STUDIES OF A MURINE STRAIN OF POLIOMYELITIS VIRUS IN COTTON RATS AND WHITE MICE

A recent view about encephalomyocarditis virus circulating in compartmentalised animal population in Northern Italy

Encephalomyocarditis virus (Picornaviridae, Cardiovirus A) is the causative agent of the homonymous disease, which may induce myocarditis, encephalitis and reproductive disorders in various mammals, especially in swine. Despite the disease occurred endemically in pig farms since 1997, the recent increase of death experimented in Northern Italy prompted to furtherly investigate the evolution of the virus and the actual spread of the infection. Italian EMC viruses, collected between 2013 and 2019, showed an overall antigenic stability. The in-house ELISA Monoclonal Antibodies based, able to reveal changes in seven different antigenic sites, showed only sporadic and occasional mutations in considered samples and the subsequent phylogenetic analysis confirmed antigenic panel's remarks. All the isolates could be classified within a unique lineage, which comprise other European strains and confirm that the viruses currently circulating in Italy developed from a unique common ancestor. Despite the demonstrated stability of virus, some putative newly emerged variants were detected through antigenic profile analysis and phylogenesis. Finally, the serosurvey proved that spread of EMCV is greater than the diffusion of fatal infections would suggest, due to subclinical circulation of EMCV. It demonstrated an increase in the proportion of seropositive farms, if compared with previous data with no remarkable differences between farms with and without clinical evidence of disease.

Epithelial cell lines of the cotton rat (Sigmodon hispidus) are highly susceptible in vitro models to zoonotic Bunya-, Rhabdo-, and Flaviviruses

Background

Small mammals such as bats and rodents have been increasingly recognized as reservoirs of novel potentially zoonotic pathogens. However, few in vitro model systems to date allow assessment of zoonotic viruses in a relevant host context. The cotton rat (Sigmodon hispidus) is a New World rodent species that has a long-standing history as an experimental animal model due to its unique susceptibility to human viruses. Furthermore, wild cotton rats are associated with a large variety of known or potentially zoonotic pathogens.

Methods

A method for the isolation and culture of airway epithelial cell lines recently developed for bats was applied for the generation of rodent airway and renal epithelial cell lines from the cotton rat. Continuous cell lines were characterized for their epithelial properties as well as for their interferon competence. Susceptibility to members of zoonotic Bunya-, Rhabdo-, and Flaviviridae, in particular Rift Valley fever virus (RVFV), vesicular stomatitis virus (VSV), West Nile virus (WNV), and tick-borne encephalitis virus (TBEV) was tested. Furthermore, novel arthropod-derived viruses belonging to the families Bunya-, Rhabdo-, and Mesoniviridae were tested.

Results

We successfully established airway and kidney epithelial cell lines from the cotton rat, and characterized their epithelial properties. Cells were shown to be interferon-competent. Viral infection assays showed high-titre viral replication of RVFV, VSV, WNV, and TBEV, as well as production of infectious virus particles. No viral replication was observed for novel arthropod-derived members of the Bunya-, Rhabdo-, and Mesoniviridae families in these cell lines.

Conclusion

In the current study, we showed that newly established cell lines from the cotton rat can serve as host-specific in vitro models for viral infection experiments. These cell lines may also serve as novel tools for virus isolation, as well as for the investigation of virus-host interactions in a relevant host species.

INTERFERENCE BETWEEN THE INFLUENZA VIRUSES : I. THE EFFECT OF ACTIVE VIRUS UPON THE MULTIPLICATION OF INFLUENZA VIRUSES IN THE CHICK EMBRYO

Reciprocal interference between influenza A, influenza B, and swine influenza viruses has been demonstrated in the chick embryo. Certain temporal and quantitative factors which influence the production of interference in this host-virus system have been studied. The implications of these observations in relation to the mechanism by which interference is produced are discussed.

STUDIES IN RODENT POLIOMYELITIS : I. FURTHER EXPERIMENTS WITH THE MURINE STRAIN OF SK POLIOMYELITIS VIRUS

1. SK murine virus maintained over more than 200 serial mouse passages increased in virulence for mice from an initial intracerebral titer of about 1:1 million to a maximum titer of not less than 1:1 billion dilution activity. 2. Following intracerebral injection with murine virus of remote mouse passages, 5 of 13 rhesus monkeys developed a characteristic encephalitic syndrome. Repeated intravenous injection of massive doses of virus caused localized flaccid paralysis in 2 of 14 monkeys. 3. Intracerebral injection of graded doses of murine virus into mice of different age groups caused fatal paralysis in young and old animals alike. Infection with small doses of virus by peripheral routes, while uniformly fatal to young mice, was followed by survival of almost half of the old mice. 4. The incubation period of the disease in young mice infected intracerebrally with a standard dose of murine virus, when studied throughout the period of 1 year, was found considerably lengthened during the summer months. 5. Cross neutralization tests furnished no evidence for any serological relationship between SK murine virus and lymphocytic choriomeningitis virus. Theiler's virus was found to be neutralizable by antimurine horse serum and, to a lesser extent, by concentrated antipoliomyelitis horse serum; however, such inactivation, in both cases, was distinctly inferior to that occurring with SK murine virus. On the other hand, no neutralization whatsoever was obtained between SK murine virus and normal adult mouse serum, whereas the same serum completely neutralized Theiler's virus. Mice surviving infection with Theiler's virus, though acquiring immunity to this virus, remained fully susceptible to reinfection with SK murine virus. 6. Neutralization tests with SK murine virus against poliomyelitis-convalescent monkey sera gave irregular results, but neutralization of murine virus occurred regularly with a hyperimmune antipoliomyelitis horse serum. Hyperimmune antimurine horse and rabbit sera, on the other hand, failed to inactivate three strains of monkey poliomyelitis virus (SK, RMV, Aycock) by intracerebral tests in monkeys. The same sera inactivated murine virus in mice by intraperitoneal, but not by intracerebral injection of virus-serum mixtures. 7. The identity of SK murine virus and its relation to other rodent strains of poliomyelitis virus is discussed on the basis of the available data.

STUDIES IN RODENT POLIOMYELITIS : II. CULTIVATION OF THE MURINE STRAIN OF SK POLIOMYELITIS VIRUS

1. The murine virus may be grown in embryonic mouse brain-serum ultrafiltrate cultures. The virus fails to grow in embryonic chick tissue cultures or in fertilized egg preparations. 2. Some relationship can be demonstrated between the amount of nervous tissue and the infectivity of the culture. 3. Optimum titers of virus potency (10^–6) can be obtained by adjusting the pH of the growing culture at 7.3 to 7.6. 4. A simple pH inactivation curve for virus alone and for virus when actively growing in tissue culture has been obtained. 5. The rate of virus propagation, as determined by potency tests in mice, has been established for cultures which were seeded with large or small amounts of virus. The murine virus "grows" relatively fast. The optimum titer for a large inoculum was reached in 19 hours, for a small inoculum in 72 hours. 6. With extended subcultivation in vitro the cultured virus shows a loss of infectivity for mice by peripheral injection. However, potency as determined by intracerebral injection remains constant. 7. Mice surviving inoculation of culture virus by routes other than intracerebral acquire a relative resistance to reinfection with mouse passage virus. 8. The murine culture virus passes without difficulty through collodion membranes with an A.P.D. of 30 mµ. Its particle size may therefore be estimated as lying between 10 to 15 mµ. 9. On a basis of in vitro activity and cross infection, the murine culture virus is distinct from the virus of lymphocytic choriomeningitis. 10. Murine culture virus may be used as an interfering agent to block infection with poliomyelitis virus in monkeys. The interaction between the two viruses seems to be quantitatively limited. Such interference, with the present potency of culture virus, operates effectively only against comparatively small doses of monkey virus.

ACTIVE IMMUNITY TO POLIOMYELITIS IN CHIMPANZEES FOLLOWING SUBCLINICAL INFECTION

This report is concerned with the production of subclinical poliomyelitis in the chimpanzee, and immunity to such infection. Subclinical infection was produced in eight chimpanzees by the administration of virus orally or intracutaneously, and was measured by the production of a virus carrier state and the development of neutralizing antibodies. Immunity was tested by attempting to induce the carrier state again by a challenge dose of virus by the same routes. With an homologous strain as the challenge virus, immunity as defined here was produced in each of four chimpanzees tested. Neutralizing antibodies to the homologous strain were present about 1 month following the first exposure to virus by either the oral or cutaneous route. In another group of two chimpanzees, one developed neutralizing antibodies and immunity to heterologous strains, and one did not.

STUDIES IN RODENT POLIOMYELITIS : IV. THE PATHOLOGY OF MURINE AND CAVIAN POLIOMYELITIS

A description has been given of the lesions produced in mice and guinea pigs by inoculation of the Jungeblut-Sanders virus. The histopathological findings, although in themselves not conclusive, would tend to support the opinion that Jungeblut and Sanders have transmitted the SK poliomyelitis virus to mouse and guinea pig. In mice the virus apparently retains its affinity for the anterior horns of the spinal cord, but in a moderate degree. Associated with a marked increase in virulence of the virus, a strong affinity for the cerebral tissues, more particularly the olfactory centers, develops. On transmitting this murine variant of the virus to guinea pigs, however, the original character of the virus is again revealed. There is a reversion to a predominant affinity for the nerve cells of the anterior horns of the spinal cord.

MULTIPLE VIRUS INFECTION OF SINGLE HOST CELLS

Evidence is presented to show that two or more viruses can simultaneously manifest their characteristic activities within individual epithelial cells of the normal rabbit's cornea. This evidence, together with that previously presented (1, 5, 6), makes plain that multiple virus infection of a single host cell can take place in corneal cells, in the cells of chick embryos, and in those of rabbit tumors, both benign (Shope's papilloma) and malignant. Certain implications of the findings are discussed.

STUDIES IN RODENT POLIOMYELITIS : V. INTERFERENCE BETWEEN MURINE AND MONKEY POLIOMYELITIS VIRUS

1. The murine strain of SK poliomyelitis virus interferes with the propagation in rhesus monkeys of SK, Aycock, and RMV poliomyelitis monkey virus. 2. This interference is demonstrable by intracerebral injection of mixtures of murine and monkey virus prepared in vitro as well as by separate injection of the two viruses by diverse routes. 3. Mixture tests carried out with graded doses of murine and monkey virus show that 0.5 cc. of a 10 per cent suspension prepared from the brains of paralyzed mice is capable of counteracting at least 100 minimal paralyzing doses of two strains of monkey virus. 4. No interference was demonstrable with suspensions of brains infected with murine virus which had been inactivated by heating for (1/2) hour at 75 degrees C., or with suspensions prepared from normal mice, or with brain suspensions prepared from mice infected with herpes virus. 5. When murine virus is introduced into monkeys by the intravenous route, before or after intracerebral infection with monkey virus, distinct prophylactic or therapeutic results may be obtained. 6. Analysis of the figures shows that the success of interference depends upon (a) the size of the infecting dose of monkey virus, (b) the amount of murine virus injected, and (c) the choice of proper intervals between the injection of monkey and murine virus. 7. The mechanism of the interference phenomenon here described is discussed in the light of the available data.

The susceptibility of the hamster to mouse encephalomyelitis virus

The OT strain of mouse encephalomyelitis virus induces an inapparent infection in suckling hamsters associated with lesions of the central nervous system and skeletal muscles. The virus increases in pathogenicity after alternating mouse-hamster transfers and then induces both paralysis and encephalitis. Pathogenicity is lost through serial hamster passages but is restored by a single mouse transfer.

Serologic responses of Barbary sheep (Ammotragus lervia), Indian antelope (Antilope cervicapra), wallaroos (Macropus robustus), and chimpanzees (Pan troglodytes) to an inactivated encephalomyocarditis virus vaccine

Encephalomyocarditis virus (EMCV) is a picornavirus with a worldwide distribution, capable of infecting a wide range of species. Episodes of EMCV-associated mortality have been reported in zoos and national parks around the world, including sporadic cases at Taronga Zoo, Sydney. An inactivated EMCV vaccine was evaluated by inoculating Barbary sheep (Ammotragus lervia), Indian antelope (Antilope cervicapra), Eastern wallaroos (Macropus robustus), and chimpanzees (Pan troglodytes). A proportion of the vaccinated ungulates were administered a second vaccination 4 wk after the initial dose. Neutralizing antibody titers were monitored for a period of 12 mo. One month after vaccination, all vaccinated groups had developed significant antibody titers that persisted for at least 6 mo. Animals receiving two doses of vaccine had higher titers 3, 6, and 12 mo after the initial vaccination compared with animals receiving a single vaccine dose.

Poliovirus and poliomyelitis: a tale of guts, brains, and an accidental event

Nearly 100 years after its discovery poliovirus remains one of most thoroughly studied and best understood virus models for the molecular virologist. While poliovirus has been of vital importance for our insight into picornavirus biology at the cellular and biochemical level, it is ironic to note that, due to the early success in defeating poliomyelitis in the developed world through vaccination, many of the basic aspects of poliovirus pathogenesis remain poorly understood. This is chiefly due to the lack of an adequate and affordable animal model, save of old world monkeys. Fundamental questions, such as the identity of the target cells during the enteric phase of infection, or mechanisms of systemic spread are still unanswered. This review will attempt to summarize our current knowledge of the molecular biology of poliovirus, its pathogenesis, as well as recent advances in the areas of cell and tissue tropism and mechanisms of central nervous system invasion.

The neuropathology observed in wild-type mice inoculated with human poliovirus mirrors human paralytic poliomyelitis

Human paralytic poliomyelitis results from the destruction of spinal cord anterior horn motor neurons by human poliovirus (PV). CNS disease pathology similar to human poliomyelitis has been observed in experimentally infected chimpanzees, monkeys and wild-type mice. In this study we present a detailed examination of the clinical and histopathological features in the wild-type mouse after intracranial (i.c.) and novel intramuscular (i.m.) injection of poliovirus. Either route of poliovirus administration results in a clinical disease characterized predominately by flaccid paralysis. The observed histopathological features are compared with the histopathology reported for human paralytic poliomyelitis, experimentally infected chimpanzees, monkeys and transgenic mice expressing the human poliovirus receptor (hPVR). The observation of flaccid paralysis and anterior horn motor neuron destruction mirrors what is observed in human paralytic poliomyelitis. Our results suggest that the neuropathology observed in the wild-type mouse model is similar to what has been observed in both the human disease and in other experimental animal models, with the possible exception of the transgenic mouse model. The observed neuropathology of the wild-type mouse model more closely reflects what has been observed in human poliomyelitis, as well as in experimentally infected chimpanzees and monkeys, than does the hPVR transgenic mouse model. The previously reported poliovirus-induced white matter demyelinating disease was not observed.

Molecular analysis of encephalomyocarditis viruses isolated from pigs and rodents in Italy

Partial nucleotide sequences of encephalomyocarditis (EMC) viruses isolated from five, apparently independent, outbreaks of fatal myocarditis in pigs in Italy were compared with three EMC viruses isolated from wild rodents from a different geographic region in the same country. These viruses were also compared with EMC viruses isolated from pigs in other European countries and three historical strains. All the Italian EMC viruses were closely related (> 94.6% nucleotide identity), but were distinct from viruses occurring in Belgium in 1991 (< 80.5% nucleotide identity), Greece in 1990 (< 83.3% nucleotide identity) and the three older viruses (< 82.9% nucleotide identity). An EMC virus isolated from pigs in the Netherlands in 1988, was closely related to the Italian viruses (95.3-99.3% nucleotide identity). It is suggested that pigs may play a role in the movement of EMC viruses between different geographic regions.

INTERFERENCE BETWEEN VIRUSES IN TISSUE CULTURE

The influence of one virus on the growth of another in tissue culture was investigated. The 17DD High strain of yellow fever virus was found capable of completely suppressing the growth of both the Asibi strain of the same virus and of the heterologous West Nile virus, even when these were added to the cultures in large amounts. The 17DD High strain of yellow fever virus and the West Nile virus produced either partial or complete suppression of growth of the Venezuelan equine encephalomyelitis virus, depending upon the quantity of the latter inoculated into the cultures. Owing to lack of methods for the detection of interference except in a single direction, reciprocal interference with these viruses could not be investigated. The 17DD High strain of yellow fever virus and the West Nile virus were able to suppress completely, or almost completely, the growth of influenza A virus added to the infected cultures in maximal amounts. Interference in the reverse direction, even with the use of small amounts of the neurotropic viruses, was not demonstrable. Cultures infected with the 17DD High strain of yellow fever virus were examined for the presence of neutralizing antibodies and non-specific antiviral substances; neither was found present.