Virulence and pathogenesis of the MSW and MSD strains of Californian myxoma virus in European rabbits with genetic resistance to myxomatosis compared to rabbits with no genetic resistance

Rabbit Dermatology

The majority of rabbit skin disease presentations can be divided into pruritus, alopecia, scaling, and nodules. Some disease will have more than one of these clinical signs. Ectoparasites, bacterial and fungal infections, and neoplasia account for most of the causes seen. Diagnostic tests include skin scraping and cytology, microbial culture, and biopsy. Therapy is dependent on cause. In addition to discussing the various causes and their treatments, important clinical care points are noted.

Pathological changes and viral antigen distribution in tissues of Iberian hare (Lepus granatensis) naturally infected with the emerging recombinant myxoma virus (ha-MYXV)

BACKGROUND

A cross-species jump was confirmed in 2018, when a novel recombinant myxoma virus (MYXV) (ha-MYXV) caused high mortality in Iberian hare (Lepus granatensis) in the Iberian Peninsula.

METHOD

The aim of this study was to evaluate the main lesions, tissular distribution and target cells of ha-MYXV in Iberian hare. Gross postmortem examinations and histological and immunohistochemical studies to detect ha-MYXV were carried out in 28 animals that were confirmed as ha-MYXV positive by PCR.

RESULTS

The main macroscopic lesions were bilateral blepharoconjunctivitis, epistaxis, intense congestion and oedema in several organs and some internal haemorrhages. Visible myxomas were not found. Histopathological examination revealed hyperplastic epidermis with predominant hyperkeratosis and myxoid matrix in the dermis. ha-MYXV-positive keratinocytes showed hydropic degeneration and cytoplasmic inclusion bodies. Alveolar oedema, interstitial pneumonia, dramatic lymphoid depletion in the spleen and necrosis in the liver and testis were observed. ha-MYXV was mainly detected in epithelial and myxoma cells in the skin, and also in macrophages, lymphocytes, fibroblasts and endothelial cells in several organs, as well as in hepatocytes and Leydig cells.

LIMITATIONS

A non-homogeneous number of samples were included in all the animals. Future experimental studies with controlled variables are necessary.

CONCLUSION

These findings correspond to an unusual form of myxomatosis, characterised by an acute or hyperacute presentation.

Capripoxviruses, leporipoxviruses, and orthopoxviruses: Occurrences of recombination

Poxviruses are double-stranded DNA viruses with several members displaying restricted host ranges. They are genetically stable with low nucleotide mutation rates compared to other viruses, due to the poxviral high-fidelity DNA polymerase. Despite the low accumulation of mutations per replication cycle, poxvirus genomes can recombine with each other to generate genetically rearranged viruses through recombination, a process directly associated with replication and the aforementioned DNA polymerase. Orthopoxvirus replication is intimately tethered to high frequencies of homologous recombination between co-infecting viruses, duplicated sequences of the same virus, and plasmid DNA transfected into poxvirus-infected cells. Unfortunately, the effect of these genomic alterations on the cellular context for all poxviruses across the family Poxviridae remains elusive. However, emerging sequence data on currently circulating and archived poxviruses, such as the genera orthopoxviruses and capripoxviruses, display a wide degree of divergence. This genetic variability cannot be explained by clonality or genetic drift alone, but are probably a result of significant genomic alterations, such as homologous recombination, gene loss and gain, or gene duplications as the major selection forces acting on viral progeny. The objective of this review is to cross-sectionally overview the currently available findings on natural and laboratory observations of recombination in orthopoxviruses, capripoxviruses, and leporipoxviruses, as well as the possible mechanisms involved. Overall, the reviewed available evidence allows us to conclude that the current state of knowledge is limited in terms of the relevance of genetic variations across even a genus of poxviruses as well as fundamental features governing and precipitating intrinsic gene flow and recombination events.

Divergent Evolutionary Pathways of Myxoma Virus in Australia: Virulence Phenotypes in Susceptible and Partially Resistant Rabbits Indicate Possible Selection for Transmissibility

To characterize the ongoing evolution of myxoma virus in Australian rabbits, we used experimental infections of laboratory rabbits to determine the virulence and disease phenotypes of recent virus isolates. The viruses, collected between 2012 and 2015, fell into three lineages, one of which, lineage c, experienced a punctuated increase in evolutionary rate. All viruses were capable of causing acute death with aspects of neutropenic septicemia, characterized by minimal signs of myxomatosis, the occurrence of pulmonary edema and bacteria invasions throughout internal organs, but with no inflammatory response. For the viruses of highest virulence all rabbits usually died at this point. In more attenuated viruses, some rabbits died acutely, while others developed an amyxomatous phenotype. Rabbits that survived for longer periods developed greatly swollen cutaneous tissues with very high virus titers. This was particularly true of lineage c viruses. Unexpectedly, we identified a line of laboratory rabbits with some innate resistance to myxomatosis and used these in direct comparisons with the fully susceptible rabbit line. Importantly, the same disease phenotype occurred in both susceptible and resistant rabbits, although virulence was shifted toward more attenuated grades in resistant animals. We propose that selection against inflammation at cutaneous sites prolongs virus replication and enhances transmission, leading to the amyxomatous phenotype. In some virus backgrounds this creates an immunosuppressive state that predisposes to high virulence and acute death. The alterations in disease pathogenesis, particularly the overwhelming bacterial invasions that characterize the modern viruses, suggest that their virulence grades are not directly comparable with earlier studies.

IMPORTANCE The evolution of the myxoma virus (MYXV) following its release as a biological control for European rabbits in Australia is the textbook example of the coevolution of virus virulence and host resistance. However, most of our knowledge of MYXV evolution only covers the first few decades of its spread in Australia and often with little direct connection between how changes in virus phenotype relate to those in the underlying virus genotype. By conducting detailed experimental infections of recent isolates of MYXV in different lines of laboratory rabbits, we examined the ongoing evolution of MYXV disease phenotypes. Our results reveal a wide range of phenotypes, including an amyxomatous type, as well as the impact of invasive bacteria, that in part depended on the level of rabbit host resistance. These results provide a unique insight into the complex virus and host factors that combine to shape disease phenotype and viral evolution.

Viruses for Landscape-Scale Therapy: Biological Control of Rabbits in Australia

Viral diseases, whether of animals or humans, are normally considered as problems to be managed. However, in Australia, two viruses have been used as landscape-scale therapeutics to control European rabbits (Oryctolagus cuniculus), the preeminent invasive vertebrate pest species. Rabbits have caused major environmental and agricultural losses and contributed to extinction of native species. It was not until the introduction of Myxoma virus that effective control of this pest was obtained at a continental scale. Subsequent coevolution of rabbit and virus saw a gradual reduction in the effectiveness of biological control that was partially ameliorated by the introduction of the European rabbit flea to act as an additional vector for the virus. In 1995, a completely different virus, Rabbit hemorrhagic disease virus (RHDV), escaped from testing and spread through the Australian rabbit population and again significantly reduced rabbit numbers and environmental impacts. The evolutionary pressures on this virus appear to be producing quite different outcomes to those that occurred with myxoma virus and the emergence and invasion of a novel genotype of RHDV in 2014 have further augmented control. Molecular studies on myxoma virus have demonstrated multiple proteins that manipulate the host innate and adaptive immune response; however the molecular basis of virus attenuation and reversion to virulence are not yet understood.

Outbreaks of myxomatosis in Egyptian domestic rabbit farms

<p class="Default">Myxomatosis is an endemic infectious, severe and often fatal disease of rabbit caused by myxoma virus. In the present study, myxomatosis outbreaks were reported in 7 domestic rabbit farms in Egypt. Rabbits showed oedema of the eyelids, facial oedema and blepharoconjunctivitis. The morbidity and lethality rates were 18-100% and 20-80%, respectively. The myxomatosis diagnosis was based on histopathology, virus isolation on rabbit kidney cell line (RK-13), polymerase chain reaction (PCR) and sequence analysis. Histopathological examination revealed the presence of epidermal hyperplasia, dermal necrosis and intracytoplasmic eosinophilic inclusion bodies. The virus was isolated on RK-13 cells and induced cytopathic effect. Using PCR, a band of 471 base pair corresponding to the M071L gene was amplified from extracted DNA. Sequence alignment of four out of the 7 isolates revealed that these isolates were 98-99% identical to European and Australian rabbit myxoma reference viruses. In conclusion, rabbit myxomatosis outbreaks and virus isolation procedures are reported herein for the first time in Egypt. Preventive policies against disease circulation should be adopted by the national authorities.</p>

Next step in the ongoing arms race between myxoma virus and wild rabbits in Australia is a novel disease phenotype

In host-pathogen arms races, increases in host resistance prompt counteradaptation by pathogens, but the nature of that counteradaptation is seldom directly observed outside of laboratory models. The best-documented field example is the coevolution of myxoma virus (MYXV) in European rabbits. To understand how MYXV in Australia has continued to evolve in wild rabbits under intense selection for genetic resistance to myxomatosis, we compared the phenotypes of the progenitor MYXV and viral isolates from the 1950s and the 1990s in laboratory rabbits with no resistance. Strikingly, and unlike their 1950s counterparts, most virus isolates from the 1990s induced a highly lethal immune collapse syndrome similar to septic shock. Thus, the next step in this canonical case of coevolution after a species jump has been further escalation by the virus in the face of widespread host resistance.

Genomic and phenotypic characterization of myxoma virus from Great Britain reveals multiple evolutionary pathways distinct from those in Australia

The co-evolution of myxoma virus (MYXV) and the European rabbit occurred independently in Australia and Europe from different progenitor viruses. Although this is the canonical study of the evolution of virulence, whether the genomic and phenotypic outcomes of MYXV evolution in Europe mirror those observed in Australia is unknown. We addressed this question using viruses isolated in the United Kingdom early in the MYXV epizootic (1954-1955) and between 2008-2013. The later UK viruses fell into three distinct lineages indicative of a long period of separation and independent evolution. Although rates of evolutionary change were almost identical to those previously described for MYXV in Australia and strongly clock-like, genome evolution in the UK and Australia showed little convergence. The phenotypes of eight UK viruses from three lineages were characterized in laboratory rabbits and compared to the progenitor (release) Lausanne strain. Inferred virulence ranged from highly virulent (grade 1) to highly attenuated (grade 5). Two broad disease types were seen: cutaneous nodular myxomatosis characterized by multiple raised secondary cutaneous lesions, or an amyxomatous phenotype with few or no secondary lesions. A novel clinical outcome was acute death with pulmonary oedema and haemorrhage, often associated with bacteria in many tissues but an absence of inflammatory cells. Notably, reading frame disruptions in genes defined as essential for virulence in the progenitor Lausanne strain were compatible with the acquisition of high virulence. Combined, these data support a model of ongoing host-pathogen co-evolution in which multiple genetic pathways can produce successful outcomes in the field that involve both different virulence grades and disease phenotypes, with alterations in tissue tropism and disease mechanisms.

Myxoma virus and the Leporipoxviruses: an evolutionary paradigm

Myxoma virus (MYXV) is the type species of the Leporipoxviruses, a genus of Chordopoxvirinae, double stranded DNA viruses, whose members infect leporids and squirrels, inducing cutaneous fibromas from which virus is mechanically transmitted by biting arthropods. However, in the European rabbit (Oryctolagus cuniculus), MYXV causes the lethal disease myxomatosis. The release of MYXV as a biological control for the wild European rabbit population in Australia, initiated one of the great experiments in evolution. The subsequent coevolution of MYXV and rabbits is a classic example of natural selection acting on virulence as a pathogen adapts to a novel host species. Slightly attenuated mutants of the progenitor virus were more readily transmitted by the mosquito vector because the infected rabbit survived longer, while highly attenuated viruses could be controlled by the rabbit immune response. As a consequence, moderately attenuated viruses came to dominate. This evolution of the virus was accompanied by selection for genetic resistance in the wild rabbit population, which may have created an ongoing co-evolutionary dynamic between resistance and virulence for efficient transmission. This natural experiment was repeated on a continental scale with the release of a separate strain of MYXV in France and its subsequent spread throughout Europe. The selection of attenuated strains of virus and resistant rabbits mirrored the experience in Australia in a very different environment, albeit with somewhat different rates. Genome sequencing of the progenitor virus and the early radiation, as well as those from the 1990s in Australia and Europe, has shown that although MYXV evolved at high rates there was no conserved route to attenuation or back to virulence. In contrast, it seems that these relatively large viral genomes have the flexibility for multiple pathways that converge on a similar phenotype.

Skin diseases in pet rabbits: a retrospective study of 334 cases seen at the University of California at Davis, USA (1984-2004)

BACKGROUND

Rabbits are growing in popularity as companion animals, and dermatology problems are often the presenting complaint when seeing a veterinarian.

HYPOTHESIS/OBJECTIVES

To document skin diseases and their prevalence in pet domestic rabbits (Oryctolagus cuniculus) in northern California, USA; to investigate predilections for breed, age or sex for the most common conditions over a 20 year period.

ANIMALS

Three hundred and thirty-four pet rabbits from the overall hospital population met inclusion criteria.

METHODS

A retrospective study was carried out by searching the computerized medical records of rabbits seen at the Veterinary Medical Teaching Hospital (VMTH) from 1 January 1984 to 31 December 2004 using key words relevant to dermatology.

RESULTS

Twenty-nine per cent of pet rabbits seen at the VMTH had skin disease. Lop-eared rabbits were over-represented, whereas mixed-breed rabbits and castrated males were under-represented in the dermatology caseload compared with the hospital population. Pododermatitis was the most common skin disease; abscesses, alopecia, otitis externa and ectoparasites also were common. Several species of bacteria other than Pasteurella spp. were isolated from abscesses. Despite the frequent mention of myxomatosis and venereal spirochetosis in the rabbit literature, few cases (three and two, respectively) were diagnosed. Castrated males were 3.7 times more likely to present with alopecia than intact males; rabbits <1 year of age were 3.6 times more likely to present with Psoroptes infestation/otitis than adult rabbits.

CONCLUSIONS AND CLINICAL IMPORTANCE

Cutaneous conditions in pet rabbits in the USA are common. Bacterial culture should be performed and antibiotic susceptibility determined in all rabbits with abscesses, particularly those not responding to typical anti-Pasteurella antibiotics.

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.

Comparative analysis of the complete genome sequence of the California MSW strain of myxoma virus reveals potential host adaptations

Myxomatosis is a rapidly lethal disease of European rabbits that is caused by myxoma virus (MYXV). The introduction of a South American strain of MYXV into the European rabbit population of Australia is the classic case of host-pathogen coevolution following cross-species transmission. The most virulent strains of MYXV for European rabbits are the Californian viruses, found in the Pacific states of the United States and the Baja Peninsula, Mexico. The natural host of Californian MYXV is the brush rabbit, Sylvilagus bachmani. We determined the complete sequence of the MSW strain of Californian MYXV and performed a comparative analysis with other MYXV genomes. The MSW genome is larger than that of the South American Lausanne (type) strain of MYXV due to an expansion of the terminal inverted repeats (TIRs) of the genome, with duplication of the M156R, M154L, M153R, M152R, and M151R genes and part of the M150R gene from the right-hand (RH) end of the genome at the left-hand (LH) TIR. Despite the extreme virulence of MSW, no novel genes were identified; five genes were disrupted by multiple indels or mutations to the ATG start codon, including two genes, M008.1L/R and M152R, with major virulence functions in European rabbits, and a sixth gene, M000.5L/R, was absent. The loss of these gene functions suggests that S. bachmani is a relatively recent host for MYXV and that duplication of virulence genes in the TIRs, gene loss, or sequence variation in other genes can compensate for the loss of M008.1L/R and M152R in infections of European rabbits.

The HIV-1 pandemic: does the selective sweep in chimpanzees mirror humankind's future?

An HIV-1 infection progresses in most human individuals sooner or later into AIDS, a devastating disease that kills more than a million people worldwide on an annual basis. Nonetheless, certain HIV-1-infected persons appear to act as long-term non-progressors, and elite control is associated with the presence of particular MHC class I allotypes such as HLA-B*27 or -B*57. The HIV-1 pandemic in humans arose from the cross-species transmission of SIV_cpz originating from chimpanzees. Chimpanzees, however, appear to be relatively resistant to developing AIDS after HIV-1/SIV_cpz infection. Mounting evidence illustrates that, in the distant past, chimpanzees experienced a selective sweep resulting in a severe reduction of their MHC class I repertoire. This was most likely caused by an HIV-1/SIV-like retrovirus, suggesting that chimpanzees may have experienced long-lasting host-virus relationships with SIV-like viruses. Hence, if natural selection is allowed to follow its course, prospects for the human population may look grim, thus underscoring the desperate need for an effective vaccine.

Myxomatosis in Australia and Europe: a model for emerging infectious diseases

Myxoma virus is a poxvirus naturally found in two American leporid (rabbit) species (Sylvilagus brasiliensis and Sylvilagus bachmani) in which it causes an innocuous localised cutaneous fibroma. However, in European rabbits (Oryctolagus cuniculus) the same virus causes the lethal disseminated disease myxomatosis. The introduction of myxoma virus into the European rabbit population in Australia in 1950 initiated the best known example of what happens when a novel pathogen jumps into a completely naïve new mammalian host species. The short generation time of the rabbit and their vast numbers in Australia meant evolution could be studied in real time. The carefully documented emergence of attenuated strains of virus that were more effectively transmitted by the mosquito vector and the subsequent selection of rabbits with genetic resistance to myxomatosis is the paradigm for pathogen virulence and host-pathogen coevolution. This natural experiment was repeated with the release of a separate strain of myxoma virus in France in 1952. The subsequent spread of the virus throughout Europe and its coevolution with the rabbit essentially paralleled what occurred in Australia. Detailed molecular studies on myxoma virus have dissected the role of virulence genes in the pathogenesis of myxomatosis and when combined with genomic data and reverse genetics should in future enable the understanding of the molecular evolution of the virus as it adapted to its new host. This review describes the natural history and evolution of myxoma virus together with the molecular biology and experimental pathogenesis studies that are informing our understanding of evolution of emerging diseases.

Genome sequence of SG33 strain and recombination between wild-type and vaccine myxoma viruses

Myxomatosis in Europe is the result of the release of a South America strain of myxoma virus in 1952. Several attenuated strains with origins in South America or California have since been used as vaccines in the rabbit industry. We sequenced the genome of the SG33 myxoma virus vaccine strain and compared it with those of other myxoma virus strains. We show that SG33 genome carries a large deletion in its right end. Furthermore, our data strongly suggest that the virus isolate from which SG33 is derived results from an in vivo recombination between a wild-type South America (Lausanne) strain and a California MSD-derived strain. These findings raise questions about the use of insufficiently attenuated virus in vaccination.

Host-specificity of myxoma virus: Pathogenesis of South American and North American strains of myxoma virus in two North American lagomorph species

The pathogenesis of South American and North American myxoma viruses was examined in two species of North American lagomorphs, Sylvilagus nuttallii (mountain cottontail) and Sylvilagus audubonii (desert cottontail) both of which have been shown to have the potential to transmit the South American type of myxoma virus. Following infection with the South American strain (Lausanne, Lu), S. nuttallii developed both a local lesion and secondary lesions on the skin. They did not develop the classical myxomatosis seen in European rabbits (Oryctolagus cuniculus). The infection at the inoculation site did not resolve during the 20-day time course of the trial and contained transmissible virus titres at all times. In contrast, S. audubonii infected with Lu had very few signs of disseminated infection and partially controlled virus replication at the inoculation site. The prototype Californian strain of myxoma virus (MSW) was able to replicate at the inoculation site of both species but did not induce clinical signs of a disseminated infection. In S. audubonii, there was a rapid response to MSW characterised by a massive T lymphocyte infiltration of the inoculation site by day 5. MSW did not reach transmissible titres at the inoculation site in either species. This might explain why the Californian myxoma virus has not expanded its host-range in North America.

Genome comparison of a nonpathogenic myxoma virus field strain with its ancestor, the virulent Lausanne strain

One of the best-studied examples of host-virus coevolution is the release of myxoma virus (MV) for biological control of European rabbits in Australia and Europe. To investigate the genetic basis of MV adaptation to its new host, we sequenced the genome of 6918, an attenuated Spanish field strain, and compared it with that of Lausanne, the strain originally released in Europe in 1952. Although isolated 43 years apart, the genomes were highly conserved (99.95% identical). Only 32 of the 159 MV predicted proteins revealed amino acid changes. Four genes (M009L, M036L, M135R, and M148R) in 6918 were disrupted by frameshift mutations.

Experimental manipulation of immune-mediated disease and its fitness costs for rodent malaria parasites

BACKGROUND

Explaining parasite virulence (harm to the host) represents a major challenge for evolutionary and biomedical scientists alike. Most theoretical models of virulence evolution assume that virulence arises as a direct consequence of host exploitation, the process whereby parasites convert host resources into transmission opportunities. However, infection-induced disease can be immune-mediated (immunopathology). Little is known about how immunopathology affects parasite fitness, or how it will affect the evolution of parasite virulence. Here we studied the effects of immunopathology on infection-induced host mortality rate and lifetime transmission potential - key components of parasite fitness - using the rodent malaria model, Plasmodium chabaudi chabaudi.

RESULTS

Neutralizing interleukin [IL]-10, an important regulator of inflammation, allowed us to experimentally increase the proportion of virulence due to immunopathology for eight parasite clones. In vivo blockade of the IL-10 receptor (IL-10R) with a neutralizing antibody resulted in a shorter time to death that was independent of parasite density and was particularly marked for normally avirulent clones. This suggests that IL-10 induction may provide a pathway to avirulence for P. c. chabaudi. Despite the increased investment in transmission-stage parasites observed for some clones in response to IL-10R blockade, experimental enhancement of immunopathology incurred a uniform fitness cost to all parasite clones by reducing lifetime transmission potential.

CONCLUSION

This is the first experimental study to demonstrate that infection-induced immunopathology and parasite genetic variability may together have the potential to shape virulence evolution. In accord with recent theory, the data show that some forms of immunopathology may select for parasites that make hosts less sick.

The future biological control of pest populations of European rabbits, Oryctolagus cuniculus

European rabbits are exotic pests in Australia, New Zealand, parts of South America and Europe, and on many islands. Their abundance, and the damage they cause, might be reduced by the release of naturally occurring or genetically modified organisms (GMOs) that act as biological control agents (BCAs). Some promising pathogens and parasites of European rabbits and other lagomorphs are discussed, with special reference to those absent from Australia as an example of the range of necessary considerations in any given case. The possibility of introducing these already-known BCAs into areas where rabbits are pests warrants further investigation. The most cost-effective method for finding potentially useful but as-yet undiscovered BCAs would be to maintain a global watch on new diseases and pathologies in domestic rabbits. The absence of wild European rabbits from climatically suitable parts of North and South America and southern Africa may indicate the presence there of useful BCAs, although other explanations for their absence are possible. Until the non-target risks of deploying disseminating GMOs to control rabbits have been satisfactorily minimised, efforts to introduce BCAs into exotic rabbit populations should focus on naturally occurring organisms. The development of safe disseminating GMOs remains an important long-term goal, with the possible use of homing endonuclease genes warranting further investigation.

Immunopathogenesis of poxvirus infections: forecasting the impending storm

Variola virus, the causative agent of smallpox, is a member of the poxvirus family and one of the most virulent human pathogens known. Although smallpox was eradicated almost 30 years ago, it is not understood why the mortality rates associated with the disease were high, why some patients recovered, and what constitutes an effective host response against infection. As variola virus infects only humans, our current understanding of poxvirus infections comes largely from historical clinical data from smallpox patients and from animal studies using closely related viruses such as ectromelia, myxoma and monkeypox. The outcome of an infection is determined by a complex interaction between the type of immune response mounted by the host and by evasion mechanisms that the virus has evolved to subvert it. Disease pathogenesis is also a function of both host and viral factors. Poxviruses are not only cytopathic, causing host tissue damage, but also encode an array of immunomodulatory molecules that affect the severity of disease. The ability of the host to control virus replication is therefore critical in limiting tissue damage. However, in addition to targeting virus, the immune response can inadvertently damage the host to such a degree that it causes illness and even death. There is growing evidence that many of the symptoms associated with serious poxvirus infections are a result of a 'cytokine storm' or sepsis and that this may be the underlying cause of pathology.