Novel bivalent vectored vaccine for control of myxomatosis and rabbit haemorrhagic disease

Vaccination against Rabbit Hemorrhagic Disease Virus 2 (RHDV2) Using a Baculovirus Recombinant Vaccine Provides Durable Immunity in Rabbits

Rabbit hemorrhagic disease virus 2 (RHDV2) emerged in the United States in 2018 and has spread in both domestic and wild rabbits nationwide. The virus has a high mortality rate and can spread rapidly once introduced in a rabbit population. Vaccination against RHDV2 provides the best protection against disease and should be considered by all rabbit owners. Here, we investigate the duration of immunity provided by vaccination with the Medgene Platform conditionally licensed commercial vaccine 6 months following the initial series. Rabbits received either the vaccination or a placebo and were challenged with RHDV2 6 months later. All vaccinated rabbits survived challenge whereas 18/19 non-vaccinated controls succumbed to infection within 10 or fewer days post-challenge. These results demonstrate lasting immunity following vaccination with the Medgene RHDV2 vaccine.

Development and Evaluation of a Duplex Lateral Flow Assay for the Detection and Differentiation between Rabbit Haemorrhagic Disease Virus Lagovirus europaeus/GI.1 and /GI.2

Simple Summary

Rabbit Haemorrhagic Disease is caused by a virus that affects the liver, the spleen and the lungs of rabbits, causing hepatitis, splenomegaly and haemorrhages. A new genotype of the virus was first reported in France in 2010 and has spread globally since then, replacing most of the circulating former viruses in many countries. The detection of the virus and the differentiation of both genotypes is of crucial importance for disease surveillance. In this article, a rapid test for antigen detection is described and evaluated, providing the first description of a quick and easy-to-use test that allows for the simultaneous detection and differentiation of the genotypes. A total of 136 samples, rabbit liver samples and liver exudates (liquid collected after freeze–thawing) classified as infected and non-infected, were analysed, with good results. These data confirm that the developed rapid test can be used as a reliable diagnostic test for disease surveillance, especially in farms and the field.

Abstract

Rabbit Haemorrhagic Disease Virus 2 (RHDV2, recently named Lagovirus europaeus/GI.2) was first reported in France in 2010 and has spread globally since then, replacing most of the circulating former RHDV (genotype GI.1) in many countries. The detection and differentiation of both genotypes is of crucial importance for the surveillance of the disease. In this article, a duplex lateral flow assay (LFA) for antigen detection is described and evaluated, providing the first description of a quick and easy-to-use test that allows for the simultaneous detection and differentiation of RHDV genotypes GI.1 and GI.2. A panel of GI.1- or GI.2-infected and non-infected rabbit liver samples and liver exudates (136 samples) was analysed, obtaining a total sensitivity of 94.4% and specificity of 100%. These data confirm that the developed duplex LFA can be used as a reliable diagnostic test for RHD surveillance, especially in farms and the field.

Evaluation of Commercial Myxomatosis Vaccines against Recombinant Myxoma Virus (ha-MYXV) in Iberian Hare and Wild Rabbit

The recent emergence of a new myxoma virus capable of causing disease in the Iberian hare (Lepus granatensis) has resulted in numerous outbreaks with high mortality leading to the reduction, or even the disappearance, of many local populations of this wild species in the Iberian Peninsula. Currently, the available vaccines that prevent myxomatosis in domestic rabbits caused by classic strains of myxoma virus have not been assessed for use in Iberian hares. The main objective of this study was to evaluate the efficacy of commercial rabbit vaccines in Iberian hares and wild rabbits against the natural recombinant myxoma virus (ha-MYXV), bearing in mind its application in specific scenarios where capture is possible, such as genetic reserves. The study used a limited number of animals (pilot study), 15 Iberian hares and 10 wild rabbits. Hares were vaccinated with Mixohipra-FSA vaccine (Hipra) and Mixohipra-H vaccine (Hipra) using two different doses, and rabbits were vaccinated with the Mixohipra-H vaccine or the Nobivac Myxo-RHD PLUS (MSD Animal Health) using the recommended doses for domestic rabbits. After the vaccination trials, the animals were challenged with a wild type strain of ha-MYXV. The results showed that no protection to ha-MYXV challenge was afforded when a commercial dose of Mixohipra-FSA or Mixohipra-H vaccine was used in hares. However, the application of a higher dose of Mixohipra-FSA vaccine may induce protection and could possibly be used to counteract the accelerated decrease of wild hare populations due to ha-MYXV emergence. The two commercial vaccines (Mixohipra-H and Nobivac Myxo-RHD PLUS) tested in wild rabbits were fully protective against ha-MYXV infection. This knowledge gives more insights into ha-MYXV management in hares and rabbits and emphasises the importance of developing a vaccine capable of protecting wild populations of Iberian hare and wild rabbit towards MYXV and ha-MYXV strains.

Novel Vaccine Technologies in Veterinary Medicine: A Herald to Human Medicine Vaccines

The success of inactivated and live-attenuated vaccines has enhanced livestock productivity, promoted food security, and attenuated the morbidity and mortality of several human, animal, and zoonotic diseases. However, these traditional vaccine technologies are not without fault. The efficacy of inactivated vaccines can be suboptimal with particular pathogens and safety concerns arise with live-attenuated vaccines. Additionally, the rate of emerging infectious diseases continues to increase and with that the need to quickly deploy new vaccines. Unfortunately, first generation vaccines are not conducive to such urgencies. Within the last three decades, veterinary medicine has spearheaded the advancement in novel vaccine development to circumvent several of the flaws associated with classical vaccines. These third generation vaccines, including DNA, RNA and recombinant viral-vector vaccines, induce both humoral and cellular immune response, are economically manufactured, safe to use, and can be utilized to differentiate infected from vaccinated animals. The present article offers a review of commercially available novel vaccine technologies currently utilized in companion animal, food animal, and wildlife disease control.

Novel Trivalent Vectored Vaccine for Control of Myxomatosis and Disease Caused by Classical and a New Genotype of Rabbit Haemorrhagic Disease Virus

Myxoma virus (MV) and rabbit haemorrhagic disease virus (RHDV) are the major causes of lethal viral diseases in the European rabbit. In 2010, a new RHDV genotype (RHDV2) emerged in the field that had limited cross-protection with the classical RHDV (RHDV1). For optimal protection of rabbits and preventing spread of disease, a vaccine providing protection against all three key viruses would be ideal. Therefore, a novel trivalent myxoma vectored RHDV vaccine (Nobivac Myxo-RHD PLUS) was developed similar to the existing bivalent myxoma vectored RHDV vaccine Nobivac Myxo-RHD. The new vaccine contains the Myxo-RHDV1 strain already included in Nobivac Myxo-RHD and a similarly produced Myxo-RHDV2 strain. This paper describes several key safety and efficacy studies conducted for European licensing purposes. Nobivac Myxo-RHD PLUS showed to be safe for use in rabbits from five weeks of age onwards, including pregnant rabbits, and did not spread from vaccinated rabbits to in-contact controls. Furthermore, protection to RHDV1 and RHDV2 was demonstrated by challenge, while the serological response to MV was similar to that after vaccination with Nobivac Myxo-RHD. Therefore, routine vaccination with Nobivac Myxo-RHD PLUS can prevent the kept rabbit population from these major viral diseases.

Seasonality and risk factors for myxomatosis in pet rabbits in Great Britain

Myxomatosis is a highly contagious, frequently fatal viral disease affecting both wild and domesticated European rabbits across many areas of the world. Here we used electronic health records (EHRs) collected from pet rabbits attending a sentinel voluntary network of 191 veterinary practices across Great Britain (GB) between March 2014 and June 2019 to identify new features of this disease's epidemiology. From a total of 89,408 rabbit consultations, text mining verified by domain experts identified 207 (0.23 %) cases where myxomatosis was the only differential diagnosis recorded by the attending practitioner. Cases occurred in all months but February and were distributed across the country. Consistent with studies in wild rabbits, the majority of cases occurred between August and November. However, there was also evidence for considerable variation between years. A nested case control study identified important risk factors for myxomatosis within this pet animal population including season, sex, age, vaccination status and distance to likely wild rabbit habitats. Female entire rabbits were twice as likely to be a case (odds ratio (OR) 1.98, 95 % confidence interval (CI) 1.26-3.13, p = 0.003), suggesting a novel role for behaviour in driving transmission from wild to domesticated rabbits. Vaccination had the largest protective effect with vaccinated rabbits being 8.3 times less likely to be a case than unvaccinated rabbits (OR = 0.12, 95 % CI 0.06-0.21, p = <0.001).

Myxomatosis and Rabbit Haemorrhagic Disease: A 30-Year Study of the Occurrence on Commercial Farms in Spain

In this retrospective study, we describe the relative occurrence of clinical myxomatosis, and rabbit haemorrhagic disease (RHD), on 1714 commercial farms visited in Spain, between 1988 and 2018. We determined the annual prevalence based on 817 visits to 394 farms affected by myxomatosis. Myxomatosis was more prevalent from August to March, being lowest in June (3%) and highest in September (8.9%). With regard to RHD, we assessed 253 visits to 156 affected farms. We analyzed mean annual and monthly incidence. Two important RHD epidemics occurred; the first in 1988-1989 due to RHDV GI.1 (also known as RHDV), and the second from 2011 to 2013 due to RHDV GI.2 (RHDV2 or RHDVb). These epidemics occurred at times when effective vaccination had not been carried out. Relative monthly incidence in 2011-2018 was higher from April to August (p < 0.001). The results we obtained from 1404 necropsies on 102 farms did not clearly relate serosanguinous nasal discharge in rabbits with disease caused by GI.2 infection. We also assessed vaccination schedules used on 200 doe farms visited from the end of 2014 to 2018; 95.5% vaccinated against myxomatosis and 97.5% against RHD. Both diseases remain prevalent; however, effective vaccination has produced a steady decline in myxomatosis and RHDV GI.1 and GI.2 on-farm detection. The maintenance of high hygienic standards will be needed to continue and improve this control. However, further studies are required to investigate the causes of sustained virus presence and vaccine breaks.

Rabbit haemorrhagic disease: experimental study of a recent highly pathogenic GI.2/RHDV2/b strain and evaluation of vaccine efficacy

In 2010, a variant of the rabbit haemorrhagic disease virus (RHDV) belonging to a new GI.2 genotype was identified in France and rapidly spread worldwide. Due to antigenic difference, new vaccines including G1.2 strains have been developed to confer adequate protection. An increase in the pathogenicity of the circulating strains was recently reported. The objective of this experimental study was to characterise the infection with a highly pathogenic GI.2/RHDV2/b isolate (2017) and assess the efficacy of Filavac VHD K C+V vaccine (Filavie) against this strain. Four and 10-wk-old specific pathogen-free rabbits were inoculated with a recommended dose of vaccine. After 7 d, controls and vaccinated rabbits were challenged and clinically monitored for 14 d. All animals were necropsied and blood, organs and urine were sampled for quantitative reverse transcription polymerase chain reaction (RT-qPCR) analysis. In adult groups, regular nasal and rectal swabbing were performed, and faeces were collected after death to monitor RNA shedding. In control groups, the challenge strain induced acute RHD between 31 and 72 h post-inoculation, with a mortality rate of 100% for kits and 89% for adult rabbits. Except for a shorter mean time to death in kits, similar clinical signs and lesions were observed between age groups. The vaccination significantly prevented all mortality, clinical signs, detection of viral RNA in serum and gross lesions in kits and adult rabbits. In adult groups, we also demonstrated that vaccine significantly protected from detectable RNA shedding via naso-conjunctival and rectal routes. Two weeks after challenge, RNA copies were not detected by PCR in the liver, spleen, lungs, kidneys, faeces and urine of vaccinated adult rabbits. The findings for kits were similar, except that very low levels of RNA were present in the liver and spleen of a few rabbits. These data show that immunisation prevented any significant viral multiplication and/or allowed a rapid clearance. We concluded that, despite the quick evolution of GI.2/RHDV2/b strains, the protection conferred by the vaccine remains adequate. In the context of coexistence of both GI.1 and GI.2 genotypes in some countries, with the circulation of multiples recombinant viruses, the vaccination should be based on the association of strains from both genotypes.

Humoral immune response to different routes of myxomatosis vaccine application

The aim of our study was to monitor the dynamics of the serological response to different application routes of live attenuated myxomatosis vaccine. The study included 42 Californian breed rabbits, aged 3 mo, of both sexes. They were separated into 7 groups: 6 experimental and 1 control. All experimental groups were vaccinated on day 0 with a single dose of myxomatosis vaccine (min 10^3.3 tissue culture infective dose 50 [TCID₅₀], max 10^5.8 TCID₅₀). Three of the groups were injected with monovalent attenuated myxomatosis vaccine using different types of application: intradermal (i.d.), intramuscular (i.m.) and subcutaneous (s.c.). The other 3 groups were injected with bivalent attenuated vaccine against myxomatosis and rabbit haemorrhagic disease; again the routes of administration were i.d., i.m. and s.c.. There were no clinical signs or serious side effects after vaccination. The serological response was evaluated on days 7, 15 and 30 with a monoclonal antibody based-competition enzyme-linked immunosorbent assay (cELISA). More rapid and potent humoral response was detected in groups with i.d. inoculation in comparison to i.m. and s.c. routes. Vaccination with monovalent vaccine against myxomatosis induced higher antibody titre in comparison to bivalent vaccine. Our study showed that the vaccine application route and the type of vaccine used influence the speed and intensity of antibody response.

Recent Advances in Vaccine Technologies

This brief review discusses some recent advances in vaccine technologies with particular reference to their application within veterinary medicine. It highlights some of the key inactivated/killed approaches to vaccination, including natural split-product and subunit vaccines, recombinant subunit and protein vaccines, and peptide vaccines. It also covers live/attenuated vaccine strategies, including modified live marker/differentiating infected from vaccinated animals vaccines, live vector vaccines, and nucleic acid vaccines.

Biology and Diseases of Rabbits

Beginning in 1931, an inbred rabbit colony was developed at the Phipps Institute for the Study, Treatment and Prevention of Tuberculosis at the University of Pennsylvania. This colony was used to study natural resistance to infection with tuberculosis (Robertson et al., 1966). Other inbred colonies or well-defined breeding colonies were also developed at the University of Illinois College of Medicine Center for Genetics, the Laboratories of the International Health Division of The Rockefeller Foundation, the University of Utrecht in the Netherlands, and Jackson Laboratories. These colonies were moved or closed in the years to follow. Since 1973, the U.S. Department of Agriculture has reported the total number of certain species of animals used by registered research facilities (1997). In 1973, 447,570 rabbits were used in research. There has been an overall decrease in numbers of rabbits used. This decreasing trend started in the mid-1990s. In 2010, 210,172 rabbits were used in research. Despite the overall drop in the number used in research, the rabbit is still a valuable model and tool for many disciplines.

Cutaneous lesions in pet rabbits following subcutaneous administration of a novel bivalent vaccine against myxomatosis and rabbit haemorrhagic disease

BACKGROUND

A novel bivalent vaccine to protect against myxomatosis and rabbit haemorrhagic disease is commercially available for pet rabbits.

HYPOTHESIS/OBJECTIVES

To describe the appearance of cutaneous lesions arising in pet rabbits positive for myxoma virus (MV) by RT-PCR evaluation shortly after vaccination.

ANIMALS

Four pet rabbits presenting with papular, crusting skin lesions ~10 days after vaccination.

METHODS

Histological evaluation of formalin-fixed skin biopsies obtained from lesional skin (case 1). Real-time polymerase chain reaction (RT-PCR) evaluation of paraffin-embedded tissue from skin biopsies (case 1) and crusts obtained from the lesion surface (cases 2-4) for myxoma virus are reported as cycle threshold (Ct ) values.

RESULTS

Lesions affecting the ear pinna, dorsal aspect of the nose, vulva and/or conjunctiva are reported. Histopathological findings included severe ulcerative, necrotizing dermatitis and intralesional cytoplasmic inclusion bodies in myxoma cells. DNA was amplified from all the paraffin-embedded skin biopsies (Ct  = 34-35) and crusts (Ct  = 20-24).

CONCLUSIONS AND CLINICAL IMPORTANCE

Although a wild virus challenge cannot be definitively excluded, veterinarians and pet-owners should be aware that cutaneous lesions have been observed after vaccination with this novel vaccine in low numbers of rabbits.

Viral skin diseases of the rabbit

This article describes the viral skin diseases affecting the domestic rabbit, the most important being myxomatosis. Transmission and pathogenesis, clinical signs, diagnosis, treatment, and control are described and the article will be of interest to veterinary practitioners who treat rabbits. Shope fibroma virus, Shope papilloma virus, and rabbitpox are also discussed.

Viral infections of rabbits

Viral diseases of rabbits have been used historically to study oncogenesis (e.g. rabbit fibroma virus, cottontail rabbit papillomavirus) and biologically to control feral rabbit populations (e.g. myxoma virus). However, clinicians seeing pet rabbits in North America infrequently encounter viral diseases although myxomatosis may be seen occasionally. The situation is different in Europe and Australia, where myxomatosis and rabbit hemorrhagic disease are endemic. Advances in epidemiology and virology have led to detection of other lapine viruses that are now recognized as agents of emerging infectious diseases. Rabbit caliciviruses, related to rabbit hemorrhagic disease, are generally avirulent, but lethal variants are being identified in Europe and North America. Enteric viruses including lapine rotavirus, rabbit enteric coronavirus and rabbit astrovirus are being acknowledged as contributors to the multifactorial enteritis complex of juvenile rabbits. Three avirulent leporid herpesviruses are found in domestic rabbits. A fourth highly pathogenic virus designated leporid herpesvirus 4 has been described in Canada and Alaska. This review considers viruses affecting rabbits by their clinical significance. Viruses of major and minor clinical significance are described, and viruses of laboratory significance are mentioned.

DNA vaccination with a gene encoding VP60 elicited protective immunity against rabbit hemorrhagic disease virus

Rabbit hemorrhagic disease (RHD) is a contagious disease in adult rabbits, with high mortality, that occurs throughout the world. The VP60 protein has been implicated as main protein antigen in virus diagnosis and vaccine design. In this report, we describe the construction of a novel DNA vaccine (pcDNA-VP60) expressing the RHDV capsid protein (VP60), and the expression of the recombinant protein was identified through indirect immunofluorescence assay (IFA) and Western blot assay. VP60 protein self-assembled to form virus-like particles (VLPs) observed by electron microscopy were morphologically similar to native virions. For the evaluation of vaccine efficacy, rabbits were inoculated with PBS, pcDNA3.1((+)), pcDNA-VP60 or RHDV inactive vaccine. They were challenged with RHDV-TP isolate four weeks after last boost immunization. In all cases, the rabbits immunized with pcDNA-VP60 developed high level of RHDV-specific antibodies and cellular immune response. The rabbits injected with DNA vaccine were completely protected against RHDV challenge like commercial RHDV inactive vaccine, moreover, RHDV viral load was significantly reduced in the liver samples from immunized rabbits. The recombinant DNA vaccine may provide a novel strategy for the immunization of rabbits for the control of RHDV.