The invasion routes of neurovirulent A/Hong Kong/483/97 (H5N1) influenza virus into the central nervous system after respiratory infection in mice

Influenza A virus (H5N1) infection in cats causes systemic disease with potential novel routes of virus spread within and between hosts

The ongoing outbreak of avian influenza A virus (subtype H5N1) infection in Asia is of great concern because of the high human case fatality rate and the threat of a new influenza pandemic. Case reports in humans and felids suggest that this virus may have a different tissue tropism from other influenza viruses, which are normally restricted to the respiratory tract in mammals. To study its pathogenesis in a mammalian host, domestic cats were inoculated with H5N1 virus intratracheally (n = 3), by feeding on virus-infected chicks (n = 3), or by horizontal transmission (n = 2) and examined by virological and pathological assays. In all cats, virus replicated not only in the respiratory tract but also in multiple extra-respiratory tissues. Virus antigen expression in these tissues was associated with severe necrosis and inflammation 7 days after inoculation. In cats fed on virus-infected chicks only, virus-associated ganglioneuritis also occurred in the submucosal and myenteric plexi of the small intestine, suggesting direct infection from the intestinal lumen. All cats excreted virus not only via the respiratory tract but also via the digestive tract. This study in cats demonstrates that H5N1 virus infection causes systemic disease and spreads by potentially novel routes within and between mammalian hosts.

Avian influenza and the brain--comments on the occasion of resurrection of the Spanish flu virus

Recent incidences of direct passage of highly pathogenic avian influenza A virus strains of the H5N1 and H7N7 subtypes from birds to man have become a major public concern. Although presence of virus in the human brain has not yet been reported in deceased patients, these avian influenza subtypes have the propensity to invade the brain along cranial nerves to target brainstem and diencephalic nuclei following intranasal instillation in mice and ferrets. The associations between influenza and psychiatric disturbances in past epidemics are here commented upon, and the potentials of influenza to cause nervous system dysfunction in experimental infections with a mouse-neuroadapted WSN/33 strain of the virus are reviewed. This virus strain is closely related to the Spanish flu virus, which is characterized as a uniquely high-virulence strain of the H1N1 subtype. The Spanish flu virus has recently been reconstructed in the laboratory and it passed once, most likely, directly from birds to humans to cause the severe 1918-1919 pandemic.

Olfactory transmission of neurotropic viruses

Olfactory receptor neurons are unique in their anatomical structure and function. Each neuron is directly exposed to the external environment at the site of its dendritic nerve terminals where it is exposed to macromolecules. These molecules can be incorporated into by olfactory receptor neurons and transported transsynaptically to the central nervous system. Certain neurotropic pathogens such as herpes simplex virus and Borna disease virus make use of this physiological mechanism to invade the brain. Here the authors review the olfactory transmission of infectious agents and the resulting hazards to human and animal health.

Lethality to ferrets of H5N1 influenza viruses isolated from humans and poultry in 2004

The 2004 outbreaks of H5N1 influenza viruses in Vietnam and Thailand were highly lethal to humans and to poultry; therefore, newly emerging avian influenza A viruses pose a continued threat, not only to avian species but also to humans. We studied the pathogenicity of four human and nine avian H5N1/04 influenza viruses in ferrets (an excellent model for influenza studies). All four human isolates were fatal to intranasally inoculated ferrets. The human isolate A/Vietnam/1203/04 (H5N1) was the most pathogenic isolate; the severity of disease was associated with a broad tissue tropism and high virus titers in multiple organs, including the brain. High fever, weight loss, anorexia, extreme lethargy, and diarrhea were observed. Two avian H5N1/04 isolates were as pathogenic as the human viruses, causing lethal systemic infections in ferrets. Seven of nine H5N1/04 viruses isolated from avian species caused mild infections, with virus replication restricted to the upper respiratory tract. All chicken isolates were nonlethal to ferrets. A sequence analysis revealed polybasic amino acids in the hemagglutinin connecting peptides of all H5N1/04 viruses, indicating that multiple molecular differences in other genes are important for a high level of virulence. Interestingly, the human A/Vietnam/1203/04 isolate had a lysine substitution at position 627 of PB2 and had one to eight amino acid changes in all gene products except that of the M1 gene, unlike the A/chicken/Vietnam/C58/04 and A/quail/Vietnam/36/04 viruses. Our results indicate that viruses that are lethal to mammals are circulating among birds in Asia and suggest that pathogenicity in ferrets, and perhaps humans, reflects a complex combination of different residues rather than a single amino acid difference.

Gene expression changes in brains of mice exposed to a maternal virus infection

In this study, we tested the hypothesis that exposure to a maternal infection during fetal life can lead to the appearance of alterations in the brain later in life. C57BL/6 mice were infected intranasally with influenza A/WSN/33 virus on day 14 of gestation. The levels of transcripts encoding neuroleukin and fibroblast growth factor 5 were significantly elevated in the brains of the virus-exposed offspring at 90 and 280 days of age, but not at earlier time-points. For neuroleukin, this difference could also be observed at the protein level. Thus, a maternal influenza A virus infection can give rise to alterations in gene expression in the brain that become apparent only after a prepubertal latency period.

Maternal influenza infection is likely to alter fetal brain development indirectly: the virus is not detected in the fetus

Epidemiological studies have shown that maternal infection can increase the risk for mental illness in the offspring. In a mouse model of maternal respiratory infection with influenza virus, the adult offspring display striking behavioral, pharmacological and histological abnormalities. Although influenza primarily infects the respiratory system, there are reports of viral mRNA and protein in the fetus of infected pregnant animals. To determine the extent of viral spread following maternal respiratory infection, we used RT-PCR to assay various maternal and fetal tissues for influenza A mRNAs coding for neuraminidase, non-structural protein 2, nuclear protein and matrix protein. While infected maternal lungs exhibit uniformly very strong signals, placentae are only rarely positive, and viral RNAs are not detectable in fetal brains from infected mothers. Thus, the effects of maternal infection on fetal brain development are likely to be indirect, probably involving the maternal inflammatory response.

In vitro demonstration of neural transmission of avian influenza A virus

Neural involvement following infections of influenza viruses can be serious. The neural transport of influenza viruses from the periphery to the central nervous system has been indicated by using mouse models. However, no direct evidence for neuronal infection has been obtained in vitro and the mechanisms of neural transmission of influenza viruses have not been reported. In this study, the transneural transmission of a neurotropic influenza A virus was examined using compartmentalized cultures of neurons from mouse dorsal root ganglia, and the results were compared with those obtained using the pseudorabies virus, a virus with well-established neurotransmission. Both viruses reached the cell bodies of the neurons via the axons. This is the first report on axonal transport of influenza A virus in vitro. In addition, the role of the cytoskeleton (microtubules, microfilaments and intermediate filaments) in the neural transmission of influenza virus was investigated by conducting cytoskeletal perturbation experiments. The results indicated that the transport of avian influenza A virus in the neurons was independent of microtubule integrity but was dependent on the integrity of intermediate filaments, whereas pseudorabies virus needed both for neural spread.

Influenza A virus infection causes alterations in expression of synaptic regulatory genes combined with changes in cognitive and emotional behaviors in mice

Epidemiological studies have indicated a link between certain neuropsychiatric diseases and exposure to viral infections. In order to examine long-term effects on behavior and gene expression in the brain of one candidate virus, we have used a model involving olfactory bulb injection of the neuro-adapted influenza A virus strain, WSN/33, in C57Bl/6 mice. Following this olfactory route of invasion, the virus targets neurons in the medial habenular, midline thalamic and hypothalamic nuclei as well as monoaminergic neurons in the brainstem. The mice survive and the viral infection is cleared from the brain within 12 days. When tested 14-20 weeks after infection, the mice displayed decreased anxiety in the elevated plus-maze and impaired spatial learning in the Morris water maze test. Elevated transcriptional activity of two genes encoding synaptic regulatory proteins, regulator of G-protein signaling 4 and calcium/calmodulin-dependent protein kinase IIalpha, was found in the amygdala, hypothalamus and cerebellum. It is of particular interest that the gene encoding RGS4, which has been related to schizophrenia, showed the most pronounced alteration. This study indicates that a transient influenza virus infection can cause persistent changes in emotional and cognitive functions as well as alterations in the expression of genes involved in the regulation of synaptic activities.

Fatal avian influenza A (H5N1) in a child presenting with diarrhea followed by coma

In southern Vietnam, a four-year-old boy presented with severe diarrhea, followed by seizures, coma, and death. The cerebrospinal fluid contained 1 white cell per cubic millimeter, normal glucose levels, and increased levels of protein (0.81 g per liter). The diagnosis of avian influenza A (H5N1) was established by isolation of the virus from cerebrospinal fluid, fecal, throat, and serum specimens. The patient's nine-year-old sister had died from a similar syndrome two weeks earlier. In both siblings, the clinical diagnosis was acute encephalitis. Neither patient had respiratory symptoms at presentation. These cases suggest that the spectrum of influenza H5N1 is wider than previously thought.

Effects on synaptic activity in cultured hippocampal neurons by influenza A viral proteins

Certain viruses can infect neurons and cause persistent infections with restricted expression of viral proteins. To study the consequences of such viral proteins on synaptic functions, the effects of two influenza A virus proteins, the nonstructural protein 1 (NS1) and the nucleoprotein (NP), were analyzed in cultures of rat hippocampal neurons. Transduction of the NS1 and NP proteins into the neurons was performed by applying the 11-amino acid peptide transduction domain (PTD) of human immunodeficiency virus (HIV) TAT coupled to the viral proteins. Neurons exposed to the NS1 and NP fusion proteins (NS1-PTD and NP-PTD, respectively) for 4 h were immunopositive for these proteins as diffuse cytoplasmic and nuclear distribution. After exposure for 48 h to NP-PTD, a punctate pattern of the immunolabel appeared in dendritic spinelike processes. Electrophysiologically, a reduction in both the frequency of spontaneous excitatory synaptic activity and in the amplitude of the miniature excitatory postsynaptic currents were recorded after exposing the hippocampal neurons to NP-PTD between 17 and 22 days in culture. These changes may reflect disturbances in postsynaptic functions. No such alterations in synaptic activities were recorded after exposure to NS1-PTD or to green fluorescent protein-PTD, which was used as a control. Based on these findings the authors hypothesize that the viral NP, by its localization to dendritic spinelike structures, interferes with the expression or anchoring of postsynaptic glutamate receptors and thereby disturbs synaptic functions. Thus a persistent viral infection in the brain may be associated with functional disturbances at the synaptic level.

Productive infection in the murine central nervous system with avian influenza virus A (H5N1) after intranasal inoculation

The H5N1 type of influenza A virus isolated from human patients in 1997 has a characteristic hemagglutinin and was considered to be directly transmitted from birds. Although neuropathogenicity of this virus was not demonstrated in human autopsy cases, some experimental studies using mice have disclosed that this virus infects the central nervous system (CNS) after intranasal inoculation. In this study we focused on the topographical localization of virus-infected cells in the murine CNS after intranasal inoculation. We immunohistochemically examined virus-infected cells in mouse tissues using a rabbit antiserum recognizing the nucleoprotein of influenza A virus. The virus-infected cells appeared initially in the respiratory tract. Thereafter, the virus antigen-positive cells appeared in the olfactory system and the cranial nerve nuclei innervating the facial region. This suggests that this virus is principally transmitted from the nasal cavity to CNS through the cranial nerves. Neurons were frequently infected and glial and ependymal cells were also infected. Transneuronal transmission of the virus might play the important role of viral spread within the CNS.

The vagus nerve is one route of transneural invasion for intranasally inoculated influenza a virus in mice

Intranasally inoculated neurotropic influenza viruses in mice infect not only the respiratory tract but also the central nervous system (CNS), mainly the brain stem. Previous studies suggested that the route of invasion of virus into the CNS was via the peripheral nervous system, especially the vagus nerve. To evaluate the transvagal transmission of the virus, we intranasally inoculated unilaterally vagectomized mice with a virulent influenza virus (strain 24a5b) and examined the distribution of the viral protein and genome by immunohistochemistry and in situ hybridization over time. An asymmetric distribution of viral antigens was observed between vagal (nodose) ganglia: viral antigen was detected in the vagal ganglion of the vagectomized side 2 days later than in the vagal ganglion of the intact side. The virus was apparently transported from the respiratory mucosa to the CNS directly and decussately via the vagus nerve and centrifugally to the vagal ganglion of the vagectomized side. The results of this study, thus, demonstrate that neurotropic influenza virus travels to the CNS mainly via the vagus nerve.

[A case of Influenza virus encephalitis in south of France]

INTRODUCTION

Influenza virus outbreaks occur each year, in France, during autumn and winter. Influenza-associated acute encephalitis were reported during epidemics or pandemics. Sporadic cases are rarely identified probably because influenza virus is not searched among etiology of febrile encephalitis.

EXEGESIS

We report a case of influenza-associated encephalitis complicated by adrenal insufficiency in a young woman. Diagnosis was based on seroconversion of serum influenza virus A antibodies (complement fixation test). Follow up of the patient showed a total recovery.

CONCLUSION

Influenza must be searched for any febrile encephalitis occurring during winter. Reverse transcriptase polymerase chain reaction (RT-PCR) on cerebrospinal fluid should be assessed. It is not actually a routine technique and we do not know yet if it is accurate enough for diagnosis. So, it is important to identify influenza virus and obtain documentary evidence concerning neurological impairment. Nevertheless, a better understanding of pathogenesis and use of vaccination are needed to improve prognosis.

Persistence of viral RNA segments in the central nervous system of mice after recovery from acute influenza A virus infection

One-hundred thirty-seven BALB/c mice were intranasally inoculated with neurotropic avian influenza A virus (H5N3). Thirty-nine of these mice died within 16 days post-inoculation (PID) and 98 of the mice recovered from the infection. To investigate whether viral antigens and genomes persist in the central nervous system (CNS) of recovered mice, immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) methods were performed. Histopathologically, mild interstitial pneumonia and non-suppurative encephalomyelitis restricted to the basal part of the frontal lobe of the cerebrum, brain stem and thoracic spinal cord were observed in BALB/c mice until 40 PID. Small amounts of viral antigens were detected in the brain and spinal cord and some viral RNA segments (NA, NP, M, PA, HA, NS, PB1) were intermittently detected in the CNS until 48 PID. Immunosuppression of these mice by dexamethazone (DEX) treatment did not increase the frequency of detection of the lesions, viral antigens or genomes. These findings suggest that viral genomes of neurovirulent influenza virus persist with restricted transcriptive activity in the CNS of the mice even after clinical recovery from the infection.

Experimental models of pulmonary infection

Experimental models of pulmonary infection are being discussed, focused on various aspects of good experimental design, such as choice of animal species and infecting strain, and route of infection/inoculation techniques (intranasal inoculation, aerosol inoculation, and direct instillation into the lower respiratory tract). In addition, parameters to monitor pulmonary infection are being reviewed such as general clinical signs, pulmonary-associated signs, complication of the pulmonary infection, mortality rate, and parameters after dissection of animals. Examples of pulmonary infection models caused by bacteria, fungi, viruses or parasites in experimental animals with intact or impaired host defense mechanisms are shortly summarized including key-references.

Invasion and persistence of the neuroadapted influenza virus A/WSN/33 in the mouse olfactory system

Invasion and persistence of the neuroadapted influenza virus A/WSN/33 in the mouse olfactory system was studied. WSN/33 instilled intranasally infected neurons in the olfactory epithelium and was transported in axons to the olfactory bulbs in wild type mice that survived the infection. In adult mice lacking the recombination activating gene 1 (RAG-1-/-), infected neurons occurred in the olfactory bulbs for 22-65 days after which the mice developed a rapidly progressive lethal infection affecting neurons in olfactory projection pathways, i.e. primary olfactory cortex, raphe in upper brainstem and hypothalamus. Adult mice without genes for interferon (IFN)-alpha/beta receptor, IFN-gamma receptor, inducible nitric oxide synthase (iNOS), IgH, the transporter associated with antigen processing 1 (TAP1), and natural killer cell-depleted mice, all survived the infection. Viral RNA was found in the olfactory bulbs in more than 80 per cent of the surviving iNOS-/-, IFN-gamma receptor-/-, and TAP1-/- mice. Taken together, this study shows that influenza A virus can invade the brain through the olfactory pathways and that the cellular immune responses prevent establishment of persistent infections in the olfactory bulbs. Furthermore, innate responses in olfactory bulbs may for a period of time keep the infection under control.