Derivation of the nucleoproteins (NP) of influenza A viruses isolated from marine mammals

Pinnipeds and avian influenza: a global timeline and review of research on the impact of highly pathogenic avian influenza on pinniped populations with particular reference to the endangered Caspian seal (Pusa caspica)

This study reviews chronologically the international scientific and health management literature and resources relating to impacts of highly pathogenic avian influenza (HPAI) viruses on pinnipeds in order to reinforce strategies for the conservation of the endangered Caspian seal (Pusa caspica), currently under threat from the HPAI H5N1 subtype transmitted from infected avifauna which share its haul-out habitats. Many cases of mass pinniped deaths globally have occurred from HPAI spill-overs, and are attributed to infected sympatric aquatic avifauna. As the seasonal migrations of Caspian seals provide occasions for contact with viruses from infected migratory aquatic birds in many locations around the Caspian Sea, this poses a great challenge to seal conservation. These are thus critical locations for the surveillance of highly pathogenic influenza A viruses, whose future reassortments may present a pandemic threat to humans.

Zoonotic Animal Influenza Virus and Potential Mixing Vessel Hosts

Influenza viruses belong to the family Orthomyxoviridae with a negative-sense, single-stranded segmented RNA genome. They infect a wide range of animals, including humans. From 1918 to 2009, there were four influenza pandemics, which caused millions of casualties. Frequent spillover of animal influenza viruses to humans with or without intermediate hosts poses a serious zoonotic and pandemic threat. The current SARS-CoV-2 pandemic overshadowed the high risk raised by animal influenza viruses, but highlighted the role of wildlife as a reservoir for pandemic viruses. In this review, we summarize the occurrence of animal influenza virus in humans and describe potential mixing vessel or intermediate hosts for zoonotic influenza viruses. While several animal influenza viruses possess a high zoonotic risk (e.g., avian and swine influenza viruses), others are of low to negligible zoonotic potential (e.g., equine, canine, bat and bovine influenza viruses). Transmission can occur directly from animals, particularly poultry and swine, to humans or through reassortant viruses in "mixing vessel" hosts. To date, there are less than 3000 confirmed human infections with avian-origin viruses and less than 7000 subclinical infections documented. Likewise, only a few hundreds of confirmed human cases caused by swine influenza viruses have been reported. Pigs are the historic mixing vessel host for the generation of zoonotic influenza viruses due to the expression of both avian-type and human-type receptors. Nevertheless, there are a number of hosts which carry both types of receptors and can act as a potential mixing vessel host. High vigilance is warranted to prevent the next pandemic caused by animal influenza viruses.

Pathogen Exposure in White Whales (Delphinapterus leucas) in Svalbard, Norway

The Svalbard white whale (Delphinapterus leucas) population is one of the smallest in the world, making it particularly vulnerable to challenges such as climate change and pathogens. In this study, serum samples from live captured (2001−2016) white whales from this region were investigated for influenza A virus (IAV) antibodies (Abs) (n = 27) and RNA (n = 25); morbillivirus (MV) Abs (n = 3) and RNA (n = 25); Brucella spp. Abs; and Toxoplasma gondii Abs (n = 27). IAV Abs were found in a single adult male that was captured in Van Mijenfjorden in 2001, although no IAV RNA was detected. Brucella spp. Abs were found in 59% of the sample group (16/27). All MV and T. gondii results were negative. The results show that Svalbard white whales have been exposed to IAV and Brucella spp., although evidence of disease is lacking. However, dramatic changes in climate and marine ecosystems are taking place in the Arctic, so surveillance of health parameters, including pathogens, is critical for tracking changes in the status of this vulnerable population.

A Brief Introduction to Influenza A Virus in Marine Mammals

Influenza A infection has been detected in marine mammals going back to 1975, with additional unconfirmed outbreaks as far back as 1931. Over the past forty years, infectious virus has been recovered on ten separate occasions from both pinnipeds (harbor seal, elephant seal, and Caspian seal) and cetaceans (striped whale and pilot whale). Recovered viruses have spanned a range of subtypes (H1, H3, H4, H7, H10, and H13) and, in all but H1N1, show strong evidence for deriving directly from avian sources. To date, there have been five unusual mortality events directly attributed to influenza A virus; these have primarily occurred in harbor seals in the Northeastern United States, with the most recent occurring in harbor seals in the North Sea.There are numerous additional reports wherein influenza A virus has indirectly been identified in marine mammals; these include serosurveillance efforts that have detected influenza A- and B-specific antibodies in marine mammals spanning the globe and the detection of viral RNA in both active and opportunistic surveillance in the Northwest Atlantic. For viral detection and recovery, nasal, rectal, and conjunctival swabs have been employed in pinnipeds, while blowhole, nasal, and rectal swabs have been employed in cetaceans. In the case of deceased animals, virus has also been detected in tissue. Surveillance has historically been somewhat limited, relying largely upon opportunistic sampling of stranded or bycaught animals and primarily occurring in response to a mortality event. A handful of active surveillance projects have shown that influenza may be more endemic in marine mammals than previously appreciated, though live virus is difficult to recover. Surveillance efforts are hindered by permitting and logistical challenges, the absence of reagents and methodology optimized for nonavian wild hosts, and low concentration of virus recovered from asymptomatic animals. Despite these challenges, a growing body of evidence suggests that marine mammals are an important wild reservoir of influenza and may contribute to mammalian adaptation of avian variants.

Influenza Virus Infection of Marine Mammals

Interspecies transmission may play a key role in the evolution and ecology of influenza A viruses. The importance of marine mammals as hosts or carriers of potential zoonotic pathogens such as highly pathogenic H5 and H7 influenza viruses is not well understood. The fact that influenza viruses are some of the few zoonotic pathogens known to have caused infection in marine mammals, evidence for direct transmission of influenza A virus H7N7 subtype from seals to man, transmission of pandemic H1N1 influenza viruses to seals and also limited evidence for long-term persistence of influenza B viruses in seal populations without significant genetic change, makes monitoring of influenza viruses in marine mammal populations worth being performed. In addition, such monitoring studies could be a great tool to better understand the ecology of influenza viruses in nature.

Human pathogens in marine mammal meat – a northern perspective

Only a few countries worldwide hunt seals and whales commercially. In Norway, hooded and harp seals and minke whales are commercially harvested, and coastal seals (harbour and grey seals) are hunted as game. Marine mammal meat is sold to the public and thus included in general microbiological meat control regulations. Slaughtering and dressing of marine mammals are performed in the open air on deck, and many factors on board sealing or whaling vessels may affect meat quality, such as the ice used for cooling whale meat and the seawater used for cleaning, storage of whale meat in the open air until ambient temperature is reached, and the hygienic conditions of equipment, decks, and other surfaces. Based on existing reports, it appears that meat of seal and whale does not usually represent a microbiological hazard to consumers in Norway, because human disease has not been associated with consumption of such foods. However, as hygienic control on marine mammal meat is ad hoc, mainly based on spot-testing, and addresses very few human pathogens, this conclusion may be premature. Additionally, few data from surveys or systematic quality control screenings have been published. This review examines the occurrence of potential human pathogens in marine mammals, as well as critical points for contamination of meat during the slaughter, dressing, cooling, storage and processing of meat. Some zoonotic agents are of particular relevance as foodborne pathogens, such as Trichinella spp., Toxoplasma gondii, Salmonella and Leptospira spp. In addition, Mycoplasma spp. parapoxvirus and Mycobacterium spp. constitute occupational risks during handling of marine mammals and marine mammal products. Adequate training in hygienic procedures is necessary to minimize the risk of contamination on board, and acquiring further data is essential for obtaining a realistic assessment of the microbiological risk to humans from consuming marine mammal meat.

The genome of an influenza virus from a pilot whale: relation to influenza viruses of gulls and marine mammals

Influenza virus A/whale/Maine/328B/1984 (H13N2) was isolated from a diseased pilot whale. Since only a partial sequence was available, its complete genome was sequenced and compared to the sequences of subtype H13 influenza viruses from shorebirds and various influenza viruses of marine mammals. The data reveal a rare genotype constellation with all gene segments derived of an influenza virus adapted to gulls, terns and waders. In contrast, the phylogenetic trees indicate that the majority of influenza viruses isolated from marine mammals derived from influenza viruses adapted to geese and ducks. We conclude that A/whale/Maine/328B/1984 is the first record of an infection of a marine mammal from a gull-origin influenza virus.

Natural history of highly pathogenic avian influenza H5N1

The ecology of highly pathogenic avian influenza (HPAI) H5N1 has significantly changed from sporadic outbreaks in terrestrial poultry to persistent circulation in terrestrial and aquatic poultry and potentially in wild waterfowl. A novel genotype of HPAI H5N1 arose in 1996 in Southern China and through ongoing mutation, reassortment, and natural selection, has diverged into distinct lineages and expanded into multiple reservoir hosts. The evolution of Goose/Guangdong-lineage highly pathogenic H5N1 viruses is ongoing: while stable interactions exist with some reservoir hosts, these viruses are continuing to evolve and adapt to others, and pose an un-calculable risk to sporadic hosts, including humans.

Pandemic H1N1 influenza isolated from free-ranging Northern Elephant Seals in 2010 off the central California coast

Interspecies transmission of influenza A is an important factor in the evolution and ecology of influenza viruses. Marine mammals are in contact with a number of influenza reservoirs, including aquatic birds and humans, and this may facilitate transmission among avian and mammalian hosts. Virus isolation, whole genome sequencing, and hemagluttination inhibition assay confirmed that exposure to pandemic H1N1 influenza virus occurred among free-ranging Northern Elephant Seals (Mirounga angustirostris) in 2010. Nasal swabs were collected from 42 adult female seals in April 2010, just after the animals had returned to the central California coast from their short post-breeding migration in the northeast Pacific. Swabs from two seals tested positive by RT-PCR for the matrix gene, and virus was isolated from each by inoculation into embryonic chicken eggs. Whole genome sequencing revealed greater than 99% homology with A/California/04/2009 (H1N1) that emerged in humans from swine in 2009. Analysis of more than 300 serum samples showed that samples collected early in 2010 (n = 100) were negative and by April animals began to test positive for antibodies against the pH1N1 virus (HI titer of ≥1∶40), supporting the molecular findings. In vitro characterizations studies revealed that viral replication was indistinguishable from that of reference strains of pH1N1 in canine kidney cells, but replication was inefficient in human epithelial respiratory cells, indicating these isolates may be elephant seal adapted viruses. Thus findings confirmed that exposure to pandemic H1N1 that was circulating in people in 2009 occurred among free-ranging Northern Elephant Seals in 2010 off the central California coast. This is the first report of pH1N1 (A/Elephant seal/California/1/2010) in any marine mammal and provides evidence for cross species transmission of influenza viruses in free-ranging wildlife and movement of influenza viruses between humans and wildlife.

Connecting the study of wild influenza with the potential for pandemic disease

Continuing outbreaks of pathogenic (H5N1) and pandemic (SOIVH1N1) influenza have underscored the need to understand the origin, characteristics, and evolution of novel influenza A virus (IAV) variants that pose a threat to human health. In the last 4-5years, focus has been placed on the organization of large-scale surveillance programs to examine the phylogenetics of avian influenza virus (AIV) and host-virus relationships in domestic and wild animals. Here we review the current gaps in wild animal and environmental surveillance and the current understanding of genetic signatures in potentially pandemic strains.

Influenza A viruses: an ecology review

In humans, influenza A viruses cause yearly outbreaks with high morbidity and excess fatality rates as a direct effect. Placed in its ecological niche, however - in dabbling ducks - avian influenza virus (AIV) induces quite a mild disease. It is when the virus crosses the species barrier that pathogenic traits are attributed to infection. When infecting phylogenetically more distant species (i.e. chicken and turkeys), the AIV can cause high morbidity and may in some cases change the virus into a highly pathogenic variant with nearly 100% fatality rate. Being a very adaptable virus, these spill-over events are frequent and numerous species are susceptible to influenza virus. When a subtype of AIV that has not previously infected humans crosses the species barrier, adapts to humans, and spreads easily, a pandemic event is imminent. There is no cure for influenza infection and vaccination is a cumbersome endeavor so, currently, the strategy when a pandemic strikes is damage control. The interest in AIV ecology has increased dramatically since the beginning of the millennium as a key factor for preventive work for future pandemics. This review gives a broad overview of influenza A virus ecology: in the natural host, accidental hosts, new endemic hosts, and humans.

Susceptibility of laughing gulls (Larus atricilla) to H5N1 and H5N3 highly pathogenic avian influenza viruses

This investigation detailed the clinical disease, gross and histologic lesions, and distribution of viral antigen in juvenile laughing gulls (Larus atricilla) intranasally inoculated with either the A/tern/South Africa/61 (H5N3) (tern/SA) influenza virus or the A/chicken/Hong Kong/220/97 (H5N1) (chicken/HK) influenza virus, which are both highly pathogenic for chickens. Neither morbidity nor mortality was observed in gulls inoculated with either virus within the 14-day investigative period. Gross lesions resultant from infection with either virus were only mild, with the tern/SA virus causing decreased lucency of the air sacs (2/6), splenomegaly (2/6), and pancreatic mottling (1/6) and the chicken/HK virus causing only decreased lucency of the air sacs (2/8) and conjunctival edema (2/8). Histologic lesions in the tern/SA-inoculated gulls included a mild to moderate heterophilic to lymphoplasmacytic airsacculitis (6/6), mild to moderate interstitial pneumonia (3/6), and moderate necrotizing pancreatitis and hepatitis at 14 days postinoculation (DPI) (2/6). Immunohistochemical demonstration of viral antigen occurred only in association with lesions in the liver and pancreas. In contrast, viral antigen was not demonstrated in any tissues from the chicken/HK-inoculated gulls, and inflammatory lesions were confined to the air sac (3/8) and lungs (3/8). Both viruses were isolated at low titers (<10(1.68) mean embryo lethal dose) from oropharyngeal and cloacal swabs up to 7 days postinoculation (DPI), from the lung and kidney of one of two tern/SA-inoculated gulls at 14 DPI, and from the lung of one of two chicken/HK-inoculated gulls at 7 DPI. Antibodies to influenza viruses as determined with the agar gel precipitin test at 14 DPI were detected only in the two tern/SA-inoculated gulls and not in the two chicken/HK-inoculated gulls.

Antigenic heterogeneity of N2 neuraminidases of avian influenza viruses isolated in Israel

Twenty one N2 neuraminidase (NA)-containing viruses isolated in Israel from different avian hosts during 1971-1984 were studied comparatively by means of the panel of 7 monoclonal antibodies (MAB) against A/Guiyang/57(H2N2) virus. Fifteen from the 21 viruses were studied in comprehensive cross reaction NA inhibition (NI) tests with the corresponding polyclonal antisera. The principal result of the studies is that all the isolates can be distributed into two main groups. The 1st group includes the majority of the isolates whose NA shows close relatedness to the "early" (1957 type) N2 NA by NI tests with polyclonal antisera, and demonstrates remarkable stability in the NI tests by reacting with the same 6 from 7 MABs of the panel. The 2nd group does not show any special kinship to either "early" or "late" (1968 type) N2 when analyzed with polyclonal antisera and demonstrates heterogeneity by the analysis with the MABs. A hypothetical explanation of the phenomenon of co-circulation in the local avian reservoir of viral strains displaying either remarkable stability or wide heterogeneity of their NAs is suggested. In accordance with it, the viruses with "stable" ("conservative") N2 NA did not leave the avian reservoir and, hence, did not drift because of very low antibody "selection pressure". Contrary to it, the viruses with heterogeneous N2 NA had been circulating in the human (mammalian) reservoir during various periods before their transfer into the avian reservoir; they drifted accordingly and, being then isolated from birds and designated as "avian" viruses, demonstrate heterogeneity of their NAs which is typical for human viruses.

Analysis of influenza A virus nucleoproteins for the assessment of molecular genetic mechanisms leading to new phylogenetic virus lineages

The nucleoprotein (NP) gene of influenza A viruses is decisive for separating two large individually evolving reservoirs in birds and humans. A phylogenetic analysis of the NP gene revealed that all mammalian influenza viruses originated--directly or indirectly--from an avian ancestor. The stable introduction of an avian influenza A virus into a mammalian species seems to be a relatively rare event, the latest one occurred in 1979 when such an avian virus was introduced into pigs in Northern Europe which gave rise to a new lineage. At least two concomitant events are required for such a new and stable introduction: (1) The new species has to become infected, and (2) a mutation in the polymerase complex has to establish a labile variant, which is prone to provide a large number of different variants, from which some can adapt rapidly to the new host (or to any unusual environments). Since such mutator mutations might be advantageous only during stress periods, variants with a less error prone polymerase might emerge again after adaptation. Examples for such fluctuations in terms of mutational and evolutionary rates are discussed in this brief review.

Recent advances in avian virology

Selected, recent research on the following avian diseases, and their causative viruses, has been reviewed: chicken anaemia, infectious bursal disease, turkey rhinotracheitis, avian nephritis, fowlpox, influenza, infectious bronchitis and turkey enteritis.

Characterization of a new avian-like influenza A virus from horses in China

In March 1989 a severe outbreak of respiratory disease occurred in horses in the Jilin and Heilongjiang provinces of Northeast China that caused up to 20% mortality in some herds. An influenza virus of the H3N8 subtype was isolated from the infected animals and was antigenically and molecularly distinguishable from the equine 2 (H3N8) viruses currently circulating in the world. The reference strain A/Equine/Jilin/1/89 (H3N8) was most closely related to avian H3N8 influenza viruses. Sequence comparisons of the entire hemagglutinin (HA), nucleoprotein (NP), neuraminidase (NA), matrix (M), and NS genes along with partial sequences of the three polymerase (PB1, PB2, PA) genes suggest that six of the eight gene segments (PA, HA, NP, NA, M, NS) are closely related to avian influenza viruses. Since direct sequence analysis can only provide a crude measure of relationship, phylogenetic analysis was done on the sequence information. Phylogenetic analyses of the entire HA, NP, M, and NS genes and of partial sequences of PB1, PB2, and PA indicated that these genes are of recent avian origin. The NP gene segment is closely related to the gene segment found in the newly described H14 subtype isolated from ducks in the USSR. The A/Equine/Jilin/1/89 (H3N8) influenza virus failed to replicate in ducks, but did replicate and cause disease in mice on initial inoculation and on subsequent passaging caused 100% mortality. In ferrets, the virus caused severe influenza symptoms. A second outbreak of influenza in horses in Northeast China occurred in April 1990 in the Heilongjiang province with 48% morbidity and no mortality. The viruses isolated from this outbreak were antigenically indistinguishable from those in the 1989 outbreak and it is probable that the reduced mortality was due to the immune status of of the horses in the region. No influenza was detected in horses in Northern China in the spring, summer, or fall of 1991 and no influenza has been detected in horses in adjacent areas. Our analysis suggests that this new equine influenza virus in horses in Northeast China is the latest influenza virus in mammals to emerge from the avian gene pool in nature and that it may have spread to horses without reassortment. The appearance of this new equine virus in China emphasizes the potential for whole avian influenza viruses to successfully enter mammalian hosts and serves as a model and a warning for the appearance of new pandemic influenza viruses in humans.(ABSTRACT TRUNCATED AT 250 WORDS)

Subtype H7 influenza viruses: comparative antigenic and molecular analysis of the HA-, M-, and NS-genes

Antigenic analysis of the haemagglutinin and matrix protein with corresponding sets of monoclonal antibodies as well as sequence analysis of HA-, M-, and NS-genes were carried out to establish antigenic and genetic relationships between four fowl plague virus (FPV) strains of H7 subtype. The data obtained revealed close genetic relatedness between the oldest known influenza A virus, A/chicken/Brescia/1902 (H7N7), and two FPV strains, A/FPV/Dobson (H7N7) and A/FPV/Weybridge (H7N7). These three strains apparently differ in all genes investigated from the A/FPV/Rostock isolate.

Sequence changes in the live attenuated, cold-adapted variants of influenza A/Leningrad/134/57 (H2N2) virus

Nucleotide sequences were determined for the RNA segments coding for proteins other than the hemagglutinin and neuraminidase of the A/Leningrad/134/57 (H2N2) wild-type (A/Len/wt) virus and its two cold-adapted (ca) and attenuated variants, A/Leningrad/134/17/57 (A/Len/17/ca) and A/Leningrad/134/47/57 (A/Len/47/ca) that are used in the U.S.S.R. in the preparation of reassortant live attenuated vaccines. Ten nucleotide differences were detected between the sequences of the A/Len/wt and A/Len/17/ca viruses; of these, eight were deduced to encode amino acid (aa) substitutions. One aa substitution each was predicted for the PB2, M1, M2, and NS2 proteins, whereas two aa substitutions each were predicted for the PB1, and PA proteins of the A/Len/17/ca virus. Four additional nucleotide changes were found in the genome of the A/Len/47/ca virus; three of these were detected to code for one additional aa substitution each for the PB2, PB1, and NP proteins.

Genetic relatedness of the nucleoprotein (NP) of recent swine, turkey, and human influenza A virus (H1N1) isolates

The sequences of nucleoprotein (NP) genes of recent human and turkey isolates of influenza A viruses, which serologically could be correlated to contemporary swine viruses, were determined. These sequences were closely related to the NPs of these swine viruses and they formed a separate branch on the phylogenetic tree. While the early swine virus from 1931 resembled the avian strains in consensus amino acids of the NP and in its ability to rescue NP ts mutants of fowl plague virus in chicken embryo cells, the later strains on that branch were different: at 15 positions they have their own amino acids and they rescued the NP ts mutants only poorly. Of the NPs of the human New Jersey/76 isolates analysed, one clustered with the recent H1N1 swine viruses of the U.S.A., the other one with contemporary human strains. Since the NP is one of the main determinants of species specificity it is concluded that, although the H1N1 swine isolates from the U.S.A. form their own branch in the phylogenetic tree, they can be transmitted to humans and turkeys, but they do not spread further in these populations and so far have not contributed to human pandemics. It is not very likely that they will do so in future, since its branch in the phylogenetic tree develops further away from the human and avian branch.

Phylogenetic relationship of the nonstructural (NS) genes of influenza A viruses

Phylogenetic trees were constructed using 38 sequences of the A group and 10 sequences of the B group of the NS gene of influenza A viruses. Within the A group we found avian as well as mammalian influenza a viruses, while within the B group exclusively avian strains were found. The avian and human NS genes of the A group were derived from a common ancestor existing at about 1912. At 13 positions of the amino acid sequences of the NS1 protein two subtypes of the A group can be differentiated, a human and a non-human subtype. Starting at the time of the introduction of an avian PB1 gene into human strains during the antigenic shift at 1957 the NS1 protein of the human strains came under an enhanced selection pressure which might indicate a cooperation of the NS1 protein with and adaptation of the NS1 protein on the newly introduced PB1 gene. Such a selection pressure on the NS2 protein is completely missing. Comparison of all sequences of the NS1 protein revealed four highly conserved regions within the amino-terminal half of the molecule. One of this regions seems to contain the nuclear migration signal. The carboxy-terminal half is completely variable and seems to be dispensable.

Evolution of influenza A virus nucleoprotein genes: implications for the origins of H1N1 human and classical swine viruses

A phylogenetic analysis of 52 published and 37 new nucleoprotein (NP) gene sequences addressed the evolution and origin of human and swine influenza A viruses. H1N1 human and classical swine viruses (i.e., those related to Swine/Iowa/15/30) share a single common ancestor, which was estimated to have occurred in 1912 to 1913. From this common ancestor, human and classical swine virus NP genes have evolved at similar rates that are higher than in avian virus NP genes (3.31 to 3.41 versus 1.90 nucleotide changes per year). At the protein level, human virus NPs have evolved twice as fast as classical swine virus NPs (0.66 versus 0.34 amino acid change per year). Despite evidence of frequent interspecies transmission of human and classical swine viruses, our analysis indicates that these viruses have evolved independently since well before the first isolates in the early 1930s. Although our analysis cannot reveal the original host, the ancestor virus was avianlike, showing only five amino acid differences from the root of the avian virus NP lineage. The common pattern of relationship and origin for the NP and other genes of H1N1 human and classical swine viruses suggests that the common ancestor was an avian virus and not a reassortant derived from previous human or swine influenza A viruses. The new avianlike H1N1 swine viruses in Europe may provide a model for the evolution of newly introduced avian viruses into the swine host reservoir. The NPs of these viruses are evolving more rapidly than those of human or classical swine viruses (4.50 nucleotide changes and 0.74 amino acid change per year), and when these rates are applied to pre-1930s human and classical swine virus NPs, the predicted date of a common ancestor is 1918 rather than 1912 to 1913. Thus, our NP phylogeny is consistent with historical records and the proposal that a short time before 1918, a new H1N1 avianlike virus entered human or swine hosts (O. T. Gorman, R. O. Donis, Y. Kawaoka, and R. G. Webster, J. Virol. 64:4893-4902, 1990). This virus provided the ancestors of all known human influenza A virus genes, except for HA, NA, and PB1, which have since been reassorted from avian viruses. We propose that during 1918 a virulent strain of this new avianlike virus caused a severe human influenza pandemic and that the pandemic virus was introduced into North American swine populations, constituting the origin of classical swine virus.

Evolution of pig influenza viruses

There is evidence that the nucleoprotein (NP) gene of the classical swine virus (A/Swine/1976/31) clusters with the early human strains at the nucleotide sequence level, while at the level of the amino acid sequence, as defined by consensus amino acids and in functional tests, its NP is clearly "avian like." Therefore it was suggested that the Sw/31 NP had been recently under strong selection pressure, possibly caused by reassortment with other avian influenza genes, whose gene products have to cooperate intimately with NP (Gammelin et al., 1989. Virology 170, 71-80). This suggestion has been investigated by sequencing the genes of internal and nonstructural proteins of Sw/31. The data on these sequences and on the phylogenetic trees are not in accordance with that suggestion: all these genes cluster with the early human strains at the nucleotide level while, at the level of the amino acid sequence, most of them are more closely related to the avian strains, thus resembling NP in this respect. This indicates that these genes rather evolved concomitantly with the NP gene. Our data are in agreement with the suggestion that, at about the time of the Spanish Flu (1918/19), a human influenza A (H1N1) virus entered the pig population. Furthermore, it is known that the NP of the human influenza A viruses--in contrast to that of the avian and swine strains--has been under strong selection pressure to change (Gammelin et al., 1990. Mol. Biol. Evol. 7, 194-200. Gorman et al., 1990a. J. Virol. 64, 1487-1497). Thus, after transfer of a human strain into pigs, the selection pressure might be released, enabling the NP and the other genes of the swine virus to evolve back to the optimal avian sequences, especially at the functionally important consensus positions. The swine influenza viruses circulating since 1979 in Northern Europe--represented by A/Swine/Germany/2/81 (H1N1)--have all genes, so far examined, derived from an avian influenza virus pool and are different from the classical swine viruses.

Mutants and revertants of an avian influenza A virus with temperature-sensitive defects in the nucleoprotein and PB2

ts19 is a temperature-sensitive (ts) mutant of the influenza A fowl plague virus with a defect in the nucleoprotein (NP). In ts19-infected chicken embryo cells all viral components are synthesized in normal yields at the nonpermissive temperature, but infectious virus is not formed. Under these conditions the migration of the NP and M of ts19 from the cell nucleus to the cytoplasm is affected. This ts defect is due to a single amino acid replacement (R162K) in a completely conserved region of the NP. Another mutant with a different defect in the NP is ts81. After infection with ts81 at 40 degrees no vRNA is being synthesized. By backcross of a revertant derived from ts81 many isolates with a ts defect in the PB2 protein were obtained. This ts defect seems to extragenically suppress the ts defect in the NP gene and to be dominant in a wild-type background.