Comparison of RNA hybridization, hemagglutination assay, titration of infectious virus and immunofluorescence as methods for monitoring influenza virus replication in vitro

Rapid and sensitive methods for the monitoring of influenza virus replication in vitro are needed to address several research questions. Four methods based on different principles were compared: the hemagglutination (HA) assay, the measurement of virus infectivity titers in culture supernatants, the enumeration of infected cells by immunofluorescence and RNA hybridization techniques using digoxigenin (DIG) labeled RNA probes. To this end, MDCK cells were infected at different multiplicities of infection (moi) with a recent influenza A virus (A/Netherlands/18/94 H3N2) and the kinetics of virus replication were monitored with these four assays. At high moi, virus released into the culture supernatant of infected cells was detected by the HA assay 12 h post infection, whereas at lower moi (< or = 0.01) the first HA activity was not detected before 24 h post infection. The measurement of infectious viruses in the culture supernatant proved to be more sensitive, since 4-12 h post infection newly produced virus was detected depending on the moi used. This finding was in agreement with results obtained by the immunofluorescence assay using an antibody preparation specific for the nucleoprotein: single infected cells could be detected as early as 4 h post infection. At this time point, positive signals were also obtained when mRNA/cRNA specific hybridization was carried out for the NP gene segment, but not for viral NP RNA or RNA specific for the hemagglutinin, which were only detected at later time points after infection. Thus, besides direct measurement of infectious virus and immunofluorescence, RNA hybridization proved to be a sensitive assay for monitoring influenza virus replication in vitro.

[Influenza A (H5N1) in Hong Kong: Forerunner of a pandemic or just a scientifically interesting phenomenon and a useful exercise in pandemiology?]

In 1997, 18 influenza patients were detected who were infected with influenza A(H5N1) virus. Six patients died. Presumably most of the patients had acquired the infection directly from chickens with the fowl plague prevalent in China in 1997. These are the first reported cases of isolation of influenza viruses belonging to one of the H4-H15 subtypes from human influenza patients. Man-to-man transmission of the virus has not been demonstrated but cannot be excluded in every case. Genetic analyses of seven of these virus isolates showed that no reassortment with a human or porcine influenza virus had occurred. It is unpredictable whether the H5N1-virus in question will start a pandemic in the next few years.

[Influenza A(H5N1) in Hong Kong: forerunner of a pandemic or an only scientifically interesting phenomenon and a useful exercise in pandemiology?]

From a three-year old boy in Hong Kong who died in May 1997 with an extensive influenza pneumonia an influenza A virus has been isolated which was, first at the National Influenza Centre of the Netherlands, identified as belonging to subtype H5N1. Presumably the patient had acquired the infection directly from an outbreak of fowl plague among chickens. As far as is known this is the first case of the isolation of an influenza virus belonging to one of the subtypes H4-H15 from a human influenza patient. At the end of 1997 seventeen more cases of human A (H5N1) influenza have been detected in Hong Kong, including five fatal cases. Genetic analyses of seven of these virus isolates did not reveal the occurrence of reassortment with a human or porcine influenza virus, which could have rendered the virus potentially pandemic. Man-to-man transmission of the virus has not been demonstrated but cannot be excluded either. This event has shown that the WHO surveillance of influenza viruses, although perhaps not perfect, has functioned well.

Human influenza virus A/HongKong/156/97 (H5N1) infection

Introduction of influenza viruses with gene segments of avian origin into the human population may result in the emergence of new pathogenic human influenza viruses. The recent infection of a 3-year-old boy with an influenza A (H5N1) virus of avian origin can be considered as an example of such an event. However, this virus, influenza A/Hong Kong/156/97 (H5N1) and the 17 additional H5N1 viruses isolated from humans by the end of 1997 lack the ability to spread efficiently amongst humans and therefore have limited pandemic potential. However, the possibility of reassortment of these viruses with currently circulating human viruses illustrates the need for pandemic preparedness.

Human influenza A H5N1 virus related to a highly pathogenic avian influenza virus

BACKGROUND

In May, 1997, a 3-year-old boy in Hong Kong was admitted to the hospital and subsequently died from influenza pneumonia, acute respiratory distress syndrome, Reye's syndrome, multiorgan failure, and disseminated intravascular coagulation. An influenza A H5N1 virus was isolated from a tracheal aspirate of the boy. Preceding this incident, avian influenza outbreaks of high mortality were reported from three chicken farms in Hong Kong, and the virus involved was also found to be of the H5 subtype.

METHODS

We carried out an antigenic and molecular comparison of the influenza A H5N1 virus isolated from the boy with one of the viruses isolated from outbreaks of avian influenza by haemagglutination-inhibition and neuraminidase-inhibition assays and nucleotide sequence analysis.

FINDINGS

Differences were observed in the antigenic reactivities of the viruses by the haemagglutination-inhibition assay. However, nucleotide sequence analysis of all gene segments revealed that the human virus A/Hong Kong/156/97 was genetically closely related to the avian A/chicken/Hong Kong/258/97.

INTERPRETATION

Although direct contact between the sick child and affected chickens has not been established, our results suggest transmission of the virus from infected chickens to the child without another intermediate mammalian host acting as a "mixing vessel". This event illustrates the importance of intensive global influenza surveillance.

[Influenza in the 1996/'97 season; vaccine composition for the 1997/'98 season]

The first indication of flu activity in the Netherlands in the 1996/'97 season was the isolation of an A/H3N2 influenza virus in week 48 of 1996. In subsequent weeks influenza viruses were isolated sporadically. The clinical influenza activity increased from week I of 1997 and reached its peak in week 4 of 1997. Simultaneously with the increase of clinical influenza activity, an increasing number of influenza viruses were isolated. The epidemic had a relatively small extent. Initially, A/H3N2 influenza viruses were predominant, but in the second half of the epidemic an increasing number of influenza B viruses were isolated as well. The A/H3N2 viruses were antigenically fairly strongly distinct from the variants prevalent in the preceding years. This season influenza A/HINI viruses did not play a significant role and only one virus of this subtype was isolated. All influenza A/H3N2, A/HINI and B viruses isolated were antigenically similar to the vaccine strains.

Respiratory syncytial virus specific serum antibodies in infants under six months of age: limited serological response upon infection

The decline of maternal respiratory syncytial virus (RSV) specific serum antibodies was studied in 45 children during the first 6 months of life, using a virus neutralization assay and competition ELISAs measuring fusion protein and glycoprotein specific antibodies. In all children RSV neutralizing antibodies were demonstrated at birth, with titers ranging from 33 to 1382. The calculated mean half life of these antibodies was 26 days. Furthermore, in a group of 38 children with suspected RSV infection, all younger than 6 months of age on admission, the diagnostic value of serological assays was evaluated. In 32 children RSV infection was confirmed by virus isolation, direct immune fluorescence and RT-PCR. In 7 patients of this group a significant titer rise in virus neutralization assay was demonstrated. Six additional RSV infected children could be identified by showing the presence of RSV-specific IgM or IgA serum antibodies or by showing an increase in fusion protein or glycoprotein specific antibodies. All serological tests together identified 13 (41%) of the 32 RSV infected patients. It is concluded that in children of this age group, which represent the majority of patients hospitalized with RSV infections, serological assays not only have a limited diagnostic value but are of limited value for sero-epidemiological studies.

Induction of protective immunity against influenza virus in a macaque model: comparison of conventional and iscom vaccines

Cynomolgus macaque monkeys (Macaca fascicularis) were immunized twice intramuscularly, either with a conventional non-adjuvanted subunit vaccine or with a candidate immune-stimulating complex (iscom) vaccine, each containing 10 micrograms envelope glycoprotein of a recent human influenza A(H3N2) virus (A/Netherlands/18/94). In contrast to the macaques vaccinated with the classical subunit vaccine, those immunized with the iscom vaccine developed high titres of specific IgM, IgA and IgG serum antibodies, as well as high titres of haemagglutination-inhibiting and virus-neutralizing serum antibodies. Also, specific proliferative T cell responses were only found in the iscom-vaccinated monkeys and their levels were similar to those found in monkeys experimentally infected with the homologous virus. Upon intratracheal challenge with the homologous virus, the iscom-vaccinated monkeys were completely protected from detectable virus replication in lungs, pharynx and nose, whereas those vaccinated with the classical subunit vaccines were not, or were only partially protected. The kinetics of specific serum antibody development in the iscom-vaccinated monkeys after challenge were quite similar to those of monkeys after secondary infection with the same virus. In contrast, the post-challenge kinetics of serum antibody development in the monkeys vaccinated with the classical subunit vaccines resembled those of naive monkeys, confirming that these vaccines only provided limited protection in such animals.

A pandemic warning?

Introduction of new influenza type-A viruses, carrying different combinations of the viral envelope glycoproteins haemagglutinin (H) and neuraminidase (N), have led to three major pandemics of influenza in humans this century. Phylogenetic evidence suggests that these viruses have originated from avian influenza A viruses, either unchanged or after reassortment with humaninfluenza A viruses. In aquatic birds, all of the known H and N antigenic varieties (15 varieties carry H, nine carry N envelope glycoproteins) apparently circulate in a genetically conserved fashion. Viruses carrying the H1N1, H2N2 and H3N2 combinations were responsible for the Spanish flu of 1918, the Asian flu in 1957 and Hong Kong flu in 1968, respectively¹. An influenza A virus of the H5N1 subtype has now been identified in a human patient, raising discussions about its potential to spark a new human influenza pandemic.

[Influenza in the 1995/'96 season; vaccine composition for the 1996/'97 season]

The 1995/'96 season in the Netherlands was marked by an influenza A/H3N2 epidemic that peaked in week 5I. In this week, 39 patients with influenza-like illness per 10,000 inhabitants contacted the sentinel physicians. With two exceptions, influenza A/H3N2 viruses exclusively were isolated during this epidemic period. In the first few months of 1996, a substantial number of influenza A/H1N1 and influenza B viruses were isolated as well. Serological characterization of the circulating viruses revealed that they all resembled the virus strains of the influenza vaccine of 1995/'96, which therefore probably will have provided good protection. Based on the epidemiological data from other countries and the fact that similar H3N2 viruses have been circulating since 1993, the World Health Organization has recommended to exchange the H3N2 component of the 1996/'97 vaccine for a Wuhan/353/95 (H3N2)-like strain.

Genetic reassortment in pandemic and interpandemic influenza viruses. A study of 122 viruses infecting humans

The human influenza pandemics of 1957 and 1968 were caused by reassortant viruses that possessed internal gene segments from avian and human strains. Whether genetic reassortment of human and avian influenza viruses occurs during interpandemic periods and how often humans are infected with such reassortants is not known. To provide this information, we used dot-blot hybridization, partial nucleotide sequencing and subsequent phylogenetic analysis to examine the 6 internal genes of 122 viruses isolated in humans between 1933 and 1992 primarily from Asia, Europe, and the Americas. The internal genes of A/New Jersey/11/76 isolated from a human fatality at Fort Dix, New Jersey in 1976 were found to be of porcine origin. Although none of the geographically and temporally diverse collection of 122 viruses was an avian-human or other reassortant, cognizance was made of the fact that there were two isolates from children from amongst 546 influenza A isolates obtained from The Netherlands from 1989-1994 which were influenza A reassortants containing genes of avian origin, viruses which have infected European pigs since 1983-1985. Thus, genetic reassortment between avian and human influenza strains does occur in the emergence of pandemic and interpandemic influenza A viruses. However, in the interpandemic periods the reassortants have no survival advantage, and the circulating interpandemic influenza viruses in humans do not appear to accumulate avian influenza virus genes.

[Influenza in the 1994/95 season; composition of vaccine for the 1995/96 season]

The 1994/'95 season in the Netherlands was marked by a limited influenza activity which only emerged in late February. The influenza activity remained elevated until the end of April, which is unusually late, and epidemic activity was only reported in the south of the country. Both influenza A/H3N2 and B viruses were isolated in this period. In addition, influenza A/HINI viruses were isolated for the first time since March 1993, from two patients. The majority of the influenza A strains that circulated in the Netherlands in 1994/'95 reacted well with ferret antiserum raised against the strains of the 1994/'95 influenza vaccine, which therefore probably offered good protection. The reactivity of the B strains to antiserum raised against the vaccine strain, B/Panama/45/90, was only moderate, which implies that the protection against the Dutch influenza B strains was not optimal. Based on the results of the worldwide influenza surveillance, the World Health Organization (WHO) has recommended an alteration in both the A/H3N2 and the B component for the vaccine of 1995/1996.

Interspecies transmission of influenza viruses

In this report we examine the hypothesis that aquatic birds are the primordial source of all influenza viruses in other species. Two partly overlapping reservoirs of influenza A viruses exist in migrating water-fowl and shorebirds throughout the world. These species harbor influenza viruses of all the known hemagglutinin and neuraminidase subtypes. In contrast to the rapid, progressive changes in both the nucleotide and amino acid sequences of mammalian virus gene lineages, avian virus genes show far less variation and, in most cases, appear to be in evolutionary stasis. There are periodic exchanges of influenza virus genes or whole viruses between species, giving rise to pandemics of disease in humans, lower animals, and birds. The periodic exchange of influenza viruses between species has been illustrated by the appearance of new pandemic influenza viruses in humans, including the Spanish influenza of 1918, the Asian influenza of 1957, and the Hong Kong influenza of 1968. Transmission of avian influenza viruses to swine in Europe in 1979 has resulted in the appearance of human-avian reassortant influenza viruses in pigs in Italy and in children in the Netherlands. These studies provide evidence supporting the possibility that pigs serve as a mixing vessel for reassortment between influenza viruses in mammalian and avian hosts and raise the question of whether the avian influenza viruses now circulating in European swine are the precursors of the next human pandemic virus.

Infection of children with avian-human reassortant influenza virus from pigs in Europe

Pigs have been proposed to act as the intermediate hosts in the generation of pandemic human influenza strains by reassortment of genes from avian and human influenza virus strains. The circulation of avian-like H1N1 influenza viruses in European pigs since 1979 and the detection of human-avian reassortants in pigs raises the question of whether these viruses actually have the potential to transmit and cause disease in humans. We now report the serologic and genetic characterization of two human influenza A viruses (A/Netherlands/5/93 [H3N2] and A/Netherlands/35/93 [H3N2]) that caused influenza in children in The Netherlands in 1993. The results show that these viruses are human-avian ressortants that were generated and currently still are circulating in European swine. This shows the pivotal role that pigs can play in the generation and transmission of avian influenza virus genes to humans and their potential to generate a new human pandemic strain.

[Influenza in the 1993/'94 season; composition of the vaccine for the 1994/'95 season]

The influenza season 1993/'94 in the Netherlands and the rest of Northwestern Europe was marked by an influenza A/H3N2 epidemic. The morbidity of this epidemic was moderate, but a high mortality rate was observed. The epidemic viruses, represented by A/Netherlands/241/93 (H3N2), were characterised by haemagglutination inhibition assays and nucleotide sequence analysis. The viruses were related to A/Beijing/32/92 (H3N2), the vaccine strain for 1993/'94, but clear antigenic differences were detected. Therefore, the WHO has recommended a new A/H3N2 component, A/Shangdong/9/93, for the vaccine of 1994/'95. The onset of the epidemic was unusually early in the influenza season. An increase in the influenza activity was already noticed in the second week of November and it reached its peak in week 49. As a result of the early epidemic, the influenza vaccination programme had not been completed yet. Therefore, the point of time for vaccinating people at risk may have to be reconsidered and moved up in order to complete the vaccination programme earlier.

Detection of respiratory syncytial virus by RNA-polymerase chain reaction and differentiation of subgroups with oligonucleotide probes

The polymerase chain reaction (RNA-PCR) was used for specific detection of respiratory syncytial virus (RSV) genomes in clinical specimens. A set of primers was selected from conserved regions of the 1B and N genes for detection of both subgroups. The primers were found to be RSV specific, all RSV strains generated a 218 bp product, and no RSV specific amplified product was obtained when nucleic acids from a variety of micro-organisms from the respiratory tract were subjected to the RNA-PCR. We took advantage of the sequence heterogeneity of the amplified products to discriminate between the A and B strains by hybridisation with subgroup specific oligonucleotide probes. This additional hybridisation assay increased the sensitivity of the RNA-PCR tenfold. The RNA-PCR was tested on clinical specimens from children with symptoms of an infection of the respiratory tract. The results were compared with isolation of RSV in cell culture and direct immunofluorescence. From 93 specimens tested, 31 were found positive by all three techniques. Six additional positive results were detected using RNA-PCR. From these 37 RSV positive specimens 33 (92%), including all 6 additional positives, were subgroup A and only 4 were subgroup B strains. Thus, the RNA-PCR is a specific and sensitive technique for the detection and subgroup classification of RSV genomes.

Detection of infections of the eye with Chlamydia trachomatis by the polymerase chain reaction

The aim of this study was to test the diagnostic feasibility of the polymerase chain reaction (PCR) for detection of infections with Chlamydia trachomatis in eye swabs from patients with conjunctivitis, and to establish the basic technique of the PCR for epidemiological survey. The results of the PCR were compared with the Mikro Trak immunofluorescence assay (IFA). From 49 specimens of patients with conjunctivitis, 31 were found positive by PCR (63%) and 23 by IFA (47%). On the other hand, in 10 normal eye specimens and 10 non-Chlamydia trachoma conjunctivitis specimens no Chlamydia trachomatis was detected.

Prospective application of reverse transcriptase polymerase chain reaction for diagnosing influenza infections in respiratory samples from a children's hospital

A prospective clinical evaluation of the reverse transcriptase polymerase chain reaction (RNA PCR) for detection of influenza viruses was carried out with specimens from 342 patients of a children's hospital in The Netherlands. The RNA PCR, carried out directly on the specimens without an organic extraction, showed a sensitivity and specificity which are superior to those of direct immunofluorescence and comparable to those of cell culture combined with immunofluorescence (culture/IF). Negative results can be obtained within 2 days by the RNA PCR but may take up to 14 days by culture/IF. Because culturing is the standard technique for the detection of respiratory viruses, at this moment there are no strong arguments to replace culture/IF with RNA PCR for the detection of influenza A virus.

Type-specific identification of influenza viruses A, B and C by the polymerase chain reaction

The aim of this study was to develop a polymerase chain reaction for specific detection of influenza A, B, and C RNA genomes. Three primer sets were selected from conserved regions of the genome coding for the non-structural proteins and were tested on 61 influenza A (22 H1N1, 9 H2N2, and 30 H3N2), 11 influenza B, and three influenza C isolates. Specific amplified products were obtained with all these strains after electrophoresis on a 2% agarose gel. The specificity of the reaction was increased by hybridization with oligonucleotide probes. When nucleic acids from a variety of micro-organisms from the respiratory tract were subjected to the PCR with these primers, no specific amplified products were generated. The sensitivity of the technique was found to be at the subpicogram level. The RNA-PCR was applied to 21 clinical specimens from patients with a culture/IF proven influenza infection. Six influenza A positive patients and 13 influenza B positive patients could be confirmed in the RNA-PCR. In two cases, influenza B positive IF specimens were found negative by the PCR. No virus could be isolated on eggs or tissue culture from these samples. RNA-PCR is a specific and sensitive technique for the detection of influenza virus genomes.

Infections of the cervix uteri with human papillomavirus and Chlamydia trachomatis

Apart from infection with human papillomavirus (HPV), other microorganisms may be involved in the development of cervical neoplasia. To study concomitant infections with HPV and Chlamydia trachomatis, cervical specimens from 4 groups of women were examined for the presence of these microorganisms by the polymerase chain reaction. The first group consisted of 143 consecutive samples from women with no cytological abnormalities who participated in a triennial screening program to prevent cervical cancer. In this group 2 samples were found positive for HPV and 2 additional samples were found positive for C. trachomatis. In the second group of 46 cytologically abnormal smears, HPV DNA was detected in 71.7% of the samples and C. tra chomatis in 4.3%. In a third group of 94 histological abnormal biopsies, the HPV prevalence ranged from 15% in mild dysplastic lesions up to 92% in invasive cervical carcinomas. Only 2 biopsies of this group (2.1%) were found positive for C. trachomatis. Finally, a group of cervical scrapes was obtained from women attending a clinic for sexually transmitted diseases. In 52 samples positive for C. trachomatis and 60 samples negative for C. trachomatis, no significant (P = 0.57) difference in the frequency of HPV infections was found (11.5% and 8.3%, respectively). The data show that in these study groups HPV and C. trachomatis are independently occurring agents.