First isolation: Uganda

ZIKV was first isolated in April 1947 from a pyrexial sentinel rhesus monkey native to the Zika forest in Uganda, in an attempt to study yellow fever [Reference Dick, Kitchen and Haddow15]. The second isolation took place in January 1948, this time from a pool of Aedes africanus mosquitoes obtained in the same forest [Reference Lanciotti16]. Due to its being native to Zika forest, this newfound virus became known as the ‘Zika virus’ [Reference Dick, Kitchen and Haddow15]. The first report of a human infection of ZIKV occurred in Uganda in 1964, in which a European man developed a 5-day febrile syndrome with myalgia and maculopapular rash, that was eventually confirmed by convalescent and acute serum samples that showed neutralising ZIKV antibodies [Reference Beckham17]. Throughout the 1960s and 1970s, multiple ZIKV human isolates were obtained, confirming infections in several Asian and African countries (Egypt, Nigeria, Uganda, India, Malaysia, Indonesia, Pakistan, Thailand, North Vietnam and the Philippines) [Reference Dick, Kitchen and Haddow15–Reference Rabaan18]. Non-structural gene 5 (NS5) sequencing demonstrated the existence of three lineages of ZIKV which originated infections in East Africa, West Africa and Asia [Reference Chibueze11]. Nonetheless, the first sample of human ZIKV infection outside of Africa and Asia was identified in 2007, in Yap Island in the Federated States of Micronesia, Oceania [Reference Dick, Kitchen and Haddow15–Reference Rabaan18].

First outbreak: French Polynesia and association with neurological affections

The previous largest documented ZIKV outbreak took place in French Polynesia in 2013 [Reference Rabaan18]. It was during this Zika outbreak, alongside types 1 and 3 Dengue fever co-epidemics, that the first associated cases of GBS following a ZIKV infection were recorded [8]. This was reported in a case–control study in French Polynesia that took place between October 2013 and April 2014, which found a possible association between ZIKV and GBS [Reference Cao-Lormeau19]. There were two control groups: control group 1 (n = 98), containing patients who presented at the hospital with non-febrile illness; and control group 2 (n = 70), containing patients with acute ZIKV disease and no neurological symptoms. A total of 42 cases of GBS showed electrophysiological findings consistent with acute motor axonal neuropathy (AMAN) and exhibited a rapidly evolving pathology [Reference Cao-Lormeau19]. This study found an odds ratio for GBS of 34.1, with a wide but significant confidence interval (CI) (5.8, Inf). Likewise, research by Salinas et al. and Styczynski et al., found odds ratios of 4.6 (95% CI 1.1–19.0) and 6.45 (95%CI 1.88–22.10), further reinforcing the association between ZKV and GBS and confirming the early results from the Polynesian studies [Reference Salinas20, Reference Styczynski21].

The pandemic arrives to America: Brazil, Zika and nervous system affections

The latest outbreak documented occurred in Brazil close to the end of 2014, with between 0.4 and 1.3 million people estimated to have been infected during 2015 [1]. There are several theories as to how the disease arrived in the South American country. A two-step protocol DNA analysis of the virus showed its similarity with the Asian lineage, therefore suggesting its arrival during the 2014 FIFA World Cup tournament, held in June and July of that year, and also from the Va'a World Sprint Championship canoe race, held the following August [Reference Younger12, Reference Al-Qahtani24, Reference Faye25]. The explanation of the disease's arrival during the football tournament seems to be imprecise, as no Zika-endemic Pacific countries were participants [Reference Wikan and Smith26]. Even so, it is possible that spectators from Zika-endemic countries might have been present [Reference Wikan and Smith26]. The canoe race, on the other hand, included participants from four Pacific countries in which ZIKV was circulating: French Polynesia, New Caledonia, Cook Islands and Easter Island [Reference Musso27]. In either case, the outbreak is generally assumed to be a consequence of such sporting events [Reference Musso27]. Other researchers from Canada, Brazil, UK and USA published a study suggesting the virus entered Brazil in 2013 through someone infected in that year's Zika epidemic in French Polynesia or from another Zika-endemic country [Reference Vogel28]. This theory is also supported by the increase in air travel from Zika-endemic areas to Brazil by almost 50% during 2013 and by mathematical modelling of the outbreak [Reference Vogel28, Reference Massad29]. The growing number of infected people by local transmission made it necessary for Brazil to report ZIKV as a noteworthy disease in 2016 [2]. Additionally, GBS was temporally and geographically associated with the presence of ZIKV in Brazil, Colombia, El Salvador, Martinique, Panama, Puerto Rico, Suriname and Venezuela [Reference Broutet30, Reference Song31]. In contrast to the AMAN subtype found in patients in French Polynesia, case reports in Latin America suggest AIDP as the predominant subtype in this region [Reference Anaya32].

Zika and other clinical manifestations around the world

Approximately 80% of patients infected with ZIKV are asymptomatic [Reference Jamali Moghadam14]. Hospitalisations are uncommon, and death is rare [Reference Paixão36]. However, these features are not pathognomonic for the disease as they may also be present in other Flaviviridae infections and therefore cannot be relied upon in isolation for diagnosis [Reference Wong, Poon and Wong37].

The incubation period is between 2 and 14 days [Reference Chan38], with clinical manifestations consisting of: low fever (<38.5 °C), headache, retro-orbital pain, bilateral non-purulent conjunctivitis, maculopapular rash, arthralgia with oedema on extremities, anorexia and occasionally abdominal symptoms such as diarrhoea and pain [Reference Ioos39, Reference Chang40]. Symptoms that may aid a clinical practitioner in differential diagnosis are conjunctivitis and limb oedema, which manifest more commonly after ZIKV infection in comparison to Dengue virus (DENV) and Chikungunya infection. Hepatomegaly, leukopenia and thrombocytopenia are also less common in ZIKV than in DENV [Reference Ioos39].

In 2016, sensorineural hearing loss was reported in a case of congenital ZIKV, implicating a possible complication other than microcephaly in newborns [Reference de Leal41]. Additionally, a prospective case study in Colombia and Venezuela including 43 patients found that babies born with ZIKV infection presented optic disc abnormalities, optical nerve hypoplasia, glaucoma, pigment mottling and chorioretinal atrophy with a predilection for the macula [Reference Yepez42]. Intrauterine and neonatal death may also occur [Reference Li9]. In fact, Chauchemez et al. reported, in a retrospective study, eight cases of microcephaly, five of which were aborted and three were born [Reference Song31].

The clinical spectrum of ZIKV infection remains a matter of investigation. Other manifestations have been reported such as: ocular flutter, meningoencephalitis, haematospermia, hearing difficulties, subcutaneous bleeding and acute myelitis [Reference Song31, Reference Carteaux43–Reference Karam45].

Diagnosis

Diagnosis of ZIKV infection is based on three pillars: first, the evaluation of clinical symptoms; second, the finding of an epidemiological association between infection and endemic zones; and third, confirmation by serological and molecular laboratory findings [Reference Sharma46]. However, each of the pillars alone does not suffice to diagnose ZIKV infection, as the virus is very similar in structure, clinical features and geographical location to another flavivirus, especially Dengue, and togavirus like Chikungunya [Reference Wong, Poon and Wong37, Reference Lee, Kim and Shin47]. The gold standard for diagnosis is a real-time PCR (RT–PCR) on body fluids chosen depending on each patient's characteristics [Reference Li9, Reference Ferreira48]. ZIKV can be found in semen, saliva, blood, cerebrospinal fluid, amniotic fluid and urine. Serum samples should be taken no more than 10 days after symptom onset, as identification of the ZIKV RNA becomes more difficult. Nevertheless, ZIKV infection during pregnancy is associated with prolonged viraemia and the virus has been detected in the serum of pregnant women up to 53 days post-exposure [Reference Meaney-Delman49]. Saliva samples may also be taken within this range of time with an increased rate of molecular detection of the virus. However, it is especially used in patients with whom serum samples are difficult to take, such as the paediatric population [Reference Li9]. On the other hand, new investigations regarding urine samples of infected individuals have shown positivity for ZIKV RNA after ten days of symptom onset. This discovery brings an essential tool for diagnosis in cases in which obtaining samples is not possible within ten days of symptom onset [Reference Gourinat50]. Finally, diagnosis of ZIKV infection in newborns or aborted foetuses can be achieved by confirming viral RNA (vRNA) in their brain or serum and mother's placenta [Reference Song31], with a positive RT–PCR result being conclusive for ZIKV diagnosis. If results are negative, serum antibodies and IgM ELISA should be collected to confirm or discard the diagnosis [Reference Lee, Kim and Shin47]. Importance of early diagnosis lies in ZIKV's sexual transmission, foetal health problems and neurological complications that can be prevented [Reference Li9].

Transmission

The rapid spread of the pathogen occurs due to diverse transmission pathways; however, the primary mode of transmission is mosquito-borne [51]. A sylvatic transmission cycle involves nonhuman primates and forest species of Aedes mosquitoes. In urban and suburban environments, ZIKV is transmitted in a cycle of human–mosquito–human bites by two species of the Aedes subgenus: Ae. aegypti and Ae. albopictus. The vectors usually bite humans during daytime, and can be found both inside and outside houses. Ae. aegypti mosquitoes are spread extensively in the Americas (excluding Chile), where the climatic conditions are appropriate for vector breeding. This is partially responsible for the ZIKV epidemic reported since 2015. Ae. albopictus is distributed in regions with cooler temperatures, such as New York and Chicago in northern USA and in parts of southern Europe [Reference Song31]. The Aedes mosquito species are found widely around the world, and in consequence, the likelihood of the outbreak to spread to new countries remains high [Reference Aziz6, Reference Petersen52]. Furthermore, climate change must be accounted for, as it expands vectorial capacity [Reference Liu-Helmersson53]. Species distribution modelling techniques have predicted that, due to the broadly tropical and sub-tropical regions worldwide with suitable environmental conditions, which also have a larger susceptible human population, there is a high risk of introducing and establishing new autochthonous transmission [Reference Song31]. Some studies also assess the possible competence of Culex species as a vector. However, due to conflicting results, additional evidence is needed to confirm these preliminary findings [54].

Substantial evidence also exists of non-vector transmission. Transplacental transmission during pregnancy (from mother to foetus) has been identified by detection of ZIKV RNA in the amniotic fluid of mothers whose children show cerebral abnormalities in ultrasonography, as well as the identification of the virus in brain tissues and placentas of children born with microcephaly. Peripartum transmission was documented in two infants: in one case the child exhibited symptoms, while the other was asymptomatic [Reference Besnard55]. ZIKV RNA load has also been identified in breast milk, with much higher loads as compared with serum, which represents a risk of viral transmission while breastfeeding [Reference Dupont-Rouzeyrol56]. Between the 16 October 2016 and 9 March 2017, 13 countries reported sexual transmission of the ZIKV [57]. These reports comprise probable transmission from male to female, and male to male, through vaginal, anal and oral sexual intercourse [Reference Harrower58]. The virus can be passed from an infected person before their symptoms start, while they are symptomatic and after their symptoms end [51]. So far, the maximum documented virus time of survival in semen of symptomatic men is 125 days [Reference Paz-Bailey59]. Further evidence is needed, but the virus may also be passed by someone infected, who never develops symptoms [Reference Moghadas60]. Multiple reports of blood transfusion transmission cases in Brazil have been published [Reference Motta61–Reference Slavov63]. In spite of testing positive for vRNA, four probable cases of transfusion transmission did not develop symptoms compatible with the virus [Reference Motta61, Reference Barjas-Castro62]. Previous to the current outbreak, four documented cases of infection were known to have occurred through laboratory exposure [51]. Direct transmission can happen through the skin or mucous membranes, although it is uncommon [Reference Wikan and Smith26]. Finally, one case of an Australian man bitten by a monkey in Indonesia, and who subsequently developed ZIKV 5 days after the incident, led to speculation of this as a potential route of transmission [Reference Leung64]. However, it is still undetermined due to the possibility of his also being bitten by a mosquito during his stay [Reference Leung64].

Between January 2015 and 9 March 2017, 70 countries and territories have reported evidence of mosquito-borne transmission of the virus according to the ECDC; 13 countries have reported evidence of person-to-person transmission of the virus, via a sexual route [57]. Moreover, 31 countries have reported microcephaly and other CNS malformations in newborns potentially associated with ZIKV infection [57]. Four additional countries – Saint Martin, Bolivia, Trinidad and Tobago and Curaçao – have reported GBS associated with ZIKV infection [57].

ZIKV in animal models

As the Zika epidemic progresses, the study of its pathogenesis has gained great interest in the medical community. Development of numerous animal-based models to understand its pathophysiology, its action in pregnancy and foetal development, as well as the possibility of developing a vaccine and a treatment, are on the cutting-edge of science. The most often used models for these investigations have been the murine models (both immunocompetent and immunosuppressed), guinea-pigs and non-human primates [Reference Bierle68].

Recent murine models have shown potential for determining neurovirulence of different ZIKV strains. A recent study on immunocompetent CD1/ICR murine models showed an 80–100% age-independent mortality rate after intracranial inoculation of MR766 – a 1947 Ugandan ZIKV prototype strain [Reference Duggal69]. It is important to underline that the route of inoculation was definitive for the initiation of disease. While the intra-peritoneal inoculation route did not show any symptoms of the disease, the intracranial route showed affection of the CNS confirmed by histopathology. Widespread variations have been observed, with inflammatory changes in the cerebral cortex showing mixed leptomeningeal and parenchymal inflammation with extensive glial proliferation and neuronal cell death [Reference Song31, Reference Duggal69]. Other studies have shown neuronal degeneration, softening, infiltration of round cells, infiltration of Crowdy Type A bodies and encephalitis in post-mortem histopathological studies of murine brains infected with the virus [Reference Boorman and Porterfield70]. Regarding other clinical manifestations, no mice showed any retinal changes [Reference Duggal69]. Similarly, a study showing intracranial inoculation of 35-day-old immunocompetent mice with this same prototype demonstrated signs of disease during a 5–15-day time-lapse after inoculation [Reference Maxian65].

Guinea-pigs have also been used to understand the course of other transplacental viral infections. ZIKV studies on guinea-pigs have shown low viraemia on non-pregnant animals [Reference Bierle68]. No viraemia was detected on pregnant guinea-pigs or their pup's blood, plasma or tissues, or other weight manifestations. However, antibodies were found in pups and their mothers, thereby posing immunocompetent guinea-pigs as an ideal model for the ZIKV-associated immune response but not ideal for the pathogenesis of complications of ZIKV infection [Reference Bierle68].

Experiments in macaques have shown that neutralising antibodies are produced in response to the injection of ZIKV RNA into plasma [Reference Dudley71]. This protects the organism from further infection and permits the development of specific T-cell response against the virus. Short-term viraemia follows infection in non-pregnant macaques, which usually resolves 10 days post-infection. However, infections in pregnant macaques show a more prolonged viraemia. ZIKV has been identified in macaque blood up to 57 days post-infection [Reference Dudley71]. Finally, an important finding on rhesus monkey models was that despite blood clearance of vRNA, the virus was still detected in saliva, urine and in cerebrospinal fluid, indicating the presence of ZIKV in some tissues at low levels [Reference Shankar, Patil and Skariyachan72]. The vRNA was also found in the brain, eye and placenta of foetal tissues of macaques during pregnancy [Reference Nguyen73].

Social determinants of disease

All illnesses are immersed in a context that determines in an extensive manner the characteristics of the disease's course. The transmission of the ZIKV, for example, partly depends on conditions which perpetuate the vector cycle. This includes standing water reservoirs where eggs laid by the mosquito can find an optimal environment to develop, such as buckets, rainwater gathered in puddles, pools, drums, flower pots, empty cans, etc. [Reference Aziz6, Reference Petersen52]. Rapid urbanisation in conditions of extreme poverty contributes to deforestation, exposing populations to different vectors and pathogens with greater frequency. Furthermore, despite widespread exposure to the vector in endemic areas, evidence has shown that poverty and inequality are factors that make people more vulnerable to develop ZIKV-related microcephaly (as shown in the economically deprived northeast region of Brazil) [Reference Possas74]. This social determinant of disease relates to the lack of basic services, in particular the persistent absence of water services, as well as problems with the establishment of sewage, residue collection and the use and disposal of non-biodegradable material [75]. Finally, phenomena such as population growth, migration, uncontrolled urbanisation and poverty belts in cities facilitate vector breeding and the perpetuation of the disease cycle [Reference Possas74, 75].

Lessons learned

On 18 November 2016, the Director-General of the International Health Regulations (IHR) accepted the recommendations of the Emergency Committee and declared the end of the Public Health Emergency of International Concern (PHEIC) [76]. However, ZIKV infection and its complications are still a concern for public health. Despite a decline in cases of ZIKV infection registered in many territories, the virus continues to spread geographically to areas where vectors are found [1]. Therefore, in order to prevent the establishment of new transmission chains, it is necessary to undertake several strategic public health interventions:

• • Intensify vector-control measures and the strategic intervention in public health risk communication, community engagement activities, personal protection against mosquitoes and appropriate advice for the caregivers of those affected [1].

• • Surveillance needs to be assured through an optimal preparedness plan by local, national, and international authorities, as well as mosquito control professionals and national laboratories. A documented example of the success of such plans occurred in the Chikungunya outbreak in Montpellier, France 2014 [Reference Delisle77]. The outbreak was rapidly contained thanks to a prompt attribution of responsibilities, an aggressive communication campaign, and continued vector control interventions. This included the use of general measures to avoid mosquito bites, as well as the use of disinfectants to which the virus is vulnerable such as 1% sodium hypochlorite, 70% ethanol or 6% hydrogen peroxide, among others [Reference Delisle77, Reference Müller and Harms78].

• • The screening of TORCH, dengue and other arboviruses during pregnancy, taking into account that the synergic potential is given by the presence of immunity and/or seropositivity of another virus can cause atypical presentations of the disease or absence of the symptoms, as well as an extension of the spectrum of the maternal–foetal presentation [Reference Coyne and Lazear79].

• • Enable the ‘massification’ of education, in particular regarding the epidemiology of the disease and the general measures that must be taken to prevent transmission by the public health authorities of each country [Reference Chang40]. Regarding travellers, it is important to take into account the differential diagnosis of other travel-related illnesses, the use of measures to avoid mosquito bites and the fact that travellers who plan to become pregnant soon should refrain from travelling to areas where the transmission of the ZIKV may be active.