Zika Virus: Background, Politics, and Prospects

Ebola, MERS, and Hepatitis C viruses dominated virology news during the past year (2015). Now, early in 2016, Zika virus has taken center stage. The reasons are clear. This once seemingly innocuous virus, initially restricted to Equatorial Africa, has of late spread to the Western Hemisphere, and is now suspected (but not proven) to cause microcephaly—an otherwise rare condition in which babies have unusually small heads and incomplete brain development—in transplacentally infected fetuses of infected pregnant woman. Moreover there is evidence which links Zika virus to Guillain–Barré syndrome—a potentially severe autoimmune attack on peripheral nerves that may occur after signs of a viral infection. We begin with some background.

Zika virus is a member of the flavivirus family of plus-strand RNA viruses. The family also includes several notable human pathogens, including yellow fever, dengue, hepatitis C, and West Nile viruses. Like most other flaviviruses, Zika virus too is spread by an arthropod vector; in this instance Aedes mosquitoes. 80% of Zika virus infections are asymptomatic and, prior to recent developments, symptomatic infections were seen as mild, acute febrile illnesses, similar to dengue.

Zika virus was discovered by accident in the Zika Forest of Uganda in 1947. The discovery was made by scientists who had been studying yellow fever. They isolated Zika virus from one of their rhesus macaques, which was suffering from an unknown fever. The following year the same virus was found in Aedes mosquitos from the same Ugandan forest, thus identifying the mosquito as a vector for Zika virus. Zika virus was detected for the first time in humans in 1954, in Nigeria.

The Aedes aegypti mosquito, the Zika virus vector

Until recently, Zika virus infections were rare and were reported only within equatorial Africa and Southeast Asia. Then, in 2007, an outbreak occurred in Yap Island, Micronesia. The Yap Island Zika outbreak was the first one outside of Africa and Asia. None of the Yap Island cases, which included 49 in which Zinka virus was confirmed by the presence of Zinka RNA, resulted in either hospitalization or death.

The Yap Island outbreak was followed by epidemics in Polynesia, Easter Island, the Cook Islands and New Caledonia. The Polynesian outbreak was notable for being the first in which Zika infection was associated with Guillain–Barré syndrome.

Concern over Zika virus was heightened, particularly in the Americas, when, in April 2015, a large and still ongoing outbreak of Zika virus occurred in Brazil. The Brazilian outbreak marked the first appearance of Zika virus in the Western Hemisphere. It is not clear how Zika virus made its way to Brazil, but it is widely believed that the virus made the leap from Polynesia to Brazil during the 2014 World Cup soccer tournament.

Apprehension over Zika virus increased in November 2015 when the virus was isolated from a Brazilian newborn with microcephaly. By December 2015 many more cases of this generally rare disorder were reported. The European Center for Disease Prevention and Control then warned of a possible association between Zika virus infection and congenital microcephaly, and with Guillain–Barré syndrome as well.

More than a million Brazilian people since been infected with Zika virus, and the number of Brazilian children born with microcephaly jumped from 147 in 2014 to nearly 4,000 in 2015. There is no anti-Zika vaccine, nor is there an effective therapy.

The first Zika virus-associated case of microcephaly in the United States occurred in early January 2016 in a baby born in Oahu, Hawaii. The baby and its mother each tested positive for a past Zinka infection; probably acquired in May 2015 when the mother, then pregnant, had been traveling in Brazil.

On January 24, 2016 the World Health Organization warned that Zika virus will likely spread to every nation in the Western Hemisphere (possibly excepting Canada and Chile), since its Aedes aegypti vector can thrive in tropical and sub-tropical climates here. The Aedes mosquito has long been present in the United States, ranging as far north as New York and west into Indiana and Illinois. [An earlier posting reported that Aedes aegypti may have been brought from Africa to the New World by slave ships in 1596 (1). Mosquito larvae, present in the water casks of the sailing ships of the day, also carried yellow fever to the New World.]

Global concern over the Brazilian Zika outbreak was heightened by the fact that Brazil is scheduled to host the Olympic Games this summer, and about 500,000 people are expected to attend from all over the world, including 200,000 Americans. Some of these attendees will, of course, be bringing the virus back to their home countries.

Brazilian officials no doubt are concerned that their Zika outbreak will affect attendance at the upcoming Olympic Games. Consequently, commercial considerations may be one of the motives behind Brazil’s extensive campaign to eradicate its mosquitoes. Unfortunately, standard approaches, such as using insecticides and removing standing water where mosquitoes breed, have not done the job. Thus, the Brazilian Zika outbreak may not be under control by the start of the Olympic Games. [Brazil also experienced more than 1.6 million cases of dengue during 2015, with 863 people dying from the disease, underscoring that the Aedes mosquito vector is not well contained in that country.]

The failure of Brazilian vector-control approaches suggests that new strategies may be needed to contain the outbreak. Apropos that, this past January Colombia began releasing mosquitoes treated with bacteria, which are hoped might limit the mosquitoes’ capacity to spread disease. Note that insecticides have limited effectiveness. Not only are they toxic to humans, but after decades of overexposure to them, many mosquitoes are now resistant.

Zika virus is now present in the continental United States. Thus, it is timely to consider how grave a threat Zika virus might impose here. To that point, consider that yellow fever, dengue and chikungunya viruses are dangerous pathogens that also are spread by Aedes mosquitoes. Yet these viruses are not regarded as important threats in the United States. That is so because our vector control measures have thus far been able to contain them. Those measures might likewise be expected to contain local transmission of Zika virus here.

But, what if Zika virus has a mode of transmission other than via its mosquito vector? To that point, there is a single reported case of Zika transmission via a blood transfusion. Also, it was suggested that Zika virus might have a sexual route of transmission, as per the finding of high levels of the virus in the semen of a man from French Polynesia. In addition, there is a report of an American scientist, Brian D. Foy, who contracted Zika virus while working in Senegal in 2008, and who transmitted the virus to his wife after returning home (2). Serologic analyses of the couple’s convalescent serum confirmed that they had been infected with Zika. Sexual transmission is implicated in this instance since neither Foy nor his wife passed the infection to their children or to other close relatives. Moreover, Foy and his wife observed signs of hematospermia (red–brown fluid in his ejaculate).

Foy notes in his scientific report (2), “If sexual transmission could be verified in subsequent studies, this would have major implications toward the epidemiology of Zika virus and possibly other arthropod-borne flaviviruses.” [Human sexual transmission of an arthropod-borne virus has not yet been documented.] Foy has been trying to get funds to investigate sexual transmission of Zika. However, according to a January 26, 2016 article in the N.Y. Times, the CDC says that the “theoretical risk” of sexual transmission in the above instances is insufficient to justify a warning (and funding?). But, see the following paragraph.

As I’m sitting at my computer on the evening of February 2, 2016, NPR, CNN, BBC News, the N.Y. Times, etc., are reporting a case of Zika virus infection in Texas that appears to have been sexually transmitted. According to the Dallas County Health and Human Services Department, a patient with the Zika virus was infected after having sex with someone who returned from Venezuela, where Zika is circulating. The CDC appears to give credence to the Texas report, since it quickly responded to it by advising men having sex after traveling to these areas to “consider” wearing condoms, and advised pregnant women to avoid “contact with semen” from men recently exposed to the virus.

Sexual transmission will probably account for only a very small fraction of Zika cases, but that isn’t known for certain. As in instances of mosquito-borne transmission, its contribution will depend in part on how long the virus might persist in infected individuals.

Since the vast majority of Zika virus infections are likely transmitted via its mosquito vector, and since Zika virus mainly threatens fetuses infected in utero, the most severe consequences of Zika virus infection can be largely avoided if pregnant women, or women planning to become pregnant, avoid traveling to places where Zika virus remains prevalent (a fact which doesn’t help individuals living in those regions). For that reason, on January 15, 2016, the United States Centers for Disease Control and Prevention (CDC) released a list of countries—Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela, and Puerto Rico—where mosquitoes are spreading the Zika virus, and which pregnant women should avoid at this time. On February 1, 2016, the World Health Organization added Costa Rica and Jamaica.

Political and commercial considerations may have been behind the Brazilian minister of tourism taking exception to the CDC’s warning, claiming that measures adapted by Brazilian health authorities are bringing the Zika outbreak under control, and that Brazil is, in fact, a safe destination for pregnant women. The Brazilian health minister added, “Zika virus doesn’t worry us…,” calling it a “benign disease.” Those pronouncements were made despite the fact that Brazilian health authorities were at the same time investigating more than 3,500 cases of microcephaly. But at least some Brazilian health professionals did endorse the CDC announcement.

On February 1, 2016 the World Health Organization took the further step of declaring that Zika virus and its suspected link to birth defects constitute an international public health emergency. Yet the WHO stopped short of advising pregnant women not to travel to affected regions. Some public health experts claimed that the WHO’s silence on that point was more about politics than public health. Any travel ban—even one aimed only at pregnant women—would be embarrassing and costly to Brazil, which is moving ahead with its plans to host the Olympic Games this summer. And, while there have been calls to cancel, postpone, or move the Rio games, the International Olympic Committee (IOC) hasn’t expressed any concerns over the Games taking place as planned.

Meanwhile, the governments of Columbia, El Salvador, Ecuador, and Jamaica have taken the rather extraordinary step of recommending that women avoid getting pregnant until the Zika outbreak might be brought under control in their countries. This advisory was not well received by many El Salvadoran women, especially in view of the strict abortion laws and high levels of sexual violence against women in that country.

And, as I’m putting the final touches on this piece, an article in today’s (February 4, 2016) N.Y. Times reports that the Zika virus/microcephaly link is causing a fierce debate in Brazil over its strict abortion laws; under which abortion is illegal under most circumstances. [Remarkably, Brazil’s strict abortion laws are actually less restrictive than those in other Latin American countries.] Some Brazilian doctors are already seeing pregnant women who are seeking abortions because they fear microcephaly. Yet conservative Brazilian lawmakers actually want to make the restrictions against abortion more stringent than they already are. [The Times article says that their position reflects “the influence of Roman Catholic leaders and the increasingly powerful preachers at the helm of a growing evangelical Christian movement.”] Regardless, individuals on both sides of the debate might be troubled by the fact that microcephaly can not be detected by ultrasound scans until the end of the second trimester, when the “child” is already very much formed. Moreover, the criteria for diagnosing microcephaly are rather non-specific, and it is difficult to predict what its consequences might be.

A crucially important question regarding Zika virus concerns determining its true pathologic potential, particularly its role in microcephaly—a role that is strongly inferred (but not proven) by the geographic and temporal relationship between microcephaly and Zika infection. To that point, no increase in microcephaly has been linked to Zika virus outside of Brazil. For instance, Colombia is the second-most Zika-affected country, with around 20,000 confirmed cases. More than 2,000 of the Columbian cases have been pregnant women. Yet none of their fetuses have been diagnosed with microcephaly.

Did Zika virus become an etiologic agent for microcephaly only after reaching Brazil? If so, how did that happen? Was it because of the emergence of a new strain of the virus? Or, does Zika virus cause microcephaly only if the mother has had a previous infection, like dengue? Alternatively, was the link simply missed in the past because, until now, the virus has not invaded a country where there are a large enough number of non-immune individuals, who also are living under conditions that are ideal for the virus to spread? Or, were previous cases merely under-reported, such that the 147 Brazilian cases in 2014 were a vast underestimate?

The flip side is that the current extraordinarily high number of reported cases of microcephaly in Brazil might merely be due to a heightened awareness of that condition; a possibility that is favored by some Brazilian officials. A supporting argument is that the criteria for diagnosing microcephaly are relatively unspecific. However, others point out that physicians were reporting a rise in cases as early as November 2015, before the increased attention from health authorities and the media.

Another unexplained yet key factor is the unusually severe congenital deformities—extensive loss of brain tissue, unusually smooth, wrinkleless brains, many calcium deposits, and smaller cerebellums—seen in the Brazilian microcephaly cases. These features are not characteristic of microcephaly caused by other pathogens, such as toxoplasmosis, cytomegalovirus, or rubella.

And, presuming that Zika virus indeed causes microcephaly, how or why is it able to cross the human placenta and enter the fetal brain? [In December 2015, the Pan American Health Organization reported that Zika virus RNA was identified by reverse transcription-polymerase chain reaction (RT-PCR) in amniotic fluid samples from two pregnant women whose fetuses were found to have microcephaly by prenatal ultrasound. Moreover, Zika virus RNA was identified in multiple fetal body tissues, including the brain of an infant with microcephaly (3).] Remarkably, only a handful of viruses cross the human placenta and infect the fetus with any notable frequency (4). These include rubella virus, cytomegalovirus, and HIV; none of which is related to Zika virus. Yellow fever, dengue, and West Nile viruses, which are related to Zika virus, are not known to harm embryos.

Since most Zika virus infections are either asymptomatic, or present with flu-like symptoms that mimic other infections, a rapid diagnostic test for Zika infection is needed to accurately measure the prevalence of the virus in a population, and to measure its spread. Such a test might also help sort out whether the Brazilian microcephaly cases indeed have been due to Zika, rather than to another virus, such as the related dengue virus. Efforts are currently underway to develop Zika-specific immunological reagents for these purposes.

Vaccine researchers say that a vaccine against Zika virus may be available for testing by the end of 2016. But, even if the vaccine were effective, how long might it take for it to gain approval?

Meanwhile, an increasing, but still small number of Zika virus infections are being detected in the continental United States. With the exception of the Texas case noted above, all cases have thus far involved travelers who recently returned from overseas. Thus, with the exception of the Texas case, there is no evidence yet for local transmission here. But that well might change as summer approaches.

So, Zika now joins Lyme, West Nile, Chagas, dengue, and chikungunya on the list of recently emergent arthropod-borne diseases. Still, as we’ve noted, it is not yet clear how much of a threat Zika virus actually poses. Regardless, until that is known, it will be necessary to prepare for the worst. Even if the threat of Zika has been vastly overblown, progress towards its containment will pay important dividends in the containment of established threats, such as dengue and chikungunya.

And, if Zika is indeed a dangerous pathogen that is responsible for severe birth defects, then current conditions—global warming, more people traveling worldwide on jet airliners, cities in tropical countries becoming larger and ever more crowded—don’t portend well for the future. Stand by for new developments.

References:

1. The Struggle Against Yellow Fever: Featuring Walter Reed and Max Theiler, Posted on the blog May 13, 2014.

2. Foy, B.D., K. C. Kobylinski, J.L. Foy, et al., 2011. Probable Non–Vector-borne Transmission of Zika Virus, Colorado, USA, Emerg Infect Dis. 17: 880–882.

3. Pan American Health Organization. Neurological syndrome, congenital malformations, and Zika virus infection. Implications for public health in the Americas—epidemiological alert. Washington DC: World Health Organization, Pan American Health Organization; 2015. This paper is in Spanish.

4. Norkin, L.C., Virology: Molecular Biology and Pathogenesis, ASM Press, 2010.

The Origin and Natural History of Measles Virus

Our last three postings focused on measles, mumps, and rubella, and the trivalent measles, mumps, and rubella (MMR) vaccine. The first of these postings considered the controversy surrounding the MMR vaccine, provoked by a discredited paper which alleged a link between the vaccine and autism (1). The second of these postings highlighted mumps and rubella, the “other” two diseases targeted by the MMR vaccine (2). The third posting told how Australian ophthalmologist, Norman McAlister Gregg, discovered the link between fetal birth defects and rubella infection of the mother during pregnancy (3). The current posting considers the origin and natural history of measles virus.

We begin by asking why measles virus, unlike Ebola virus, is one of the most contagious viruses known. Measles is a member of the paramyxovirus family of RNA viruses, which also includes mumps virus, respiratory syncytial virus, and the parainfluenza viruses. Each of these viruses is similarly highly contagious, mainly because each is efficiently transmitted via the respiratory route. That is, these viruses can be transmitted between individuals through the air via a cough or a sneeze. In contrast to measles virus, the inefficiently transmitted Ebola virus (a member of the filovirus family of RNA viruses that also includes the Marburg virus) is spread only by direct contact with the blood or body fluids (e.g., urine, diarrhea, and vomit) from a person who is expressing Ebola symptoms (4, 5).

Measles is hardly unusual in being transmitted by the respiratory route. Indeed, that route is the most common means of viral transmission between humans. A majority of viruses evolved to use the respiratory route because of several key features of the respiratory mucosa (the epithelium that lines the respiratory tract). Most importantly, the respiratory mucosa encompasses a very large surface area of about 140 square meters. For comparison, the entire surface area of the skin is a mere 2 square meters. Also, the respiratory mucosa is delicate and in intimate contact with the vasculature. Another factor which contributes to the communicability of measles is that an infected individual can unknowingly transmit the virus for several days before showing signs of illness.

The rhinoviruses and coronaviruses, which give rise to mild common colds, are likewise transmitted by the respiratory route. Why then do measles infections have the potential to be severe, even fatal? The answer, in part, is that the mild rhinoviruses and coronaviruses remain in the respiratory tract during infection, whereas measles virus is able to disseminate widely throughout the body. One reason it can do so is that it can bind to several different receptors, including one called CD46, which is present on most cells of the body. Because measles virus can disseminate widely, serious complications from measles can occur in almost every organ system.

Nonetheless, most measles fatalities actually result from secondary bacterial and viral infections of the respiratory and digestive tracts. These secondary infections are facilitated by a prolonged immunosuppressed state brought on by the primary measles infection. Paradoxically, the specific immune response against the measles infection is accompanied by a not-yet-understood suppressed immune response to other new antigens.

Our next (and perhaps most interesting) issue concerns the origin of measles virus and its establishment in the human population. We begin this topic by first defining the contagiousness of measles virus in quantitative terms.

The contagiousness of any virus is quantitatively defined by its reproduction number (R0), which is the average number of new individuals who are infected by each previously infected individual in a completely susceptible population. The R0 of measles virus is important to our analysis since, as we will see, measles virus was once an emerging virus in humans, and an emerging virus can successfully establish itself in a new host only when its R0 in that host is greater than 1. If the R0 is less than 1, then outbreaks in the new host will be self-limiting, even if the virus emerges in the host on multiple independent occasions. [Although Ebola virus is extremely virulent in humans, it has not established itself in humans, since its R0 in humans is less than 1.]

What then is the R0 of measles virus in humans? The answer is 15! On the one hand, this implies that if the contemporary measles virus were to emerge in an isolated and completely susceptible human population, it would readily spread into that population. On the other hand, consider the following. Since measles virus is so highly transmissible in humans, nearly every individual in that wholly susceptible population would be infected. Moreover, each of those infected individuals would either succumb to measles and die, or survive and develop lifelong immunity to the virus. This creates a dilemma for measles virus. How would it be able to continually find new susceptible individuals in order to sustain itself in the population?

The answer to the above dilemma is that measles virus, as it now exists, can survive only in populations that are large enough to continually generate a sufficient number of new susceptible individuals. Mathematical modeling by epidemiologists has led to the estimate that a community of at least 500,000 persons is necessary to sustain measles virus. This leads to the somewhat astonishing conclusion that measles virus, as we know it, has existed in the human population for no longer than the last several millennia, since communities large enough to sustain measles virus did not exist before then, and the much smaller communities that did exist had only occasional contact with other human groups. [By the same reasoning, measles virus is not present in other primates, since their population sizes are not nearly large enough to sustain this highly contagious virus.]

The above line of argument also explains why, in the pre-vaccine era, measles outbreaks or epidemics occurred every several years, with many fewer cases occurring between epidemics. Moreover, it leads to the notion that epidemic diseases in humans are a relatively recent development. To these points, consider the following. Measles virus shares the following three essential features with other epidemic viruses (e.g., poliovirus). First, each of these viruses is highly contagious, such that each spreads quickly through the susceptible individuals in the population. Second, epidemic viruses give rise to acute infections. [In contrast, non-epidemic viruses, like the herpesviruses, give rise to life-long persistent infections.] Individuals who recover from an acute infection are immune to re-infection by that agent. Third, epidemic human viruses have no animal reservoir to sustain them.

At the end of an epidemic, the virus persists in the human population, but only at a low incidence, and only if the population is large enough. Then, as a sufficient number of new susceptible individuals are born into the population, the epidemic threshold is reached, and the cycle repeats. The above argument also explains why very young children are the targets of epidemic viruses; they are the only non-immune individuals in the population.

Since measles virus, as it now exists, only recently became established in the human population, and since measles virus has no known animal reservoir, where did it come from? Viruses do not emerge from out of nowhere, nor do they leave fossils that might provide insights into their natural history. Nevertheless, the origin of measles virus can be inferred with a fair degree of certainty. First, there is the general principal that animals are the source of the emerging viruses that become established in humans. Second, nucleotide sequence analysis can identify the closest relative of measles virus among zootic viruses. That virus turns out to be rinderpest virus, which infects cattle, sheep, and goats.

Most studies estimate that  rinderpest virus emerged in humans about 8,000 to 9,000 years ago, around the time that some human groups began to domesticate large herd animals. Clearly, the development of animal husbandry meant that these human groups would live in close proximity to herd animals. Moreover, and extremely important, the development of animal husbandry (as well as the emergence of agriculture) enabled human populations to grow enormously in size, since population size was no longer constrained by the limited availability of food which, previously, was either gathered, or killed on the hunt. Thus, the development of animal husbandry created the conditions for the emergence of rinderpest virus in humans, and for its subsequent evolution into measles virus.

The emergence of rinderpest virus in humans is generally believed to have occurred in the Fertile Crescent of the Middle East; one of the first areas where humans developed animal husbandry. A probable sequence of events was as follows. At first, rinderpest virus was transmitted to humans only from the animal host. Then, as the virus adapted to humans, it eventually evolved to the point where it could be transmitted from human-to-human, but only inefficiently. At this stage in the evolution of measles virus, outbreaks in humans would have been self-limiting and brief. In time, measles virus evolved to sustain longer outbreaks in humans. It then became so well adapted to humans that it was no longer able to infect its original animal hosts. As measles virus was evolving from its rinderpest precursor, it spread from the Middle East throughout Europe and Asia. See Asides 1 and 2.

[Aside 1: The developments of animal husbandry and agriculture made it possible, even necessary for some humans to settle in one place. This in turn made it possible for civilization to thrive and produce art, science, technology, the development of cities, etc., which could not happen under the earlier conditions of a nomadic existence.]

[Aside 2: Domestication of large herd animals developed in the Old World, rather than elsewhere, simply because that is where these animals were available. Old World populations eventually developed widespread resistance to many of these “new” human pathogens (e.g., smallpox, influenza, and measles). Then, when Europeans first came into contact with native groups in the Americas and Oceana, which had no previous exposure to Old World pathogens, the consequences were devastating to the native populations. Indeed, the germs carried by these European newcomers played a far greater role in their conquests of native populations than any advantages the Europeans had in weapons or technology (6).]

In the past, rinderpest virus caused severe disease in domestic cattle, with death rates during outbreaks actually approaching 100%. Indeed, the term rinderpest is adapted from German, meaning cattle-plague. Such cattle plagues were common, with documented outbreaks dating back to the time of the Roman Empire.

Rinderpest does not readily infect people. Nonetheless, rinderpest outbreaks in animals had enormous consequence for humans. For instance, when the virus spread to Africa from India in the late 19th century, it killed an estimated 90% of domestic cattle. This, in turn, led to the deaths from starvation of one third of the people of Ethiopia and two thirds of the Maasai people. The virus also spread to wild ungulates in sub-Saharan Africa and was considered a greater threat to the survival of these animals than human encroachment on their habitats.

Thanks to a rinderpest vaccine developed by Walter Plowright (at the Animal Virus Research Institute in Pirbright Laboratory in Surrey, England), and a world-wide eradication program, the last confirmed case of rinderpest was reported in Kenya in 2001. In June of 2011, the United Nations Food and Agriculture Organization declared that the disease had been eradicated, making rinderpest only the second disease in history, after smallpox, to be eradicated.

The stamping out of rinderpest might well be the greatest achievement in the history of veterinary medicine. Compare the rinderpest case to the situation that prevails in the case of measles (1)! But, as stated by English rinderperst virologist, Michael Baron: “When it came to vaccination, the cows never had a choice.”

Some final points: First, just as measles virus requires a large human population to sustain itself, its rinderpest progenitor likewise needed large herds to sustain itself. However, cattle population size was not as severe a hindrance to rinderpest virus as human population size is to measles virus. That was so, at least in part, because of the more rapid turnover rate of cattle, which led to the more rapid emergence of new susceptibles.

Second, more than half of all human infectious diseases, including influenza (from wild birds), and HIV/AIDS (from chimpanzees and gorillas), originated in animals; domestic and wild. And, pathogens may jump the other way as well, since humans have in fact transmitted measles virus to mountain gorillas, and poliovirus to chimpanzees.

Third, RNA viruses (e.g., Ebola, HIV, SARS, H5N1 influenza) are disproportionately represented among emerging viruses. This is explained in part by the facts that RNA viruses must use their own RNA polymerases to copy their RNA genomes, and that RNA polymerases do not have the proofreading activity of DNA polymerases. Consequently, RNA virus genomic replication is vastly more error-prone than DNA virus genomic replication. Indeed, RNA virus genomic replication may result in as many as one mutation per every thousand nucleotides. The higher mutation rate of RNA viruses is a key reason why they are much more likely than DNA viruses to adapt to a new host. In addition, an RNA virus may only need to recognize a new cell surface receptor to adapt to its new host, while a DNA virus may also need to adapt to the DNA replication “machinery” of a new host.

References:

(1) Andrew Wakefield and the Measles Vaccine Controversy, Posted on the blog February 9, 2015.

(2) Andrew Wakefield and the MMR Vaccine Controversy: What about Mumps and Rubella?, Posted on the blog February 18, 2015.

(3) Norman McAlister Gregg and the Discovery of Congenital Rubella Syndrome, Posted on the blog March 4, 2015.

(4) The American Public’s Response to the 2014 West African Ebola Outbreak, posted on the blog August 10, 2014.

(5) The American Public’s Response to the 2014 West African Ebola Outbreak: Update, Posted on the blog October 7, 2014.

(6) Smallpox in the New World: Vignettes featuring Hernan Cortes, Francisco Pizarro, and Lord Jeffrey Amherst, Posted on the blog February 25, 2014.

Peter Piot: The Discovery of Ebola Virus

Peter Piot co-discovered Ebola virus in a laboratory in Belgium in 1976. Two weeks after the discovery, he risked his life in Zaire, now the Democratic Republic of the Congo, studying the Ebola outbreak at its source.

Piot went on to have a distinguished career and is now best known for his advocacy of HIV/AIDS control and prevention, particularly in Africa. Toward that end, Piot participated in the first international project on AIDS in Africa, leading the way to understanding the African AIDS epidemic. He helped found and then served as Executive Director of the Joint United Nations Program on HIV/AIDS (UNAIDS) and then as Assistant-Secretary-General of the United Nations. It is safe to say that Piot helped to save hundreds of thousands, and perhaps even millions of lives. He currently serves as director of the London School of Hygiene & Tropical Medicine.

As a boy growing up in Belgium, Piot fantasized of having exotic adventures. One of his dreams was to travel to Africa, where he might lend a hand to underprivileged people. A more mature Piot, aware that infectious diseases caused most deaths in the developing world, thought that medicine might be an ideal means of fulfilling his fantasies. But, in medical school, one of his professors emphatically advised him: “There’s no future in infectious diseases…Just don’t do that, that’s a waste of time, we have antibiotics, we have vaccines, it’s all solved.”

However, Piot chose to ignore his professor’s advice. “But I wanted to go to Africa. I wanted to help save the world. And it seemed to me that infectious disease might be just the ticket and full of unresolved scientific questions. So I ignored him.”

[Aside 1: The view expressed by Piot’s professor was not altogether rare in the day. As noted in Virology: Molecular Biology and Pathogenesis (1), “Shortly before the emergence of HIV/AIDS in 1981, many health professionals held the opinion that the development of antibiotics against bacteria, and vaccines against poliovirus and other viruses, relegated the threat of infectious disease epidemics to history. Indeed, in 1962, Sir MacFarlane Burnet, who won the Nobel Prize in 1960 for his work on immunological tolerance, stated “…one can think of the middle of the 20th century as the end of one of the most important social events in history; the virtual elimination of the infectious disease as a significant factor in social life.”

AIDS and Ebola were not the only “new” infectious diseases to shatter the kind of optimism voiced by Burnet and others. There was also Lyme disease (caused by the bacterium Borrelia burgdorferi), Legionnaires’ disease (caused by the bacterium Legionella pneumophila), SARS, and West Nile virus. What’s more, rotaviruses are now a major cause of death, especially in children in the developing world, and hepatitis C virus may now affect more people worldwide than even HIV. Moreover, earlier pathogens, such as Mycobacterium tuberculosis, Corynebacterium diphtheriae, the dengue hemorrhagic fever virus, and yellow fever virus, have reemerged over the past 30 years. Making matters worse still, increasing resistance of bacterial pathogens to antibiotics, due in part to misuse of these once “miracle” drugs, is now a major threat to public health.]

The following, sometimes spine-tingling excerpts from Piot’s 2012 memoir, No Time to Lose (2), tell how 27-year-old Piot, two years out of medical school, helped to discover Ebola virus in a laboratory in Antwerp in 1976.

“On the last Tuesday in September 1976 my boss at the microbiology lab was alerted that a special package was on its way to us from Zaire. It was flying in from Kinshasa: samples of blood from an unusual epidemic that seemed to be stirring in the distant Équateur region, along the river Congo.”

[My notes: The blood sample was from a Flemish nun who had been carrying out missionary work in Zaire. She was stricken with a mysterious illness that was killing scores of people there. Piot was then working towards a PhD in Microbiology, awarded to him in 1980 by the University of Antwerp.]

“Nothing quite like this had happened in the two years I had so far been working in a junior position at a lab at the Prince Leopold Institute of Tropical Medicine in Antwerp, Belgium. But I knew it was part of the job. We sometimes took in strange samples of bodily fluids and tried to work out what they were. Our lab was certified to diagnose all kinds of diseases, including arbovirus infections like yellow fever, and the working hypothesis for this epidemic was reported to be “yellow fever with hemorrhagic manifestations.”

“I never actually worked with any suspected yellow fever. It wasn’t every day we received samples from as far away as equatorial Zaire. And it was clear this was an unusual sample, and that something pretty curious had occurred, because several Belgian nuns apparently died of the disease even though their vaccinations were completely up to date.”

“The next day—September 29—the package arrived: a cheap plastic thermos flask, shiny and blue. I settled down with Guido Van Der.” [The italics here, and in the following excerpts, are mine, for emphasis.]

“Groen—a shy, funny, fellow Belgian aged about thirty, a few years older than I—and René Delgadillo, a Bolivian postdoc student, opened it up on the lab bench. Nowadays it makes me wince just to think of it. Sure, we were wearing latex gloves—our boss insisted on gloves in the lab but we used no other precautions, no suits or masks of any kind.”

“We didn’t even imagine the risk we were taking. Indeed, shipping those blood samples in a simple thermos, without any kind of precautions, was an incredibly perilous act. Maybe the world was a simpler, more innocent place in those days, or maybe it was just a lot more reckless.”

“Unscrewing the thermos, we found a soup of half-melted ice: it was clear that subzero temperatures had not been constantly maintained. And the thermos itself had taken a few knocks, too. One of the test tubes was intact, but there were pieces of a broken tube—its lethal content now mixed up with the ice water—as well as a handwritten note, whose ink had partially bled away into the icy wet.”

“It was from Dr. Jacques Courteille, a Belgian physician who worked at the Clinique Ngaliema in Kinshasa. He described the thermos’s contents as two vials, each containing 5 milliliters of clotted blood from a Flemish nun who was too ill to be evacuated out of Zaire. … She was suffering from a mysterious epidemic that had so far evaded identification, possibly yellow fever.”

“I was still trying to find my way in the labyrinth of infectious diseases research, and this kind of thing made my heart beat faster. Guido and René picked out the one remaining test tube of blood from the thermos and set to work. We needed to look for antibodies against the yellow fever virus, and other causes of hemorrhagic or epidemic fever such as typhoid. To isolate any virus material, we injected small amounts of the blood samples into VERO cells, an easily replicable cell lineage that is used a lot in labs. We also injected some into the brains of adult mice and newborn baby mice. (I never liked this aspect of the work. Sometimes we needed to inject patient tissue into the testicles of rats, to isolate Mycobacterium ulcerans, the cause of Buruli ulcers, and it made me cringe.)”

“All this work was done with no more precautions than if we had been handling a routine case of salmonella or tuberculosis. It never occurred to us that something far more rare and much more powerful might have just entered our lives.”

[Aside 2: The italicized excerpts take me back to my postdoctoral days in the early 1970’s, when my colleagues and I routinely mouth-pipetted the much less (but still) dangerous SV40 (3), and the monkey kidney cell cultures that the virus was grown in. Indeed, we were more concerned with contaminating our cultures with mycoplasma, than with infecting ourselves with a simian virus.]

“In the next few days, the antibody tests for yellow fever, Lassa fever, and several other candidates all came up negative, and it seemed likely that the samples had been fatally damaged by their transportation at a semi-thawed temperature. We bustled nervously around the mice and checked our cell cultures four times a day instead of two. On the weekend, each of us popped in to check the samples. All of us, I think, were hoping something would grow.”

“Then it happened. On Monday morning, October 4, we found that several adult mice had died. Three days later all the baby mice had also died—a sign that a pathogenic virus was probably present in the blood samples that we had used to inoculate them.”

“By this time our boss, Professor Stefaan Pattyn, had also gleaned a little more information about the epidemic in Zaire. It seemed to be centered on a village called Yambuku, where there was a mission outpost run by Flemish nuns—the Sisters of the Sacred Heart of Our Lady of s’Gravenwezel. (S’Gravenwezel is a small town north of Antwerp.) The epidemic had been raging for three weeks, since September 5, and at least 200 people had died. Although two Zairean doctors who had been to the region had diagnosed the malady yellow fever, the patients suffered violent hemorrhagic symptoms, including extensive bleeding from the anal passage, nose, and mouth as well as high fever, headache, and vomiting.”

“Previously I had been excited about the work we were doing; now I was inflamed. If we were hunting for signs of a hemorrhagic virus, this was outbreak investigation of the most stirring variety. I truly loved the detective thrill of working in infectious disease. You came in and figured out what the problem was. And if you managed to figure it out quickly enough—before the patient died, basically—then you could almost always solve it, because, just like my medical school professor of social medicine had said, solutions had by this time been found for almost every kind of infectious illness.”

Piot relates that Pattyn “ knew we were not equipped to do the work in safety. In 1974 there were only three labs outside the Soviet Union that could handle hemorrhagic viruses: Fort Detrick, a military lab in Maryland that did high-security bioterrorism work on anthrax and other highly lethal diseases; the Army High Security Laboratory in Porton Down, in England; and the so-called hot lab at the Centers for Disease Control (CDC), in Atlanta.”

“Nonetheless, we continued to bustle around like amateurs in our cotton lab coats and latex gloves, checking our VERO cell lines. The cells began detaching from the glass sides of their containers: it was either a toxic effect or an infection, but either way, cytotoxicity had kicked in. That meant we might be close to isolating a virus, and we began extracting cells to cultivate them in a second line of VERO cells. And Pattyn had been told we should expect more samples from Zaire in the next few days.”

At this point in the story, Pattyn received a message from the Viral Diseases Unit of the World Health Organization (WHO), instructing him to ship all of the Belgium lab’s samples of the mysterious new virus to Porton Down in Britain.

“Pattyn was furious, and I too was upset. It looked as though our outbreak investigation was over before it had even begun. Glumly, we prepared to pack everything in tightly sealed containers: the patient serum, the inoculated cell lines, and the autopsied mouse brains and samples. But then Pattyn told us to keep some of the material back. He claimed that we needed a few more days to ready it for transport. So we kept a few tubes of VERO cells, as well as some of the newborn mice, which were dying. Perhaps it was a stubborn rebellion against the whole Belgian history of constantly being forced to grovel to greater powers. That material was just too valuable, too glorious to let it go. It was new, it was exciting—just too exciting to hand it over to the Brits or, in particular, to the Americans.” [A few days after Porton Down received the samples from Antwerp, the British lab passed a portion on to the CDC in Atlanta, which was the world’s reference lab for hemorrhagic viruses.]

So, with some of their sample material held back, the misadventures in the Antwerp lab kept on. “There was a rack of secondary tubes in the lab, which we had inoculated after the first VERO cell line was killed. We knew there was something in there—something that was trouble—but still, we had taken out the rack so we could examine the tubes under the microscope. Doing that kind of work wasn’t Pattyn’s job. He was a micro-manager but he wasn’t a technician, and in fact he could be rather clumsy. But impulsively he reached for one of the precious tubes, to check it out himself under the scope, and as he did so it slipped from his hand and crashed on the floor.”

“Little René Delgadillo was the one who got his shoes splashed. They were good, solid leather shoes but René bleated, “Madre de Dios” (Mother of God!) while Pattyn swore, “Godverdomme” (Goddamn!)—and there was a moment, just a beat, of blank fear. Immediately we whisked into action: the floor was disinfected and the shoes removed. It was just a small incident. But it struck me only then how lethal this thing really might be and the huge risks we had been taking in handling it so cavalierly.”

Piot then relates that electron micrographs of the new virus, prepared at the university hospital, showed it to be strikingly similar morphologically to the Marburg virus.

“Marburg was clearly a very scary illness, and as we did not have Marburg virus–specific antibodies, we could not definitely conclude whether our isolate was Marburg. Perhaps it was a different virus with similar morphology.”

“Pattyn was not suicidal. Once he had established that ‘our’ virus was—at the very least—closely related to the terrifying Marburg, he had the sense to shelve all further work on it and sent the remaining samples directly to the high-security lab at the CDC.”

Shortly afterwards, the CDC informed the Belgium lab that the mystery virus did not react with Marburg antibodies, and was indeed a new, previously unknown agent. Recalling the earlier advice from his medical school professor, Piot remarked, perhaps with some satisfaction, “… my professors were wrong.”

Piot then tells us, “I was still very excited. It felt as though my childhood fantasy of exploration was almost within my reach. I kept arguing that we had to follow up our work, go to Zaire and check out the epidemic. I felt strongly that we shouldn’t hand this world-class discovery over to some other team. We had identified this virus, after all, so we should be the ones to establish its lethality and its real effects on the ground.”

Piot then relates the political machinations that enabled him to fulfill his dream of going to Africa, where he would help to analyze and contain the deadly epidemic. In Zaire, in the quarantine zone, Piot lived among dying Zairians, repeatedly risking his life collecting blood samples from victims and gradually helping put together a picture of how the virus (eventually known as Ebola, after the river where its first outbreak occurred ), was transmitted.

Piot’s experience in Zaire led him to believe that he might benefit from additional training in infectious diseases. So, he came to the United States to receive further training on sexually transmitted diseases. Upon returning to Belgium, he became the go-to doctor for people arriving from Africa with exotic tropical infections. The emergence of the African AIDS epidemic led Piot back to Africa. He was now well prepared to lead the world’s response against the newer and eventually much graver HIV/AIDS epidemic there (4, 5).

(1) Virology: Molecular Biology and Pathogenesis, by Leonard C. Norkin, ASM Press, 2010.

(2) No Time to Lose: A Life in Pursuit of Deadly Viruses, by Peter Piot, W. W. Norton & Company, 2012.

(3) SV40-Contaminated Polio Vaccines and Human Cancer, posted on the blog, July 24, 2014

(4) The American Public’s Response to the 2014 West African Ebola Outbreak, posted on the blog, August 10, 2014.

(5) Thabo Mbeki and the South African AIDS Epidemic, posted on the blog, July 3, 2014.