Viruses and rickettsiae

Understanding Enterovirus 71 Neuropathogenesis and Its Impact on Other Neurotropic Enteroviruses

Enterovirus A71 (EV-A71) belongs to the species group A in the Enterovirus genus within the Picornaviridae family. EV-A71 usually causes self-limiting hand, foot and mouth disease or herpangina but rarely causes severe neurological complications such as acute flaccid paralysis and encephalomyelitis. The pathology and neuropathogenesis of these neurological syndromes is beginning to be understood. EV-A71 neurotropism for motor neurons in the spinal cord and brainstem, and other neurons, is mainly responsible for central nervous system damage. This review on the general aspects, recent developments and advances of EV-A71 infection will focus on neuropathogenesis and its implications on other neurotropic enteroviruses, such as poliovirus and the newly emergent Enterovirus D68. With the imminent eradication of poliovirus, EV-A71 is likely to replace it as an important neurotropic enterovirus of worldwide importance.

Neuropathology of viral infections of the central nervous system

Many viral infections of the nervous system cause stereotyped pathologic features and overlapping clinical and imaging features. Neuroimaging usually offers neuroanatomical localization of the pathology, degree of involvement of the nervous system, and response to therapy during follow up in a few instances. Neuroimaging is a useful adjunct for diagnosis.

Inflammation in the nervous system: the human perspective

Many basic aspects of brain inflammation, recently disclosed in experimental models, are reflected in the pathology of human inflammatory brain diseases. Examples include the key role of T lymphocytes in immune surveillance and in the regulation of the inflammatory response, the essential contributions of adhesion molecules, proinflammatory cytokines, chemokines, and proteases in the recruitment of inflammatory cells into the nervous tissue, the modulating effect of glia cells on the inflammatory process and the termination of T-cell-mediated inflammation by apoptotic cell death. Despite this progress in our understanding of the pathogenesis of brain inflammation, there are still major unresolved questions. Because of technical constraints, most of our knowledge on central nervous system inflammation so far relates to the role of a specific T-cell subset, the so-called T-helper-1 cells. Other T-cell subsets, in particular cytotoxic class I MHC-restricted T lymphocytes, however, appear to be of major importance in human disease. Furthermore, the detailed mechanisms, which are responsible for the profound differences in the patterns of tissue damage in different human inflammatory brain diseases, such as multiple sclerosis or various forms of virus encephalitis, are largely unresolved. We discuss the open questions to be addressed in the future, which, when answered, may help to design novel therapeutic strategies.

Emerging and re-emerging epidemic encephalitis: a tale of two viruses

Two major epidemics of viral encephalitis occurred in Asia in 1997 and 1998. The first was a re-emergence of neurovirulent strains of enterovirus 71, which caused severe encephalomyelitis in children in Malaysia, Taiwan and Japan, on a background of hand, foot and mouth disease. Necropsy studies of patients who died of enterovirus 71 infection showed severe perivascular cuffing, parenchymal inflammation and neuronophagia in the spinal cord, brainstem and diencephalon, and in focal areas in the cerebellum and cerebrum. Although no viral inclusions were detected, immunohistochemistry showed viral antigen in the neuronal cytoplasm. Inflammation was often more extensive than neuronal infection, suggesting that other factors, in addition to direct viral cytolysis, may be involved in tissue damage. The second epidemic of viral encephalitis was the result of a novel paramyxovirus called Nipah, which mainly involved pig handlers in Malaysia and Singapore. Pathological evidence suggested that the endothelium of small blood vessels in the central nervous system was particularly susceptible to infection. This led to disseminated endothelial damage and syncytium formation, vasculitis, thrombosis, ischaemia and microinfarction. However, there was also evidence of neuronal infection by the virus and this may also have contributed to the neurological dysfunction in Nipah encephalitis. Some patients who seemed to recover from the acute symptoms have been re-admitted with clinical findings suggestive of relapsing encephalitis. As these two epidemics indicate, the emergence and re-emergence of viral encephalitides continue to pose considerable challenges to the neuropathologist, in establishing the diagnosis and unravelling the pathogenesis of the neurological disease.

Morphological features of Murray Valley encephalitis virus infection in the central nervous system of Swiss mice

We have examined the histological and ultrastructural features of CNS infection with Murray Valley encephalitis (MVE) virus in mice inoculated with a virulent parental strain (BH3479). Light microscopic examination revealed neuronal necrosis in the olfactory bulb and hippocampus of MVE-infected brains by 5 days post-infection (pi). Electron microscopy of these regions showed endoplasmic reticulum membrane proliferation, and tubular and spherical structures in the cisternae of the endoplasmic reticulum, Golgi complex and nuclear envelope. At seven to eight days pi, infected neurones exhibited chromatin condensation and extrusion, nuclear fragmentation, loss of segments of the nuclear envelope, reduced surface contact with adjacent cells and loss of cytoplasmic organelles. This cell injury was particularly noticeable in the proximal CA3 and distal CA1 regions of the hippocampus. The inflammatory cell profile consisted of macrophages, lymphocytes and especially neutrophils, and many of these inflammatory cells were apoptotic. High mortality rates in the BH3479-infected population of mice correlated with the intense polymorphonuclear and mononuclear leucocyte inflammatory infiltrate in the CNS.

Electron microscopic diagnosis of human flavivirus encephalitis: use of confocal microscopy as an aid

The distinction between intracranial viral infections and inflammatory conditions requiring immunosuppression is important. Although specific laboratory reagents are readily available for some viruses, diagnosis of arbovirus infection is more difficult. Transmission electron microscopy (TEM) theoretically allows identification of viral particles independent of reagent availability, but it has limited sensitivity. We report two cases of human flavivirus encephalitis diagnosed by TEM. Laser scanning confocal microscopy (LSCM) was used in one case to survey unembedded tissue slices for focal abnormalities, from which fragments smaller than 1 mm2 were excised for epoxy embedding. This facilitated TEM identification of intracytoplasmic, budding, 35-40 nm spherical virus particles, confirmed by serology as St. Louis encephalitis. In contrast to mosquitoes and newborn mice, in which high viral loads are associated with minimal tissue responses, these biopsies showed florid angiodestructive inflammation and microgliosis, with rare virions in necrotic perivascular cells and astrocytes. To our knowledge, this represents the first ultrastructural study of St. Louis encephalitis in humans, indicating the potential value of LSCM-aided TEM.