In vitro selection and characterisation of influenza B/Beijing/1/87 isolates with altered susceptibility to zanamivir

We describe the in vitro selection and characterisation of virus derived from B/Beijing/1/87 passaged in the presence of zanamivir. During zanamivir passage, the phenotype of virus isolates was either drug dependent or drug resistant in plaque reduction assays. The susceptibility of the neuraminidase of the drug-dependent isolates was unchanged from that of the wild-type enzyme. The drug-dependent isolates contained two mutations in the viral haemagglutinin: V90A, close to the proposed secondary sialic acid-binding site, and L240Q, close to the primary sialic acid-binding site. Virus isolates that were drug resistant contained the same mutations in the haemagglutinin but also contained the mutation E116G in the neuraminidase. For the drug-dependent viruses, zanamivir susceptibility could not be measured because plaque numbers increased with increasing drug concentration. The in vitro zanamivir susceptibility of drug-resistant viruses was lower than that of the wild-type virus by a factor of 275- to >2532-fold. Neuraminidase containing the E116G mutation has a 33-fold lower affinity for zanamivir than the wild-type enzyme. The finding that the same haemagglutinin mutations are found in both drug-dependent and drug-resistant viruses confirms that the same changes to the receptor binding function can contribute to both phenotypes. This observation demonstrates the interplay between the influenza virus haemagglutinin and neuraminidase in escape from zanamivir inhibition in vitro.     

Neuraminidase inhibitor susceptibility of swine influenza A viruses isolated in Germany between 1981 and 2008

European swine influenza A viruses donated the matrix protein 2 as well as the neuraminidase (NA) gene to pandemic influenza A (H1N1) viruses that emerged in 2009. As a result, the latter became amantadine resistant and neuraminidase inhibitor (NAI) susceptible. These recent developments reflecting the close connection between influenza A virus infection chains in humans and pigs urge an antiviral surveillance within swine influenza A viruses. Here, NAI susceptibility of 204 serologically typed swine influenza A viruses of subtypes H1N1, H1N2, and H3N2 circulating in Germany between 1981 and 2008 was analyzed in chemiluminescence-based NA inhibition assays. Mean 50% inhibitory concentrations of oseltamivir and zanamivir indicate a good drug susceptibility of tested viruses. As found for human isolates, the oseltamivir and zanamivir susceptibility was subtype-specific. So, swine influenza A (H1N1) viruses were just as susceptible to oseltamivir as to zanamivir. In contrast, swine H1N2 and H3N2 influenza A viruses were more sensitive to oseltamivir than to zanamivir. Furthermore, reduction in plaque size and virus spread by both drugs was tested with selected H1N1 and H1N2 isolates in MDCK cells expressing similar amounts of α2.3- and α2.6-linked sialic acid receptors. Data obtained in cell culture-based assays for H1N1 isolates correlated with that from enzyme inhibition assays. But, H1N2 isolates that are additionally glycosylated at Asn158 and Asn163 near the receptor-binding site of hemagglutinin (HA) were resistant to both NAI in MDCK cells. Possibly, these additional HA glycosylations cause a misbalance between HA and NA function that hampers or abolishes NAI activity in cells.     

A new and rapid genotypic assay for the detection of neuraminidase inhibitor resistant influenza A viruses of subtype H1N1, H3N2, and H5N1

The neuraminidase of influenza viruses is the target of the inhibitors oseltamivir and zanamivir. Recent reports on influenza viruses with reduced susceptibility to neuraminidase inhibitors (NAI) are a cause for concern. Several amino acid substitutions, each as a consequence of one single nucleotide mutation, are known to confer resistance to NAI. An increase of NAI-resistant viruses appears to be likely as a result of a wider application of NAI for treatment and prophylaxis of seasonal influenza infections. Monitoring the occurrence and spread of resistant viruses is an important task. Therefore, RT-PCR assays were developed with subsequent pyrosequencing analysis (PSQ-PCR). These assays allow a rapid, high-throughput and cost-effective screening of subtype A/H1N1, A/H3N2, and A/H5N1 viruses. Various specimens such as respiratory swabs, allantoic fluid, or cell-propagated viruses can be used and results are available within hours. Several A/H1N1, A/H3N2, and A/H5N1 viruses isolated from human and avian specimens were tested to evaluate the method. Positive controls encoding resistance-associated mutations were created using site-directed mutagenesis. The results obtained with these controls showed that the assay can discriminate clearly the wild-type virus from a mutant virus. The detection limit of minor virus variants within the viral quasispecies amounts to 10%.     

In vitro activity of favipiravir and neuraminidase inhibitor combinations against oseltamivir-sensitive and oseltamivir-resistant pandemic influenza A (H1N1) virus

Few anti-influenza drugs are licensed in the United States for the prevention and therapy of influenza A and B virus infections. This shortage, coupled with continuously emerging drug resistance, as detected through a global surveillance network, seriously limits our anti-influenza armamentarium. Combination therapy appears to offer several advantages over traditional monotherapy in not only delaying development of resistance but also potentially enhancing single antiviral activity. In the present study, we evaluated the antiviral drug susceptibilities of fourteen pandemic influenza A (H1N1) virus isolates in MDCK cells. In addition, we evaluated favipiravir (T-705), an investigational drug with a broad antiviral spectrum and a unique mode of action, alone and in dual combination with the neuraminidase inhibitors (NAIs) oseltamivir, peramivir, or zanamivir, against oseltamivir-sensitive pandemic influenza A/California/07/2009 (H1N1) and oseltamivir-resistant A/Hong Kong/2369/2009 (H1N1) virus. Mean inhibitory values showed that the tested virus isolates remained sensitive to commonly used antiviral drugs, with the exception of the Hong Kong virus isolate. Drug dose-response curves confirmed complete drug resistance to oseltamivir, partial sensitivity to peramivir, and retained susceptibility to zanamivir and favipiravir against the A/Hong Kong/2369/2009 virus. Three-dimensional analysis of drug interactions using the MacSynergy^TM II program indicated an overall synergistic interaction when favipiravir was combined with the NAIs against the oseltamivir-sensitive influenza virus, and an additive effect against the oseltamivir-resistant virus. Although the clinical relevance of these drug combinations remains to be evaluated, results obtained from this study support the use of combination therapy with favipiravir and NAIs for treatment of human influenza virus infections.     

Evaluation of neuraminidase enzyme assays using different substrates to measure susceptibility of influenza virus clinical isolates to neuraminidase inhibitors: report of the neuraminidase inhibitor susceptibility network

The increasing use of influenza virus neuraminidase (NA) inhibitors (NIs) necessitates the development of reliable methods for assessing the NI susceptibility of clinical isolates. We evaluated three NA inhibition assays against a panel of five clinical isolates each of influenza virus A/H1N1, A/H3N2, and B strains and four viruses with a defined resistance genotype (R292K, H274Y, R152K, and E119V). For fluorometric enzyme assay (FA) 1 (FA-1), 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid (MUNANA) at 100 microM was used as the substrate, with pretitration of the virus input. For FA-2, MUNANA at 200 microM was used as the substrate, with a fixed 1:10 dilution of input virus. For the chemiluminescence (CL) assay, the 1,2-dioxetane derivative of sialic acid at 100 microM was used as the substrate, with pretitration of the virus. Four different operators repeated the assays several times in a blinded fashion with both zanamivir and oseltamivir carboxylate (GS4071) to determine intra- and interassay variations. Mean 50% inhibitory concentration (IC(50)) values were lower and generally less variable with the CL assay. FA-1 displayed greater variation than the CL assay or FA-2 and the highest IC(50) values with zanamivir; FA-2 showed the highest values with oseltamivir, particularly for influenza virus B, and was more variable with zanamivir than was the CL assay. All three assays detected 40-fold or greater changes in IC(50) values for the resistant viruses with at least one drug. Mixing experiments, whereby increasing fractions (0, 20, 40, 60, 80, and 100%) of NA from a known NI-resistant virus were mixed with the corresponding NI-sensitive parental NA, indicated that the resolution of IC(50) values was clearer with the CL assay than with FA-2 for two of the resistant variants (R152K and E119V). The FA and CL methods were reliable for the detection of NI resistance, but all assays have certain limitations. Based on reproducibility, ease of automation, time required for the assay, and greater sensitivity, the CL assay was selected for future susceptibility testing of influenza virus isolates circulating globally.     

Issues in pharmacotherapy of 2009 H1N1 influenza infection

The pandemic caused by the 2009 H1N1 influenza A virus has been a cause of great concern for healthcare professionals and the scientific community worldwide. Due to the widespread resistance of the virus to adamantanes, pharmacotherapy is currently limited to neuraminidase inhibitors, oseltamivir and zanamivir. The use of neuraminidase inhibitors in India is primarily associated with issues of patient and physician awareness, variability in disease management guidelines, safety and efficacy in the Indian population, need for active drug safety monitoring, and development of resistance due to possible misuse. In addition, other issues like availability of the drugs in retail and stockpiling by the public health authorities need careful introspection. The development of influenza vaccines in India and its adequate availability to the country's populace also poses significant challenges in the management of the pandemic. In light of the limited therapeutic options available for the management of the disease, research on novel targets and pharmacological agents would also be beneficial in addressing the challenges of future outbreaks.     

Surveillance of influenza isolates for susceptibility to neuraminidase inhibitors during the 2000-2002 influenza seasons

Neuraminidase (NA) inhibitors (NI) have recently been licensed for the prophylaxis and treatment of influenza virus infection in humans. This study has utilized a new chemiluminescent (CL) neuraminidase assay to routinely monitor more than a thousand influenza field isolates collected worldwide during the 2000-2002 seasons for susceptibility to both licensed NIs, zanamivir, and oseltamivir by determining the 50% inhibitory concentration (IC50). Our data demonstrated that influenza A viruses of the N2 subtype were less susceptible to zanamivir, but not oseltamivir, than those of the N1 subtype such that 41 of 45 confirmed H1N2 isolates could be reliably differentiated from H1N1 viruses based on their zanamivir susceptibility. Pre-titration of influenza A viruses appeared to have no effect on IC50 determined for either NI, while pre-titration of influenza B viruses significantly reduced oseltamivir IC50 and increased zanamivir IC50. Influenza B viruses were less susceptible to either compound than type A isolates. The CL assay is a rapid and reliable method for screening large numbers of influenza isolates for NI susceptibility. Reassortant viruses of the H1N2 subtype that started to circulate worldwide during the 2001-2002 season can be reliably separated from H1N1 viruses based on their zanamivir susceptibility, enabling large scale screening of H1 isolates for determining the prevalence of such reassortants.     

Recovery of influenza B virus with the H273Y point mutation in the neuraminidase active site from a human patient

The H275Y oseltamivir resistance mutation confers high-level resistance to oseltamivir in isolates of human A(H1N1) influenza. We report the recovery and identification of an influenza B virus with the H273Y neuraminidase point mutation directly from a human patient. The H273Y influenza B isolate is resistant to oseltamivir and peramivir but sensitive to zanamivir.     

Management of influenza in patients with asthma or chronic obstructive pulmonary disease.

OBJECTIVE: To review the prevention and treatment of influenza in patients with asthma and/or chronic obstructive pulmonary disease (COPD). DATA SOURCES: Computer-assisted MEDLINE searches for article and manual searches of conference proceedings on influenza, influenza vaccination, rimantadine, amantadine, oseltamivir, zanamivir, asthma, and/or COPD. STUDY SELECTION: Published articles and pertinent conference abstracts in the areas mentioned in Data sources were selected. Articles included for review were studies conducted on humans. RESULTS: Annual vaccination against influenza is the currently accepted practice for influenza management in patients with asthma and/or COPD. However, despite the availability and use of vaccination, influenza continues to cause serious morbidity and increased mortality. The management of influenza in at-risk patients with the older antivirals such as amantadine or rimantadine has not been widely accepted because of the rapid emergence of resistant variants, their lack of effect against influenza B, and poor adverse event profile. A new class of influenza antivirals, the neuraminidase inhibitors, has recently become available for the management of influenza. The currently marketed neuraminidase inhibitors are zanamivir and oseltamivir. Clinical studies have shown that these neuraminidase inhibitors are effective for the treatment and chemoprophylaxis of influenza A and B. CONCLUSIONS: Vaccination against influenza remains the gold standard for the prevention of influenza in patients with asthma and/or COPD. The neuraminidase inhibitors zanamivir and oseltamivir are useful adjuncts to influenza vaccines for the management of influenza in these patients who are at high-risk of developing influenza related complications.     

Molecular mechanisms of influenza virus resistance to neuraminidase inhibitors

A wide use of inhibitors of influenza virus neuraminidase (NAIs) to control influenza in humans demands a better understanding of the mechanisms involved in the resistance emergence. In vitro studies demonstrate that both neuraminidase (NA) and hemagglutinin (HA) influence virus susceptibility to NAIs. Drug resistance conferred due to changes in the NA could be monitored in the NA inhibition assays. Zanamivir-selected viruses acquired the NA substitutions at residues 119 and 292; oseltamivir-selected--at 274 and 292; peramivir-selected--at 292; and A-315675-selected--at 119. The HA binding efficiency and therefore susceptibility to NAIs are affected by the amino acids forming the HA receptor-binding site, the location and number of oligosaccharide chains, and structure of the neuraminic acid-containing cellular receptors. The lack of suitable cell culture-based assays hampers the assessment of virus susceptibility in humans. Emergence of the viruses with the NAI-induced substitutions in the NA is uncommon in drug-treated humans, however a compromised state of the immune system promotes emergence of drug resistance. In vivo, the zanamivir-selected mutant contained a substitution at 152 (B/NA); the oseltamivir-selected mutants-at residues 119 (A/N2), 198 (B/NA), 274 (A/N1), and 292 (A/N2). Substitutions in the NA were often accompanied by impairment of virus infectivity and virulence in animal models. Because of complexity of mechanisms of virus resistance, further analysis of the viruses recovered from the drug-treated humans is warranted.     

Functional balance between haemagglutinin and neuraminidase in influenza virus infections

Influenza A and B viruses carry two surface glycoproteins, the haemagglutinin (HA) and the neuraminidase (NA). Both proteins have been found to recognise the same host cell molecule, sialic acid. HA binds to sialic acid-containing receptors on target cells to initiate virus infection, whereas NA cleaves sialic acids from cellular receptors and extracellular inhibitors to facilitate progeny virus release and to promote the spread of the infection to neighbouring cells. Numerous studies performed recently have revealed that an optimal interplay between these receptor-binding and receptor-destroying activities of the surface glycoproteins is required for efficient virus replication. An existing balance between the antagonistic HA and NA functions of individual viruses can be disturbed by various events, such as reassortment, virus transmission to a new host, or therapeutic inhibition of neuraminidase. The resulting decrease in the viral replicative fitness is usually overcome by restoration of the functional balance due to compensatory mutations in HA, NA or both proteins.     

In vitro selection of influenza B viruses with reduced sensitivity to neuraminidase inhibitors

We and others have previously isolated influenza B viruses with reduced sensitivity to neuraminidase (NA) inhibitors (oseltamivir and zanamivir) from patients who were never exposed to these drugs. It was unclear whether the NA substitutions found in these influenza B isolates arose spontaneously or were caused by selective pressure. Here, we obtained influenza B viruses with reduced NA inhibitor sensitivity by in vitro selection with NA inhibitors. We found that these viruses possessed the same NA substitutions as those previously found in viruses isolated from untreated patients. These results suggest that these NA substitutions were selected in patients who were treated with an NA inhibitor and that the resistant variants were then transmitted to others.     

Intergenic HA-NA interactions in influenza A virus: postreassortment substitutions of charged amino acid in the hemagglutinin of different subtypes

In our previous studies influenza A virus reassortants having neuraminidase (NA) gene of A/USSR/90/77 (H1N1) strain and hemagglutinin (HA) genes of H3, H4 and H13 subtypes were shown to produce a low virus yield and to exhibit a strong tendency to virion aggregation. More detailed studies with the use of a H3N1 reassortant and its high-yield non-aggregating variants revealed that NA of A/USSR/90/77 strain is inefficient in the removal of the terminal sialic acid residues from the virion components, and that the inefficiency of NA may be compensated by mutations in HA gene leading to a decrease of the receptor-binding affinity (Kaverin, N.V. , Gambaryan, A.S., Bovin, N.V., Rudneva, I.A., Shilov, A.A., Khodova, O.M., Varich, N.L., Sinitsin, B.V., Makarova, N.L., Kaverin, N.V., 1998. Postreassortment changes in influenza virus hemagglutinin restoring HA-NA functional match, Virology 244, 315-321). The present report describes studies performed with the use of H2N1 and H4N1 reassortants having HA genes of A/Pintail/Primorie/695/76 (H2N3) and A/Duck/Czechoslovakia/56 (H4N6) strains respectively and NA gene of A/USSR/90/77 strain. The low-yield reassortants and their high-yield non-aggregating variants were studied in both direct and competitive binding assays with sialic acid-containing substrates. The non-aggregating variants were shown to have a decreased affinity as compared to the initial reassortants toward high-molecular-weight sialic acid-containing substrates. The sequencing of HA genes revealed that all non-aggregating variants of H2N1 and H4N1 reassortants had amino acid substitutions increasing the negative charge of the HA molecule in the vicinity of the receptor-binding pocket. The results suggest that the influenza virus reassortants containing low-functional NA undergo similar postreassortment changes irrespective of the HA subtype: their receptor-binding activity decreased due to negatively charged amino acid substitutions in the vicinity of the receptor-binding pocket.     

Attachment of influenza A virus to ferret tracheal epithelium at different maturational stages

Influenza virus attaches primarily to ciliated cells in mature airways epithelium. This process is mediated by a viral envelope glycoprotein (hemagglutinin) that binds to sialic acid-containing receptors in the apical membrane of host cells. The purpose of this study was to determine the cellular distribution of these receptors as a function of tracheal epithelial maturation in the ferret, which is susceptible to influenza virus infection at all ages and undergoes postnatal ciliation. To assay for virus attachment, tracheal strips from ferrets at ages 0, 7, 14, and 28 d were incubated at 4 degrees C for 1 h with a concentrated suspension of influenza A virus. Transmission electron microscopy demonstrated virus attachment to the apical surface of 77 to 87% of ciliated cells, but only to 1 to 9% of nonciliated surface epithelial cells at all ages, including the newborn, which has few ciliated cells (less than 10% of total cells). Virions also attached to most of the preciliated cells identified. Pretreatment of tracheal strips with neuraminidase virtually eliminated viral attachment. These findings demonstrate preferential influenza virus binding to sialylated receptors on ciliated cells and their immediate precursors. The sparsity of ciliated cells with no evidence for increased influenza virus binding per cell in newborn ferret tracheas suggests that the previously demonstrated high risk of death from influenza infection in newborn ferrets is due to factors other than increased susceptibility to virus attachment. Influenza virus receptors appear to be selective membrane markers for ciliated cells and may be particularly useful for the identification of preciliated cells.     

Prophylaxis and Treatment of Influenza Virus Infection

Influenza virus infections remain an important cause of morbidity and mortality. Furthermore, a recurrence of pandemic influenza remains a real possibility. There are now effective ways to both prevent and treat influenza. Prevention of infection is most effectively accomplished by vaccination. Vaccination with the inactivated, intramuscular influenza vaccine has been clearly demonstrated to reduce serious morbidity and mortality associated with influenza infection, especially in groups of patients at high risk (e.g. the elderly). However, the inactivated, intramuscular vaccine does not strongly induce cell-mediated or mucosal immune responses, and protection induced by the vaccine is highly strain specific. Live, attenuated influenza vaccines administered intranasally have been studied in clinical trials and shown to elicit stronger mucosal and cell-mediated immune responses. Live, attenuated vaccines appear to be more effective for inducing protective immunity in children or the elderly than inactivated, intramuscular vaccines. Additionally, novel vaccine methodologies employing conserved com-ponents of influenza virus or viral DNA are being developed. Preclinical studies suggest that these approaches may lead to methods of vaccination that could induce immunity against diverse strains or subtypes of influenza. Because of the limitations of vaccination, antiviral therapy continues to play an important role in the control of influenza. Two major classes of antivirais have demonstrated ability to prevent or treat influenza in clinical trials: the adaman-tanes and the neuraminidase inhibitors. The adamantanes (amantadine and rimantadine) have been in use for many years. They inhibit viral uncoating by blocking the proton channel activity of the influenza A viral M2 protein. Limitations of the adamantanes include lack of activity against influenza B, toxicity (especially in the elderly), and the rapid development of resistance. The neuraminidase inhibitors were designed to interfere with the conserved sialic acid binding site of the viral neuraminidase and act against both influenza A and B with a high degree of specificity when administered by the oral (oseltamivir) or inhaled (zanamivir) route. The neuraminidase inhibitors have relatively low toxicity, and viral resistance to these inhibitors appears to be uncommon. Additional novel antivirals that target other phases of the life cycle of influenza are in preclinical development. For example, recombinant collectins inhibit replication of influenza by binding to the viral haemagglutinin as well as altering phagocyte responses to the virus. Recombinant techniques have been used for generation of antiviral proteins (e.g. modified collectins) or oligonucleotides. Greater understanding of the biology of influenza viruses has already resulted in significant advances in the management of this important pathogen. Further advances in vaccination and antiviral therapy of influenza should remain a high priority.     

Isolation and characterization of influenza C virus inhibitor in rat serum

Two hemagglutination inhibitors for influenza C virus were isolated from pooled sera of normal rats by sequential chromatography on Blue Sepharose CL 6B, Ultrogel AcA 22, and DEAE-cellulose. The two inhibitors were identified as alpha 1-macroglobulin and murinoglobulin by comparison with the authentic samples. These inhibitors abolished the hemagglutination by influenza C virus strains but did not affect the hemagglutination by influenza A and B virus strains. Hemagglutination inhibition activity of both inhibitors was completely destroyed by incubation with influenza C virus at 37 degrees C but not with the other types of influenza virus, indicating that the inhibitors are specific for influenza C virus. The inhibitory activity was also destroyed by incubation with neuraminidase from Arthrobacter ureafaciens. By contrast, no activity was lost after treatment with neuraminidase from Vibrio cholerae. These results suggest that the sialic acid residue(s) which is cleavable by the former neuraminidase but not by the latter is essential for the hemagglutination inhibition. The two inhibitors were inactivated by treating with sodium hydroxide and methylamine but not with sodium metaperiodate.     

Utilization of the embryonated egg for in vivo evaluation of the anti-influenza virus activity of neuraminidase inhibitors

Previous studies have shown that embryonated egg provides a convenient and easy to use system for in vivo screening of anti-influenza virus inhibitors. However, it is not known whether this model is suitable for testing neuraminidase (NA) inhibitors, too. Therefore, the present study describes the evaluation of the ion-channel blockers amantadine and rimantadine in comparison with the NA inhibitors oseltamivir and zanamivir by using the influenza A virus hen’s egg model. The treatment was started immediately before or after the challenge dose was placed on the chorioallantoic membrane (CAM). Differences between the survival rate of treated and untreated chick embryos infected with influenza A virus were analyzed statistically. As result, the survival rate of chick embryos could be significantly increased when the treatment with amantadine, rimantadine, oseltamivir, or zanamivir was started before the CAM was inoculated with one egg infective dose 50% (EID₅₀) influenza A virus. When the drugs were administered shortly after viral inoculation, significant antiviral efficacy was shown for rimantadine, oseltamivir, and zanamivir. Antiviral efficacy could be demonstrated exclusively for both oseltamivir and zanamivir after the embryos were infected with higher challenge doses of 10² EID₅₀influenza A virus. In conclusion, the NA inhibitors oseltamivir and zanamivir have a significantly better antiviral activity against influenza A virus than amantadine and rimantadine tested in embryonated hen’s eggs. Therefore, this model can be a valuable alternative approach for in vivo pre-testing anti-influenza virus activity of NA inhibitors.     

Neuraminidase inhibitor resistance in influenza viruses

Zanamivir and oseltamivir, the currently marketed influenza virus neuraminidase inhibitors (NAIs), are prescribed for the treatment and prophylaxis of influenza and are being stockpiled for pandemic influenza. Oseltamivir resistance has been reported in up to 2% of patients in clinical trials of oseltamivir and in up to 18% of treated children. There are also reports in at least three patients treated with oseltamivir for influenza A (H5N1) infections. At this stage, there are no reports of resistance occurring to zanamivir in immunocompetent patients. Zanamivir and oseltamivir bind differently at the neuraminidase catalytic site and this contributes to different drug resistance profiles. The magnitude and duration of NAI concentrations at the site of infection are also expected to be important factors and are determined by route and timing of drug administration, dose, and pharmacokinetic differences between patients. In addition, the type, strain, and virulence of the influenza strain and the nature of the immune response all appear to play a role in determining the likelihood of drug resistance arising. The clinical significance of a particular NAI-resistant isolate from a patient is often not clear but virus viability and transmissibility are clearly important characteristics. Early initiation of NAI treatment in suspected cases of influenza is important for maximizing efficacy and minimizing the risk of drug resistance. Higher NAI doses and longer periods of treatment may be required for patients with influenza A (H5N1) infections but further work is needed in this area.     

Intérêts des inhibiteurs de la neuraminidase dans la prise en charge des infections dues aux virus influenza

Oseltamivir and zanamivir are two neuraminidase inhibitors (NAIs) active on A and B influenza viruses. These analogues have been developed from the structure of sialic acid, the neuraminidase (NA) substrate. Resistance to NAIs have been detected. They are mainly associated to mutations located on the NA gene. The use of these antiviral drugs remains low in the context of seasonal flu, even the duration of symptoms can be reduced of one day if an antiviral treatment is started within 48 hours after disease onset. NAIs also present a significant effect when used in postexposition prophylaxis. Resistance, mainly to oseltamivir, have been detected but remained rare until the spontaneous emergence in 2007–2008 winter of a seasonal A(H1N1) variant resistant to this drug. NAIs are also interesting for the treatment of severe flu infections, specially those associated to A(H5N1). Finally, because of the pandemic A(H1N1)2009 virus, NAIs use has largely increased for prophylactic and therapeutic treatment of severe and non severe infections. This large use may be associated to an increased risk of selection of resistant viruses. Up to now, this phenomenon remains fortunately limited but has to be closely monitored.