Kitabı oku: «Virusphere», sayfa 4

When compared to the bacterial genome, the norovirus counterpart is frugal in the extreme. The viral genome comprises regulatory regions at either end of a compact linear string of RNA, which codes for a minimum of eight proteins, two of which code for the protein structures of the viral capsid, and six concerned with viral replication. A key difference between the bacterium and the virus is that the bacterium has all it needs to reproduce itself, but the virus can only replicate to produce daughter viruses by making use of the genetic and biochemical properties of its cellular host. In the case of the human strain of norovirus, these genetic and biochemical properties are those of the human target cell.

The norovirus genome codes for a singular aggressive viral protein known as the ‘protein virulence factor’, or VF1. This menacing entity localises to the human mitochondria during infection with the virus, where it antagonises the infected person’s innate immune response to the virus. While some viruses are capable of commensalism or even mutualistic interactions with their hosts, we see little evidence for this in the norovirus. Its symbiotic interaction with humans appears to be exclusively parasitic. Unlike the bacterium, it has no genes devoted to nutrition, or to internal metabolic pathways, since, unlike the bacterium, it has no internal metabolic pathways. Its genome is designed to take advantage of the physiology, metabolic pathways, genetic pathways, and even the very locomotion and life-style patterns of human behaviour in order to replicate itself and transmit its contagion as widely as possible.

So now we see that viruses are not fluids or poisons. They are organisms that follow a wide range of symbiotic interactions, each virus usually associated with a highly specific host, a tiny minority of which happen to be human. They are clearly very different in size, genomic organisation and life-cycle patterns to bacteria. The fact that most viruses do not possess their own internal metabolic processes does not imply that viruses do not utilise metabolic processes. On the contrary, viruses take advantage of their host’s metabolic pathways. This is why it is a mistake to think of viruses in isolation from their hosts. Outside their hosts viruses are biologically inactive: but this does not mean that they are inorganic chemicals.

Outside the target cells of their hosts, viruses have evolved stages that are somewhat equivalent to suspended animation. This stage is well-suited to being ejected in the aerosol created by a cough or a sneeze, or excreted in faeces, or in sexual secretions, or surviving being transferred by a secondary carrier, such as a biting insect or a rabid dog; or in the case of plant viruses, being carried to new hosts on the wind, or through water, or through a miscellany of other avenues of transmission, to find new hosts. Only when they enter into their obligate symbiotic partnership with the new host do we witness viruses behaving with the genetic and biochemical subtlety and efficiency we might expect of biological organisms.

The norovirus is no exception to such symbiotic evolutionary behaviour. So specific is the virus in its symbiotic interaction with its human host that different human-associated viral genotypes have affinities for specific ABO blood group proteins on cell membranes, these protein ‘receptors’ binding with one of the two proteins of the viral capsid as an integral step in the infectious process. On passing into the bowel, the virus has a predilection for the upper small bowel, or jejunum. How, exactly, the virus then penetrates the intestinal wall is not fully understood, but it would appear that it preferentially infects the immune lymphoid follicles in the gut wall, which are known as Peyer’s patches, while also searching out a type of intestinal cell, known as H-cells. After making its way through the gut wall, the virus is identified as alien by the innate immune defences of the gut, which might be just fine as far as the virus is concerned, since these may be its target cells. Whatever the target cells, we can anticipate that the virus will hijack their genetic and metabolic pathways in order to replicate itself, thus establishing its cycle of infection and multiplication, generation after generation.

Since we don’t yet have suitable tissue cultures or animal models to study the norovirus, we are not in a position to examine the ways in which it provokes the vomiting and diarrhoea, which play a key role in spreading the virus far and wide throughout the world. Currently there is no preventative vaccine, but trials of an oral vaccine are taking place as I write. Let us cross our fingers and hope that these trials are rewarded with an early success!

6
A Coincidental Paralysis

In the summer of 1921 the 39-year-old Franklin D. Roosevelt fell overboard from his yacht on the Bay of Fundy, a beautiful if freezing inlet between the eastern Canadian provinces of New Brunswick and Nova Scotia. The following day he was tormented by pain in his lower back and then, as the day progressed, he felt his legs grow increasingly weak until they could no longer sustain his body weight. This was the onset of Roosevelt’s poliomyelitis, at this time known as ‘infantile paralysis’. Poliomyelitis is caused by a virus that goes by the same name. In 1921 doctors were limited in their knowledge of the poliovirus, or indeed viruses as such. They might, however, have known that the virus did not infect Roosevelt while he was struggling in the cold water – the only infectious source of poliomyelitis virus is another person who has already contracted it. Once again, we are looking at an exclusively human reservoir. Moreover, the paralytic disease has an ancient pedigree.

Infantile paralysis was familiar to physicians in the time of the pharaohs of Egypt, since the effects of the disease were painted, with stunning accuracy, on the walls of their tombs. In 1921, as indeed today, there was no cure for the paralytic effects of the virus once it had afflicted a victim. Fortunately, Roosevelt was gifted with an extraordinary vitality and courage, enabling him to overcome the lifetime of paralysis that would result from his illness. It is to his credit that despite this handicap he became the 32nd President of the United States and he continued to serve the American people for an unprecedented four terms in office.

Viruses do not follow our human notions of rules and so they are apt to surprise us. One such surprise is that those viruses that replicate primarily in the gut – the so-called ‘enteroviruses’ – do not cause the usual symptoms of gastroenteritis. Instead, the viruses that do cause gastroenteritis are a miscellaneous group with members coming from widely different viral families. Of course, these include the genus of noroviruses within the family of calciviruses. Another group of gastroenteritis-associated viruses are the rotaviruses, a genus within the family of reoviruses, which cause vomiting, diarrhoea and fever in babies under the age of two years. Other similar offenders include adenoviruses, coronaviruses and astroviruses. We are sometimes inclined to joke about the clinical effects of gastroenteritis, but the truth is that this is a distressing condition in people of any age. Moreover, in less developed countries, gastroenteritis is one of the commonest causes of death in children, a tragic situation complicating poor hygiene and contaminated water supplies. As we might anticipate, these illnesses are transmitted by the faecal-oral route.

The ‘enteroviruses’ are also transmitted by the faecal-oral route and the viruses also replicate within the intestine, but curiously they do not present with the typical fever, vomiting and diarrhoea that typifies gastroenteritis. Instead they cause less predictable and often complex patterns of illness that affect various organs and tissues, for example, the brain and meninges, or the heart, skeletal muscles, skin and mucous membranes, the pancreas, and so on. The most familiar of this strange gamut of enterovirus-linked illnesses is poliomyelitis. All three ‘serotypes’ of the poliovirus, which have slight differences in their capsid proteins, are ‘enteroviruses’ within the family known as the picornaviruses. We might recall that these belong to the family of very small RNA-based viruses that includes the rhinoviruses. A cardinal feature of enteroviruses is that they are resistant to acid, so they can pass through the human stomach to replicate further down the alimentary tract. The poliovirus was the first of the enteroviruses to be discovered, earning its finders – Enders, Weller and Robbins – a Nobel Prize in 1954.

We should not be too surprised to discover that humans are the exclusive host of the poliovirus. The individual virion is a mere 18 to 30 nanometres in diameter. Under the electron microscope it has a capsid with the familiar icosahedral symmetry, which encloses a relatively simple RNA-based genome. In the small intestine, the virus binds to a specific receptor molecule in the lymphoid tissues of the pharynx and the ‘Peyer’s patches’ of the gut. Here the virus hacks its way into the interior of the cells, where it takes over the genetic processes to convert the cell into a factory for manufacturing daughter viruses. The daughter viruses are released through rupture of the infected cell, after which they re-invade neighbouring cells and repeat the process.

All of this sounds a trifle horrific and even potentially deadly. But in reality the great majority of individuals infected by poliovirus show little or no signs of disease other than, perhaps, a mild looseness of the bowels. But the stools of an infected individual will now be swarming with virus, which will be passed on to contacts through the faecal-oral route. Polio characteristically moves through populations in epidemic waves, with most of the infected unaware that they have encountered the virus. Only in a tiny minority does the virus make its way to the anterior horn nerve cells in the spinal cord, where infection and subsequent death of the nerve cells gives rise to the paralysis we saw in President Roosevelt. Bizarre as it might seem, the infection of the nerve cells appears to serve no purpose as far as virus transmission or evolutionary pathways are concerned. Indeed, this most dreaded complication of poliomyelitis appears to be coincidental.

The incubation period of poliovirus infection is usually a week to two weeks and, in the minority that show symptoms of infection, this involves a minor malaise, fever and a sore throat. These reflect the virus entering the bloodstream and will usually resolve without requiring any treatment and with no long-term consequences. Only in a small minority of those infected does polio give rise to a more severe illness. The onset is usually abrupt with headache, fever, vomiting – in some this may be accompanied by the neck stiffness typical of meningitis. Even still, the majority of symptomatic cases will go on to make a good recovery. But in the tiny but highly significant minority the paralysis of poliomyelitis sets in.

Paralytic poliomyelitis gets its name from the Greek polios for ‘grey’ and muelos, for marrow. This derives from the fact that the paralysis results from destruction of the grey marrow of the anterior horns of the spinal cord, which contain the cell bodies of the nerves that supply the muscles of arms, legs, chest and remainder of the trunk. The death of those cell bodies in the spinal cord causes a floppy style paralysis of the affected muscles, which is usually apparent within two or three days of the onset of the disease. In children affected by paralysis, this will have secondary long-term effects on limb growth and development. Bulbar poliomyelitis, a similar infection, causes damage to the nerve bodies of the cranial nerves, which results in paralysis of the pharynx and possibly accompanying difficulty with the muscles involved in breathing. This dreadful complication is why, before the advent of vaccination, some unfortunate patients ended up having to be supported by ‘iron lungs’.

We do not know why this unfortunate minority of infected individuals develop serious disease, including paralysis, from the poliovirus. There is some evidence that the virus gets into the central nervous system more commonly than is suggested by clinical signs. Indeed, as we shall see, this pattern of unwanted penetration into the central nervous system can feature in illnesses caused by other enteroviruses. One wonders if some genetic propensity might perhaps play some role, but it may be no more than bad luck. As we saw above, this pattern of paralysis in children, with its effects on limb growth, was recognised in the wall paintings of the tombs of pharaohs from Ancient Egypt. How puzzling then that such an ancient and easily recognisable disease was unfamiliar to European doctors until the latter years of the nineteenth century, when the first epidemics began in the cooler climates of industrialised Europe and the United States!

Such has been the dramatic success of vaccination programmes, using live attenuated viral vaccines taken by mouth, that polio has been largely eliminated from developed countries. In 2018, according to the Global Polio Eradication Initiative, the disease is now endemic in just three countries: Afghanistan, Nigeria and Pakistan. But, given the ease and extent of modern travel, we cannot rest assured until this historic and maiming disease is completely eradicated in these remaining pockets of potential contagion.

While poliomyelitis is now approaching global control, it is not the only enterovirus to afflict humanity. Other members of this virus family are still commonly encountered in developed countries, including viruses that can be baffling in their presentations and clinically unpredictable in the course of their illnesses. Perhaps the best known of these are the Coxsackie B viruses, which sometimes present with a condition known to doctors as epidemic pleurodynia. Also known as ‘Bornholm disease’, after the Danish island where it was first recognised, this can present as severe chest pain arising from inflammation in the intercostal muscles of the chest wall. Popularly known as ‘the devil’s grip’, the sudden onset and severity of the pain can mimic a heart attack. Coxsackie B viruses can occasionally cause inflammation of the brain, presenting as the condition known as myalgic encephalomyelitis, or ‘Royal Free disease’, named after the London teaching hospital where it first presented. The same enterovirus may also present with inflammation of the heart muscle, or myocarditis, coupled with inflammation of the membrane surrounding the heart, known as pericarditis, a combination that presents in both children and adults and can very occasionally prove fatal. Other enteroviruses, including the echoviruses and types 70 and 71 enteroviruses, can cause chest infections and various patterns of muscle, meningeal and brain infections, where the diagnosis of the causative virus may be exceedingly difficult to pin down.

Viruses and their associated illnesses can be very puzzling. Ever since we first discovered their enigmatic presence among us, questions have inevitably arisen as to the evolutionary purpose behind their behaviours. When faced with the unpleasant, sometimes life-threatening, effects of virus infections, we are inclined to wonder what possible benefit such behaviour might confer on the virus. In the case of the poliovirus we saw how it appears to be mere happenstance that the virus causes serious illness in a tiny minority of those it infects. But there are other viruses that sweep through the human population and inflict dreadful patterns of illnesses in the majority of those infected, sometimes accompanied by a high mortality. This is all the more baffling since all that matters to the virus is its survival and successful replication. Survival of the virus must surely be threatened by killing its host. When one views the same question from a medical perspective, we are inclined to question: why are some viruses so deadly?

7
Deadly Viruses

The Four Horsemen of the Apocalypse feature in the biblical Book of Revelation, where, having been released by the opening of seven seals, they ride out on red, white, black and pale horses. Theologians differ in their interpretations of what these riders might signify, but one of the four is commonly interpreted as pestilence, which, in modern terminology, would be interpreted as plague. While the common childhood infections, caused by viruses, are usually self-limiting, some viruses are truly dreadful in their capacity for death and suffering. In the recorded pages of history, two plagues of humanity would justify the term ‘apocalyptic’: these are the bacterial pandemics known as bubonic plague, as seen in the Black Death in the Middle Ages, and its viral counterpart, the plague of smallpox. Both have tormented humanity from ancient times, bequeathing a grim legacy in historical records and grave pits.

The Black Death was named after the festering swellings, or ‘buboes’, where lymph glands in the groin or armpit became swollen with pus and erupted onto the skin of victims. The causative bacterium, Pasturella pestis, is transmitted by the bite of an infected rat flea. Although the public commonly assumes that bubonic plague has gone away, in fact a milder form of the illness is still endemic in rural parts of the United States, South America, Asia and Africa. The viral apocalypse, smallpox, was named after the rash that accompanied the disease, which resulted from pustular blistering in the skin that healed with deep circular scars, or ‘pocks’.

It is comforting to use the past tense here since, mercifully, smallpox has been eradicated as a plague. The clinical term for smallpox was ‘variola’, and the disease followed two very different patterns of virulence, depending on the causative virus. Variola major and Variola minor are species within the family of poxviruses. The poxviruses infect a wide variety of animals, but only three species infect humans: namely the two variola viruses and a related species, Molluscum contagiosum, which causes minor blisters on the skin of children. We shall confine our attentions to the variola viruses, which have a number of unusual features.

Humans are the only hosts for smallpox, so we are the exclusive reservoir of the two variola viruses in nature. The individual ‘brick-shaped’ virions are relatively large, measuring 302 to 350 by 244 to 270 nanometres. Before being displaced by the discovery of the ‘Megaviruses’, poxviruses were the giants among the viruses, being big enough to be seen as tiny cytoplasmic inclusions under high magnification of the light microscope. This feature alone alerts us to the fact that we are dealing with a relatively complex virus. The variola genome is predictably large and DNA-based. Unusually for a virus, it contains the genetic wherewithal for the manufacture of its own virus messenger, RNA, which takes care of the manufacture of viral proteins. This virus also has its own coded enzymes and transcriptional factors which control the manufacture of daughter viruses within the cytoplasm of infected host cells.

Smallpox viruses are extremely contagious, spreading by that most infectious route of all, aerosol inhalation. The viruses are also capable of spread through skin contact with the blistering rash, or through contaminated clothing, bed linen, utensils or dust. Infection usually begins with the arrival of the virus into the air passages of the throat and lungs of a susceptible individual, where they penetrate the superficial lining cells to be ‘discovered’ by the tissue macrophages, the first line of the human immunological defences. The stage of infection within the macrophages is asymptomatic, but accompanied by stealthy advance of the virus towards its ultimate goal. By about the third day after infection, the ‘virus-factories’ within the macrophages journey on to the lymphatic stream and local lymph glands, from where the viruses spread to the other key elements of the ‘reticuloendothelial system’, in particular the bone marrow, spleen and circulating blood. This triggers a massive immune counter-attack on the virus, including cytotoxic T-cells and interferons. But, as the history, and the grave pits, suggests, this counter-attack is unsuccessful in the majority of sufferers. Symptoms begin with a severe sore throat at much the same time that blood-borne spread carries the viruses to the skin, where they produce the blistering and scarring rash, with its predilection for the face and limbs. The blisters are the result of direct viral invasion of the skin and they teem with viruses.

Historically it is thought that smallpox first arrived among humans about 10,000 years ago in the agricultural settlements in northeast Africa, spreading to India through trade with Ancient Egypt. It grieves one to imagine such a disease spreading through such populations of naïve people, and impossible to imagine exactly what they thought was among them. No doubt they had some simple rules for dealing with contagion, and, equally likely, they would have blamed some occult cause. We discover the pathognomonic pocks in the mummified skin of Ancient Egyptian mummies, such as the Pharaoh Rameses V, who died in 1156 BCE.

Smallpox, or the ‘small pocks’, was a clinical term that came into usage in the sixteenth and seventeenth centuries to differentiate it from the inch-or-more-diameter ‘great pocks’ that medical historians assume were pathognomonic of tertiary syphilis, a bacterial plague that may have been imported into Europe from the Americas. The viral plague of smallpox arrived into Europe much earlier, sometime between the fifth and seventh centuries CE, where it persisted as an infection, giving rise to repeated epidemics during the Middle Ages. Estimates suggest that it killed some 400,000 Europeans annually in the late 1700s, affecting all levels of society, including five reigning monarchs, and was responsible for a third of all cases of blindness. The same plague played a key role in the Conquistador subjugation of the Aztecs and Incas of South America, during the sixteenth and seventeenth centuries, when it may have dominated the history of encounters between Eurasian adventurers and the stricken native and hitherto ‘virgin’ peoples.

Today we can scarcely imagine the terror of living through a major epidemic of plague or smallpox sweeping through such a ‘virgin’ population. They would have been very quickly aware that a pestilence was among them, with panic-stricken populations in the grip of raging fever and, in case of smallpox, a virulent rash, which, when severe, caused the entire skin to boil with blisters and carried with it a horrific lethality, at its worst as high as 90 per cent. It must surely have seemed as if a pitiless demon had entered their world, intent on wiping out entire families, and even entire villages, towns and cities.

But smallpox was never a uniform death sentence. We cannot be certain of the actual levels of lethality of smallpox in various parts of the Americas, though we are informed that it was as high as 60 to 90 per cent in the worst-affected populations, falling to 30 to 35 per cent in some of the lesser-affected regions. This lower lethality was in fact similar with the calculated overall mortality of Variola major in concurrent Eurasian populations, suggesting that the virus had already become endemic in those regions. Meanwhile, even in the Americas, the Variola minor virus caused a much milder disease, with a mortality of about 1 per cent. It is somewhat ironic that smallpox, one of the deadliest plagues in history, was the first to be subdued by the use of a vaccine. Many readers will be familiar with the discovery of cowpox vaccine by the English physician, Edward Jenner, and this more than a century before the world even realised the existence of viruses.

In such less-enlightened times various therapies that we would now dismiss as ‘quack’ were touted as preventions or curatives for every frightening illness. In seventeenth-century England, for example, Dr Sydenham, an eminent physician in his day, treated patients in the throes of smallpox by allowing no fire in the room, leaving the windows permanently open, drawing the bedclothes no higher than the patient’s waist and administering ‘twelve bottles of small beer every twenty-four hours’. If nothing else, the beer would have dampened consciousness of the suffering – and perhaps the discomfort of the therapeutically induced hypothermia in winter. But it was famously known from ancient times that survivors of smallpox were immune to further infection. A hazardous treatment, involving inoculation of non-immune individuals with a scalpel wet with material from the ripe pustule of an infected patient, was variously employed in Africa, India and China long before Jenner introduced his vaccine.

History has it that Jenner overheard a dairymaid say, ‘I shall never have smallpox for I have had cowpox.’ Cowpox, a milder pox infection in cattle, was known as vaccinia, after the Latin, vacca, for cow. In 1796 Jenner conducted a now-famous experiment in which he inoculated an eight-year-old boy with pus from a vaccinia blister, obtained from a dairymaid with cowpox, and, having waited for the boy to develop immunity, subsequently tested this by inoculating him with smallpox. Thank goodness that the boy now proved to be immune. Although Jenner had rivals, who dismissed the importance of his discovery, the cowpox inoculation was soon taken up as a preventive measure against smallpox. We still refer to it today with the term Jenner coined for it: ‘vaccination’.

When I was a child, it was still mandatory to be vaccinated against smallpox. I still bear the scar, which is a pock-shaped irregular oval about half an inch in diameter, on the skin of my upper left arm. Today children are no longer vaccinated against smallpox because the disease was eradicated from the global human population by a ten-year international programme of smallpox vaccination, headed by the American physician, Donald Ainslie Henderson, who worked under the auspices of the World Health Organization. This was formally signed off with the confirmed eradication of the disease in 1979.

There can be no denying that the eradication of smallpox was an extraordinary achievement. Ironically, however, this very success makes our modern populations unduly susceptible to a malicious attack involving a potentially bioengineered smallpox virus that might be deliberately created to be as lethal as possible. New generations, who have never been vaccinated, would have no inbuilt protection to such a spreading lethal strain. This is why the smallpox virus is now included in the list of Category A bio-warfare agents. Following smallpox eradication, it was agreed by international treaty that samples of the smallpox virus should only be retained in two maximum security laboratories – one at the CDC in Atlanta, in the United States, and one at similar facilities in Moscow, in Russia. The plan was to allow some continuing research aimed at countering any attempt to use the virus for bio-warfare, whether through terrorism or through formal hostilities between nations. We must hope that, if the worst comes to the worst, the officially sanctioned research in this small number of biosafety laboratories will come to our rescue with a modern vaccine, which will need to be spread globally with more efficiency than we have ever seen with any previous vaccination programme.

Why then are some viruses so lethal when they infect us?

We humans are graced with the gift of knowledge, education, morality, self-awareness, enabling us to think ahead, and so largely control the many aspects of our existence. Viruses are devoid of such self-knowledge, morality or anticipation. They are exclusively driven by those familiar goals: survival and reproduction. But it would be a mistake to underestimate them; viruses are extremely efficient in achieving those goals. Surely the lethality of dangerous viruses must be linked in part to whatever mechanisms viruses have evolved in order to overcome our human immune defences to viral infection. It will come as no surprise that the study of the smallpox virus in one of the two key bio-defence centres allowed to store the virus, the Centers for Disease Control in Atlanta, Georgia, has provided an important clue as to how the smallpox virus, Variola major, overwhelms our human immunity.

When Variola enters our human tissues, the ‘innate immune response’ is the first line in our defence against this alien invasion. As part of this innate response, the infected cells produce type I interferons in response to the presence of the virus, and these then engage other immunological defences that would normally inactivate and destroy the virus. What the scientists at CDC discovered is that inside the infected human cells the invading virus produces a protein, known as type I interferon-binding protein, which inactivates the human type I interferons. As we saw with the production of a virulence factor in norovirus infection, this is another example of the same malevolent viral strategy. In other words, the smallpox virus carries in its genome the coding for a major virulence factor that explains the severity of Variola major smallpox infection. This discovery of these interferon-binding proteins may, in the future, help authorities to improve the design of new vaccines and antiviral therapies, which might, for example, also apply to related viruses, such as the monkeypox virus, which causes virulent infections in humans.