Recent years have seen an increasing, rather than decreasing, incidence of viral diseases that had been confidently supposed to be under control. Chief among these is yellow fever - the disease which almost stopped the building of the Panama Canal - which is still endemic throughout the tropics, despite there having been extensive eradication and vaccination campaigns over several decades. Now it is threatening to spread to more temperate zones, including the continental US, largely as a result of inadvertent importation of vector mosquitoes in water in used tyres transported from Asia to the US. Another factor in its recent epidemiology has been the settling of cleared forest areas in the Amazon and elsewhere, which results in disturbance of the natural mosquito-small mammal virus transmission cycle. Although there has been a vaccine for the virus for many years, it is apparently not particularly effective in terms of conferring lasting immunity, and there have been complications resulting from its use: these factors and the recent spread of the virus combine in making it essential to engineer new and more efficient (and cheap!) vaccines for combatting its spread.
Although it is not an emerging virus, hepatitis C virus was only finally identified (after years as non-A-non-B-hepatitis virus) as recently as five years ago. This was as a result of cDNA cloning of a short viral sequence from the nucleic acid in patient serum followed by sequencing, synthesis of PCR primers, and "walking" along the hardly-detectable genome, till the entire genome had been cloned and sequenced. Although the sequences of at least five distinct strains of HCV are now known - thanks to cDNA PCR - and the true incidence of the virus is finally understood, HCV has not yet ever been isolated as RNA or even as particles from patient serum. That we know so much the virus despite the fact that it never either been purified or cultured, is due entirely to modern molecular biology techniques, such as diagnosis of HCV in serum samples by PCR. It is also possible to do molecular epidemiological studies on HCV by using PCR products for molecular hybridisation and sequencing.
The recent spate of publicity surrounding what was called the "Four Corners virus" - and now known to be a hantavirus, sometimes called Muerto Canyon hantavirus - has brought home with a vengeance the realisation that even First World countries such as the USA are not immune to the sudden emergence of an indigenous disease with life-threatening potential. Muerto Canyon virus is related to the Hantaan virus which caused great problems among GIs in the Korean War. Both are members of the genus Hantavirus, family Bunyaviridae, have three segments of negative-sense single-strand RNA, and have small rodents (deer mice in the case or MC virus) as natural hosts. Humans are exposed to the virus when rodent infestation of dwellings is high, when rodent urine and faeces serves as inoculum. MC virus became prominent because of its high fatality rate - 60% of confirmed cases have been fatal - and because it was first recognised in the Navajo country of the US south-west, which sparked fears among potential tourists across the country.
Identification of the virus as a novel hantavirus had occurred within a month of the first alarm over the disease in May 1993: first attempts to grow the virus failed, but serological screening of patient samples indicated a hantavirus. Subsequent progress depended on degenerate primer PCR, which allowed amplification via cDNA of a conserved sequence which could be used as a molecular probe, and could be sequenced. Use as a probe allowed detection of virus in deer mice, and post-mortem tissue samples. Sequencing allowed molecular cloning of most of the virus genome, and further sequence proved that the virus was novel. It has also been found all across the US, indicating a wide distribution. Subsequent molecular probing of paraffin-embedded tissues proved also that the disease must have been around as long as 15 yrs. PCR amplification using newly designed degenerate primers has shown that other novel hantaviruses are present in other rodents elsewhere in the US. Within a year of the initial identification of a cluster of suspected hantavirus pulmonary syndrome has come not only a confirmation of the disease, but molecular characterisation and epidemiological studies that have done much to allow prevention of outbreaks of the same and similar viruses. This progress is thanks to the rapid application of sophisticated biotechnology after an initial serological diagnosis.
As stated earlier, this outbreak is warning that severe and previously unidentified diseases could strike at any time in any society: Joshua Lederberg warns (ASM News, 60: 233) that we may well be in line for another influenza epidemic on the scale of the 1918 Spanish 'Flu, which would spread far faster and potentially reach more people, thanks to modern transport systems and porous borders. Although we partially understand the mechanisms which give rise to epidemic 'flu strains - genetic reassortment of human, avian and porcine strains in Chinese intensive farming schemes - there is the distinct possibility that other, more lethal diseases may well be about to burst out of some ecology that we do not understand, but have somehow upset to the point that hitherto secluded/sequestered virus reservoirs, or novel vectors, are coming into contact with humans.
As an example, although monkeypoxvirus causes smallpox-like symptoms in humans, it is not nearly as contagious. However, we do not know what might make it more so - and as it gets into humans living in tropical trainforests in Africa due to contact with arboreal rodents, and pressure on rainforests is becoming more acute, it may become a matter of necessity that we find out just why it has not yet caused an epidemic. Like the HIV work that is being done here and elsewhere, the monkeypox work focussed on looking for changes on viral genomes in the human and animal populations which could explain differences in biology. In the case of monkeypox, important differences were found relative to the smallpox genome which could explain differences in human disease potential. In the case of HIV - which, thanks to molecular epidemiology and molecular phylogenetic investigations has almost certainly been shown to be an escape from monkeys - the virus exhibits far more variability in humans due to rapid mutation, than in monkeys, in which hosts it is most often temperate i its effects. However, knowledge of the variability of the virus has allowed the formulation of test kits that more accurately detect virus in African patients (in whom the variability of strains is greatest).
Although the emerging virus problem in the human sphere is more important in social terms, emerging virus diseases in crop plants should not be overlooked in terms of their potential impact on the human population. For example, a speaker at the last International Congress of Virology in Glasgow in 1993 cited HIVs, yellow fever, hantaviruses, and tomato spotted wilt and whitefly-transmitted geminiviruses as beng the most important of the emerging or re-emerging virus diseases affecting humankind.
TSWV is - coincidentally - another of the Bunyaviridae, has a very wide plant host range, causes a great deal of damge to infected plants, and is cutting a swathe through agriculture from Asia, through the Americas, in Europe, and in Africa. Its spread has been due to diseemination of its thrips insect vector, and of diseased plant material. Again, molecular biological techniques such as PCR have allowed its rapid identification and characterisation of strains and distinct related viruses. Molecular epidemiology of the virus is a logical corollary, and is proceeding by means of large-scale internation collaborations.
WT- geminiviruses have caused a great deal of crop damage in plants such as tomatoes, beans, squash, cassava and cotton, and their spread may be directly linked to the inadvertent world-wide dissemination of a particular (so-called silverleaf) biotype of the whitefly Bemisia tabaci. This happens to be a particularly indiscriminate feeder, and a very good vector, allowing rapid and efficient spread of viruses from indigenous weed species to neighbouring crops. The detection of these viruses in plants and vector has again been hugely aided by the advent of PCR and molecular hybridisation. To illustrate how rapid is the advance in our understanding of these viruses thanks to modern techniques, there are currently over 40 complete and hundreds of partial viral genomic DNA sequences known, compared to only 1 complete sequence in 1983. PCR and sequencing and restriction mapping has similarly allowed the characterisation of a wide variety of strains of the extremely agriculturally important maize streak virus, and to show that viruses infecting wheat and grasses and sugarcane are distinct from those infecting maize, meaning traditional ideas on virus spread will have to be re-examined.