100 years of virology
'100 years of virology' was published by the IBMS Historical Committee and launched at the IBMS Biomedical Science Congress 2001. You can contact the Historical Committee at 12 Coldbath Square, London, EC1R 5HL.
Early horse-driven centrifuge
The virus before virology
Mention virology in the course of the history of pathology and the question arises as to how recently it emerged as a science. Consider the nature of viruses and their existence and the story can be traced back to at least Hippocrates (fifth century BC). A bas-relief dating from the 18th century Egyptian dynasty (approx. 1500 BC) shows the shrivelled leg of a priest - a characteristic of the consequences of paralytic poliomyelitis.
The term virus was used in the Middle Ages and comes from the Greek word for poison or Latin meaning slimy, poisonous or malodorous liquid. Its application in those early days did not have the modern understanding of the word. By the late 1930s scientists were regularly using the term 'filterable virus' to describe those agents capable of passing filters fine enough to retain bacteria.
Identifying a 'father of virology' is uncertain. Beijerinck, Ivanovski and Loeffler have all been mentioned for their short term efforts but the science grew from the consistent work of many scientists interacting over long periods in different disciplines throughout the world.
In 1885 Martinus Beijerinck, working with tobacco mosaic virus (TMV) showed that the sap of diseased plants was infectious after filtration through the bacterium-proof Chamberland porcelain filter but assumed it was soluble in water and designated it 'contagium vivum fluidum' in his paper of 1898. Meanwhile in 1892 Dmitri Ivanovski working independently with the same agent also found the same phenomenon. It is interesting to note that the word 'virus' does not appear in his paper of that year. Nevertheless this plant pathogen was the first replicating member of the sub-microscopic group to be recognised.
A glance at the Milestone dates (Table 1) shows that thereafter there has been a steady progress in the identification and understanding of virus infections across the biosphere.
Virus infection in all forms of life have been known since the beginning of civilisation and probably before that yet their significance has only emerged since 1892. However this did not deter early workers from searching for treatment, cures and prevention. Edward Jenner made interesting observations in 1776 that eventually led to the successful vaccination against smallpox in 1796, although Lady Jane Wortley Montague introduced the practice of variolation to England from Turkey in 1721. Louis Pasteur became deeply involved in experiments with rabies culminating with the first human use of post exposure vaccine on a nine year old boy, Joseph Meister, in July 1885, the vaccine having previously only been shown to be efficacious with dogs.
Such ingenuity was not confined to these two pioneers. History records efforts in many directions where success did not come until much longer, inspired by a greater understanding of the possible causes and the nature of the agent and its route of transmission and site of infection.
Another glance at Table 1 shows the first account of the recovery of a human filterable agent causing yellow fever. Thereafter comes a steady flow of filterable viruses together with techniques, materials, instrumentation and systems to aid the development of virology. Sometimes technology appeared before full advantage of the scope afforded could be appreciated. The electron microscope (EM) was invented in 1932 and although tobacco mosaic virus particles were first demonstrated in 1935, by its use, it was not put to its full potential much before the 1950's when some of the first virus laboratories were established. Today the EM has a specific role alongside the latest techniques in virus diagnosis.
The first approaches to virus diagnosis
Since virology became established the question has been repeatedly asked 'What is a virus?' Three theories have emerged. The first, the regressive theory of Green 1935 and Laidlaw 1938, considers viruses to be the end product of a degenerative process from higher organisms via micro-organisms. The second theory by Andrewes 1966 and Luria 1967 suggests viruses to be modifications of cellular RNA and DNA. The third theory points to the evolvement from complex polymers - a coevolution with cellular life forms. There is yet a fourth theory put forward by Professor Sir Fred Hoyle, astronomer - that viruses and in particular influenza and AIDS, arrived here from outer space. The question still remains 'is virus a living entity?' - a suitable subject for debate.
In the meantime there is no doubt of their existence and their presence poses threats to the wellbeing of all life as we know it, unless notice is taken and steps to control them and perhaps eradicate them.
Some viruses appear to cause no harm to their host. Tulip mosaic or break virus, is the oldest known plant virus disease and although an infectious agent, has produced diseased varieties highly prized by bulb growers. Some human viruses are happy to live cheek by jowl with their hosts only to spring into pathogenic activity by one stimulus or another. Others appeared to have no effect during their passage through the human gastro-intestinal tract (eg. Enteric Cytopathic Human Orphan virus or echovirus). Zoonotic infections can turn apparently timid viruses into pathogenic monsters. None can be considered commensal in their existence.
Early observance of virus infections were sometimes seen in cellular changes, although the link with a virus remained unclear for sometime. Several workers identified inclusion bodies and associated them with particular diseases. These usually bore the name of the discoverer, e.g. Negri bodies associated with rabies, the Lipschutz bodies of herpesvirus, Bollinger bodies of fowl pox and Guarnieri bodies of smallpox. More can be found in the older texts on virology. Tinctorially they are most pleasing to see 'in situ' in a section or sometimes in a scrape where it can be extremely useful in the clinical setting.
At the beginning of the last century these findings and clinical observance were the essential tools for the diagnosis of a virus infection. At this stage laboratory techniques were based mainly on well founded bacteriological methods. Gradually the information on the biological and physio-chemical properties of viruses accumulated.
Meanwhile tissue culture was emerging with early work on cell survival leading to cell growth 'in vitro' (Table 2). These early techniques were stifled by the lack of a balanced growth medium and antibiotics. However it is recorded that Carrel demonstrated that animal cells could be grown 'in vitro' indefinitely. By applying his surgical knowledge of asepsis he kept a strain of cells in active multiplication for 34 years! His meticulous attention to asepsis was seen to be too tedious by other biologists at the time and this delayed progress in development and application for a while.
The development of the organ culture technique, by Strangeways and Fell in 1926, involving small fragments of tissue in a state close as possible to their state 'in vivo', has become one of the traditional methods. In the 1960's this technique was successfully applied to the isolation of coronavirus from respiratory infections using human embryonic trachea fragments. However limited supplies of this type of tissue has prompted alternative systems for its identification.
The next step
Perhaps the most significant single event in the development of diagnostic virology was Enders and his colleagues' success in growing poliovirus in non-neural cell culture in 1949. But before that time was reached steady progress was being made by several scientists in different ways. Collectively this knowledge was applied to the better understanding of a virus, its biological behaviour, its size and shape and some of its physico-chemical properties, thus putting it in a separate category to bacteria.
Growth of the virus was among the first considerations. Today it remains an important part of laboratory diagnosis; so much so that several laboratory procedures still regard cell culture as the 'gold standard'.
Use of the laboratory animal was the early method of virus growth from patient material. This depended on collecting the correct specimen and finding a susceptible animal. The rhesus monkey was found to be susceptible to poliomyelitis after inoculation with spinal cord material from a fatal infantile infection (Lansteiner and Popper 1908). Then came neutralisation tests in a variety of laboratory animals using convalescent serum in the first tests and later hyperimmune sera produced from antigens prepared from tissue extracts.
Animal work is a speciality in the study of viruses and has been confined to specific tasks that have played an important part in its history and continues to do so today. Animal work requires intensive husbandry and management and has not always been the first choice for laboratories. Where animal inoculation is required referral of specimen and other material is the usual procedure. Whilst alternative techniques have evolved, some virus infections still rely on the use of animals for their isolation and subsequent identity eg. Coxshackle group A viruses and rabies.
By 1932 attention was directed to the use of the developing chick embryo for virus isolation. Almost every part of the egg would eventually be utilised for virus growth. Two of the first viruses to be grown in 1932 were vaccinia and herpes simplex on the choriallantoic membrane (CAM). In 1935 variola was grown on the CAM and later, in 1940, influenza A & B were grown in the amniotic and allantoic cavities. Influenza virus had been grown previously by intra-nasal installation in ferrets. This route had the advantage over eggs in that antibody was also produced. By 1946 the use of fertile eggs for virus isolation was an important part of the laboratory's diagnosis and research.
A very useful MRC publication by B I B Beveridge and F M Burnet 'The Cultivation of Viruses and Rickettsiae in the Chick Embryo' (1946) was amongst the first publications produced with virus techniques in mind and specifically for viruses grown in eggs. Under the section Rickettsiae appears the heading Psittacosis. This group of agents is now classified as the Chlamydiacia and, in spite of being bacteria, it has traditionally fallen into the province of virology because of the obligatory cellular parasitic nature. It is interesting to note that this book refers to them as 'viruses'!
The fertile hen's egg reached its greatest importance between the 1950's and late 1970's during which time it was used almost exclusively for the isolation and, with associated techniques, the identification of two major influenza pandemics - the Asian 'flu' of 1957 and Hong Kong 'flu' of 1968. Today the fertile egg remains the primary route of growth for the production of influenza vaccines. Its popularity for other virus work waned as cell culture became more sophisticated and other procedures became simpler and more direct in their application.
An important development
The use of human embryonic tissue culture by Enders, Weller and Robbins to isolate poliovirus in 1949 heralded a big step forward in the laboratory diagnosis of virus infections. Several sources of tissue produced a wide variety of cell lines providing fresh, semi-continuous and continuous cultures. The availability of antibiotics helped ease the difficulty of growing and maintaining cell lines. Even so some were culturally demanding and it would appear that some workers had a particular flair for growing cells that went beyond standard procedures and were referred to as having 'green fingers'. The previous use of trypsin to disperse cell clumps by Rous and Jones (1916) was now applied vigorously to the preparation of single sheet cell culture from tissue and to subsequent sub-culture.
Rooms were set aside and declared 'clean areas' to separate cell culture production from laboratories where speciman inoculation took place. Success during this early period required meticulous attention to the quality and cleanliness of glassware and its subsequent handling in order to avoid toxic chemicals either - in the glass itself, or in their retention due to inadequate washing. As cell culture for virus growth became first choice the expertise in cell maintenance improved and better materials came on the market. Precious cell lines, usually at low passage and particularly sensitive to selected virus growth, could be stored at low temperatures, first at -70 degrees C and later, using liquid nitrogen, at -196 degrees C.
By the late 1950's cell culture was being applied to all infections thought to be of viral origin. The rewards were plentiful, although some diseases failed to yield the causative agent by this method. However the introduction of new techniques and acquiring more information on the virus by alternative systems has enabled some to be virtually recovered by cell culture. Some viruses remain undetectable by this method today, eg hepatitis C and hepatitis E. Choice of cell lines for the investigation of different types of infection, eg respiratory, enteric, neurotropic or cutaneous would give a preliminary identity by its affinity for growth in particular cell lines and by its pattern of cytopathic effect (CPE).
Throughout the 1960's cell culture dominated the diagnostic laboratory for it not only provided good culture medium for virus growth from a wide variety of specimens but it also served as a useful vehicle for identifying virus type and strains. The ability to acquire virus in high concentrations meant that it could be used as a source of antigen in other test systems or for raising high-titred heterologous and homologous antiserum in laboratory animals. The product was then used in a variety of tests to identify infecting viruses.
A further step
With the plethora of viruses being isolated a need to put some sort of order into their classification was urgent. Several viruses had acquired colloquial or vernacular names eg Coxsackievirus named after the town in New York State USA from where faecal material from two children yielded the first isolates in 1948. Use of the vernacular remains part of virus classification, eg Lassa and Ebola viruses. The International Committee on Nomenclature of Viruses (INCV) was established in 1973 and was subsequently renamed the International Committee on Taxonomy of Viruses (ICTV). Among its first set of rules was one which stated that 'the code of bacterial nomenclature shall not be applied to viruses'. Another ruled that 'no person's name shall be used'. Virus classification has been periodically revised since that time. Approximately 30% of some 4000 species or types of known viruses are now classified with the greatest progress in animal viruses (45%), just 6% of 2200 bacteriophagees and 55% of 1000 plant viruses.
Some of the early discoveries were now to be put to current use (1950's onwards). Alongside the establishment of virus culture was an assessment of what could be gained from applying some of the early principles. Size, shape and chemical composition would help in the classification of a virus. Filtration through membranes with graduated pore size and use of ultracentifugation and electron microscopy provided details of size, shape and symmetry, eg Tiselius and Gard (1942) were able to show the first EM images of poliovirus. Molecular biology and virology had been developing along parallel lines since the turn of the last century and in the 1940's and 1950's discoveries were made in the areas of bacteriophage and tobacco mosaic virus (TMV) showing the genetic material in reproduction to be nucleic acid. Viruses contain either RNA or DNA but not both at the same time. Different viral species contain nucleic acids which differ in length and nucleotide sequence. The genome is contained in a nucleoprotein compartment, the nucleocapsid, which also contains capsid structural proteins and replicative enzymes. A viral envelope - where present - consists of a membrane derived from the host cell containing viral protein and surrounds the capsid. These were to become important features of classification.
The physical properties of some types of virus were used to complement preliminary identification during culture. The ability of some groups to agglutinate red blood cells of various species of mammals and birds, especially the orthomyxo and paramyxoviruses, was applied during their growth in egg and cell culture, where in the latter it was adapted to a haemadsorption technique. Other properties such as optimal growth temperature, stationary or rolling cultures and chemical resistance all contributed in their identification. Strain and serotyping required the use of specific antisera.
The shape and size of the virus became an important feature in the distinction of one type from another using the electron microscope. A glance at a selection of electron micrographs will show the wide variation in shapes - enveloped or non-enveloped, helical or icosahedral. Some appear with rigid geometric symmetry, others are highly pleomorphic. Unfortunately the EM alone cannot accurately differentiate between viruses causing different infections that happen to be members of the same group, where their shape, size and disease patterns are similar, eg echoviruses and enteroviruses are all members of the picornaviridae, where several clinical symptoms are similar. Other limitations included the need to have a relative high concentration of viral particles (106 - 107 particles/ml). This was overcome by the use of ultracentrifugation, cell culture amplification or immune enhancement. Some of the exanthematous viral infections are capable of producing this level of viral concentration naturally in vesicular eruptions.
Negative staining techniques were introduced in 1959 by S Brenner and R W Horne giving a sharper contrast to the virus particle, enhancing resolution and revealing ultrastructure. This technique is now standard procedure in the examination of virus suspensions, whether prepared in the laboratory or when examining material direct from the patient eg vesicle fluid. Examination of ultra-thin sections from virus infected tissue was a further development but has been a routine first choice. Another alternative has been the use of the scanning facility that some models of EM's provide. These last two methods are usually applied in research work, being too time consuming and cumbersome for routine diagnosis.
One of the advantages of direct specimen examination in the EM is the immediacy of reporting. This was and still is, an important factor in the early diagnosis of a virus infection. As 'modern virology' established itself such a service appeased clinicians anxious for rapid reports which were not so forthcoming when using culture systems or the earlier serological methods.
Electron microscopy is a facility not available to every laboratory. It is an expensive outlay and hardly cost effective for laboratories with a low specimen turnover. It requires considerable experience from the operator for its maintenance in good working order and for specimen processing and subsequent examination and interpretation.
The need for a diagnostic virology service
The adaptation of cell culture techniques for the isolation of viruses was a tremendous boost to virus diagnosis by complementing the earlier established techniques. To extend this service further methods of identification were required. Until this stage serology played only a small part, being limited to those few viruses that were grown and from which antigen could be produced.
The fact that tests could now be used to detect antibody levels in patients from whom no virus had been isolated extended the diagnostic arm to retrospective as wel as current infections. Virus isolation depends on a great deal on the most suitable specimen being collected at the best time of infection. This tends to be during the early stages, sometimes the prodromal period when perhaps the patient has not reported sick. Early serology began to address that problem.
The four principal ways of measuring virus antibodies levels in the 1950's were the complement fixation test (CFT), haemagglutination inhibition, neutralisation and precipitin or agglutination.
The CFT was first used in bacteriology in 1909. Its first application to virological problems met with limited success due to the crudeness of the antigens available. As their sensitivity and potency improved it became an important method in both routine and research. The test depended on acute and convalescent sera being tested to determine a diagnostic rise in titre against a viral antigen over a 10-14 day period. Unfortunately this put it in the retrospective diagnosis bracket. In spite of this disadvantage it was an important tool that could be used to simultaneously test a serum against a panel of antigens such as polio, mumps, influenza, parainfluenza, adenovirus and herpes. Also it usually included psittacosis and Q fever. Although challenged for its usefulness by some workers it is still in use today.
Neutralisation and haemagglutination inhibition tests tended to have a more specific reaction. Whilst the CFT was an overnight incubation, neutralisation would usually take several days to obtain a result and required attention to detail in setting up the test. Tests for serum antibody were performed in animals and in cell culture. When the neutralisation test was 'Hobson's choice' its results were nevertheless most useful as they proved in the determination of antibody during a rubella infection in early pregnancy. Serum antibody is now determined within the working day using several alternatives to the original method with greater specificity.
Red blood cell agglutination is the property of several groups of viruses but it has frequently been applied to the ortho and paramyxoviruses. It is a short incubation period test and can be used to titrate specific serum antibody, or to determine the antigenic identity of freshly isolated virus. This is most important during an influenza outbreak and also for the selection of antigenic types for vaccine production. The principle of haemagglutination was adapted for direct use on cell cultures when searching for early virus growth or when the development of cytopathic effort was weak or hardly visible with the microscope. The paramyxoviruses benefited most from this method.
The remaining alternative of precipitin had few applications but, when based on the Ouchterlony agar gel precipitation test, gave indisputable lines of identity for variola/vaccinia versus anti-vaccinia rabbit serum. The advantage of this test was that a result could be obtained within the hour. However only a positive result was significant.
The use of coated red blood cells in the agglutination test for antigen or antibody was available in 1957 (Scott et al) and only came into regular use with the introduction of test kits that included the already sensitised rbc's. The third generation test kits for hepatitis B included the reversed passive haemagglutination test (RPHA).
From the foundation of the four basic approaches to viral serology a whole host of serological test systems emerged. Sensitivity and specificity became an important consideration. Virology was expanding at a fairly brisk rate and required a sophisticated service that was accurate and reliable, relatively simple and safe to handle, cost effective and easily available. Immunology was providing much to the design of test systems. Increasing knowledge of the virus and Its disease process was dictating what type of test was needed. The semjnal work of Kohler and Milstein in 1975 on the production of monoclonal antibodies enabled highly specific antibody to be used in these tests.
With the general availability of immune reagents for most viruses several formats of the immunological test were introduced. These included immunofluorescence (IF), radioimmunoassay (RIA), enzyme immunoassay (EIA), immunoblotting ("Western blot") and lmmunocytochemical staining. Applying a single method to a variety of diagnostic demands, where generality rather than specificity was the end result, were being superseded. These basic procedures proved very adaptable, resulting in the most suitable choice of a test. Now antigen and antibody types could be estimated; identifying early, middle or late infection, most crucial to clinical management. This was most obvious in the areas where virus could not be grown, or only with difficulty. The simplicity and speed of some of the tests also meant immediate results were available. EIA and IF remain the most popular systems with Western blot as an important confirmatory test. RIA has lost favour because of its obvious hazards during handling of some of the reagents. Here is a good example of the rapidity with which diagnostic virology was advancing in the 1970's in that a relatively new technique was already being side-lined In favour of other equally sensitive yet more practical methods.
Accompanying these scientific developments was a serious need to look at the technology required to achieve them. Microtechnology was the answer. It provided systems for precious small volume work and for easy batch testing and subsequent reading of results. Increasing numbers of specimens for serology dictated the need to streamline techniques. Manual methods moved rapidly - from single transfer to multi-transfer systems as the specimen volume used became smaller. Reading results by eye was difficult to judge for some of the new EIA test endpoints The design of instruments capable of reading results without variation and based on standards and controlled materials were now providing a reliable and repeatable system.
With the continuing development of microtechniques there was a growing knowledge of sub-cellular virology. Molecular virology was now moving into diagnostic work. In 1985 Mullis and co-workers described the polymerase chain reaction (PCA). A little later other nucleic acid techniques would emerge but PCR was the most developed and now, with amplicor sequencing, it currently plays a pivotal role in patient monitoring, especially concerning viral load during HIV treatment.
The increasingly sophisticated immunological profiles which these latest tests can provide has extended the virological service into the realms of transplantation, blood transfusion and products and oncogenic studies. This is another story in the development of virology that has only recently begun. For more comprehensive information on this period reference should be made to the numerous text books on the subject and on individual viruses.
Working with viruses
Aseptic techniques, developed out of the need to prevent cross contamination between specimens and between cultures, has remained the fundamental approach across the spectrum of microbiology. The protection of the worker at the bench was governed by the awareness of the dangers of handling infectious material and by precisely following the prescribed procedures to avoid the incidence of laboratory acquired infection. Microbiological rules were applied in a similar fashion to virology when most of the early work was conducted on the open bench. This was soon to change as more viruses were being isolated and the volume of work increased. Higher standards required designated areas for certain types of work eg. the separation of cell culture maintenance from virus cultures. Safety cabinets became the norm and eventually became imperative. From the early simple 'glove box' emerged specifically designed biological safety cabinets whose designs were first applied in the space industry and later adapted to the needs of microbiological laboratories. The cabinets are classified into three categories. Class I is the classic exhaust protective cabinet found in almost every virus laboratory, Class Ill provides a complete barrier to the operator with gloves physically attached to the unit. Class 8 cabinets are used for cell culture production and maintenance.
In the 1950's there were few published rules and recommendations for handling infectious agents. By 1972 this paucity was being addressed by the then Department of Health and Social Security. Thereafter came a succession of committees and publications, several of which carried legal status. Two laboratory leaks of smallpox in the 1970's induced further legislation. Safety is now part of all aspects of a laboratory service from the bench to stafflng and training, management, laboratory design, equipment and materials.
Control and prevention
Before virus laboratories were in a position to offer a useful service to the clinician, indeed before the modern concept of a virus, hopes of combating these diseases were raised by two particular events.
In the latter part of the 18th century Edward Jenner, with the observation of the course of cowpox infection and the apparent protection it afforded against smallpox, eventually successfully vaccinated the now famous James Phipps in 1796. This was the classic experiment but was not his first. The first experiment was the successful vaccination of his older son in 1789 who was challenged some ten months later with swinepox. Jenner first heard that cowpox protected against smallpox whilst in apprenticeship to a surgeon in Sodbury near Bristol where a milkmaid once said to him when he thought she might be incubating the disease "I cannot take that disease, for I have had the cowpox". Jenner's work was at the threshold of vaccination.
It was nearly a century later when Louis Pasteur developed rabies vaccine and in 1885 had success with his post-exposure vaccine, that he had previously used only on dogs. The patient was a nine year old boy, Joseph Meister, another famous name in the history of vaccination. Both these achievements were accomplished without either claiming to have seen the virus, although Pasteur could differentiate between rabid and non-rabid brain tissue histologically. Pasteur was also fortunate to have attenuated a virulent strain of fowl cholera from which he had produced a vaccine. With dogged determination he applied the same technique of ageing to his 'fixed' rabies virus which eventually produced attenuation. With a series of graded levels of strength of virulence of the virus over two weeks of injections the child's wounds were healing whilst he continued in perfect health. It is recorded that neither Jenner nor Pasteur slept easily during the passage of these primary experiments!
A third important event took place in the early 1950's, amidst the formative years of virology. For some time the hope of producing a vaccine against poliomyelitis had suffered a series of stumbling blocks. Finally, with the success of Enders and colleagues growing poliovirus in non-nervous cell culture, this paved the way to easier vaccine production. Large scale field trials were organised in 1954 with formalin inactivated Salk vaccine. From 1962 onwards Sabin's oral vaccine using live attenuated virus began to take over.
These three examples of successful vaccines gave every hope of further success as more viruses were identified. This was in spite of the relative difficulties of designing the polio vaccine where three antigen types were involved. In comparison Jenner's discovery was not plagued with such problems, cowpox giving protection and vaccinia and smallpox being similar serologically. This antigenic simplicity and stability has been the main feature of the ultimately successful vaccination programme that was designed and pursued by the World Health Organisation (WHO) to the eradication of smallpox from the globe by 1980.
The problems of producing an effective and safe vaccine has restricted success so far to just ten virus infections. Six are live attenuated and five are killed vaccines, (polio vaccine is available in both live and killed versions). The infections covered by these vaccines include smallpox (no longer required), rabies, yellow fever, mumps, influenza, polio, measles, rubella, hepatitis B and hepatitis A. Other candidates include rotavirus (one of the causative agents of viral gastroenteritis and a serious problem for infants and small children in developing countries), respiratory syncitial virus (RSV), varicella-zoster (VZV) and parainfluenza. The International Task Force for Disease Eradication is considering poliomyelitis, mumps and rubella as candidates to follow the demise of smallpox.
The use of immune globulins have proved useful in certain situations eg, as a prophylaxis and as an immediate post-exposure therapy, sometimes in association with the vaccine. As well as human immune globulin, hepatitis B (HBIG), rabies (HRIG) and varicella-zostar immune globulin (ZIG) are available.
While vaccines have had success, drug treatments against viruses have proved difficult to develop, since viruses have an intimate association with the host chemical complex. Toxicity and drug resistance are two major problems. Antigenic variation and instability of some viruses is also a challenge to the pharmacologists. Nevertheless this has not discouraged development and drugs are now available for most strains of influenza A and B (amantadine and rimantadlne), herpesviruses (vidarabine and acyclovir and other drugs related to acyclovir). Ribavirin has a wide spectrum of activity and has been used in the treatment of RSV. It has also been tried against influenza A and B, hepatitis A, measles and most notably Lassa fever with varying degrees of success. Cytokines. Also known as interferons, have been used in chronic hepatitis due to hepatitis B and early trials of hepatitis C (HCV) infection. Antiretroviral drugs in the shape of zidovudine have been thoroughly examined for oral and intravenous use in the treatment of HIV and AIDS. Unfortunately for various reasons including the emergence of viral resistance the efficacy is short lived. Other retrovirals used against HIV include didanosine and zalcitabine. Side effects have been recorded. Protease inhibitors promise longer suppression of viral replication which encourages an improvement in patient management. Such developments are becoming crucial now that knowledge of the viral load during a patient's illness, especially in AIDS, is becoming so critical.
The future
This brief story has shown the rapid development of the science of virology over a relatively short period. The extraordinary speed of progress since the 1950's is still fraught with difficulties. Laboratory procedures for the recovery and identification of viruses has become very sophisticated. No longer the only route to an answer is via a lengthy process, immunoserology and molecular virology have seen to that. Batch and multi-testing by automative techniques has improved standardisation and rapid throughput. Same day service, especially in HIV screening, is now commonplace.
Viruses may be small but behave in a most complex and sometimes bizarre way. Their ability to survive is the almost impossible challenge to the scientist. This is exemplified by the emergence of 'new' viruses. Many 'new' viruses are actually newly recognised. Opportunism on their part is the key word. Environment and ecology have contributed in some parts of the world. Zoonoses, international conflict, mass movement of armies and refugees and famine have all played a part. Some emerging viruses have failed to reach epidemic proportions. HIV and AIDS on the other hand has become a world wide threat. Modern virology. with its molecular and immunological ability, its attention to research and epidemiology, is well equipped to take the story onwards.
By way of an epilogue it has been projected by Matthews that over the next l0-20 years there will be a fairly constant increase of 4-6 newly detected higher plant viruses and an average of one new virus per year capable of infecting humans. With this in mind it is not surprising to hear the phrase "It is probably a virus " at times of clinical uncertainty.
Selected further reading
Calisher CH, Horznek MC. '100 Years of Virology'. Wien: Springer-Verlag, 1999. ISBN 3 211 83384 6.
De Kruif Paul. 'Microbe Hunters'. London: Jonathan Cape, 1930.
Fenner F, Henderson DA, Arita I, Jefek Z, Ladnyi ID. 'Smallpox and its Eradication'. Geneva: WHO. 1988. ISBN 92 4 156 110 6.
Grafe Alfred. 'A History of Experimental Virology'. Springer-Verlag: 1991. ISBN 3 540 51925.
Levine Arnold J. 'Viruses'. Scientific American Library, New York: 1991. ISSN 1040
3213.
Waterson AP, Wilkinson Lise. 'An Introduction to the History of Virology'. Cambridge University Press: 1978. ISBN 0 521 21917 5.
Williams Greer. 'Virus Hunters'. London: Hutchinson
Images
Felix d'Herelle
Friedrich Loeffler and Robert Knock
Louis Pasteur
Tables
100 years of virology: Table 1
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