A recombinant influenza A virus expressing an RNA-binding-defective NS1 protein induces high levels of beta interferon and is attenuated in mice

Previously we found that the amino-terminal region of the NS1 protein of influenza A virus plays a key role in preventing the induction of beta interferon (IFN-beta) in virus-infected cells. This region is characterized by its ability to bind to different RNA species, including double-stranded RNA (dsRNA), a known potent inducer of IFNs. In order to investigate whether the NS1 RNA-binding activity is required for its IFN antagonist properties, we have generated a recombinant influenza A virus which expresses a mutant NS1 protein defective in dsRNA binding. For this purpose, we substituted alanines for two basic amino acids within NS1 (R38 and K41) that were previously found to be required for RNA binding. Cells infected with the resulting recombinant virus showed increased IFN-beta production, demonstrating that these two amino acids play a critical role in the inhibition of IFN production by the NS1 protein during viral infection. In addition, this virus grew to lower titers than wild-type virus in MDCK cells, and it was attenuated in mice. Interestingly, passaging in MDCK cells resulted in the selection of a mutant virus containing a third mutation at amino acid residue 42 of the NS1 protein (S42G). This mutation did not result in a gain in dsRNA-binding activity by the NS1 protein, as measured by an in vitro assay. Nevertheless, the NS1 R38AK41AS42G mutant virus was able to replicate in MDCK cells to titers close to those of wild-type virus. This mutant virus had intermediate virulence in mice, between those of the wild-type and parental NS1 R38AK41A viruses. These results suggest not only that the IFN antagonist properties of the NS1 protein depend on its ability to bind dsRNA but also that they can be modulated by amino acid residues not involved in RNA binding.

[Antiviral activity of oral ultralow doses of antibodies to gamma-interferon: experimental study of influenza infection in mice]

Course intragastric administration of ultralow doses of human gamma-interferon antibodies (ULD anti-IFN-gamma) to intact mice resulted in an increase of endogenous IFN-gamma production by the animal lymphocytes. Oral prophylactic administration of ULD anti-IFN-gamma significantly lowered the influenza virus concentration in the animal lungs at the initial stage of the aerogenous infection: in 2 (p = 0.05) and 3 (p = 0.07) days after the contamination. The therapeutic antiviral effect of ULD anti-IFN-gamma in mice with influenza was evident from a significant decrease of the influenza virus concentration in the lungs of the animals on the 4th (p = 0.05) and 5th (p = 0.07) days after the contamination. The antiviral effect of ULD anti-IFN-gamma after the prophylactic and therapeutic use is likely provided by induction of endogenous IFN-gamma.

[Pulmonary cell susceptibility in mice and rats to influenza virus when infected in vivo and in vitro]

The purpose of the case study was to evaluate comparatively the relative contribution of cell susceptibility and the inhibiting effect of factors of pulmonary epithelial lining in mice and rats to influenza virus A/Aichi/2/68 (H3N2) adapted to mice as related with the development of infection process in the lungs of experimental animals when infected in vivo and in vitro. Mice and rats were infected aerogenically with different doses of influenza virus. The primary cell-culture suspensions sampled from the lungs of mice and rats were used to study the adsorption and dynamics of influenza virus production in infection by different dose of influenza virus in vitro. The cell suspensions were shown to be able to produce the influenza virus for as long as 48 hours after infection. It was for the first time that the results denoted the identical susceptibility of primary pulmonary cells in mice and rats to influenza virus. A lower pulmonary susceptibility to influenza virus in rats versus mice could be indicative of that the surface factors of epithelial lining contribute essentially to shaping the pulmonary susceptibility to influenza virus since there is no difference of the susceptibility of pulmonary cells to influenza virus between the two above animals' species.

Mechanisms of pathogenicity of influenza A (H5N1) viruses in mice

Avian-like H5N1 influenza viruses isolated from humans in 1997 were shown to have two distinct pathogenic phenotypes in BALB/c mice, after intranasal inoculation and without prior adaptation to this host. To further understand the mechanisms of H5N1 pathogenicity, we investigated the consequences of the mute of viral inoculation on morbidity and mortality, viral replication in pulmonary and systemic organs, and lymphocyte depletion. This study demonstrates the importance of extrapulmonary spread and replication, particularly in the brain, for the lethality of H5N1 viruses.

A primate model to study the pathogenesis of influenza A (H5N1) virus infection

Cynomolgus macaques (Macaca fascicularis) infected with influenza virus A/HongKong/156/97 (H5N1) developed acute respiratory distress syndrome (ARDS) with fever. Reverse transcriptase/polymerase chain reaction (RT/PCR) and virus isolation showed that the respiratory tract is the major target of the virus. The main lesion observed upon necropsy, performed 4 or 7 days postinfection, was a necrotizing bronchointerstitial pneumonia, similar to that found in primary influenza pneumonia in human beings. By immunohistochemistry, influenza virus antigen proved to be limited to pulmonary tissue and tonsils. The data indicate that ARDS and multiple organ dysfunction syndrome (MODS), observed in both humans and monkeys infected with this virus, are caused by diffuse alveolar damage from virus replication in the lungs alone.

Emergence and maintenance of infectious salmon anaemia virus (ISAV) in Europe: a new hypothesis

The present study describes the use of molecular methods in studying infectious salmon anaemia virus (ISAV), an important pathogen of farmed salmon in Norway, Scotland, the Faeroe Islands, Canada, USA and Chile. The nucleotide sequences of the haemagglutinin gene (HA) from 70 ISAV isolates have been analysed for phylogenetic relationship and the average mutation rate of nucleotide substitutions calculated. The isolates constitute 2 major groups, 1 European and 1 North American group. The isolate from Chile is closely related to the North American isolates. The European isolates can be further divided into 3 separate groups reflecting geographical distribution, time of collection, and transmission connected with farming activity. Based on existing information about infectious salmon anaemia (ISA) and new information emerging from the present study, it is hypothesised that: (1) ISAV is maintained in wild populations of trout and salmon in Europe; (2) it is transmitted between wild hosts mainly during their freshwater spawning phase in rivers; (3) wild salmonids, mainly trout, possibly carry benign wild-type ISAV isolates; (4) a change (mutation) in virulence probably results from deletions of amino acid segments from the highly polymorphic region (HPR) of benign wild-type isolates; (5) ISA emerges in farmed Atlantic salmon when mutated isolates are transmitted from wild salmonids or, following mutation of benign isolates, in farmed salmon after transmission from wild salmonids; (6) farming activity is an important factor in transmission of ISAV between farming sites in addition to transmission of ISAV from wild salmonids to farmed salmon; (7) transmission of ISAV from farmed to wild salmonids probably occurs less frequently than transmission from wild to farmed fish due to lower frequency of susceptible wild individuals; (8) the frequency of new outbreaks of ISA in farmed salmon probably reflects natural variation in the prevalence of ISAV in wild populations of salmonids.

Visualizing infection of individual influenza viruses

Influenza is a paradigm for understanding viral infections. As an opportunistic pathogen exploiting the cellular endocytic machinery for infection, influenza is also a valuable model system for exploring the cell's constitutive endocytic pathway. We have studied the transport, acidification, and fusion of single influenza viruses in living cells by using real-time fluorescence microscopy and have dissected individual stages of the viral entry pathway. The movement of individual viruses revealed a striking three-stage active transport process that preceded viral fusion with endosomes starting with an actin-dependent movement in the cell periphery, followed by a rapid, dynein-directed translocation to the perinuclear region, and finally an intermittent movement involving both plus- and minus-end-directed microtubule-based motilities in the perinuclear region. Surprisingly, the majority of viruses experience their initial acidification in the perinuclear region immediately following the dynein-directed rapid translocation step. This finding suggests a previously undescribed scenario of the endocytic pathway toward late endosomes: endosome maturation, including initial acidification, largely occurs in the perinuclear region.

Alterations in EEG activity and sleep after influenza viral infection in GHRH receptor-deficient mice

Viral infections induce excess non-rapid eye movement sleep (NREMS) in mice. Growth hormone-releasing hormone receptor (GHRH receptor) was previously identified as a candidate gene responsible for NREMS responses to influenza challenge in mice. The dwarf lit/lit mouse with a nonfunctional GHRH receptor was used to assess the role of the GHRH receptor in viral-induced NREMS. After influenza A virus infection the duration and intensity [electroencephalogram (EEG) delta power] of NREMS increased in heterozygous mice with the normal phenotype, whereas NREMS and EEG delta power decreased in homozygous lit/lit mice. Lit/lit mice developed a pathological state with EEG slow waves and enhanced muscle tone. Other influenza-induced responses (decreases in rapid eye movement sleep, changes in the EEG high-frequency bands during the various stages of vigilance, hypothermia, and decreased motor activity) did not differ between the heterozygous and lit/lit mice. GH replacement failed to normalize the NREMS responses in the lit/lit mice after influenza inoculation. Decreases in NREMS paralleled hypothermia in the lit/lit mice. Lung virus levels were similar in the two mouse strains. Lit/lit mice had a higher death rate after influenza challenge than the heterozygotes. In conclusion, GHRH signaling is involved in the NREMS response to influenza infection.

Quantitative analysis of long-term virus-specific CD8+-T-cell memory in mice challenged with unrelated pathogens

The consequences for the long-term maintenance of virus-specific CD8+-T-cell memory have been analyzed experimentally for sequential respiratory infections with readily eliminated (influenza virus) and persistent (gammaherpesvirus 68 [gammaHV68]) pathogens. Sampling a broad range of tissue sites established that the numbers of CD8+ T cells specific for the prominent influenza virus D(b)NP(366) epitope were reduced by about half in mice that had been challenged 100 days previously with gammaHV68, though the prior presence of a large CD8+ D(b)NP366+ population caused no selective defect in the gammaHV68-specific CD8+ K(b)p79+ response. Conversely, mice that had been primed and boosted to generate substantial gammaHV68-specific CD8+ D(b)p56+ populations did not show any decrease in prevalence for this set of CD8+ memory cytotoxic T lymphocytes (CTL) at 200 days after respiratory exposure to an influenza A virus. However, in both experiments, the total magnitude of the CD8+-T-cell pool was significantly diminished in those that had been infected with gammaHV68 and the influenza A virus. The broader implications of these findings, especially under conditions of repeated exposure to unrelated pathogens, are explored with a mathematical model which emphasizes that the immune effector and memory "phenome" is a function of the overall infection experience of the individual.

Influenza type A in humans, mammals and birds: determinants of virus virulence, host-range and interspecies transmission

The virulence of a virus is determined by its ability to adversely affect the host cell, host organism or population of host organisms. Influenza A viruses have been responsible for four pandemics of severe human respiratory disease this century. Avian species harbour a large reservoir of influenza virus strains, which can contribute genes to potential new pandemic human strains. The fundamental importance of understanding the role of each of these genes in determining virulence in birds and humans was dramatically emphasised by the recent direct transmission of avian influenza A viruses to humans, causing fatal infection but not community spread. An understanding of the factors involved in transmission between avian and mammalian species should assist in the development of better surveillance strategies for early recognition of influenza A virus strains having human pandemic potential, and possibly in the design of anti-viral strategies.

[Host cellular proteases trigger the infectivity of the influenza virus in the airway and brain]

The pathogenesis of the influenza and Sendai viruses is primarily determined by host cellular trypsin-type processing proteases that activate viral fusion activity and infectivity. We isolated three secretory trypsin-type proteases from rat lungs, such as tryptase Clara, mini-plasmin, and ectopic anionic trypsin, candidates for the processing proteases of viral envelope glycoproteins. These enzymes specifically cleave the precursor of fusion glycoprotein hemagglutinin (HA) of influenza virus at Arg(325) and the F(0) of Sendai virus at Arg(116) in the consensus cleavage motif, Gln(Glu)-X-Arg, resulting in the induction of infectivity of these viruses. These proteases show different localization in the airway and susceptibility for the processing of various subtypes of influenza virus HA, suggesting that these processing proteases determine the viral pathogenicity. Influenza virus readily infects and replicates in the airway epithelial cells but occasionally replicates in the central nervous system, particularly in children below 5-6 years of age and Reye's syndrome patients. We found an invasion by a non-neurovirulent influenza virus in cerebral capillaries with progressive brain edema of mice having impaired mitochondrial fatty acid metabolism congenitally or posteriorly in the newborn period. In the brain of these mice, mini-plasmin, which potentiates viral-multiplication in vivo and destroys the blood-brain barrier, accumulated with virus antigen in the brain capillaries but only a little in the control mice without impaired mitochondrial fatty acid metabolism.

Diagnostic methods applied to analysis of an outbreak of equine influenza in a riding school in which vaccine failure occurred

An outbreak of equine influenza H3N8 in a riding school is described retrospectively with emphasis on diagnosis and putative vaccine failure. In March 1995 an outbreak of equine influenza occurred among 11 horses in a riding school, where most horses had received basic primary immunizations and several booster vaccinations against influenza. Six of the 11 diseased horses had received their last booster vaccination within 5 months of the outbreak. Nevertheless, the influenza infection spread rapidly and clinical manifestations were prominent with frequent, harsh, dry coughing often accompanied by high fever. Nasal swabs were taken from 11 diseased horses. Influenza A virus of the equine H3N8 (equi-2) subtype was isolated from five nasal swab extracts. Stored nasal swab extracts were also retrospectively investigated in two different enzyme immunoassays designed to detect the type-specific conserved nucleoprotein of influenza A viruses, and in a single-tube reverse transcription-PCR (RT-PCR) using a set of primers based on highly conserved regions of the matrix gene of influenza A viruses. Five nasal swab extracts were found positive in a DAS-ELISA and seven in the Directigen((R)) Flu A (DFA) assay, respectively. Two nasal swab extracts from which virus was isolated did not give a positive result in the DAS-ELISA, and one of these also did not give a positive result in the DFA assay. Nine nasal swab extracts were found positive by RT-PCR. Moreover, all virus isolation and/or ELISA positive nasal swab extracts were confirmed by RT-PCR. Three nasal swab extracts were negative by virus isolation, PCR and ELISA. A significant rise in HI titre against influenza A/eq/Miami/63 (H3N8) virus was detected in seven of the nine paired sera available. In acute phase serum samples from 10 horses, SRH antibody levels varied widely. However, some horses with high, or at least putatively clinically protective SRH antibody levels, showed clinical signs and infection was confirmed. Antigenic analysis of two isolates showed that A/eq/Holland/1/95 (H3N8) and A/eq/Holland/2/95 (H3N8) cluster with the UK isolate Osgodsby/92, the Swedish isolate Borlänge/91 and some other European isolates, with H/2/95 identical in reactivity to Borlänge/91 and H/1/95 more similar in reactivity to Osgodsby/92 than H/2/95. Nucleotide and deduced amino-acid sequences showed large differences of both isolates as compared with Miami/63, Fontainebleau/79 and Kentucky/81, the influenza A H3N8 subtype strains incorporated in the vaccines used in this riding school. The role of antigenic drift in vaccine breakdown is discussed in the light of evidence for vaccine breakdown in the UK in 1989, Sweden in 1991 and in the USA since 1991.

The classification of viruses infecting the respiratory tract

Following the boom in respiratory virology in the 1960s, species of rhinoviruses, coronaviruses, enteroviruses, adenoviruses, parainfluenza viruses and respiratory syncytial virus were added to influenza and measles viruses as causes of respiratory tract infection. In restricted patient groups, such as the immunocompromised, members of the family of herpesviruses including herpes simplex, cytomegalovirus, varicella-zoster virus, Epstein-Barr virus and human herpes virus 6 have also been associated with respiratory disease. This list of pathogens was extended last year with the discovery of a novel virus, the human metapneumovirus. More than 200 antigenically distinct viruses have been documented as causes of sporadic or epidemic respiratory infections in infants, children and adults. However, this varied and diverse group can be divided among six distinct families. Understanding some of the basic biology of these families gives an insight into possible strategies for diagnosis, control and therapy.

Natural influenza A virus infection of mice elicits strong antibody response to HA2 glycopolypeptide

Two influenza viruses, A/Dunedin/4/73 (H3N2) and A/Mississippi/1/85 (H3N2) were adapted to BALB/c mice. Groups of BALB/c mice were intranasally (i.n.) infected with either single dose of particular virus strain or successively with both virus strains and titers of serum antibodies against influenza virus antigens ("influenza virus antibodies") and those just against the HA2 part of hemagglutinin (HA) ("HA2 antibodies") were determined. Successive infection with virus strains Dunedin and Mississippi in interval of 21 days led to the strong increase of the proportion of anti-HA2 antibodies in sera, though whole antiviral titres remained in general unchanged. These observations confirmed that the HA2 glycopolypeptide (gp) part of influenza virus HA is very strong immunogen in natural infection.

Infection-triggered regulatory mechanisms override the role of STAT 4 in control of the immune response to influenza virus antigens

Accurate control of the balance of the T1 and T2 cells during antiviral immunity is essential for optimizing immune effector functions and for avoiding potentially severe immunopathology. We examined the in vivo role of the signal transducer and activator of transcription (STAT) 4 in regulating the T1/T2 balance during the response to live influenza virus and isolated viral proteins. We found that the differentiation of gamma interferon (IFN-gamma)-producing Th1 and Tc1 cells after inoculation of live virus occurred independently of STAT 4 expression. Influenza virus-specific T2 and Tc2 responses were well controlled in such STAT 4-deficient mice unless IFN-gamma was eliminated as well. In contrast, the STAT 4-dependent signaling pathway played a more essential role in regulating the T1/T2 balance after immunization with viral proteins and, in particular, inactivated nonreplicating virus. Pulmonary infection was cleared even in the absence of both functional STAT 4 genes and functional IFN-gamma genes because virus-neutralizing antibodies were still generated, consistent with a substantial redundancy in different antiviral effector pathways. Thus, replicating agents such as live influenza virus can elicit IFN-gamma and control T2 immunity independently of STAT 4, whereas the profile of immunity to isolated proteins is more reliant on an intact STAT 4 signaling pathway.

Influenza A virus infection inhibits the efficient recruitment of Th2 cells into the airways and the development of airway eosinophilia

Most infections with respiratory viruses induce Th1 responses characterized by the generation of Th1 and CD8(+) T cells secreting IFN-gamma, which in turn have been shown to inhibit the development of Th2 cells. Therefore, it could be expected that respiratory viral infections mediate protection against asthma. However, the opposite seems to be true, because viral infections are often associated with the exacerbation of asthma. For this reason, we investigated what effect an influenza A (flu) virus infection has on the development of asthma. We found that flu infection 1, 3, 6, or 9 wk before allergen airway challenge resulted in a strong suppression of allergen-induced airway eosinophilia. This effect was associated with strongly reduced numbers of Th2 cells in the airways and was not observed in IFN-gamma- or IL-12 p35-deficient mice. Mice infected with flu virus and immunized with OVA showed decreased IL-5 and increased IFN-gamma, eotaxin/CC chemokine ligand (CCL)11, RANTES/CCL5, and monocyte chemoattractant protein-1/CCL2 levels in the bronchoalveolar lavage fluid, and increased airway hyperreactivity compared with OVA-immunized mice. These results suggest that the flu virus infection reduced airway eosinophilia by inducing Th1 responses, which lead to the inefficient recruitment of Th2 cells into the airways. However, OVA-specific IgE and IgG1 serum levels, blood eosinophilia, and goblet cell metaplasia in the lung were not reduced by the flu infection. Flu virus infection also directly induced AHR and goblet cell metaplasia. Taken together, our results show that flu virus infections can induce, exacerbate, and suppress features of asthmatic disease in mice.

Clinical and laboratory study of newborns with lower respiratory tract infection due to respiratory viruses

OBJECTIVES

To determine the prevalence of lower respiratory tract infection due to respiratory viruses in the neonatal period at admission to the neonatal intensive care unit and to compare the clinical, laboratory and radiological aspects of the clinical course, according to the etiological agent, in the neonatal period.

METHODS

Ninety newborns were studied, from January 1999 to January 2001, with bronchiolitis and/or pneumonia. The newborns were classified into three groups, according to the etiological agent identified initially: viral infection (group A), mixed viral-bacterial infection (group B), and bacterial infection (group C).

RESULTS

The virus was identified in 72 newborns (80.0%); the most prevalent was respiratory syncytial virus (RSV) (44.4%), followed by influenza A virus (22.2%). Coughing, wheezing and an interstitial infiltrate were significantly more frequent in newborns with viral infection. Mixed infection was more associated with sepsis. There was a correlation between viral infection and low values of initial and subsequent white blood cell count and C-reactive protein. RSV was the most important virus in these patients.

CONCLUSIONS

It was observed that, although the majority of viral respiratory infections had a favorable course, some patients presented a serious and prolonged clinical manifestation, especially when there was concomitant bacterial infection.

Drift in the nucleoprotein gene of swine influenza virus (H1N1) causing respiratory disease in pigs

The nucleoprotein (NP) gene of swine influenza H1N1 variant, A/Sw/Quebec/5393/91 (SwQc91) was sequenced. When compared with other H1N1 strains, 12 amino acid (aa) replacements were observed in the 101-484 aa region of the NP protein including two aas, 345 and 430, representing the unique lineage of swine viruses. Phylogenetic analysis showed a drift in the NP gene.

Local immune responses to influenza virus infection in mice with a targeted disruption of perforin gene

The role of perforin in the local defense mechanisms against influenza virus infection was investigated. Mice deficient in the perforin gene (perforin(-/-)) were more susceptible to influenza virus infection than the ordinary wild-type C57BL/6 mice, showing an increased mortality with elevated virus growth and prolonged virus shedding. The lung parenchyma cells of perforin(-/-) mice showed no cytolytic activities of natural killer cells or virus-specific cytotoxic T lymphocytes in vitro, although the local antibody production system in the respiratory tract functioned well. In perforin(-/-) mice, the appearance of apoptotic degeneration in virally infected lung cells was delayed. This might cause cellular infiltration, including CD4, CD8, and CD19 positive cells, in the lung, peaking at day 8 after infection and maintaining a high level for a longer period. In the virus-induced local cytokine production, interferon-gamma (IFN-gamma) was prominent. The adoptive transfer of immune-competent spleen cells from wild-type C57BL/6 mice achieved a complete protection of perforin(-/-) mice against lethal challenge infection. These results suggest that perforin plays a crucial role in the host defense system against influenza virus infection, especially in its early stage, by inducing apoptosis of virus-infected cells.

Effect of maternally derived antibodies on the clinical signs and immune response in pigs after primary and secondary infection with an influenza H1N1 virus

The aim of this study was to determine the role of maternally derived antibodies (MDA) against an influenza H1N1 virus in the clinical protection of piglets and especially their effect on the development of the active immunity after an infection with a homologous influenza H1N1 virus. Twenty piglets with MDA and 10 piglets without MDA were housed together and inoculated twice with influenza H1N1 virus, at 7 and 15 weeks of age. Nine piglets without MDA were added to these groups at 12 weeks of age to be inoculated at 15 weeks of age only. Clinical signs, body temperature, growth performance, virus excretion, antibody responses, and influenza-specific T-cell response were monitored. It was shown that MDA protect piglets against the clinical consequences of a primary influenza infection, but that this protection is not complete. A short but significant rise in body temperature was observed and growth seemed to be inhibited due to the infection. Piglets with MDA shed virus for a longer period after an infection than piglets without MDA. Piglets with and without MDA were protected against the clinical consequences of a secondary infection. However, both after primary and secondary infection significant differences in immune responses were observed that indicated that pigs with MDA developed a weaker immunity than pigs without MDA. Furthermore, overall growth performances from weaning to slaughter show a trend in favour of pigs without maternal antibodies, compared to pigs with maternal antibodies, mainly caused by a significant better performance in the second half of the finishing period. The results of this study provide us insight in the role of MDA in clinical protection and their influence on active immunity after an influenza virus infection of pigs. Furthermore, it leads us to the discussion about the profitability of massive sow herd vaccinations in an attempt to increase MDA levels in piglets, taking into account the overall performance of these piglets and the possible effects on antigenic drift.

Semi-quantitative detection of viral RNA in influenza A virus-infected mice for evaluation of antiviral compounds

The aim of the study was to establish a murine model for sensitive screening of potential compounds with in vitro anti-influenza A virus activity. The evaluation in this in vivo model is based on semi-quantitative detection of viral RNA using one-step reverse transcriptase polymerase chain reaction (RT-PCR). After intranasal infection of fully-conscious mice with influenza A virus, the viral load of the respiratory tract tissues was investigated. Peaks were observed in the nasopharynx between Days 1 and 4, in the trachea on Day 4, and in the lungs between Days 4 and 7 post infection. The elimination of virus correlated with the appearance of specific serum antibodies. After 4 days of treatment with zanamivir, trachea and lungs revealed negative RT-PCR results, whereas viral load in the nasopharynx was significantly reduced. In conclusion, the virus spread in the described murine model is similar to upper respiratory tract infection with influenza virus in human. Viral load measurement by semi-quantitative detection of viral RNA allows rapid and sensitive screening of potential compounds with in vitro anti-influenza A virus activity.