Genetic variation among strains of wild-type yellow fever virus from Senegal.

We have examined and compared at the molecular level three strains of wild-type yellow fever (YF) virus isolated from Senegal in 1927, 1953 and 1965, termed French viscerotropic virus, Rendu and Dak1279 respectively. Over the structural protein genes, Rendu differed from the other two strains by 8% at the nucleotide level. Rendu also differed antigenically, possessing a 'vaccine'-specific envelope (E) protein epitope (i.e. an epitope previously shown to be found on 17D and French neurotropic vaccine viruses only and not wild-type strains of YF virus). Consequently, we propose that at least two distinct genotypes of wild-type YF virus have been present in Senegal. Since Rendu virus was isolated from a fatal case of YF, it would indicate that the vaccine-specific epitope on the E protein is not associated with attenuation of the viscerotropism of wild-type YF virus.

Comparison of the genomes of the wild-type French viscerotropic strain of yellow fever virus with its vaccine derivative French neurotropic vaccine.

The French neurotropic vaccine, or FNV, was used extensively in Africa to control yellow fever (YF). Although efficacious, the vaccine caused an unacceptable rate of post-vaccinal complications in children and was subsequently replaced by the 17D vaccine. Here we report that the genomes of the wild-type YF virus French viscerotropic virus and its attenuated vaccine derivative, FNV virus from the Institut Pasteur, Paris, (FNV-IP) differ by 77 nucleotides encoding 35 amino acid substitutions. Comparison of FNV-IP and three other isolates of FNV with other YF vaccine strains (17D-204 and 17DD derived from wild-type strain Asibi) revealed that during the two attenuation processes two common nucleotide changes arose that encode two amino acid substitutions: one is in the membrane protein at amino acid 35 (M-35), the other in non-structural (NS) protein 4B at NS4B-95. These common substitutions may be important in the process of attenuation of viscerotropic disease for humans and monkeys, and/or may be involved in loss of mosquito competence of the vaccine viruses.

Analysis of a yellow fever virus isolated from a fatal case of vaccine-associated human encephalitis.

The virulence of a yellow fever (YF) virus (P-16065) isolated from a fatal case of vaccine-associated viral encephalitis was investigated. P-16065 appeared identical to its parent vaccine virus (17D-204 USA, lot 6145) when examined with monoclonal antibodies except that YF wild type-specific MAb S24 recognized P-16065 but not 17D-204 USA 6145. Thus, a mutation of at least one epitope on the envelope (E) protein had occurred. Unlike 17D-204 USA 6145 and other 17D vaccine viruses, P-16065 was neuroinvasive and virulent for mice after intranasal inoculation, and neurovirulent for monkeys after intracerebral inoculation. The E protein of P-16065 differed from 17D-204 USA by two amino acids at positions 155 and 303. Changes at amino acid position 155 are found in other YF vaccine viruses that are not neurovirulent, and it is therefore postulated that the change at position 303 is involved in the alteration of the phenotype of P-16065 and may be important for virulence of YF virus.

Rapid detection of viruses of the tick-borne encephalitis virus complex by RT-PCR of viral RNA.

Studies were performed to identify a pair of primers, specific for the tick-borne encephalitis (TBE) virus complex of the Flaviviridae, with which to develop a rapid and specific identification system based on reverse transcription and the polymerase chain reaction (RT-PCR). The specificity of a putative primer pair was examined by RT-PCR of representative viruses from other antigenic complexes of the Flaviviridae and by computer sequence homology checks. All viruses of the TBE complex tested, with a single exception, were identified by RT-PCR using the identified primer pair. Accumulated data suggest that one of the putative primers identified in these studies may have flavivirus group specificity. The advantages of such a primer in the development of identification systems for all virus complexes of the Flaviviridae is discussed.

Nucleotide sequence of the envelope protein gene of the tick-borne flavivirus, Kumlinge A52.

The envelope protein gene of the tick-borne flavivirus, Kumlinge A52, the proto-type Finnish strain, has been amplified and sequenced.* The nucleotide and deduced amino acid sequence has been analyzed and compared with the closely related tick-borne encephalitis (TBE) virus, Western subtype, strain Neudoerfl, isolated in Austria. Although these two virus strains were isolated 12 years apart from different hosts and in different countries, the envelope proteins only differed by a single amino acid. It is likely, therefore, that strong selection pressures against antigenic variation exist. The possible reasons for this are discussed.

Nucleotide sequence of the envelope protein of a Turkish isolate of tick-borne encephalitis (TBE) virus is distinct from other viruses of the TBE virus complex.

Turkish tick-borne encephalitis (TTE) virus causes an acute form of meningoencephalomyelitis in sheep in the north-western region of Turkey. The clinical syndrome resembles louping ill (LI) and the viruses responsible for both LI and TTE are members of the tick-borne encephalitis (TBE) complex of the Flaviviridae. The envelope protein gene of TTE virus was reverse-transcribed, amplified, cloned and sequenced. Alignment of the resultant sequence with those from other viruses of the TBE complex reveals that TTE virus is more closely related, at both nucleotide and amino acid levels (84.6% and 96% respectively), to the Central European (CEE) subtype of the TBE virus, usually associated with human disease. The relationship with LI virus is more distant (83% and 93.5% respectively). These studies support the assertion that the ovine encephalomyelitis found in Turkey is caused by a virus that is genetically distinct from known strains of both LI and CEE viruses and from a number of other known viruses of the TBE complex.

Comparison of the nucleotide and deduced amino acid sequences of the envelope protein genes of the wild-type French viscerotropic strain of yellow fever virus and the live vaccine strain, French neurotropic vaccine, derived from it.

The envelope (E) protein genes of the wild-type yellow fever (YF) virus French viscerotropic virus and its' vaccine derivative, French neurotropic virus (FNV), were compared and found to differ by 13 nucleotides that coded for 8 amino acid substitutions. Comparison of the E proteins of FNV and 17D, the vaccine strain derived from wild-type strain Asibi, showed that there was no common nucleotide change or amino acid substitution between these two vaccine strains. However, changes are clustered around amino acid positions 52-56 and may represent the common vaccine epitope shared by 17D and FNV vaccine viruses. The molecular basis of any difference in neurotropism and viscerotropism of YF virus, attributable to the E protein, remains unclear.

Comparison of the nucleotide and deduced amino acid sequences of the structural protein genes of the yellow fever 17DD vaccine strain from Senegal with those of other yellow fever vaccine viruses.

The nucleic acid and deduced amino acid sequences of the structural proteins of the 17DD vaccine strain of yellow fever (YF) virus originating from Senegal are compared with those published for other vaccine strains of YF virus. Even though the 17D-204 and 17DD substrains of 17D diverged at passage 195 of 17D, they show a very high degree of nucleotide (99.5%) and amino acid (99.5%) homology over this region. The sequences are discussed with respect to monoclonal antibodies that can distinguish between 17DD and 17D-204 substrains of YF vaccines.

Variation in the biological function of envelope protein epitopes of yellow fever vaccine viruses detected with monoclonal antibodies.

Three different virus strains (17D-204, 17DD and the French neurotropic vaccine) have been used as live attenuated yellow fever (YF) vaccines and are manufactured in different centres around the world. The envelope proteins of these vaccine viruses were examined and compared using mouse monoclonal antibodies (MAbs) in haemagglutination inhibition (HAI) and neutralization (N) tests. The epitopes eliciting HAI and/or N were found to vary depending on the virus examined. Such variation was also found between vaccine viruses of the same strain manufactured in different centres. These data were confirmed by the use of mouse polyclonal antisera. On the basis of the MAb results in HAI tests a dendrogram of the similarity coefficients between the viruses was constructed and showed that the viruses could be placed into three major groups. Thus, it is concluded that YF vaccines manufactured in different centres are antigenically distinct as recognized by the mouse immune system.

Physical characteristics of an enclosed afferent reservoir breathing system.

We have assessed the characteristics of the Ohmeda Enclosed Afferent Reservoir Breathing System (EAR) using simulated spontaneous ventilation and controlled ventilation. The additional work of breathing through the system was measured and shown to be comparable to that of a modified Mapleson D breathing system (Bain) for fresh gas flows producing similar end-tidal carbon dioxide concentrations. It was shown under conditions of simulated controlled ventilation that end-tidal gas concentration was relatively insensitive to variations in inspired to expired ratio (I: E), tidal volume (VT) and deadspace (VD). Measurement of the volume of carbon dioxide rebreathed using simulated spontaneous ventilation led to the prediction that rebreathing of carbon dioxide would begin to occur in the EAR when fresh gas flow to total ventilation ratio (VF: VE) was approximately 0.87. However, comparison of the results of model lung tests and clinical data suggests that great caution should be taken in extrapolating such results into clinical advice.

Fresh gas requirements of an enclosed afferent reservoir breathing system in anaesthetized, spontaneously ventilating children.

We have determined the minimum fresh gas flow rate (VF) for use with the Ohmeda enclosed afferent reservoir breathing system (EAR) in 10 anaesthetized children breathing spontaneously. First, we determined the VF required to prevent rebreathing as detected by increased total ventilation (VE) and end-tidal carbon dioxide partial pressure. Second, we used a mathematical model to calculate the degree of rebreathing occurring at each VF. A VF equal to the predicted alveolar ventilation was sufficient to prevent clinically detectable rebreathing in all patients. From the model, no rebreathing occurred when VF/VE was 0.78 or more. We have shown previously that the EAR functions efficiently during controlled ventilation with a VF = 0.6 x weight 0.5. As this VF is slightly greater than the predicted alveolar ventilation, we suggest that the EAR may be used with a VF = 0.6 x weight 0.5 regardless of the mode of ventilation.

Immunogenicity of experimental live attenuated Japanese encephalitis vaccine viruses and comparison with wild-type strains using monoclonal and polyclonal antibodies.

A total of 105 hybridomas secreting anti-Japanese encephalitis (JE) virus monoclonal antibodies (mAbs) were generated from six fusions against four strains of JE virus: wild-type strains SA14 and G8924 and live attenuated vaccines SA14-5-3 and SA14-14-2 (PDK-9). Most of the mAbs (87%) elicited haemagglutination inhibition activity while only a minority (24%) elicited neutralization. None of the mAbs prepared against SA14-5-3, parent of SA14-14-2, elicited neutralization while the only mAbs prepared against SA14-14-2 that elicited neutralization recognized flavivirus cross-reactive epitopes. In comparison, mAbs raised against wild-type strains showed that a spectrum of epitopes with different specificities, including JE type-specific epitopes, elicited neutralizing activity. Two mAbs, prepared against SA14-5-3 virus, were found to be vaccine-specific and five, prepared against strains SA14 and G8924, were wild-type-specific.

Fresh gas requirements of an enclosed afferent reservoir breathing system during controlled ventilation in children.

An enclosed afferent reservoir breathing system (EAR) designed by Ohmeda was evaluated during anaesthesia with controlled ventilation in 104 healthy children. Carbon dioxide production and arterial carbon dioxide tension were measured in 12 children in order to determine the proportion of fresh gas (VF) involved in gas exchange. When the ratio of minute volume ventilation to fresh gas flow (VE:VF) exceeded 1.5, fractional utilization of fresh gas with the EAR was 0.92. This value and values of carbon dioxide production obtained from 43 children were used to derive a simple formula relating fresh gas flow requirements to body weight. The formula, VF = 0.6 x weight 0.5, was assessed in 49 children weighing 10-70 kg. The mean end-tidal partial pressure of carbon dioxide in these patients was 4.5 kPa (range 3.8-5.2 kPa). We conclude that the EAR has an efficiency of 92% in the use of fresh gas during controlled ventilation in healthy children, provided the VE:VF ratio is greater than 1.5. Under these conditions, normocapnia to mild hypocapnia was produced accurately using the formula VF = 0.6 x weight 0.5.