You can find TWiV #387 at microbe.tv/twiv, or listen below.
It has been speculated that the development of neurological disease and fetal abnormalities after Zika virus infection may be due to the presence of antibodies against other flaviruses that enhance disease. In support of this hypothesis, it has been shown that antibodies to dengue virus enhance infection of cells by Zika virus.
There are four serotypes of dengue virus, and infection with one of the serotypes generally leads to a self-resolving disease. When a different serotype is encountered, antibodies to the first serotype bind virus but do not block infection. Dengue virus then enters and replicates in cells that it does not normally infect, such as macrophages. Entry occurs when Fc receptors on the cell surface bind antibody that is attached to virus particles. The result is higher levels of virus replication and more severe disease. This phenomenon is called antibody-dependent enhancement, or ADE.
Because dengue and Zika viruses are antigenically related, an important question is whether antibodies to dengue virus can enhance infection with Zika virus. To answer this question, the authors used two broadly neutralizing anti-dengue virus monoclonal antibodies that had been previously isolated from patients who recovered from infection. These antibodies recognize the viral E glycoprotein; specifially, a loop of the protein involved in fusion of the viral and cell membranes. The amino acid sequence of this fusion loop region is the same in dengue virus and in Zika virus.
The two anti-dengue virus mAbs bind the Zika virus E glycoprotein and also recognize Zika virus infected cells. However, when mixed with Zika virus, they do not neutralize infectivity in a cell line made from rhesus monkey kidneys. But when the antibodies were tested in Fc receptor-bearing cells, Zika virus infection was enhanced by over 100 fold. In the absence of dengue virus antibodies, levels of Zika virus RNA are very low.
Serum from four patients who had recovered from dengue virus infection was also examined for enhancement of Zika virus infection. All four sera contain antibodies that neutralized all four serotypes of dengue virus, but only two neutralized Zika virus infection. All four human sera enhanced Zika virus infection of Fc receptor-bearing cells. Enhancement of Zika virus infection could be blocked when Fc receptors were blocked with anti-Fc receptor antibodies before virus infection. A control serum from a patient in Canada that did not contain antibodies to dengue or Zika viruses did not enhance Zika virus infection.
These findings have one caveat: enhancement of Zika virus infection by antibodies against dengue virus was measured by PCR amplification of infected cell RNA, not by measuring the yield of infectious virus. The assumption is that increased intracellular viral RNA means more virus released from the cell, but this remains to be confirmed.
It will be important to confirm these findings in animal models of Zika virus infection, and in humans. If true, they have wide implications. If antibodies against dengue virus enhance Zika virus infection in humans, more severe disease might be observed in areas such as Brazil where both viruses co-circulate. It will be necessary to determine if Guillain-Barré syndrome, other neurological complications, and birth defects correlate with antibodies to dengue virus. Perhaps Fc receptors on the placenta and neural tissues allow entry of Zika virus only when bound to dengue virus antibody. It is also possible that antibodies to Zika virus might enhance dengue virus disease.
These observations do not bode well for Dengvaxia, a tetravalent dengue virus vaccine that has been recently licensed in Brazil, Mexico, and the Philippines. Might anti-dengue virus antibodies induced by this vaccine make Zika virus disease more severe? This outcome would be a tragedy, as many years of work has gone into making this vaccine to prevent severe disease caused by dengue virus infections. Second generation dengue virus vaccines such as TV003 are already moving through clinical trials.
It is essential to determine as soon as possible if antibodies induced by Dengvaxia and TV003 enhance Zika virus disease. If so, it will be necessary to assess whether deployment of this vaccine should proceed.
Dengvaxia consists of the yellow fever virus vaccine strain 17D in which the E and prM viral membrane proteins are substituted with those of dengue virus. In contrast, the attenuated TV003 vaccine has only the dengue virus genome. Would a vaccine consisting of TV003 plus an attenuated Zika virus vaccine solve potential problems of antibody dependent enhancement of disease?
Update 4/28/16: Over on Twitter someone asked, “any idea why we DON’T see severe disease in parts of Africa/Asia where dengue and Zika co-circulate”? Good question, too long an answer for Twitter. The easiest is that in humans, there is no antibody depencent enhancement of Zika virus infection by dengue antibodies. But if there is ADE, then there are a number of possible explanations. First, there have not been enough cases in Africa, nowhere near the numbers in the Pacific and South America.
There were certainly some cases of Guillain-Barré syndrome associated with some of the Pacific outbreaks – which I would consider more serious disease and could be potentiated by dengue antibodies.
I also think that Brazil is hyperendemic for dengue virus, with multiple serotypes circulating and people having multiple infections.
But the recent outbreaks are much larger than before, and dengue antibody induced complications of Zika virus infection might only be observed in larger outbreaks.
The authors of the paper discussed in this post suggest that the introduction of Zika virus into a completely naive population could also be a factor, as the age of exposure. Maybe a robust anti-Zika virus antibody response, in a non-naive population, can temper any effects of dengue mediated ADE.
Zika virus RNA has been detected in New World monkeys from the Northeast region of Brazil. This finding suggests that primates may serve as a reservoir host for the virus, as occurs in Africa.
The results of numerous serological surveys have shown that different Old World monkeys in Africa and Asia, including Rhesus macaques, Grivets, Redtail monkeys, and others, have antibodies that react with Zika virus. In these areas Zika virus is probably transmitted among monkeys in what is called a sylvatic cycle. Periodic outbreaks (epizootics) of Zika virus infections in nonhuman primates have been documented.
Where monkey reservoirs of Zika virus are present, humans may be infected with virus transmitted from a monkey. When non-human primates are absent, as on Yap Island, where an outbreak occurred in 2007, mosquitoes transmit the virus from human to human.
The Zika virus outbreak in Brazil has been thought to have been mainly transmitted between humans by mosquitoes. However, the results of this new study suggests that nonhuman primates could also be involved. The authors used polymerase chain reaction (PCR) to detect Zika virus RNA in sera or oral swabs from 15 marmosets and 9 capuchin monkeys in Ceará State where the virus is currently circulating. Four marmosets and three capuchins tested positive for Zika virus in this test.
Nucleotide sequence analysis of the PCR products from one marmoset and one capuchin monkey showed 100% identity with the strain of Zika virus that is circulating in Brazil.
The sampled animals were obtained from distant regions of the State. The marmosets were all free-ranging but had contact with humans, while 8 capuchins were pets and one was kept in a screening center for wild animals.
If these findings are confirmed and extended to other parts of Brazil, they would suggest that Zika virus might be spreading through non-human primates in that country. If so, they could serve as a reservoir for infection of humans via mosquito vectors.
An interesting question is when Zika virus entered monkeys in Brazil. It has been suggested that the virus entered Brazil in 2013 or 2014, and might have spread first in monkeys, first in humans, or both at the same time. I also wonder whether monkey to human transmission leads to a different disease than when virus circulates among humans.
Did you know that the evolution of ancient retroviruses, millions of years ago, can be traced by studying their genomes in the chromosomes of contemporary animals? Ted Diehl and Welkin Johnson join the TWiV team to tell us how they did it with mammals. All without a single wet experiment! They also join in the discussion about virus dispersal by hand dryers.
You can find TWiV #386 at microbe.tv/twiv, or listen below.
For the first time since April of 1955, recipients of poliovirus vaccine will no longer receive all three serotypes. This past Sunday the World Health Organization orchestrated a synchronized switch from trivalent to bivalent oral poliovirus vaccine (OPV) in 150 countries.
The reason for the switch is clear: type 2 poliovirus was declared eradicated last year, and the only remaining cases are cause by vaccine-derived type 2 polioviruses. After oral administration of poliovirus vaccine, the virus replicates in the intestine, conferring immunity to subsequent infection. In all recipients of the vaccine the viruses lose the mutations that make them safe for humans. Consequently a small number of recipients, and their contacts, contract poliomyelitis from the vaccine.
To prevent further cases of poliomyelitis caused by circulating vaccine-derived polioviruses, WHO planned a synchronized, global switch from trivalent OPV to bivalent OPV on 17 April 2016. By July of 2016 all remaining stocks of the Sabin type 2 poliovirus strains, which are used to produce OPV, will also be destroyed.
My concern with this strategy is that type 2 vaccine-derived polioviruses continue to circulate. Whether they will continue to do so long enough to cause an outbreak of paralytic disease in the cohort of new infants that do not receive type 2 vaccine is a mattern of conjecture. In case there is an outbreak, monovalent type 2 oral poliovirus vaccine is being stockpiled by WHO. Of course, re-introduction of this vaccine will be accompanied by more circulating vaccine-derived poliovirus in the environment, and vaccine-associated disease, the very event WHO is trying to end with the trivalent to bivalent switch.
Type 3 poliovirus has not been isolated since 2012. Only type 1 poliovirus still causes outbreaks in two countries: Pakistan and Afghanistan. The inability to vaccinate in those countries, due to conflict, is delaying eradication. The recent killing of seven police officers who were protecting polio vaccinators by the Pakistani Taliban is an example of this difficulty.
Developing a great vaccine is not the only requirement for preventing infectious disease: you also have to be able to deploy it.
Image: Antibodies bound to poliovirus by Jason Roberts.
A major new feature of the fourth edition of Principles of Virology is the inclusion of 26 video interviews with leading scientists who have made significant contributions to the field of virology. These in-depth interviews provide the background and thinking that went into the discoveries or observations connected to the concepts being taught in this text. Students will discover the personal stories and twists of fate that led the scientists to work with viruses and make their seminal discoveries.
For the chapter on Infections of Populations, Vincent spoke with Thomas London, MD, of the Fox Chase Cancer Center, about his career and his work on hepatitis B virus.
Stuart Firestein, Columbia University neuroscientist and author of the book Ignorance, returns to TWiV for a chat about his latest work, Failure. This book is all about how experiments that don’t work, or provide the wrong conclusions, are essential for the progress of science.
You can find TWiV #385 at microbe.tv/twiv, or listen below.
Dr. Susan Nasif is a virologist and part of the team at Cimaza Comics that produces science-themed comics. In their latest creation, Zanzare, we are plunged head-first into the global mystery of Zika virus. We meet the mosquitoes (in Italian: zanzare) implicated in its spread; but the insects plead their innocence, saying it’s all a misunderstanding. They lay their case before the gods and demons of Zika’s victims, and ask for divine help. Will the mosquitoes be vindicated? Or will it all turn out that the zanzare are to blame after all?
Not even the authors know where Zanzare is heading. The comics follow weekly developments in the Zika investigation as it unfolds. The story is told through the lens of world mythology, but the virology presented comes straight from reputable journals. Thrilling and funny, Zanzare is a visionary mixture of ancient legend and up-to-the-minute fact.
The video below is an excerpt from this series, which is not yet released in book form. Their previous creation, Adventures of the Regatjes, is available here.
Not long after the appearance of an outbreak of viral disease, first scientists, and then newswriters, blame it all on mutation of the virus. It happened during the Ebolavirus outbreak in West Africa, and now it’s happening with Zika virus.
The latest example is by parasitologist Peter Hotez, who writes in the New York Times:
There are many theories for Zika’s rapid rise, but the most plausible is that the virus mutated from an African to a pandemic strain a decade or more ago and then spread east across the Pacific from Micronesia and French Polynesia, until it struck Brazil.
After its discovery in 1947 in Uganda, Zika virus caused few human infections until the 2007 outbreak on Yap Island. The virus responsible for this and subsequent outbreaks in Pacific Islands is distinct from the African genotype, but there is no experimental evidence to suggest that sequence differences in the Asian genotype were responsible for the spread of the virus. For this reason I disagree with Dr. Hotez’ conclusion that mutation of the virus is the ‘most plausible’ explanation for its global spread. It is just as likely that the virus was in the right place at the right time to spark an outbreak in the Pacific.
We will never have experimental evidence that emergence of the Asian genotype allowed pandemic spread of Zika virus, because we cannot test the effect of individual mutations on spread of the virus in humans. Consider this experiment: infect a room of humans (and mosquitoes) with either the African or Asian genotype of Zika virus, then measure virus replication and transmission. If there is a difference between the two viruses, engineer specific mutations into the virus, reinfect another batch of humans, and continue until the responsible mutations are identified. Obviously we cannot do such an experiment! We could instead use animal models, but these have limitations in extrapolating results to humans. For this reason we have never identified any specific mutation that allows an animal virus to replicate more efficiently in humans.
The same experimental limitations do not apply to animals. An example is Chikungunya virus, spread by Aedes ageyptii mosquitoes. Before 2004, outbreaks of infection were largely confined to developing countries in Africa and Asia. The virus subsequently spread globally, due to a single amino acid change in the envelope glycoprotein which allows efficient replication in Aedes albopictus, a mosquito with a greater range than A. ageyptii. It was possible to prove this point by assessing the effects of changing this single amino acid on virus replication in mosquitoes. The same experiment cannot be done in humans.
There is no evidence that the Asian genotype of Zika virus is any more competent to replicate in mosquitoes than the African strain. Results of a study of replication of Asian genotypes of Zika virus revealed that Aedes aegypti and Aedes albopictus are not very good vectors for transmitting ZIKV. The authors smartly suggest that “other factors such as the large naïve population for ZIKV and the high densities of human-biting mosquitoes contribute to the rapid spread of ZIKV during the current outbreak.” In other words, don’t blame the Zika virus genome for the expanded range of the virus.
The Zika virus that has been spreading in Brazil, and which has been associated with microcephaly, shares a common ancestor with the Asian genotype. In a recent study of the genomes of 7 Brazilian isolates, there was no evidence that specific mutations are associated with microcephaly. Those authors conclude (also smartly):
Factors other than viral genetic differences may be important for the proposed pathogenesis of ZIKV; hypothesized factors include co-infection with Chikungunya virus, previous infection with Dengue virus, or differences in human genetic predisposition to disease.
It’s easy to blame mutations in the viral genome for novel patterns of transmission or pathogenesis. Viral mutations arise during every replication cycle, due to errors made by viral enzymes as they copy nucleic acids. RNA viruses are the masters of mutation, because, unlike the polymerases of DNA viruses, RNA polymerases cannot correct any errors that arise. As viruses spread globally through different human populations, it is not surprising that different genotypes are selected. These may reflect adaptation to various selective pressures, including different humans, vectors, climate, or geography. There is no reason to assume that such changes influence virulence, disease patterns, or transmission in humans. Whether they do so can never be tested in humans.
Blaming the viral genome is nothing new. At the onset of the 2014 Ebolavirus outbreak in West Africa there were many claims that the unprecedented size of the outbreak was a consequence of mutations in the viral genome. Genomic analysis of isolates early in the epidemic suggested that the large number of infections was leading to rates of mutation not previously observed. This work lead to dubious claims of “Ebolavirus mutating rapidly as it spreads” and Ebolavirus is mutating (Time Magazine). Richard Preston, in the New Yorker article Ebola Wars quoted scientist Lisa Hensley:
In the lab in Liberia, Lisa Hensley and her colleagues had noticed something eerie in some of the blood samples they were testing. In those samples, Ebola particles were growing to a concentration much greater than had been seen in samples of human blood from previous outbreaks. Some blood samples seemed to be supercharged with Ebola. This, too, would benefit the virus, by enhancing its odds of reaching the next victim. “Is it getting better at replicating as it goes from person to person?” Hensley said.
And let’s not forget the absurd speculation, fueled by these data, that Ebolavirus would go airborne.
Within a year all this nonsense was proven wrong. Ebolavirus had not sustained mutations any faster than in previous outbreaks. Furthermore, the observed mtuations did not change the virus into a more dangerous strain.
Go back to any viral outbreak – MERS-coronavirus, SARS-coronavirus, influenza virus, HIV-1 – and you will find the same story line. Mutation of the virus is leading to more virulence, transmission, spread. But in no case has cause and effect been proven.
Let’s stop blaming viral mutation rates for altered patterns of virus spread and pathogenesis. More likely determinants include susceptibility of human populations, immune status, vector availability, and globalization, to name just a few. Not as spectacular as ‘THE VIRUS IS MUTATING!’, but nearer to the truth.
Mass die-offs of tilapia by a novel orthomyxo-like virus, Ian Lipkin’s editorial on the movie Vaxxed, and new vaccines to prevent dengue virus infections, including a human challenge model, are topics of episode #384 of the science show This Week in Virology. With guests Ian Lipkin and Nischay Mishra from the Center for Infection and Immunity.
You can find TWiV #384 at microbe.tv/twiv, or listen below.