Rapid risk assessment: Zika virus disease epidemic: potential association with microcephaly and Guillain–Barré syndrome - 4th update, 9 March 2016

Risk assessment
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European Centre for Disease Prevention and Control. Rapid Risk Assessment. Zika virus disease epidemic: potential association with microcephaly and Guillain–Barré syndrome. Fourth update, 9 March 2016. Stockholm: ECDC; 2016.

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​This update assesses the risks associated with the Zika virus epidemic currently affecting countries in the Americas. It assesses the association between Zika virus infection and congenital central nervous system malformations, including microcephaly, as well as the association between Zika virus infection and Guillain–Barré syndrome.

Considering the continued spread of Zika virus in the Americas and Caribbean, the strong evidence of an association between Zika virus infection during pregnancy and congenital central nervous system malformations, the association between Zika virus infection and Guillain–Barré syndrome, and the risk of establishment of local vector-borne transmission in Europe during the 2016 summer season, EU/EEA Member States are recommended to consider a range of mitigation measures.

Executive summary

The Zika epidemic in the Americas continues to evolve and expand geographically. There is now strong evidence of an association between Zika virus infection during pregnancy and congenital central nervous system malformations as well as between Zika virus infection and Guillain–Barré syndrome. Over the past two months, autochthonous cases of Zika virus infection were reported in 39 countries or territories worldwide.

Zika virus and pregnancy
 
At present, there is no evidence which stages of pregnancy are most vulnerable to Zika virus. Therefore the entire duration of pregnancy should be considered at risk, concludes the risk assessment published today. Pregnant women and women who are planning to become pregnant should consider postponing non-essential travel to affected areas until after delivery. The evidence in favour of a causal link between transplacental infection and congenital central nervous system (CNS) malformations is substantial and accumulating. It is still uncertain how many pregnant women who become infected will transmit the virus to the foetus, and how many foetuses will develop brain damage or other malformations. It is likely that the risk of transplacental infection as well as the risk of developing congenital CNS malformations depends on the gestational age at the time of infection. It is also plausible that other risk factors, such as the mother’s age and nutritional status, also influence the risk of transplacental transmission, but there are currently no data available to explore such risk factors.

Zika virus and Guillain–Barré syndrome
 
Cases of Guillain–Barré syndrome (GBS) continue to be reported from the affected countries. GBS is also known to be associated with other infectious diseases that are prevalent in the Americas and the Caribbean. Since October 2015, five countries or territories have reported increased GBS incidence.

Sexual transmission
 
The presence of infectious Zika virus in semen has been detected up to three weeks after onset of disease. The longest interval reported between the onset of symptoms in a male and the subsequent onset of the disease thought to be due to sexual transmission in a female partner is 19 days.
 
Several cases of sexual transmission from males to their partners have been reported or are under investigation in recent weeks. So far, no sexual transmission of Zika virus from infected women to their partners and from persons who are asymptomatically infected have been reported. The role of asymptomatic males in the sexual transmission to women is unknown.

Risk of establishment of local vector-borne transmission in Europe 

The risk of transmission of Zika virus infection in Europe is variable and depends on several co-factors, for example:

  • The presence of a potential mosquito vector; Zika virus disease is caused by a virus transmitted to humans by Aedes mosquitoes, especially by the Aedes aegypti species.
  • The competence of Aedes albopictus to transmit Zika virus, which depends on characteristics of the pathogen (strain-specific vector competence) and of the mosquito species;
  • The capacity of the vector to transmit the infection is determined by a number of factors such as vector competence, the mosquito population density, feeding host preferences, biting rates and survival of the mosquito population.

The risk of transmission of Zika virus infection is extremely low in the EU during the winter season as the climatic conditions are not suitable for the activity of the Aedes albopictus mosquito. During the summer season, autochthonous transmission in the EU following the introduction of the virus by a viraemic traveller is possible in areas where Aedes albopictus is established. Aedes aegypti mosquitoes are present in the EU outermost regions (OMR) and overseas countries and territories (OCT) in the Americas and the Caribbean. Other EU OMRs and OCTs outside of the Caribbean where mosquito vectors are present such as La Réunion and Madeira are at risk of establishment of local transmission should the virus be introduced.
 
EU/EEA Member States should consider mitigation measures, such as advising travellers visiting affected countries to take measures to prevent mosquito bites, as well as enhancing vigilance towards the early detection of imported cases of Zika virus in infections in the EU.
 
The rapid risk assessment also includes options on risk mitigation concerning substances of human origin, disease surveillance and outbreak preparedness.