by Daniel Keays

The World’s Greatest Unintended Consequence

The smallest virus

Causes such immense misery.

Benumbed cruelty.

 

            Anyone who lived in the 1940s and 50s would react much differently to the word polio than someone born later. The very name of the disease is enough to elicit painful memories for those of us who lived through the epidemic. To see bright, active, happy children and young adults brought down by this mysterious ailment was something you never forget. Every community, and just about every neighborhood, was affected—a girl in your school, a boy in your Little League, your cousin’s best friend. Everyone knew of someone who was stricken. History books show how the world united to fight aggression in World War II. The united fight against polio was no less compelling.

            But it wasn’t always that way. Before 1900 polio was a rare disease. It wasn’t even called polio, but infantile paralysis. Only a few hundred scattered cases a year occurred in the U.S. in the late 1800s. In the early 20^th century, a few more cases started to appear. Doctors had to consult their medical books to refresh their memories to make a diagnosis. As the years passed, more cases occurred, and small epidemics were noticed in various communities. Then, the big one, New York and the northeastern United States, 1916. Over 27,000 cases of paralytic polio were reported, with over 6,000 deaths. The pandemic had begun.

            With anything so profound and sudden, wild explanations are inevitable. The most common and pernicious reasoning blamed the war raging in Europe and refugees arriving in the U.S.—"the ‘foreigners’ were bringing the disease to America.” Open hostility toward European immigrants was not unusual, and a difficult situation was made even worse. Science wasn’t much help, since a virus causes the disease, and the technology of the time had very little information on those microorganisms. The disease was thought to be infectious, but the cause and means of spread were unknown.  

            As time went on, the disease spread. It was mainly observed in summer, but sporadic outbreaks would occur throughout the year. Soon the entire country was affected. There was no cure.  

 

            Now that polio is under control in developed countries and its cause has been elucidated, the reason for its sudden emergence in the early 1900s has become apparent. Astoundingly, it was the introduction of sanitation.

            Throughout history, the virus has been widespread. The lack of proper sanitation exposed virtually everyone to the polio virus. Water was often contaminated; there were no flushable toilets and very little running water. People would rarely even wash their hands except to remove large amounts of dirt, and folks often shared the same wash basin. The polio virus spread nearly universally, and just about everyone contracted it.

            When a woman became pregnant, she had already developed antibodies to the virus, protecting her. These antibodies would go from the mother to her baby in the uterus, a process known as passive immunity. Passing these antibodies into the baby in utero makes the newborn immune to the virus for about six months after birth. Since the virus was so common, the baby would encounter it during the first few months of life and develop its own immunity while still protected by its mother’s antibodies. This is referred to as naturally acquired immunity.

            What changed was the introduction of sanitation. Around the turn of the century, scientists discovered the link between contaminated water and infectious diseases like cholera, typhoid, and dysentery. The introduction of good plumbing and the establishment of a public mindset about personal hygiene significantly reduced the exposure of the turn-of-the-century youngster to naturally occurring viruses. Their mother’s antibodies still protected them, but those dissipated after about six months. Because of sanitary practices, the children weren’t exposed to the virus and didn’t develop their own active immunity. These kids grew up without naturally acquired immunity, making them susceptible to the virus. It seems ironic, but the introduction of sanitation was primarily responsible for the onslaught of polio in the modern world.

 

            The virus that causes polio is a member of a large group called the enteroviruses. About 80 different enteroviruses are known. Some cause severe disease, others are more benign. Individual enteroviruses are made up of a core of ribonucleic acid, or RNA, surrounded by a protein wrapping. The protein coat allows the virus to attach to a specific receptor on a human cell, sort of like microbial Velcro. The virus wanders around until it finds its receptor, attaches, and the infection begins. The only thing that prevents attachment are antibodies specific to the virus, preventing its attachment.

            Polio is caused by three different viruses, called simply enough poliovirus 1, 2, and 3. All three can cause neurologic disease. Resistance to one does not convey resistance to the others.  

            Poliovirus enters the body through the mouth, either by food or drink. The enteroviruses are very stable in acid, so they withstand stomach acid. In the days of the polio epidemics, it was commonly held that kids should avoid swimming pools, which was probably good advice, even though the virus is rapidly killed by chlorine. Once the virus enters the intestinal tract, the infectious process begins. Where it stops is nearly impossible to predict.

             

            Which cells the polio virus initially enters is not absolutely known, but we have a pretty good idea. Experimental evidence is conclusive that the virus enters the tissues mainly through the cells lining the distal small intestine, the ileum. 

          The host cell receptor for the virus is a protein that is usually referred to as simply PVR, for poliovirus receptor. Its actual name is CD155. This very interesting protein has multiple uses. For one, it forms part of the “glue” that holds cells together, preventing fluid leakage. It is especially important for the tight junctions between adjoining cells. It helps form the sides of endothelial and epithelial cells. Another function is less well-defined, but given its ubiquity, it is obviously very important. Many cells of the body, not just those of the intestinal tract, express CD155 as a molecule that is anchored in the cytoplasm but extends beyond the cell membrane into the environment. It seems to act as a signal to cells of the immune system as to the health of the cell—it is over-expressed when conditions such as cancer or infection occur, letting immune cells, such as natural killer lymphocytes, destroy the cell before it gets out of control. A soluble portion of the CD155 molecule also resembles an antibody. It can break off and end up in the serum. To date, we don’t know its function since it is non-specific.

            The CD155 protein has three sections, or domains, outside the cell, designated D1, D2, and D3. All three types of polioviruses attach to the D1 part of the molecule. The attachment protein on polio virus is a bit unusual. It is not a spike of a molecule out from the surface of the virus. Instead, it is a crevice, or “canyon,” in the viral coat (capsid). The D1 molecule of the CD155 protein sticks down into the viral canyon, and the attachment is firm. The viral coat proteins break apart immediately after hooking onto its receptor. The single strand of viral RNA makes its way into the cell, then goes to the cell’s ribosomes to begin transcription and the production of viral proteins.  

            All three of the polio types enter the cell and replicate similarly. Most of our cells contain the CD155 molecule, so poliovirus is not restricted to a single cell type like most viruses are, but they initially enter the body through cells in the small intestine.

            Poliovirus is a member of the enterovirus family, which is classified as a picornavirus. “Pico” is a mathematical term meaning very small, and rna means, well, RNA. The standard measurement for the size of viruses is the number of kilo base pairs, abbreviated kb. Polio and the other enteroviruses are among the smallest human-associated viruses, at about 7-8 kb. By comparison, influenza, a negative sense segmented RNA virus, weighs in at around 12-15 kb. The leader in size is the smallpox virus, about 200 kb.

             The tiny naked strand of messenger RNA from the virus is capable of great things, virally speaking. The first order of business is for it to make its way to a cellular ribosome for transcription. It carries the correct origination codes, so as soon as it meets an intact ribosome, it gets transcribed, yielding one long chain of amino acids called a polypeptide. This polypeptide is made up of all eleven virus proteins stuck together. It is much more efficient for the virus to construct its proteins this way rather than individually; it requires fewer enzymes and RNA initiation sequences.

            One of the viral proteins, known as Protein 2A, is an absolute marvel. It is like the Swiss Army knife of viral proteins, having multiple uses. Its first order of business is to cleave the long polypeptide of viral proteins into individual proteins, first into four parts, then into all the individual proteins. The virus is now cranking out all its proteins using the ribosomal machinery of the cell.

            The virus then must shut down the cell’s ability to make its own proteins.  Protein 2A is active here as well, shutting down the host cell’s RNA. The virus has now completely controlled the cell's manufacturing capability. 

            After several hours of viral transcription, tens of thousands of viral proteins and nucleic acid strands are made, a somewhat chaotic situation. To bring some order to the process, the virus has a protein that effectively hijacks the cellular autosomes. Autosomes are the cell’s “garbage dump,” small vesicles designed to eliminate cellular waste. The virus has ways of incorporating all its parts, protein and nucleic, into these autosomes for easier viral assembly. True efficiency.

            The cell, meanwhile, is just serving as this little virus factory, producing tens of thousands of virions while its own business is put on hold. This cannot be allowed to go on.  Human cells have a vast array of chemicals at their disposal to put an end to all of this. The very best one against viruses is called interferon.

            The interferons are powerful medicine. While they don’t directly kill the invader, they have the power to help protect virtually every cell in the body from further attack. Our cells are rich in interferon receptors, and when a flood of interferon is released, all our cells go into protective mode. Once cells are activated by interferon, viruses do not as easily cross into them. When they do, they are met with an onslaught of anti-viral proteins and defensive mechanisms, preventing their reproduction. The infected cells are much more likely to send chemical signals to their surface, instructing killer lymphocytes to destroy them before they become virus factories. After the release of interferons alpha and beta, virus reproduction goes way down, even before the onset of acquired immunity and the release of specific antibodies. Interferons are part of the innate immune system.

            There is a downside to interferon release, however. It makes us feel sick. The fever, muscle aches, headaches, diarrhea, mucus release, and the like do not come directly from the presence of a virus. It is due to the body’s response to the viral invasion in the form of interferons and cytokines like tumor necrosis factor-alpha (TNF-alpha). The untoward release of large amounts of interferon can bring about a significant amount of tissue damage. Its storage and release must be carefully guarded.

            About one hundred and fifty genes code for interferons and similar proteins the body uses to fight off invading microbes. Because of the damage they can cause, they are sequestered; there is a chemical overlay of the DNA section on which the genes reside. It’s like locking up armaments in an armory until you are sure you really need them. When the cell is under attack, a chemical signal is sent from the cytoplasm through the nuclear membrane and into the DNA region. This signaling molecule unlocks the protective overlay of the section of DNA harboring these important genes. The interferon and cytokine genes are then copied, and messenger RNA is sent to the cell’s ribosomes to be decoded and the proteins made.  

            The defense system is very sophisticated and highly developed, but there is a vulnerability in the scheme that the polioviruses have exploited. Both the signaling compounds going into the nucleus and messenger RNA bearing the coding for the proteins coming out must pass through a channel in the nuclear membrane. There are three protein parts of these channels: the structure of the channel (or pore), the proteins that anchor it to the nuclear membrane, and the proteins needed to escort the signal proteins into the nucleus and messenger RNA out. It is this set of escort proteins that the poliovirus attacks so that very little can get into the nucleus and very little out. It’s like a burglar who cuts the alarm and phone lines while inside the building, cutting down communication.

            The nuclear transporting proteins are severed by poliovirus protein 2A, the same one that is used to chop up the viral polyprotein and shut down cellular RNA transcription. Without chemical communication with the nucleus, the host cell’s ability to produce proteins is significantly curtailed. This includes the manufacture and release of interferon. The virus has completely controlled the cellular apparatus and can propagate without interference.

            The amount of virus produced in a single cell is amazing.  Over a hundred thousand new viruses can be made in a relatively short amount of time. Each virus has a coat of 240 proteins plus its RNA payload. Upon release, each of these can infect other cells, either close by or in some remote area of the body.  

            The release of all those newly made polio virions from the cell is made possible by a protein similar to Protein 2A: Protein 2B.  This enzyme increases the permeability of cellular membranes. Just a few don’t have much of an effect, but when they are produced by the tens of thousands, their development reaches critical mass, and the host cell’s membrane bursts, releasing the virus.

 

            Once released from the host cell, the virus has three directions it can go. One is to infect nearby cells, creating many more virions. The second is to enter the lumen of the bowel and be excreted with the infected person’s stool to perhaps infect another individual. Thirdly, they can passively diffuse into the deeper tissues to eventually be picked up by lymphoid tissues and make their way into the bloodstream. All three occur, and how much of each seems to depend on random chance.

            The virus greatly curtails the body’s immediate immune response, but the response is not zero. A small amount of interferon is always made, and a few lymphocytes are activated. However, so much virus is made that viral elimination is substantially delayed, allowing the poliovirus’ presence in the body to continue for some days.

            The first phase of viral infection is usually not accompanied by symptoms in the patient. The symptoms of fever, muscle aches, diarrhea, and the like are created by the body’s immune response to the virus, not the local damage the virus causes. But these immune response symptoms are greatly mitigated by the activity of viral proteins, so we can excrete rather large amounts of the virus without knowing we are infected. This can go on for some weeks. One reason polio is so infectious is that asymptomatic carriers outnumber sick ones. Around 70% of people infected with the poliovirus have this asymptomatic type of infection. Once infected, even without symptoms, we become immune for life to the infecting strain. However, being immune to one type of polio virus does not confer immunity to the other two types.  

            The virions that enter the circulation are randomly carried around the body, but they tend to eventually enter the tissues of the reticuloendothelial system, organs such as the spleen, liver, and bone marrow. The initial entry of the virus into the bloodstream is called a viremia, the “vir” part from virus, the “emia” from the Greek haima, meaning blood. The poliovirus in the blood at this stage of the disease does not result in very profound symptoms, perhaps just a mild fever. This stage is referred to as minor viremia. About 25% of polio-infected patients display this condition only. It is called abortive poliomyelitis.

            In most cases, once the virus enters the tissues of the reticuloendothelial system, its days are numbered. These organs are rich in anti-viral immune cells, aided by complement, interferon, and other cytokines, as well as newly formed IgM antibodies to eliminate the virus. It is not understood why it happens, but in a few individuals, less than 5% of those infected, the virus isn’t stopped at this stage. Maybe infected individuals didn’t mount enough of an interferon or antibody response. At any rate, the tissues of the infected organs all contain the poliovirus receptor, CD155, on their surface and, therefore, can become infected. If this happens, there is a massive release of the virus, many of which enter the bloodstream for a second time, known as a major viremia. If this happens, the patient becomes very ill. High fever, prostration, rapid pulse, profound muscle aches. Following major viremia, the virus can enter the central nervous system.  

            Poliovirus is not the only member of the enterovirus group that can enter the central nervous system. In fact, most cases of viral meningitis (usually called aseptic meningitis because it doesn’t involve bacteria) are caused by an enterovirus. Aseptic meningitis is usually not a life-threatening disease. The patient feels terrible for a few days with headaches, fever, and perhaps some disorientation, but symptoms usually resolve without requiring intensive medical intervention.

            Some, probably the majority, of polio victims having central nervous system involvement have this self-limited meningitis, much like that caused by other enteroviruses. It generally lasts a few days to a week, and the patient recovers completely. Most enterovirus meningitis cases do not require hospitalization—just a spinal tap to send fluid to the lab to confirm the diagnosis and some palliative care.

            In all these conditions, the poliovirus does not differ significantly from other enteroviruses: many cases of asymptomatic infection with a few considerable illnesses, to even fewer with a limited form of meningitis. But then there is the bad one: paralytic polio.  

            The nerves that signal the movement of our muscles, the motor neurons, originate in a portion of the spinal cord called the anterior, or ventral, horn. The similar structure on the back, or dorsal side, is called the posterior horn, which functions in a sensory capacity. When the poliovirus makes it into this area, it can only infect and damage the neurons of the anterior horn, the motor neurons. The sensory nerves of the posterior horn are not involved.  The reason for this needs to be clarified. Perhaps the motor neurons contain more of the poliovirus receptors than the sensory neurons.

            The motor neurons are composed of cells commonly called gray matter because of their color, which occurs because their axons are mainly unmyelinated. The white matter has mainly myelinated axons and appears white. The Greek word for “gray” is polios, and their word for spinal cord, or marrow, is myelos, hence poliomyelitis.  

            For many years, there was debate and uncertainty about how the polio virus entered the anterior horn motor neurons. Was it from the bloodstream after the virus had caused a major viremia, or did it enter at the tip of the axon, where it meets the muscle? The two are not mutually exclusive, and both may occur. The axons of motor neurons do not contain ribosomes for the virus to replicate. However, they have a “transportation system,” which allows for the carriage of material back to the cell body in the anterior horn, the part of the cell where the virus can replicate. The proteins dynein and kinesin attach to and move proteins and tiny organelles around the axon, dynein going from the distal axon to the cell body, and kinesin the other way. It is certainly possible that poliovirus may enter the neuron at the junction of the muscle and the axon, encase itself in a small autosome-like structure, then be pulled back to the ribosome-containing cell body for replication, with the resultant cell-destructive consequences. It has been known for some time that if an individual suffered a muscle injury coincident with the onset of polio, the injured muscle was commonly the first one affected, suggesting the virus may jump from the infected injured muscle to the adjoining neuron.

            The paralysis that ensues is what is known as flaccid, that is, limp or lacking firmness.  The word flaccid, which may be pronounced in two ways: "flak-sid” or “flass-sid,” comes from the Latin flaccus, meaning flabby. The sensory neurons are not damaged, so the patient can feel the affected limb but cannot move it because of the damaged motor neurons. The number of motor neurons involved and the muscles they innervate occurs randomly, although the legs are often the most commonly affected. Paralysis almost always occurs while the patient is febrile; when the fever stops, so does the progression of neuronal involvement. Once that happens, it is a waiting game. Most patients recover at least part of the function of the paralyzed limb; others do not. If the muscular damage has not abated after about a month, the chances are it will be permanent.  

            Even worse than the paralytic form of the disease is bulbar. The “bulb” refers to the medulla oblongata, the bulb-like portion of the spinal cord connecting it to the brain. It contains the cranial nerves, and it’s usually the ninth and tenth cranial nerves that become involved. These control the pharynx and larynx, among other things, so swallowing and breathing are impacted, making bulbar poliomyelitis life-threatening. 5% to 35% of paralytic polio cases also involve the bulbar form. The lungs themselves and the diaphragm and intercostal muscles are not affected. Usually, the damaged cranial nerves recover within a week to ten days, but respiratory assistance and intravenous feeding are required to get over the immediate crisis.

 

            In summary:  the polio virus enters us through our water and food. It primarily infects us through the cells lining the distal small intestine, the ileum, although it may enter through similar cells in the mouth. The virus binds to the polio virus receptor, CD155, a long three-headed molecule.  One part of it, the D1 portion, extends into a crevice in the protein coat of the polio virus. This triggers the break-up of the coat and the injection of the positive-sense single-strand of RNA into the cell. This viral RNA is decoded by a ribosome in the cell, yielding one long chain of the virus’ eleven proteins, all stuck together. One of the viral proteins, 2A, breaks off and cuts free the other 10. Host cell protein synthesis is cut off by the action of Protein 2A, damaging one of the cell’s proteins necessary for RNA transcription. The chemical signals warning of the microbe’s presence are usually sent from the cytoplasm to the nucleus. But the polio viral protein 2A significantly interferes with this process, and the manufacture of cellular proteins, like interferon, is much reduced. Protein 2A takes out the molecules that escort these chemical signals and host cell RNA through the nuclear membrane. The virus normally develops in the cell’s autosomes.

            After reaching a critical mass, the assembled virus escapes, numbering in the tens or hundreds of thousands. Some enter nearby cells, others are eliminated in the stool, and some make their way into the bloodstream, causing a minor viremia. When these blood-borne virions reach the tissues of the reticuloendothelial system, like the spleen, liver, and bone marrow, they are usually destroyed. But in a few individuals, they multiply to reach new high levels and are released into the bloodstream, giving a major viremia. They can then reach the neurons of the spinal cord’s anterior horn and cause neural damage. Some enter the ninth and tenth cranial nerves causing problems in the pharynx and larynx, including swallowing and breathing.  

            The time frame for the progression of symptoms varies, but typically is:

            Incubation time for the virus, from presumed first contact to excretion of the virus in the stool, is three days.

            Abortive poliomyelitis, characterized by fever, perhaps a headache, vomiting, and listlessness, 5-6 days after presumed first contact.

            Non-paralytic poliomyelitis, similar to meningitis caused by other enteroviruses, occurs around a week to ten days after exposure.

            Paralytic and bulbar polio take place 12-15 days after exposure.

            There is no specific anti-viral treatment for the poliovirus.  

 

            One of the more excruciating parts of polio is the “waiting game” that accompanies the paralytic and bulbar forms. Some patients displaying these symptoms recover most or even all of their muscular activity after a week to ten days of paralysis. Some don’t, although polio’s bulbar form usually reverses if the patient can be kept alive. If the paralytic symptoms don’t subside after a month, the chances are high that the damage is permanent.  

            A condition known as the post-polio syndrome is an interesting sequela of patients who have recovered some or all their muscular function. It occurred in about 20-30% of paralytic polio patients years after the initial disease. It presented as a gradual onset of weakness, pain, and fatigue in the muscles initially involved in the disease. Neither the virus nor antibody to it is found, so the most reasonable explanation is that the neurons that took up the job of innervating these damaged areas become overworked with age and give out. The condition is not usually severely disabling but can impact the quality of life.

 

            When someone today thinks about people and polio, the name Jonas Salk is the first, and sometimes only, name that comes to mind. Iron lungs and physical therapy were beneficial aids in treating afflicted persons, but the ideal is not to get the disease in the first place. Vaccines and their effectiveness had been known for some time, since 1800, in fact, in the use of the smallpox vaccine. By the late 1930s, several vaccines were widely available, including those for diphtheria, tetanus, typhoid, and others. Directly treating a virus with an anti-viral agent, as an antibiotic treats a bacterial infection, was out of the question. The only medical approach to eliminating the threat of the disease was the employment of a suitable vaccine.

            Two schools of thought prevailed on vaccine development in the 1940s: live, attenuated strains given in the same manner as a natural infection but unable to induce disease, and “killed,” or inactivated virus injected into the body. Emotions ran high on both approaches. In the case of a “killed” virus, can we always be sure it is really “dead?” Are viruses alive in the first place? They can’t reproduce on their own, and without a cell to invade and do their metabolism, they just sit there. The inactivation of a virus by chemical agents may result in a virus that may still be viable under certain circumstances, or the inactivation process alters its chemical structure to make the antibody response irrelevant to the actual wild-type virus. For live, attenuated viruses, the potential problem lay in the possibility that a suitable type would never be found, or the attenuated type could revert to the wild type and initiate the disease.  

            Journalists and historians tend to focus on a particular individual when a remarkable feat is accomplished. Edward Jenner is commonly referred to as the conqueror of smallpox, and Alexander Fleming the discoverer of penicillin. They deserve great praise, but others were involved in their breakthrough discoveries. Many of the local farmers in Jenner’s time were well aware that milkmaids who got “the cowpox” on their hands were immune to smallpox. Jenner listened to their tale, tested it, then wrote a letter to London’s Royal Society about his results. He had the wherewithal to test and reports his findings, but the actual discovery of the process lay with others. Fleming did indeed observe and test the ability of “mold juice” to kill bacteria. Still, the discovery lay dormant in his lab until Ernst Chain and Howard Florey at Oxford did the work to make penicillin a usable pharmaceutical product.

            Jonas Salk certainly deserves the accolades that have been his through the years. But we mustn’t forget those whose work preceded and accompanied his to make his accomplishment possible. The existence of three distinct types of poliovirus was confirmed by the work of John R. Paul and James Trask in 1931. The three types were initially called Brunhilde (type 1), Lansing (type 2), and Leon (type 3). They also confirmed that polio was an enteric infection. Albert Sabin and Peter Olitsky were the first to successfully cultivate the polio viruses in cell culture, using human embryonic neuronal tissue, in 1936. Until then, it was impossible to grow the virus in artificial culture. Unfortunately, this cell line was difficult to perpetuate and maintain.  The enormous breakthrough by John Enders, Thomas Weller, and Frederick Robbins in 1949 led to the polio virus's cultivation in non-neuronal cell lines, which led to the ability of Jonas Salk to do his work. Until then, one couldn’t get enough virus from cell cultures to produce enough to do vaccine experiments. The work of Enders, Weller, and Robbins allowed this to happen. They were jointly awarded the Nobel Prize in medicine in 1954 for their work.  

            Jonas Salk’s inactivated vaccine was first tested in 1954.  All three types of the polio virus harvested from monkey kidney cells were mixed in about equal quantities and inactivated by suspending them in formalin (dilute formaldehyde). About 1.6 million children from the U.S., Canada, and Finland were enrolled in the field trial. Some got the inactivated virus, some got a placebo (saline injected into the arm), and some got nothing. This was the most extensive field trial of a vaccine in history. The number of children in the trial was so large because the number of paralytic polio victims in an infected population was small, less than 1%, and it was necessary to vaccinate a vast number of children to give a result with statistical relevance. The trial was a great success, and the vaccine, with great fanfare, was approved for use in the United States in April 1955.  

            The success of the use of the vaccine was unmistakable. In 1954 the incidence of paralytic polio was 13.9 cases per 100,000 population. By 1961 that number had decreased to 0.8 cases per 100,000. There was no question that the vaccine was working, but there were problems. The most obvious one was the “Cutter incident,” which occurred in 1955. The Cutter Laboratories in California contracted to make large amounts of the polio vaccine. Unfortunately, the vaccine it produced was defective, with significant amounts of virus remaining viable. Injecting this live virus into the arms of susceptible children was a catastrophe. There were a reported 260 cases of paralytic polio and ten deaths. Either the vaccine preparation had too much foreign material in it, or some of the viral particles clumped, shielding them from the action of the formaldehyde. A second filtration step was soon added, and ensuing preparations were non-infectious, but the “Cutter incident” lives on in vaccine infamy.

            One of the problems with the early polio vaccines was the source of the cells to cultivate the virus in the laboratory. The cell line that worked best was that taken from the kidneys of monkeys, especially rhesus macaques from south and central Asia and green monkeys from west Africa. Making a million doses of the polio vaccine took about 1500 monkeys. That’s a lot of monkeys, and they were usually taken right out of the jungle. There was always the worry that even though they looked healthy, there could still be something wrong with them, and, sure enough, it was discovered a few years after vaccine production began in full force that some of the imported monkeys were infected with a virus called a polyomavirus (that’s POLYOMA, not polio). Its official name is Simian vacuolating virus 40, but most refer to it as SV40. It is a circular double-stranded DNA virus that replicates mainly in the infected cell's nucleus. The virus usually stays attached to the nuclear material of the cell it infects, although sometimes it escapes, killing the cell it resided in.

            There has long been concern about what a virus like this could do to a human. It was estimated in 1960 by Ben Sweet and Maurice Hilleman that about 10-30% of people given the polio vaccine had also been injected with the SV40 monkey virus. Usually, the virus sits on the nucleus and does nothing, but there is always a chance it can become active and cause some type of cancer. There is a human counterpart to SV40, the JK and BK viruses. They are pretty common, but neither does much unless the infected person becomes immunosuppressed by HIV infection or immunosuppressive therapy. Perhaps the SV40 virus acts in the same way. Today’s cell lines for producing the polio virus for vaccines are laboratory cell lines, not wild type, so they don’t contain any viruses other than the one being cultivated.

            Another drawback to the Salk vaccine was that the virus was inactivated and administered intramuscularly, certainly not its usual means of entering the body. The immune system to combat foreign organisms consists of two cell lines, the B-lymphocytes and the T-lymphocytes. The B-cells make the antibody, and the T-cells enhance antibody production and are primarily responsible for immune memory. The IM administration of a dead organism usually stimulates only the B-cells. Also, a single injection may fail to elicit an appropriate B-cell response, so more shots, usually three, must be administered to ensure a good antibody response. In addition, a shot to “boost” the immune response, commonly called a booster shot, should be given 4-6 years after the first series of shots. If only the B-cell line was stimulated initially, the immunity might wane after a few years. The booster shot reactivates the antibody manufacturing cells and helps ensure long-lasting immunity. All of this presupposes that a parent will dutifully bring their child to a doctor or clinic to get the shots.  

            Even in the 1950s, it was well recognized that live, attenuated (altered) viruses and bacteria give a more effective and longer-lasting immunity. The problem is coming up with one that is safe. Researchers of the time often engaged in rather heated, sometimes rancorous, debates on the correct approach to a polio vaccine: chemically inactivated or live attenuated. The first to appear was Dr. Salk’s inactivated vaccine in 1955.  Six years later, the live, attenuated trivalent vaccine of Dr. Albert Sabin became commercially available in the United States.

            Dr. Sabin was from Poland, born there in 1906. His family emigrated to the United States when he was 15, and he received his medical degree from New York University in 1931. He immediately began his polio research. One of the leading philanthropic organizations of the 1930s in the battle against polio was the Rockefeller Institute in New York. He joined that organization in 1935 and left a few years later to work at the Children’s Hospital Research Foundation in Cincinnati, Ohio. There, he and colleagues were the first researchers to grow the virus in neuronal tissue, and he confirmed that polio was an intestinal virus. During World War II, he was in the U.S. Army and helped develop vaccines for several tropical diseases.  

            The attenuated polioviruses that Dr. Sabin and his team developed could enter the host cell and multiply, just like the wild-type virus. Virus production in the cell, however, was limited. The alteration to the virus lay in the part of the virus that attaches to the host cell ribosome, the so-called initiating sequence. With an altered amino acid in the initiation sequence, the strength of the attachment to the ribosome, known as avidity (from the Latin aviditatem, or “eagerness”), was much reduced. Instead of producing hundreds of thousands of viral copies, only a few hundred were produced. The Protein 2A content was much reduced, so it was not nearly as active, especially on the nuclear membrane. The process of chemical signaling and interferon production took place close to normal. As a result, the T-cell line and the B-cells were activated, and a robust, long-lasting immunity took place. All three types of polio were included in the vaccine, as the Sabin team discovered attenuated types for each.

            The live, attenuated vaccine is clearly superior to the inactivated injected form. Only one dose is needed, and repeat clinic visits are not necessary. Also, while not present in copious amounts like the wild type, the attenuated virus is shed in the stool and can still be spread from one individual to another. When immunizing populations where reaching everyone is not likely, the vaccine can still be spread from person to person, inducing immunity like the wild type but without the disease.

            Developing an attenuated strain takes years, and a lot of luck. Put simply, you just keep passing the virus through cell cultures or a test animal over and over until you can detect a product that doesn’t infect as strongly. You keep passing that one until the virulence is reduced even more. It can take hundreds of passages, and there is no guarantee of a suitable outcome. Working with material supplied by other workers, Sabin's team developed attenuated viruses for all three types.  

             Even though the new live strains looked promising, they still had to be tested. Since the inactivated poliovirus was so commonly administered in the U.S. in the preceding years, another naïve population had to be enlisted. A study involving about 26,000 children was conducted in Cuba in 1962, but more data was needed. Sabin was from Poland, and he had made the acquaintance of a highly respected Russian virologist, Mikhail Chumakov. Dr. Chumakov was a fascinating individual. In his early days as a researcher, he worked with a Siberian virus known as tick-borne fever, which is caused by a virus related to yellow fever and dengue. He became accidentally infected, resulting in his going blind and being paralyzed on one side. Despite this, he continued as a high-level research scientist and made several important discoveries.  

            Even though the relationship between the U.S. and the Soviet Union at the time was poor, the two eminent scientists convinced the Soviet government to allow the wide-scale testing of Sabin’s live vaccine strains. Over 2 million children were vaccinated, and the results were all they had hoped for. The live vaccine was soon approved for use in the U.S.

            There’s one problem, though, and it's big. All three vaccine strains can spontaneously mutate back to the wild-type form, causing full-blown polio, either in the person receiving the vaccine or someone unfortunate enough to come into contact with the vaccinated person’s excrement. The initial mutation is in only a single nucleotide on the viral RNA: In poliovirus 1, the adenine at position 480 is altered to guanine; in poliovirus 2, the guanine at position 481 is changed to adenine, and in poliovirus 3, the cytosine at position 472 is changed to uracil. It only takes a random mutation of one of these single nucleotides to have the virus revert to wild type, a situation calculated to occur once in every 750,000 viral replications.  

            So the risk is relatively straightforward: one can immunize 750,000 people and run the risk of creating at least one case of polio, or immunize no one and run the risk of a full-blown polio epidemic. In the U.S. and other developed countries, as polio started to wane due to vaccination and herd immunity, the oral vaccine was discontinued, and only the inactivated, injected vaccine was recommended. In remote, undeveloped countries, the oral vaccine was continued because of its ability to often reach unvaccinated members of a household or community.  

            The good news is that poliovirus 2 is now considered extinct in the wild. It has not been detected since 1999; in September 2015, it was declared eradicated. Poliovirus 3 is most likely gone as well, having last been detected in a patient in November 2012; it was declared eradicated in 2019. That leaves only poliovirus 1 in the wild.  

            One could logically ask why polio vaccination is still recommended for children in the United States and other developed countries. The disease hasn’t been detected in these countries in decades. We have other vaccines that are not routinely administered unless travel is involved.  For instance, there are very good vaccines against typhoid and yellow fever, but they are not given to everyone, just those traveling to a country where the disease is endemic. Why polio vaccine for everyone? The answer, of course, has to do with the biology of the disease. Many more people infected with the virus are asymptomatic carriers than those with symptoms. A person traveling from an endemic area to a non-vaccinated one could easily, but unknowingly, carry and therefore spread the virus. Without herd immunity present in the community, it could be really bad. Several Public Health labs in the U.S. routinely test raw sewage for genomic evidence of poliovirus. Although no cases of poliomyelitis have been seen, the labs report the presence of the virus. It’s not extinct.

 

            Polio is a fascinating disease, from the biology of the virus taking over a cell and evading the immune system, and humanity’s efforts to control and eradicate it. Given its sudden emergence as the unintended consequence of the introduction of general sanitation, one cannot help but ponder what other disease agents may be lurking. Polio isn’t the only enterovirus. There are over 70 viruses in the family, and some have shown a predilection for the central nervous system. Viral meningitis is the most obvious, but two strains, D68 and A71, have given some children classic myelitis symptoms. They apparently are contracted through the respiratory route, giving rise to what appears to be a common cold, then progressing to involve the anterior horn cells. The condition is called acute flaccid myelitis, or AFM. To date, these have been rare, but with a simple mutation here or there, things could get a lot worse.

                                             The so-called "iron lung" bcame a symbol of the disease polio. (PHIL)

Prev