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.
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.
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.