Emerging Infections of CNS: Avian Influenza A Virus, Rift Valley Fever Virus and Human Parechovirus

Abstract

History is replete with emergent pandemic infections that have decimated the human population. Given the shear mass of humans that now crowd the earth, there is every reason to suspect history will repeat itself. We describe three RNA viruses that have recently emerged in the human population to mediate severe neurological disease. These new diseases are results of new mutations in the infectious agents or new exposure pathways to the agents or both. To appreciate their pathogenesis, we summarize the essential virology and immune response to each agent. Infection is described in the context of known host defenses. Once the viruses evade immune defenses and enter central nervous system (CNS) cells, they rapidly co-opt host RNA processing to a cataclysmic extent. It is not clear why the brain is particularly susceptible to RNA viruses; but perhaps because of its tremendous dependence on RNA processing for physiological functioning, classical mechanisms of host defense (eg, interferon disruption of viral replication) are diminished or not available. Effectiveness of immunity, immunization and pharmacological therapies is reviewed to contextualize the scope of the public health challenge. Unfortunately, vaccines that confer protection from systemic disease do not necessarily confer protection for the brain after exposure through unconventional routes.

Keywords: Rift Valley fever virus; avian influenza A virus; human parechovirus; infection; neuropathology; virus.

Figure 1

Diagram of influenza A virus infection of the cell. Free extracellular influenza A viruses (1) containing hemagglutinin on their envelopes bind sialylated glycoprotein receptors on the host cell surface (2). The virus enters the host cell by receptor‐mediated endocytosis (2,3). The resulting endosome becomes acidified by proton transport (3), allowing fusion of the host and viral membranes (4). Blocking acidification of the virion with small molecule drugs is one means of inhibiting viral replication. Fusion is required for metabolism of the nucleocapsid (uncoating) (5) and release of the viral ribonucleoprotein complex into host cell cytoplasm (6). The viral RNA and associated viral proteins (including three polymerases) are transported to the nucleus (7). From here, positive‐stranded mRNA templates with poly(A) tails are synthesized (with and without host splicing) at the expense of most host protein synthesis and exported to the cytoplasm for translation (8). Newly synthesized viral ribonucleoproteins are exported to the cytoplasm for eventual virion assembly (9). Influenza A virus assembles and buds from the cell surface in a polarized fashion (eg, from apical surface of epithelial cells).

Figure 2

H1N1  virus infection of human and ferret. A,B. Histopathology of the lung from a patient who succumbed to H1N1pdm09 virus infection. A. Hematoxylin & eosin (H&E)‐stained paraffin section demonstrates a severe bronchopneumonia. Necrotic debris (N) fills the lumen of a moderate size bronchiole. Surrounding alveolar tissue shows edema and severe inflammation. B. Differential interference contrast and in situ hybridization for influenza matrix protein RNA (black grains) demonstrates infected cells within the necrotic debris. At this late stage of infection, the immune response has cleared the majority of virus. C–E. Histopathology of lungs from ferrets inoculated with H1N1 virus intranasally. C. H&E‐stained paraffin section illustrates the severe bronchopneumonia at 5 days post‐infection (DPI). Necrotic debris (N) fills the lumen of a moderate size bronchiole. Surrounding alveolar tissue shows edema and severe inflammation. D. In situ hybridization for influenza matrix protein RNA (black grains) (counterstained with hematoxylin) shows infected cells in epithelial cells of the bronchi and alveoli at 3 DPI. More viruses are detected in the ferret lung suggesting the animal was sacrificed at an earlier stage of infection than when the human case died. By 8 DPI, no virus is detected in the ferret lung. E. In situ hybridization for influenza matrix protein RNA (black grains) (counterstained with hematoxylin) illustrates the severity of submucosal gland involvement as early as 1 DPI. F. The histopathology of small bowel from a ferret infected with H1N1 virus 14 days previously. In situ hybridization for influenza matrix protein RNA (black grains) (counterstained with hematoxylin) demonstrates infected cells within the lamina propria at a time when virus cannot be detected anywhere else systemically.

Figure 3

Distribution and quantitation of influenza infection in the ferret at different time points after infection. Throughout the time course of infection with H1N1pdm09 virus, viral‐infected cells are restricted to the respiratory tract except for a late chronic infection of the gut lamina propria. Infection with H5N1 virus (VN04) follows an entirely different course. While beginning in the lung, H5N1 virus infection quickly spreads to systemic organs. H5N1 virus can be detected in the liver by 2 days post‐infection (DPI) and as early as 4 DPI in the brain. At the terminal stage of infection (marked by X on the line chart), the vast majority of virus can be detected within the brain, while infection in the lung has begun to abate. Ferrets infected first with H1N1pdm09 or H3N2 virus (Vic11) followed by H5N1 virus (VN04) challenge 3 months later have different outcomes as well. Prior infection with H1N1pdm09 virus protects the ferret from H5N1 virus infection except for a late chronic infection of the gut lamina propria and liver. Prior infection with H3N2 virus leads to systemic spread of H5N1 virus to the brain and liver with lethal encephalitis by 6 DPI (marked by X on line chart).

Figure 4

H5N1  virus infection of the ferret. A,B. The histopathology of the lung from a ferret infected with H5N1 virus 5 days previously. A. Hematoxylin & eosin (H&E)‐stained paraffin section demonstrates a severe broncho‐ and alveolar pneumonia. B. Differential interference contrast and in situ hybridization for influenza matrix protein RNA (black grains) demonstrates infected alveolar cells in lower airway at 2 days post‐infection (DPI). C. Whole mount of the ferret brain 6 DPI with H5N1 virus hybridized with radioactive probes to influenza matrix protein RNA (black grains) demonstrates multifocal infection in olfactory cortex, cerebral cortex, deep gray nuclei and brainstem. D. In situ hybridization for influenza matrix protein RNA (black grains) (counterstained with hematoxylin) shows infected cells in liver surrounding intense inflammatory nodules. E. In situ hybridization for influenza matrix protein RNA (black grains) (counterstained with hematoxylin) illustrates infected cells in splenic red pulp at 18 DPI. F. Double label in situ hybridization for influenza matrix protein RNA (red) and immunohistochemistry for neurofilament (green) shows infection of neuronal elements.

Figure 5

Intraperitoneal (IP) infection of the mouse with pathogenic and nonpathogenic RVFV. IP infection of mice with nonpathogenic (MP12) RVFV leads to no significant weight loss over 4 days, while infection with pathogenic RVFV (ZH501) leads to rapid weight loss and lethal infection within 4 days. A,D. Gross photographs of livers of mice infected with MP12 and ZH501 strains of RVFV. A. MP12 infection shows no significant gross pathology at 4 days post‐infection (DPI), while livers of mice infected with pathogenic RVFV (ZH501) are pale in color (D). B,E. Hematoxylin & eosin (H&E)‐stained paraffin sections from livers of mice infected with MP12 and ZH501 strains of RVFV. B. Mice infected with nonpathogenic RVFV (MP12) show normal histology, while sections from livers of mice infected with pathogenic RVFV (ZH501) demonstrate widespread necrosis (E). C,F. Differential interference contrast and in situ hybridization for RVFV RNA (black grains) in liver. C. Mice infected with nonpathogenic RVFV MP12 show rare foci of parenchymal infection, while similar studies of mice infected with pathogenic RVFV ZH501 show multifocal necrosis of abundant viral‐infected hepatocytes (F).

Figure 6

Aerosol infection of non‐immunized and previously immunized mice with pathogenic RVFV. Non‐immunized mice show a severe hepatitis after aerosol exposure to ZH501 RVFV, while previously immunized mice show only mild hepatitis after aerosol exposure. A–E. Differential interference contrast and in situ hybridization for RVFV RNA (black grains). A. Non‐immunized mouse shows severe hepatic infection at 6 days post‐infection (DPI). Infection of the liver is delayed with aerosol infection compared with intraperitoneal infection. B. Mice immunized with alphavirus replicons expressing the Gn glycoprotein of RVFV show occasional infected foci in the liver 3 DPI. Virus is cleared from the liver by 6 DPI. C. Three days post‐infection after aerosol exposure to RVFV, enteric infection is observed in epithelial cells at the depths of small bowel crypts in mice immunized with alphavirus replicons expressing the Gn glycoprotein of RVFV. D,E. Despite modest systemic infection in mice previously immunized with DNA plasmids expressing Gn glycoprotein of RVFV fused to three copies of complement protein (C3d), aerosol exposure to RVFV ZH501 leads to lethal encephalitis 7–10 days later. D. The brain of terminally ill mice illustrates that the vast majority of neurons are infected. E. High power image of the hippocampus of (D).

Figure 7

HPeV3 infection of neonates. A. T1‐weighted non‐contrast magnetic resonance imaging (MRI) from an HPeV3‐infected infant. The child was healthy at birth but developed “neonatal sepsis” after exposure to an ill adult 30 days after delivery. Initial radiologic studies were normal; but after developing seizures, subsequent scans demonstrated cavitary deep white matter lesions. The infant died the following day. B. Gross coronal section of the infant's brain confirms the presence of deep‐seated periventricular cavitary lesions with associated hemorrhage. C. Hematoxylin & eosin (H&E)‐stained sections adjacent to the cavitary lesions demonstrate a bland gliosis with mineralization and no adaptive immune cell infiltration. D. Immunohistochemistry for glial fibrillary acidic protein confirms perilesional astrocytosis, whereas immunohistochemistry for CD68 confirms perilesional microglial activation (E).

Figure 8

HPeV3 infects blood vessel smooth muscle cells in leptomeninges and pulmonary vasculature. A,B. Paraffin‐embedded cerebellum and overlying leptomeninges probed for HPeV3 RNA using in situ hybridization (red) (counterstained with hematoxylin). Abundant HPeV3 viral RNA is confined to the modestly hypercellular leptomeninges with no evidence of infection of the brain parenchyma. Higher power (B) image of (A) confirms the presence of HPeV3 RNA in leptomeningeal cells and particularly in smooth muscle cells of blood vessel walls. C,D. Paraffin‐embedded lung probed for HPeV3 RNA using in situ hybridization (red) (counterstained with hematoxylin). HPeV3 RNAs are confined to the modestly hypercellular pulmonary arteries without evidence of lung parenchymal infection. Higher power (D) of (C) confirms the presence of HpeV3 RNA in smooth muscle cells of blood vessel walls. These observations suggest that damage noted in severe periventricular leukoencephalopathy is an indirect effect of vascular compromise to metabolically active regions.