Zika - Risk Assessment

Factors health professionals should consider when advising those travelling to Zika virus affected areas Zika - Risk Assessment

Background

Zika virus (ZIKV) is transmitted by daytime biting Aedes mosquitoes. A small number of cases have also occurred through sexual transmission.

The infection often occurs without symptoms but can also cause an illness similar to dengue or chikungunya (which are also transmitted by Aedes mosquitoes). Illness associated with ZIKV infection is usually mild and short-lived; serious complications and deaths from ZIKV are not common. Based upon a systematic review of the literature up to 30 May 2016, WHO concluded that ZIKV infection during pregnancy is a cause of congenital brain abnormalities, including microcephaly; and that ZIKV is a trigger of Guillain-Barré syndrome (GBS) [1]. 

Risk assessment prior to travel

A comprehensive risk assessment should be undertaken for any traveller going to areas with ZIKV transmission.

Countries/areas with current or past ZIKV transmission have been given one of three risk ratings (high, moderate, or low) based on ZIKV epidemiology and risk to UK travellers. The greatest likelihood of acquiring ZIKV infection is in a country/area with high or moderate risk, however the individual risk of infection may be lower especially if mosquito bite avoidance measures are followed.

A list of countries and areas and their ZIKV risk ratings can be found on the PHE website.

The following additional factors should be considered for each traveller, and will help health professionals to assess and communicate the potential risks associated with ZIKV infection, and allow the traveller to make as informed a decision as possible regarding their travel plans.

Individual factors

Is the traveller:

  • Pregnant or the male partner of a woman who is pregnant
  • Planning to become pregnant prior to travel, during travel, or during the 6 months following return from a high or moderate risk area for ZIKV transmission
  • The male partner of a woman planning pregnancy
  • Immunosuppressed or do they have any co-morbidities

A. Pregnant travellers and their partners

1. It is recommended that pregnant women should postpone non-essential travel to areas rated as ‘high risk’ of ZIKV transmission (see A-Z list) until after pregnancy.

2. Pregnant women should consider postponing non-essential travel to areas rated as ‘moderate risk’ of ZIKV transmission (see A-Z list) until after pregnancy.

3. In the event that travel to a high or moderate risk area cannot be postponed, you should ensure the pregnant woman and her partner are fully aware of the risks ZIKV may present. They should be scrupulous with mosquito bite avoidance measures both during daytime and night time hours (but especially during mid-morning and late afternoon to dusk, when the mosquito is most active). Public Health England has produced an information leaflet: mosquito bite avoidance for travellers.

4. Pregnant women who have travelled in an area rated as high or moderate risk of ZIKV transmission should seek advice from their GP or midwife on their return to the UK, even if they have not been unwell. Further information about when to perform fetal ultrasound scanning, and, if necessary, referral to the local fetal medicine service is available. Pregnant women who develop symptoms suggestive of ZIKV infection in an area rated as low risk should also seek medical advice and contact their GP on return.

5. ZIKV has been shown to be present in semen and vaginal secretions. The virus persists longer in semen than in the female genital tract, but the viral RNA detected is not necessarily infectious. Cases of sexual transmission are occasionally reported [3]. Consistent and correct use of condoms (or other barrier methods) for vaginal, anal and oral sex during and after travel, is advised to reduce the risk of the developing fetus being exposed to Zika virus.

6. Condoms (or other barrier methods) should be continued for the duration of the pregnancy if the couple both travelled, or if just the male partner travelled even in the absence of symptoms. 

B. Couples planning a pregnancy prior to, during or within 6 months after travel

1. It is recommended that couples planning pregnancy should check the ZIKV risk for their destination before booking travel and consider any travel and sexual transmission advisories.

2. Couples planning pregnancy who are travelling to an area with high or moderate risk of ZIKV transmission should be advised to consider delaying conception to reduce the risk of the developing fetus being exposed to ZIKV.

3. You should ensure that both partners are fully aware of the risks ZIKV may present. They should be scrupulous with mosquito bite avoidance measures both during daytime and night time hours (but especially during mid-morning and late afternoon to dusk, when the mosquito is most active). Public Health England has produced an information leaflet: mosquito bite avoidance for travellers.

4. Zika virus has been shown to be present in semen and vaginal secretions. The virus persists longer in semen than in the female genital tract, but the viral RNA detected is not necessarily infectious. Most cases of ZIKV are acquired via mosquito bites, but sexual transmission of ZIKV is occasionally reported [3].

5. Couples planning pregnancy who intend to travel to a high or moderate ZIKV risk area, should follow guidance on prevention of sexual transmission of ZIKV including the consistent use of effective contraception and condoms (or other barrier methods) for vaginal, anal and oral sex during and after travel. These measures should be used even in the absence of symptoms while travelling and if: 

  • Both partners travelled, for 6 months after return or after last possible Zika virus exposure*
  • Male traveller only, for 6 months after return or after last possible Zika virus exposure*
  • Female traveller only, for 8 weeks after return or after last possible Zika virus exposure*

 6. If a woman develops symptoms compatible with ZIKV infection, it is recommended she avoids becoming pregnant for 8 weeks following symptom onset.

C. Preventing sexual transmission in other travellers

The implications of sexual transmission of ZIKV are greatest for pregnant women. If couples, where the women is not pregnant or planning pregnancy, are worried about sexual transmission of ZIKV, this can be prevented by correct and consistent use of condoms (or other barrier methods) during vaginal, anal and oral sex. For those wanting to reduce the risk of sexual transmission, the consistent use of barrier methods during sexual contact should begin while travelling to high or moderate ZIKV risk countries and continue to be used for the period of time as stated for couples planning pregnancy above, depending on gender.

*Footnote: Last possible ZIKV exposure is defined as the date of leaving an area with high or moderate ZIKV risk, or the date on which unprotected sexual contact with a potentially infectious partner took place.

D. Travellers with co-morbidities, immunosuppression or at extremes of age

In the travel health consultation, these travellers should be offered advice regarding the likely impact of any travel related infection on them. More information on ZIKV infection and immunosuppression is available from Public Health England.

Destination related factors

Following an expert review, countries/areas with current or past ZIKV transmission have been given one of three risk ratings (high, moderate, or low) based on Zika virus epidemiology and risk to UK travellers. PHE/NaTHNaC uses the World Health Organization (WHO) Zika virus classification tables, the current Zika virus transmission table from the European Centre for Disease Control (ECDC), and epidemic intelligence systematically collected by PHE and NaTHNaC to produce the risk ratings for each country or area. An A to Z list of countries, and areas and their Zika virus risk ratings is available on the PHE website.

  • High risk: WHO category 1 area or country with new introduction or re-introduction of ZIKV with ongoing transmission where there is evidence of a current outbreak of ZIKV with significant transmission that may increase the risk to UK travellers

  • Moderate risk: WHO category 1 area or country with new introduction or re-introduction of ZIKV with ongoing transmission where there is no evidence of a current outbreak of Zika virus with significant transmission that may increase the risk to UK travellers OR: WHO category 2 area or country either with evidence of virus circulation before 2015 or area with ongoing transmission that is no longer in the new or re-introduction phase, but where there is no evidence of interruption

  • Low risk: WHO category 3 area or country with interrupted transmission and with potential for future transmission
Table 1: PHE/NaTHNaC Zika risk ratings and summary of travel advice
High and moderate risk

1. Pregnant women:

  • should postpone non-essential travel to high risk areas until after pregnancy
  • should consider postponing non-essential travel to moderate risk areas until after pregnancy 
  • If travel is essential, be aware of the risk, be scrupulous with insect bite avoidance, seek advice from your GP or midwife on return, even if you have not been unwell

2. All travellers should avoid mosquito bites day and night. If you experience symptoms suggestive of ZIKV infection, seek medical advice (while the symptoms are still present)

3. Male travellers: if your female partner is pregnant, consistent use of condoms or other barrier methods is advised during vaginal, anal and oral sex to reduce the risk of transmission during travel and for the duration of the pregnancy even if you did not develop symptoms compatible with ZIKV infection if your female partner is planning pregnancy: effective contraception is advised to prevent pregnancy AND consistent use of condoms or other barrier methods is advised during vaginal, anal and oral sex to reduce the risk of transmission during travel and for 6 months after return even in the absence of symptoms

4. Female travellers: avoid becoming pregnant during travel and for 8 weeks after your last possible ZIKV exposure*

Low risk
  • All travellers should avoid mosquito bites particularly between dawn and dusk.
  • Pregnant women should seek medical advice if they develop ZIKV symptoms, and contact their GP on return
* Last possible ZIKV exposure is defined as the date of leaving an area with high or moderate ZIKV risk, or the date on which unprotected sexual contact with a potentially infectious partner took place.

Information and advice for travellers is available from our Country Information pages (information is found in the ‘other risk’ section for affected areas).

When trying to ascertain the risk of ZIKV infection at any given destination, the following should also be considered:

Affected areas

1. The situation continues to evolve and the risk ratings of countries or areas may change.

2. Areas of active and significant transmission of ZIKV will vary within a country. Where specific information exists this will be provided in our Country Information pages.

3. In most cases, it will not be possible to identify specific areas within a country which are experiencing a higher or lower risk of transmission. In these situations it is prudent to assume that the whole country is high, moderate or low risk depending on what information is available.

Mosquito vector

ZIKV is transmitted predominantly by the bite of an infected Aedes mosquito, most commonly Aedes aegypti. Other species of Aedes mosquitoes may also have the potential to transmit this virus. Aedes mosquitoes bite predominantly during daylight hours.

These mosquitoes will bite both outdoors and indoors and throughout the day (and night) but are most active during mid-morning and late afternoon to dusk.

Aedes spp mosquitoes are adapted to human habitation and commonly live in urban environments, laying their eggs in collections of water in the domestic environment (e.g. buckets, vases, tyres, flower pots) [4].

Tropical Aedes mosquitoes:

  • Favour high humidity and warmth.
  • Are unlikely to be found at altitudes ≥ 2,000m (this will be relevant for some areas in Latin America where some cities/travel destinations may be at high altitude).
  • Are likely to be more abundant during seasonal rainfall. There may be regional variations in seasonal rainfall. Season alone however should not be relied upon as an indicator of risk.

Risk of exposure may be reduced in areas with good vector control programmes for Aedes infestation.

World overview

Zika Risk World Overview

Gulf of Mexico

Zika Risk Gulf of Mexico

Caribbean Islands

Zika Risk Caribbean Islands

South America

Zika Risk South America

Traveller activity and behaviour factors

A traveller’s risk will also be affected by their behaviour and activities:

1. A traveller who is aware of, and is scrupulous regarding bite avoidance measures, may reduce the risk of mosquito bites and therefore ZIKV infection.

2. In general, the risk of exposure to ZIKV will increase with the length of stay.

First Published :   10 Jan 2017
Last Updated :   29 Aug 2017

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