adjuvant only) animals. In all vaccinated animals, the mutant virus was able to induce OvHV-2 gB-specific antibody responses without causing disease, confirming the safety and immunogenicity of the vaccine candidate to cattle and bison. Comprehensive analyses of humoral and cellular responses induced by vaccination are still in progress. These were the first studies evaluating a potential vaccine to SA-MCF in natural hosts. So far, the results warrant further trials to access vaccine efficacy in protecting cattle and bison from SA-MCF upon OvHV-2 challenge. In support of Objective 2, ARS researchers continued the efforts to produce recombinant OvHV-2 gB for the development of a lateral flow assay. Researchers used mammalian and baculovirus systems to this end, but both produced insufficient yields of protein to continue the project. As another resource, they have ordered the protein from a commercial source, which they expect to have a more efficient platform for protein production than what is available in-house. Once they obtain enough protein, they will continue with downstream processes for the assay development. Alternatively, in case ARS researchers cannot produce the protein as expected, the assay will be re-designed to use another OvHV-2 protein, or a different assay will be proposed to comply with the project’s objective. ARS researchers in Pullman, Washington, also gained substantial progress in a subordinate project that aims at identifying the prevalence and risk factors associated with malignan catarrhal fever (MCF) viruses in the south of Brazil, where cattle and sheep usually co-mingle. ARS researchers worked in collaboration with researchers in Brazil to have blood samples from both cattle and sheep collected and assessed for antibodies to MCF viruses. Results indicate that viruses that cause MCF are highly prevalent in sheep and also present in a lower percentage of cattle in the sampled properties. These results are in accordance with the seroprevalence of MCF around the world. Further analysis of risk factors identified in each location and characterization of the virus and their strains in the populations will guide better management strategies to prevent MCF in cattle.

Accomplishments
1. A sheep-associated malignant catarrhal fever vaccine candidate is safe for bison. Sheep-associated malignant catarrhal fever (SA-MCF) is a fatal viral disease that affects a range of livestock and wildlife and causes a significant impact to animal health and agriculture. The bison industry is especially affected due to high susceptibility of bison to the disease. With no treatment or reliable methods to control the transmission of the virus that causes SA-MCF, ovine herpesvirus 2 (OvHV-2), development of a vaccine to protect susceptible animals is necessary to avoid losses. A critical step in vaccine development is demonstrating safety of the vaccine candidate and delivery platform. ARS researchers in Pullman, Washington, in collaboration with scientists at the University of Wyoming, Laramie, Wyoming, evaluated the safety of the first SA-MCF vaccine candidate in bison. Following three doses of the vaccine, no side effects or virus shedding were observed, and all of the animals developed specific immune responses against the OvHV-2 target. These results confirm the safety of this vaccine candidate and support vaccine-challenge trials to define its efficacy in protecting animals from SA-MCF. Overall, this represents a substantial advancement on addressing the needs of the sheep, cattle, and bison industries for a SA-MCF vaccine.

Review Publications
Tryland, M., Cunha, C.W., Fuchs, B., Breines, E., Li, H., Jokelainen, P., Laaksonen, S. 2023. A serological screening for potential viral pathogens among semi-domesticated Eurasian tundra reindeer (Rangifer tarandus tarandus) in Finland. Acta Veterinaria Scandinavica. 65. Article 8. https://doi.org/10.1186/s13028-023-00671-4.
Tryland, M., Romano, J., Nymo, I., Mørk, T., Þórarinsdóttir, R., Breines, E., Li, H., Cunha, C.W., Thórisson, S. 2023. A screening for virus infections among wild Eurasian tundra reindeer (Rangifer tarandus tarandus) in Iceland, 2017-2019. Viruses. 15(2). Article 317. https://doi.org/10.3390/v15020317.
Cunha, C.W., Baker, K.N., O'Toole, D., Cole, E., Shringi, S., Dewals, B.G., Vanderplasschen, A., Li, H. 2022. A vaccine targeting ovine herpesvirus 2 glycoprotein B protects against sheep-associated malignant catarrhal fever. Vaccines. 10(12). Article 2156. https://doi.org/10.3390/vaccines10122156.