Virulence of variola viruses for suckling mice.

We report work done in 1971 to determine the quantitative virulence for suckling mice of 26 variola virus isolates from different countries and from cases of differing severity. Strains of recognized variola major and variola minor viruses differed up to 100-fold (expressed as the harmonic mean dose of inoculum which killed mice 2-4 d old, inoculated intracranially, in 5 d). Isolates from Indonesia and from East and West Africa gave intermediate values. Unlike tests on chick embryos, this test distinguished between African and Indonesian isolates.

A variant of variola virus, characterized by changes in polypeptide and endonuclease profiles.

A variant of variola virus is described which produces a late polypeptide of 25 kDa instead of one of 27 kDa and which has an additional endonuclease cleavage site for SalI in the viral DNA. These markers were shown to be genetically independent and to characterize 14 of the 48 variola strains which were examined. The variant strains were isolated from smallpox outbreaks originating in or from Pakistan between 1961 and 1974 and also from two cases at a Mission Hospital in Vellore, India in 1964. No variant strains were found among 9 other isolates from cases of variola major occurring in other parts of India or in Bangladesh, nor among 4 isolates from Indonesia, 15 from Africa or 6 isolates of variola minor.

Evidence for incomplete replication of a penguin poxvirus in cells of mammalian origin.

The recent discovery of a novel poxvirus [penguin-pox virus (PPV)] from Jackass penguins offers the potential of a unique candidate vaccine vector for use in mammals. Infectivity studies were therefore undertaken using a number of mammalian cell lines and chick embryo fibroblasts (CEF). It was shown that the simian CV-1 cell line was able to support replication of the PPV DNA, but no infectious progeny virus could be recovered from the infected cells. Electron microscopy was used to establish the extent of virus morphogenesis in CV-1 cells as compared to that in both chorio-allantoic membranes (CAMs) of hens' eggs and CEF cells. It appears that CV-1 cells are able to support partial maturation of PPV, but that morphogenesis does not proceed to the stage of mature infectious particles. Vaccinia virus promoters were successful in achieving transient gene expression in PPV-infected cells.

DNA sequence variation as a clue to the phylogenesis of orthopoxviruses.

We have sequenced DNA equivalent to the E5R ORF of Copenhagen vaccinia virus from an additional strain of vaccinia and from cowpox (three strains), camelpox (two strains), taterapox and ectromelia viruses. None of these showed the disruptions previously reported in the equivalent region of monkeypox virus. We also constructed a viable recombinant of vaccinia virus strain Dairen in which the E5R sequence was disrupted by a 436 bp deletion and substitution of the E. coli gpt gene. Quantitative analysis of the sequences, including available sequences from monkeypox, variola and vaccinia viruses revealed four main groupings, namely cowpox, ectromelia, monkeypox and a cluster which includes variola, camelpox, taterapox and vaccinia viruses. It was noted that, at over 75 % of the positions which differentiated species. all species but one had a common nucleotide. Although the analysis covers one single gene only, the results accord with what is known of the biology of the viruses.

Evidence for recent genetic variation in monkeypox viruses.

DNA from isolates of monkeypox virus, when digested with the endonuclease PstI, gave fragment-size profiles which correlated with the geographic area from which the isolate originated. Although some of the differences were located subterminally in the genome, others mapped to the central conserved region. Further differentiation of the viral genomes was sought by analysis of a short region within the central conserved part of the genome that appeared to be a partially deleted counterpart of an intact 1024 bp open reading frame (ORF) present in variola and vaccinia virus genomes. We reasoned that this region would not be conserved by functional selection and would therefore be likely to show more variation between isolates of monkeypox virus. The deletions found in monkeypox virus isolates from Liberia and from Benin were almost the same as that which we had previously found in the Denmark strain. A much shortened ORF, potentially coding for a product of 133 amino acids, was retained in all three West African isolates, but three Zairean isolates each showed an identical series of small insertions and deletions which effectively abolish the ORF. Three deletions, present in all isolates, must pre-date the geographical separation of monkeypox virus lineages; other, presumably more recent, changes differ between the Zairean and West African isolates. In contrast, the base similarity was found to be more than 99% when all the monkeypox virus sequences were appropriately aligned. This, in a disrupted and presumably nonfunctional gene also indicates that the changes described are recent. It is suggested that insertions and deletions occur regularly during poxvirus DNA replication, but are preserved only in sequences that are not required for continued transmission in the natural host.

Two new genome types of adenovirus 7c.

Adenovirus type 7 is the type most frequently associated with serious disease. Eighteen different genome types of adenovirus type 7 had been reported up to October 1986. The genome type Ad7c, based on the restriction enzyme profiles of SmaI and BamHI, has been reported from Europe prior to 1969 and more recently from South Africa. Here, we report two new genome types of adenovirus 7 c that have not previously been identified and that have been isolated in South Africa between 1975 and 1986 from children with postmeasles pneumonia. The two new genome types differ from the prototype Ad7c virus in having two (Ad7c1) or one (Ad7c2) extra cleavage sites for the restriction endonuclease EcoRI. These sites have been located at 3.68kb and 5.32kb from the left terminus of the genome map published for the prototype Ad7c strain. A strain resembling the prototype Ad7c was also isolated in 1986 from a case of post measles pneumonia.

Characterisation of poxviruses from sporadic human infections.

An orthopoxvirus was isolated from the vesicular rash of a man in Natal who died in coma and who had not been vaccinated. Analysis of the viral DNA showed that it was a vaccinia virus, more closely related to the virus of South African smallpox vaccine than to other vaccinia viruses. DNA analysis also showed that an orthopoxvirus isolated from a sporadic case of severe pustular rash in Nigeria was a vaccinia virus closely related to the smallpox vaccine virus used there. Minor biological differences between the viruses isolated and the corresponding vaccine strains suggested that some natural transmission of the virus had occurred, but the results of DNA analysis implied that they originated from the use of smallpox vaccine. No similar cases have been detected since smallpox vaccination was discontinued.

The virology of variola minor. Correlation of laboratory tests with the geographic distribution and human virulence of variola isolates.

Several groups of variola isolates were compared in DNA structure, and by four independent biologic markers. Isolates of variola minor from Europe and South America (alastrim virus) could be distinguished from African isolates of variola minor by DNA structure and by two of the four biologic markers. Taken as a group, the properties of African isolates, in general, differed from those of variola major, but this difference was confined to properties which depended (in the laboratory) on the recent history of the virus concerned. The suggestion made previously that there was an "intermediate" or "African" variety of variola virus is discounted. Laboratory tests did not distinguish any individual African isolate from variola major virus. It is concluded that a virus which may be called "alastrim" represents a "fixed" variant of variola virus, whose distribution is consistent with the dramatic spread of variola minor through the Americas and Europe in the early part of this century, and that variola minor in Africa in recent years was due to variola virus which was not alastrim and which laboratory evidence fails to identify as an entity distinguishable from variola major virus.

The genome structure of cowpox virus white pock variants.

A previous report described restriction endonuclease analysis of white pock variants of red cowpox virus and their characterization as deletion mutants lacking certain sequences including the repetition from one specific terminus of the wild-type genome. Further analysis has confirmed the terminal deletion but demonstrated that this is compensated at the site of deletion by the presence of an inverted duplication of a variable amount of sequence from the opposite terminus, with the effect of restoring a terminal repetition and the covalent, terminal crosslink. Nine of 11 white pock variants showed a similar deletion of about 21 Mdal mapping contiguously from the right-hand terminus and extending into a 2.4 Mdal restriction fragment. Two white variants showed larger deletions of about 24 and 27 Mdal respectively. These deletions were compensated by a copy of sequences from the opposite terminus which ranged in size from 3 to 27 Mdal. No terminal deletions smaller than 21 Mdal were observed in cowpox white variants or in clones retaining the red phenotype. In contrast with other orthopoxviruses, no deletions involving the left-hand terminus were found. Some independent white isolates had similar sizes of sequence copied from the opposite terminus, but some sibling clones from a single, pock-purified white isolate with the same size of deletion had different sizes of duplicated sequence. Other siblings isolated from an independent, three times pock-purified white clone, itself derived from a single parental red pock, differed from each other in the size of both the deletion and the duplicated sequence. These observations suggest preference for deletion in a particular region, conjunction of the genome termini during DNA replication and a requirement for the preservation of symmetrical termini in orthopoxvirus genome function.

A poxvirus antigen associated with pathogenicity for rabbits.

White pock variants of cowpox virus give papular lesions on intradermal inoculation of rabbits, without the necrosis and haemorrhage that are produced by wild type cowpox viruses. Rondle & Dumbell (1962) have shown that white pock variants of cowpox virus fail to produce a specific, precipitating antigen which they called 'd' substance. In this paper it is shown that 'd' is demonstrable in soluble antigen preparations of rabbitpox virus and of neurovirulent strains of vaccinia virus but not in soluble antigens of variola viruses. Two series of recombinant viruses prepared by Dumbell & Bedson (1964) from variola and cowpox and from variola and rabbitpox viruses were tested for the production of 'd' substance. These results were compared with the previously recorded effects of these recombinants when inoculated intradermally in rabbits. It is concluded that functional genes determining the production of 'd' and of rabbit skin pathogenicity are closely linked on the pox virus genome, but that there is insufficient evidence to say that the two functions are interdependent.

The pathogenicity of variola virus. A comparison of the growth of standard strains of variola major and variola minor viruses in cell cultures from human embryos.

The international reference strains of variola major (Harvey) and of variola minor (Butler) were grown in cultures of skin and muscle cells from human embryos. The development of infective virus, complement-fixing antigen, haemagglutinin and cytological changes were followed at four temperatures between 35 and 40 degrees C. No significant difference was found in the amount of virus produced by Harvey or Butler viruses at any of the experimental temperatures, but Harvey attained the plateau titre at 16 h, some 4 h ahead of Butler in the cultures incubated at 38 degrees C. Harvey also produced a higher and more prolonged yield of virus in the extracellular medium of cultures, inoculated at low multiplicity and incubated at 37 degrees C. At 38 degrees C small inocula of Harvey produced foci which developed and spread till the whole culture was necrotic; Butler foci did not spread and remained relatively undeveloped at this temperature.Staining with acridine orange showed the development of cytoplasmic inclusions at all temperatures up to 39.5 degrees C, at which temperature most inclusions remained round and discrete and susceptible to digestion with DNase. The yield of virus at all temperatures correlated well with the number of cells in which the DNA cytoplasmic inclusions became irregular in outline, diffuse and insusceptible to digestion with DNase. It was concluded that elevated temperatures, up to 39.5 degrees C affected principally a maturation phase in the development of the virus.Equal amounts of complement-fixing antigen were produced at all temperatures and by either virus, but the late product, haemagglutinin was depressed at elevated temperatures much more in cultures infected with Butler than in cultures infected with Harvey. This was clearly shown by haemadsorption; in cultures, infected at high multiplicity and incubated at 39.5 degrees C Harvey gave semi-confluent haemadsorption, while only an occasional haemadsorbing cell could be found in the cultures infected with Butler virus.It is concluded that there were two ways in which temperature affected the growth of variola viruses in cultures of human embryo skin and muscle cells. The total yield of virus was reduced by inhibition of a stage in the maturation of the virus with a cut off point between 39.5 and 40 degrees C at which no cells produced infective virus, and with little observable differences between Harvey and Butler viruses. Changes at the surface of virus-infected cell, involving virus release and haemagglutinin, were affected independently of virus maturation; in these changes Butler was much more sensitive than Harvey to elevated temperatures. The relevance of these observations to the development of variola major and variola minor in man is discussed.

Laboratory investigation of two "whitepox" viruses and comparison with two variola strains from southern India.

Two variola-like viruses were isolated in Bilthoven in 1964 from monkey kidney tissue cultures. These viruses, coded 64/7255 and 64/7275, have been considered as two of the six "whitepox" viruses isolated from animal tissues, all of which are indistinguishable from variola virus by laboratory tests.Two specimens from suspect smallpox cases in India were examined in the Bilthoven laboratory at about the same time as the "whitepox" viruses and two strains of variola virus were isolated from them. A detailed comparison of certain biological markers of these four viruses, and of their DNAs, shows that the two "whitepox" viruses could not be distinguished from each other or from one of the two variola isolates. In view of this, and since there was a possibility of cross-contamination at the time of isolation, it is concluded that 64/7255 and 64/7275 must be regarded as genuine variola viruses and be deleted from the list of variola-like viruses isolated from animal tissues.

Comparison of white pock (h) mutants of monkeypox virus with parental monkeypox and with variola-like viruses isolated from animals.

Monkeypox mutants arising spontaneously or after serial, high multiplicity passage were characterized phenotypically and by restriction endonuclease mapping. Some resemble "whitepox" and variola viruses in several of the markers tested but all are distinguishable phenotypically from these. None resembles "whitepox" viruses in genome structure although near-terminal deletions or symmetrical, terminal rearrangements, relative to parental monkeypox, occurred. "Whitepox" viruses isolated from animals closely resemble variola in both phenotype and genome structure.

Studies on Cotia virus--an unclassified poxvirus.

This paper is a report of studies on Cotia virus; this had been first isolated in 1965 in Brazil and was subsequently shown to be a poxvirus. Cotia virus grew in a wide range of cell cultures and on the chick chorioallantois (CAM), Its growth characteristics are similar to those of other poxviruses. Microscopy showed virus factories or type B inclusions appearing before infectious progeny virus could be demonstrated. Type A inclusions appeared later, after development of progeny virus; these were shown by electron microscopy to differ from the type A inclusions of cowpox and other poxviruses and they have been termed Cotia bodies. Immunofluorescent staining also showed ring structures which appeared before the development of Cotia bodies. The growth of Cotia virus in human embryo lung (HEL) cells was sensitive to inhibitors of DNA and protein synthesis but was resistant to a concentration of rifampicin which inhibited vaccinia virus. Sharing of antigens between the Cotia virus and vaccinia virus was shown by gel precipitation tests and immunofluorescent staining. There was no cross neutralization between Cotia virus and vaccinia virus nor did anti-Cotia sera neutralize representatives of other poxvirus groups.

Concurrent smallpox and chickenpox.

Three patients showing smallpox- and chickenpox-like lesions simultaneously were investigated virologically. Both infections were confirmed in the laboratory and, in one case, by electron microscopy.