Interstitial orchitis with impaired steroidogenesis and spermatogenesis in the testes of rabbits infected with an attenuated strain of myxoma virus

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.

Molecular methods in detection and epidemiologic studies of rabbit and hare viruses: a review

Various PCR-based assays for rabbit viruses have gradually replaced traditional virologic assays, such as virus isolation, because they offer high-throughput analysis, better test sensitivity and specificity, and allow vaccine and wild-type virus strains to be fully typed and differentiated. In addition, PCR is irreplaceable in the detection of uncultivable or fastidious rabbit pathogens or those occurring in low quantity in a tested sample. We provide herein an overview of the current state of the art in the molecular detection of lagomorph viral pathogens along with details of their targeted gene or nucleic acid sequence and recommendations for their application. Apart from the nucleic acids-based methods used for identification and comprehensive typing of rabbit viruses, novel methods such as microarray, next-generation sequencing, and mass spectrometry (MALDI-TOF MS) could also be employed given that they offer greater throughput in sample screening for viral pathogens. Molecular methods should be provided with an appropriate set of controls, including an internal amplification control, to confirm the validity of the results obtained.

Infectious, inflammatory and 'autoimmune' male factor infertility: how do rodent models inform clinical practice?

BACKGROUND

Infection and inflammation of the reproductive tract are significant causes of male factor infertility. Ascending infections caused by sexually transmitted bacteria or urinary tract pathogens represent the most frequent aetiology of epididymo-orchitis, but viral, haematogenous dissemination is also a contributory factor. Limitations in adequate diagnosis and therapy reflect an obvious need for further understanding of human epididymal and testicular immunopathologies and their contribution to infertility. A major obstacle for advancing our knowledge is the limited access to suitable tissue samples. Similarly, the key events in the inflammatory or autoimmune pathologies affecting human male fertility are poorly amenable to close examination. Moreover, the disease processes generally have occurred long before the patient attends the clinic for fertility assessment. In this regard, data obtained from experimental animal models and respective comparative analyses have shown promise to overcome these restrictions in humans.

OBJECTIVE AND RATIONALE

This narrative review will focus on male fertility disturbances caused by infection and inflammation, and the usefulness of the most frequently applied animal models to study these conditions.

SEARCH METHODS

An extensive search in Medline database was performed without restrictions until January 2018 using the following search terms: 'infection' and/or 'inflammation' and 'testis' and/or 'epididymis', 'infection' and/or 'inflammation' and 'male genital tract', 'male infertility', 'orchitis', 'epididymitis', 'experimental autoimmune' and 'orchitis' or 'epididymitis' or 'epididymo-orchitis', antisperm antibodies', 'vasectomy'. In addition to that, reference lists of primary and review articles were reviewed for additional publications independently by each author. Selected articles were verified by each two separate authors and discrepancies discussed within the team.

OUTCOMES

There is clear evidence that models mimicking testicular and/or epididymal inflammation and infection have been instructive in a better understanding of the mechanisms of disease initiation and progression. In this regard, rodent models of acute bacterial epididymitis best reflect the clinical situation in terms of mimicking the infection pathway, pathogens selected and the damage, such as fibrotic transformation, observed. Similarly, animal models of acute testicular and epididymal inflammation using lipopolysaccharides show impairment of reproduction, endocrine function and histological tissue architecture, also seen in men. Autoimmune responses can be studied in models of experimental autoimmune orchitis (EAO) and vasectomy. In particular, the early stages of EAO development showing inflammatory responses in the form of peritubular lymphocytic infiltrates, thickening of the lamina propria of affected tubules, production of autoantibodies against testicular antigens or secretion of pro-inflammatory mediators, replicate observations in testicular sperm extraction samples of patients with 'mixed atrophy' of spermatogenesis. Vasectomy, in the form of sperm antibodies and chronic inflammation, can also be studied in animal models, providing valuable insights into the human response.

WIDER IMPLICATIONS

This is the first comprehensive review of rodent models of both infectious and autoimmune disease of testis/epididymis, and their clinical implications, i.e. their importance in understanding male infertility related to infectious and non-infectious/autoimmune disease of the reproductive organs.

Disorders of the Reproductive Tract of Rabbits

Disorders of the reproductive tract are common in rabbits. Conditions are different in rabbits that are farmed for their meat or fur and those that are kept as pets. Farmed rabbits suffer from infections and diseases associated with pregnancy. Congenital abnormalities are more likely to be recognized and treated in pet rabbits. Pet rabbits suffer from age-related changes to their genital tract (hyperplasia, neoplasia, or hernias). Neutering is an important part of prevention and treatment of reproductive disorders. Knowledge of normal male and female reproductive anatomy is essential to prevent complications. These are described and illustrated.

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.

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.

Immunological, paracrine and endocrine aspects of testicular immune privilege

Protection of the spermatogenic cells from the host immune response is fundamental to male fertility. Significantly, this protection extends to the tolerance of foreign tissue grafts placed within the testicular environment, a phenomenon that is called 'immune privilege'. This privilege of the testis appears to involve several levels of immune control, encompassing the normal mechanisms of immune tolerance, antigen sequestration behind the blood-testis barrier, reduced immune activation, localised immunosuppression and antigen-specific immunoregulation. Central to these regulatory processes are the somatic cells of the testis, particularly the Sertoli cells, and testicular secretions, including androgens, cytokines, peptides and bioactive lipids. Failure of these protective mechanisms, which may be precipitated by trauma, inflammation or infection, or as the consequence of genetic factors, can lead to androgen deficiency, infertility and autoimmunity.

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.

Construction and testing of a novel host-range defective myxoma virus vaccine with the M063 gene inactivated that is non-permissive for replication in rabbit cells

Deletion of the M063 gene from myxoma virus produces a virus that is unable to replicate in rabbit cells in vitro or in live rabbits but can be propagated in non-rabbit cell lines. A targeted M063 deletion mutant was constructed in the attenuated Uriarra strain of myxoma virus and the ability of this virus to act as a safe, non-transmissible vaccine against myxomatosis was tested in outbred laboratory rabbits. Immunization with the M063 deletion vaccine provided good short-term protection against lethal challenge with virulent myxoma virus. Long-term protection was similar to reported results with heterologous live virus, with some rabbits protected but others succumbing to challenge. Replication-deficient poxvirus vaccines, like the Modified Vaccinia Virus Ankara (MVA) in man and the myxoma virus vaccine described here in rabbits, are very attractive from a safety perspective. Seasonal boosting would be predicted to provide long-term protection. Targeted host-range gene deletions could have potential for rapid development of poxvirus vaccines in general.

Fibrotic lesions in the testis of bulls and relationship to semen quality

Ultrasonography of the testes was done in bulls at three locations in western Canada (n=325) and one in Argentina (n=387) to determine the prevalence of fibrotic lesions and to examine the relationship between fibrotic lesions and location, age, breed, right compared with left testes, testis size and semen quality. Fibrotic lesions were common in the testes of bulls raised under intensive rearing conditions in western Canada as well as in the more extensive rearing conditions of Argentina. Fibrotic lesions appeared as early as 5-6 months of age and the number of cases continued to increase until at least 12-14 months of age. The severity of lesions increased in some cases during this period; however, it appears that the development of lesions occurred during a finite period of pubertal development. It is unlikely that the prevalence of lesions is influenced by breed, right compared with left testes or testis size. The cause of the lesions is unknown, but there was an association between the development of fibrotic lesions and an outbreak of BRSV disease in Argentina in one group of bulls. There was some indication that during the active process that leads to fibrosis spermatogenesis is adversely affected; however, the presence of a large number of fibrotic lesions that may occupy as much as 50% of the testis parenchyma did not preclude the production of a greater percentage of sperm with normal morphology.

Prospects for fertility control in the European rabbit (Oryctolagus cuniculus) using myxoma virus-vectored immunocontraception

Research over the last 15 years has examined whether fertility control can reduce overabundant rabbit populations and whether an effective immunocontraceptive agent can be developed and delivered. The results of this research indicate that for fertility control to have an environmental impact at least 80% of females will need to be infertile and that this infertility will need to be permanent. Epidemiological studies suggest that this level of infertility may be very difficult to obtain with a recombinant myxoma virus because of competition with field strains of virus. Research with laboratory rabbits using recombinant myxoma virus to deliver an immunocontraceptive antigen demonstrated that it was possible to obtain the required level of infertility using rabbit zona pellucida C as an antigen. However, only ~50% of animals remained infertile in the medium term. Further research on delivery vector and antigen selection would be needed to produce a practical immunocontraceptive virus for laboratory testing. Such a virus would then need to be optimised for transmissibility before it would be suitable for field testing.

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

The pathogenesis of two Californian strains of myxoma virus (MSW and MSD) was examined in European rabbits (Oryctolagus cuniculus) that were either susceptible to myxomatosis (laboratory rabbits) or had undergone natural selection for genetic resistance to myxomatosis (Australian wild rabbits). MSW was highly lethal for both types of rabbits with average survival times of 7.3 and 9.4 days, respectively, and 100% mortality. Classical clinical signs of myxomatosis were not present except in one rabbit that survived for 13 days following infection. Previously described clinical signs of trembling and shaking were observed in laboratory but not wild rabbits. Despite the high resistance of wild rabbits to myxomatosis caused by South American strains of myxoma virus, the MSW strain was of such high virulence that it was able to overcome resistance. The acute nature of the infection, relatively low viral titers in the tissues and destruction of lymphoid tissues, suggested that death was probably due to an acute and overwhelming immunopathological response to the virus. No virus was found in the brain. The MSD strain was attenuated compared to previously published descriptions and therefore was only characterized in laboratory rabbits. It is concluded that Californian MSW strain of myxoma virus is at the extreme end of a continuum of myxoma virus virulence but that the basic pathophysiology of the disease induced is not broadly different to other strains of myxoma virus.

Viral diseases of the rabbit

Viral disease in the rabbit is encountered infrequently by the clinical practitioner; however, several viral diseases were reported to occur in this species. Viral diseases that are described in the rabbit primarily may affect the integument, gastrointestinal tract or, central nervous system or maybe multi-systemic in nature. Rabbit viral diseases range from oral papillomatosis, with benign clinical signs, to rabbit hemorrhagic disease and myxomatosis, which may result in significant clinical disease and mortality. The wild rabbit may serve as a reservoir for disease transmission for many of these viral agents. In general, treatment of viral disease in the rabbit is supportive in nature.

Antibody response in the female rabbit reproductive tract to influenza haemagglutinin encoded by a recombinant myxoma virus

The antibody response in serum and the reproductive tract of female rabbits to a model antigen, influenza virus haemagglutinin (HA), encoded by a recombinant myxoma virus was investigated. Strong and lasting IgG antibody responses to HA were induced in serum following intradermal, intranasal, and intravaginal immunisations. HA IgG was also detected in reproductive tract fluids but was only about 1% the titer of that in serum. HA IgA was not detected in serum of any infected groups and was occasionally detected in reproductive tract fluids at a low titer only after infections through mucosal sites. HA IgM was also detected only in some of the reproductive tract fluids at very low levels. Induction of ovulation did not change these patterns and B cell homing to the reproductive tract was not profound. In contrast, HA IgG and IgM titers in ovarian follicular fluids were comparable to that in serum. These data suggest that if this virus is used to deliver an immunocontraceptive vaccine that requires a high-level antibody response, the target antigen needs to be accessible to serum antibody or in the ovary.

Immune responses to myxoma virus

Myxoma virus causes the systemic disease myxomatosis in the European rabbit (Oryctolagus cuniculus). Originating in the South American rabbit Sylvilagus brasiliensis, where it causes a relatively localized fibroma, myxoma virus is a classic example of a virus that has jumped species to produce an exotic disease and then coevolved with its new host. Like other poxviruses, myxoma virus encodes multiple proteins capable of downregulating the host innate and acquired immune responses. Other virus-encoded proteins enable replication in host lymphocytes and monocytes, for example, by inhibiting apoptosis. Detailed studies on these proteins have demonstrated novel methods of interactions with the host immune system and added substantially to the understanding of the interaction of large DNA viruses with their hosts. Despite the increasingly detailed molecular knowledge of myxoma virus, relatively little is known about the dynamics of the interaction of the virus with the integrated host-immune system during infection and, in particular, about the evolution of resistance to the virus in wild rabbits or the species barrier. This review will focus on the detailed molecular studies that have been done with myxoma virus and discuss the more limited knowledge of the pathogenesis of myxoma virus in rabbits and the ways that the consolidated immune responses may determine genetic resistance to myxomatosis.

Serum sperm antibodies are not elevated after mumps orchitis

OBJECTIVE

To assess the level of serum sperm antibodies after mumps orchitis.

DESIGN

Controlled descriptive study.

SETTING

Academic research environment.

PATIENT(S)

Seventy-four mumps orchitis patients.

INTERVENTION(S)

Sampling of serum at different intervals after the onset of orchitis symptoms: 1 to 7 days, 31 to 60 days, and 61 to 431 days.

MAIN OUTCOME MEASURE(S)

Level of serum sperm antibodies, using Kibrick's gelatin agglutination test, Friberg's tray agglutination test, Isojima's sperm immobilization test, and ELISA.

RESULT(S)

Clinically relevant sperm antibody values were detected by the Friberg method among patients tested from 1 to 7 days (10.5%) and 61 to 431 days (10.5%) after the onset of disease. The Isojima test revealed a statistically insignificant higher incidence among patients at 61 to 431 days (31.6%) as compared with those sampled at 1 to 7 days (10.5%). None of the orchitis sera tested positive by the Kibrick and ELISA techniques. The established incidences did not differ significantly from the results for negative controls (blood donors) and were lower than the values acquired from positive controls (males with unexplained infertility).

CONCLUSION(S)

Mumps orchitis does not cause enhanced humoral immunity to spermatozoa.

Experimental inoculation of male mice with murine cytomegalovirus and effect on offspring

BACKGROUND

Murine cytomegalovirus (MCMV) was used to examine aspects of viral infection in male mice, and its possible transmission to their offspring.

METHODS AND RESULTS

FVB/N mice inoculated intratesticularly with 5x10(5) plaque forming units (PFU) of MCMV, developed peritoneal haemorrhagic exudates, spleen hypertrophy and acute local infection. Infectiousness was detected until 15 days post-inoculation (D15 PI) in the genital organs, and virus DNA up to D35 PI. Testicular endothelial and Leydig cells were infected, and peritubular cells severely damaged. Spermatogenesis was affected, but neither germ cells nor Sertoli cells were infected. No virus was found in the epididymal epithelial cells. Viral DNA was detected in cells extracted from vas deferens samples until D15 PI. Neither infectious virus nor viral DNA were found in spermatozoa recovered from uterine fluid, fertilized oocytes, blastocysts, fetal tissues or newborn animals following the mating of infected males with uninfected females.

CONCLUSIONS

MCMV harboured in the male genital organs was not transmitted to their offspring, even when mating occurred during the acute phase of CMV disease. Although the infection may have had an impact on spermatogenesis, fertility was not affected. These results do not support the hypothesis of conceptus MCMV infection by the fertilizing spermatozoon in natural conception.

Viruses in the mammalian male genital tract and their effects on the reproductive system

This review describes the various viruses identified in the semen and reproductive tracts of mammals (including humans), their distribution in tissues and fluids, their possible cell targets, and the functional consequences of their infectivity on the reproductive and endocrine systems. The consequences of these viral infections on the reproductive tract and semen can be extremely serious in terms of organ integrity, development of pathological and cancerous processes, and transmission of diseases. Furthermore, of essential importance is the fact that viral infection of the testicular cells may result not only in changes in testicular function, a serious risk for the fertility and general health of the individual (such as a fall in testosteronemia leading to cachexia), but also in the possible transmission of virus-induced mutations to subsequent generations. In addition to providing an exhaustive account of the data available in these domains, this review focuses attention on the fact that the interface between endocrinology and virology has so far been poorly explored, particularly when major health, social and economical problems are posed. Our conclusions highlight the research strategies that need to be developed. Progress in all these domains is essential for the development of new treatment strategies to eradicate viruses and to correct the virus-induced dysfunction of the endocrine system.

Coevolution of host and virus: cellular localization of virus in myxoma virus infection of resistant and susceptible European rabbits

The coevolution of myxoma virus and the wild European rabbit in Australia and the development of resistance to myxomatosis in wild rabbits have been well described. However, the mechanism of resistance to myxomatosis in wild rabbits is not understood. To determine the basis of resistance, the pathogenesis of the virulent standard laboratory strain (SLS) and the attenuated Uriarra (Ur) strain of myxoma virus were examined in Australian wild rabbits that have been naturally selected in the field for resistance to myxomatosis and in laboratory rabbits which have never been selected for resistance. Virus was localized in tissue sections by immunofluorescence. In all cases virus antigen was initially present in dendritic cells of the dermis before localizing predominantly to the epidermis by Day 6. Antigen-containing cells were detected in the lymph nodes by 24 h after inoculation. Virus replication occurred predominantly in T lymphocytes of the paracortex but SLS also replicated in germinal centers. SLS replication induced loss of most lymphocytes from the lymph nodes of susceptible rabbits. Apoptosis of lymphocytes within the lymph nodes was a major feature of all infections. These apoptotic cells did not contain detectable viral antigen but were often adjacent to infected cells. Ongoing apoptosis of lymphocytes within lymph nodes was also a feature of the recovery phase when very few or no virus-infected cells could be detected. Differences between virulent and attenuated viruses in the wild and laboratory rabbits were predominantly in the degree of tissue pathology in the draining lymph node and distal lymph node and in the type of inflammatory responses, particularly in the skin. SLS infection of laboratory rabbits was associated with a very mild inflammatory response, often distant from the site of virus replication and comprised predominantly of neutrophils. In contrast, Ur-infected rabbits and SLS-infected wild rabbits had an intense inflammatory response adjacent to the site of virus replication and this was comprised predominantly of mononuclear cells. Both the initial infection of dendritic cells and the ongoing destruction of lymphocytes provide obvious mechanisms for the suppression of the immune response by myxoma virus.

Coevolution of host and virus: the pathogenesis of virulent and attenuated strains of myxoma virus in resistant and susceptible European rabbits

Myxoma virus was introduced into the European rabbit population of Australia in 1950. Although the virus was initially highly lethal in rabbits, there was rapid selection for less virulent strains of virus and innately resistant rabbits. To investigate the basis of resistance to myxoma virus, we have compared the pathogensis of the virulent strain of myxoma virus originally released into Australia and an attenuated, naturally derived field strain of myxoma virus. This was done in laboratory rabbits, which have not been selected for resistance, and in wild rabbits that have developed significant resistance. Wild rabbits were able to recover from infection with virus that was always lethal in laboratory rabbits. Laboratory rabbits were able to control and recover from infection with attenuated virus. This virus caused a trivial disease in wild rabbits. There was little difference between laboratory and wild rabbits in titers of either virulent or attenuated virus in the skin at the inoculation site. However, resistant wild rabbits had a 10- to 100-fold lower titer of virulent virus within the lymph node draining the inoculation site and controlled virus replication in tissues distal to the draining lymph node. Replication of virus in lymphocytes or fibroblasts cultured from wild and laboratory rabbits demonstrated that resistance was not due to altered cellular permissivity for replication. Neutralizing antibodies were present in both susceptible and resistant rabbits, suggesting that these have no significant role in resistance. We hypothesise that resistance is due to an enhanced innate immune response that allows the rabbit to mount an effective cellular immune response.

Study of the virulence of five strains of amyxomatous myxoma virus in crossbred New Zealand White/Californian conventional rabbits, with evidence of long-term testicular infection in recovered animals

The virulence of five amyxomatous myxoma virus (MV) strains, the clinical and pathogenetic effects of which had been studied previously in specific pathogen-free (SPF) rabbits, was determined by inoculation of five groups of 10 crossbred New Zealand White/Californian conventional rabbits. A much more acute myxomatosis syndrome was produced in conventional rabbits than that reproduced previously in SPF animals. However, the main clinical signs were of the respiratory type. The MV strains MYX 254/95 and 801 appeared very virulent, killing all the inoculated animals. The strains MYX 217/95, MYX 555/94 and Saint Benoist were somewhat attenuated, killing only seven, six and six rabbits, respectively. Extensive lung lesions due to supervening bacterial infections were observed in 36 of the 39 rabbits that died. Lethality was found to be a better estimate of virulence than mean survival time. By 98 days after viral inoculation, all the surviving animals had completely recovered. At that time, they were immunosuppressed by treatment with adrenocorticotrophic hormone (ACTH) for 10 days to determine whether they still harboured the virus. After the ACTH treatment, eight of the 11 surviving rabbits showed clinical signs that resembled amyxomatous myxomatosis. All the virological examinations performed on naso-conjunctival exudate, on mononuclear cells, on eyelids and on ovaries remained negative but infectious virus was isolated from the testes of three of six surviving male rabbits.