Christian Drosten, Institute of Virology, University of Bonn, Germany

Please refer to the Zika facts summary by the Society for Virology in Germany

To virologists, Zika virus is a long known subject (1). Experienced labs consider the infection among the differential diagnoses in cases of fever after travel to tropical Africa and, more lately, Asia. Reports on the occurrence of this typical Old World virus in Brazil in May 2015 were admittedly surprising, but did not come totally unexpected in light of the introduction of chikungunya virus to the Americas one year earlier (2). It seemed that Zika was just another aedes-transmitted and primate-associated arbovirus that made it into the virgin soil-environment of the neotropics. Ensuing reports of cases of Guillain-Barré syndrome associated with Zika virus infection caused little concern, as the syndrome is non-specifically associated with a whole range of pathogens. A whole new and very different aspect was added in October 2015, when the Brazil Ministry of Health expressed concern about an increased incidence of microcephaly in newborns in the northeastern part of the country that followed the arrival and spread of Zika virus with a delay corresponding to the duration of a human pregnancy.

Virologists know a whole number of viruses causing this and similar neurological fetopathies in humans. Rubella virus, the most notorious representative, has been virtually eliminated as a cause of fetopathy through vaccination programs in Europe and the Americas. The One Health perspective provides epidemiological scenarios not normally considered in humans. In Europe, we have just witnessed the devastating consequences of a fetopathogenic virus that is transmitted by blood-feeding insects. The Schmallenberg virus was recognized in Germany in 2011 and spread across Europe within less than 2 years, causing dramatic malformations including severe cerebral fetopathies in ruminants (3). We did not dare at that time to project an equivalent scenario for a human disease.

The threat posed by Zika virus is difficult to express in numbers. The emotional component, the uncertainty, as well as the possibility to miss a window of opportunity for study and intervention have already triggered ad-hoc funding programs and pragmatic approaches to extract information from available data. It is not in spite of the emotional component, but because of it, that virologists should look at the problem from a scientific perspective – and communicate a conservative perspective to the public via the media.

Most virus infections that cause fetopathy have a low manifestation index. Certainly, also the Zika virus will cause harm in only a small proportion of the many pregnant women now exposed to the virus. Of note, the attack rate in the 2007 outbreak on Yap island (Micronesia) may have been as high as 73% in the population aged >3 years. In the much larger outbreak in French Polynesia the attack rate was 11.5% according to retrospective analyses (1). In the affected regions in South America, where people are similarly exposed and immunologically naïve toward the infection, very high rates of unnoticed infection are to be expected – including in pregnant women. Even if truly associated with Zika virus infection, the 5640 cases of microcephaly reported in Brazil by end of February 2016 will only represent a tiny fraction of the many pregnant women who got infected during 2015. A more severe issue is the reporting and classification of microcephaly. The perception of an increased incidence of microcephaly came up in an area notably affected by Zika virus. Beginning in autumn 2015, the Brazilian MOH strengthened and emphasized microcephaly surveillance, whereas notifications prior to this time occurred on a more routine basis. The media coverage of the Zika/microcephaly connection contributed an additional stimulus for reporting. A recent research manuscript suggests that the MOH reporting criterion for suspected cases of microcephaly, based on cranial circumference of 32 cm, may be inappropriate for the most affected population in northeastern Brazil. This criterion may trigger the formal notification of up to 10% of all newborns as suspected cases of microcephaly (4). Of the 5,640 suspected cases notified so far, 3,935 remain under investigation (5). Among the remaining 1705 cases, 950 have not been confirmed as cases of microcephaly (5).

The literature now contains several reports on Zika virus detection in amniotic fluid, blood, and even central nervous tissue of fetuses with signs of microcephaly. With every new case report that is published, we perceive the link between Zika virus infection and microcephaly to become stronger. There is probably truth to this. However, as scientists we should emphasize that case reports do not establish a causative link between virus and microcephaly. As always at the beginning of epidemics, studies tend to focus on cases but not controls. What fraction of healthy pregnancies and babies might reveal evidence for Zika virus infection if sampled at the peak of an outbreak, assuming attack rates of 10% or even higher in the adult population? Up to now, 583 cases of microcephaly in Brazil have been completely investigated including objective neurological criteria and virological laboratory tests. Only 67 (11.5%) tested positive for Zika virus infection (5). An extrapolation of this proportion to all notified cases fails to explain the reported >20-fold increase of incidence of microcephaly in northeastern Brazil compared to other populations.

The increase of microcephaly may represent a complex effect on the local population that could include other factors such as unrecognized pathogens or environmental causes. These factors may manifest as microcephaly alone or in concert with Zika virus infection. The WHO Zika Open resource carries a research manuscript that follows the incidence of microcephaly from 2012 to 2015 in Paraiba, the Brazilian federal state that was second most affected (6). Using data from prospectively-designed birth cohorts, the study reveals a stark increase of microcephaly incidence already by end of 2012 and a second peak by mid 2014, pre-dating the presumed introduction of Zika virus into the country. A third peak of incidence recorded for the second half of 2015 is the strongest peak. Only this peak plausibly correlates with Zika outbreaks. We should remain open for additional or even alternative explanations for the increased incidence of microcephaly observed in northeastern Brazil. Interestingly, the outbreak in French Polynesia, which according to ongoing retrospective analyses may have involved fetopathies, co-incided with a dengue virus outbreak (1). Dengue is also endemic in many of the regions now affected by the Zika outbreak in Brazil.

Having summarized concerns regarding the true incidence of Zika-associated microcephaly, we should not forget to mention that virally-induced fetopathies can involve many other symptoms that are not easily objectified. Along with microcephaly, retinal malformations have already been noted in 2 cases (1). Zika´s association with Guillain Barré Syndrome can be regarded as confirmed based on a recent revisit of the French Polynesia outbreak (7). However, GBS should not be regarded as predictive for other severe neurological sequelae as many other pathogens that are not specifically neurotropic are associated with the syndrome (e.g., campylobacter). Co-endemic arboviroses including dengue and chikungunya virus infection are thought to trigger Guillain Barré syndrome, typically with low incidence. Much more worrying with regard to Zika virus infection is the perspective of sequelae effective on higher neurological functions, involving deficits that may come to show as kids develop. The clear reports of fetal neurotropic infection (e.g., 8) call for neuro-psychiatric follow-up of birth cohorts.

Given the epidemiological uncertainties regarding the causation of microcephaly, animal experiments will be utilized to compensate for pressing information needs. Newborn mice can be readily infected with a range of arboviruses, but the pathology observed in adult mouse models does usually not compare to the situation in primates for most human arboviruses. Mouse-adapted adult disease models for Zika virus infection are not available. Even if they were, it is questionable whether these models could reliably depict syndromes that show a very low incidence among cases with normal courses of infection. Experiments on pregnant macaques are being conducted as this text is being written. However, the results of these experiments may not come in time to provide first evidence of causation of microcephaly. Nature is presenting us with a large-scale, cruel study scenario in humans. We will be able to observe over the next coming months whether the incidence of microcephaly will increase in areas that have been newly affected by Zika virus since the end of 2015. Next to the implementation of prophylactic measures such as mosquito control, it is our prime responsibility to secure epidemiological evidence by careful design of prospective, controlled observational trials.

References

1. Kindhauser MK, Allen T, Frank V, Santhana RS & Dye C. Zika: the origin and spread of a mosquito-borne virus [Submitted]. Bull World Health Organ E-pub: 9 Feb 2016.
2. Leparc-Goffart I, Nougairede A, Cassadou S, Prat C, de Lamballerie X. Chikungunya in the Americas. Lancet. 2014 Feb 8;383(9916):514. doi: 10.1016/S0140-6736(14)60185-9
3. Wernike K, Elbers A, Beer M. Schmallenberg virus infection. Rev Sci Tech. 2015 Aug;34(2):363-73
4. Rocha HAL, Correia LL, Leite AJM, Campos JS, Cavalcante e Silva A, Machado MMT, Rocha SGMO, Saraiva de Almeida NMG, Alves da Cunha AJL. Microcephaly: normality parameters and its determinants in northeastern Brazil: a multicentre prospective cohort study [Submitted]. Bull World Health Organ E-pub: 8 Feb 2016. doi: http://dx.doi.org/10.2471/BLT.16.171215
5. Pan American Health Organization / World Health Organization. Zika Epidemiological Update – 24 February 2016. Washington, D.C.: PAHO/WHO; 2016
6. Soares de Araújo JS, Regis CT, Gomes RGS, Tavares TR, Rocha dos Santos C, Assunção PM, et al. Microcephaly in northeast Brazil: a review of 16 208 births between 2012 and 2015 [Submitted]. Bull World Health Organ E-pub: 4 Feb 2016. doi: http://dx.doi.org/10.2471/BLT.16.170639
7. Van-Mai Cao-Lormeau, Alexandre Blake, Sandrine Mons, Stéphane Lastère, Claudine Roche, Jessica Vanhomwegen, Timothée Dub,
Laure Baudouin, Anita Teissier, Philippe Larre, Anne-Laure Vial, Christophe Decam, Valérie Choumet, Susan K Halstead, Hugh J Willison, Lucile Musset, Jean-Claude Manuguerra, Philippe Despres, Emmanuel Fournier, Henri-Pierre Mallet, Didier Musso, Arnaud Fontanet, Jean Neil, Frédéric Ghawché. Guillain-Barré Syndrome outbreak associated with Zika virus infection in French Polynesia: a case-control study. The Lancet 2016 Published online February 29, 2016 http://dx.doi.org/10.1016/S0140-6736(16)00562-6
8. Mlakar J, Korva M, Tul N, Popović M, Poljšak-Prijatelj M, Mraz J, Kolenc M, Resman Rus K, Vesnaver Vipotnik T, Fabjan Vodušek V, Vizjak A, Pižem J, Petrovec M, Avšič Županc T. Zika Virus Associated with Microcephaly. N Engl J Med. 2016 Feb 10.