No 4 - 2024

Increased incidence of Zika virus in Thailand

Zika virus spread epidemically in large parts of the world in 2015-17, EPI-NEWS 49/2015. Since then, Zika virus has been endemic in many countries, especially in countries around the equator, corresponding to the distribution of mosquitoes (Aedes species) that can transmit the virus. Infection with Zika virus is a particular concern for pregnant women, as the fetus can be infected and develop neurological damage and deformities, particularly microcephaly. There is a risk of transmission both through mosquito bites and through sexual contact. The latter is the reason why travelers who have stayed in Zika-endemic areas should use condoms for a period after returning home. For the traveler, this applies 2 months after returning for women and 3 months for men. If one in the couple is pregnant, this applies throughout the rest of the pregnancy.

Information about the current incidence and risk of infection in endemic areas is generally lacking, as Zika virus is not systematically monitored in many countries. However, there have recently been reports from Thailand of an increase in incidence by approximately 300% in 2023 compared to the previous year (around 900 confirmed cases compared to about 300 cases in 2022). Based on this, the European Centre for Disease Prevention and Control (ECDC) conducted a survey of all EU countries on the number of confirmed cases after travel to Thailand in 2023. In total, five countries have reported a total of 16 known travel-related cases from Thailand, with a majority of infections occurring in the second half of 2023. The compilation is incomplete but should be seen in relation to an average number of known travel-related cases of five per year in the period 2016-2022.

On this basis, SSI assesses that there may currently be an increased risk of Zika virus transmission when traveling to Thailand, even though the absolute risk for the individual traveller is low given the high number of travellers. To reduce the risk of Zika virus transmission as well as other mosquito-borne diseases, all travelers should use mosquito repellents, sleep under mosquito nets or in air-conditioned rooms, while pregnant women should currently consider postponing non-essential travel to Thailand. All travelers from Thailand should protect themselves against sexual transmission through the above recommendations to use condoms for a period after returning home.

There may be a similar, but unknown risk for travelers to other countries where Zika virus has established itself, and therefore the same recommendations apply to these countries, see SSI's recommendations for the protection of pregnant travelers.

(P.H. Andersen, L.S. Vestergaard, Department of Infectious Epidemiology and Prevention)

Important to remember malaria medication when traveling to Zanzibar

Statens Serum Institut (SSI) has become aware of a total of five recent cases of imported malaria of the Plasmodium falciparum type among patients returning to Denmark from a trip to the island of Zanzibar in Tanzania. Two of the cases have been severe. In some instances, it cannot be said with certainty whether the infection could have occurred elsewhere, but Zanzibar is a possibility.

Subsequently, through the European Early Warning and Response System (EWRS), SSI has informed other EU countries about the current high number of malaria cases from Zanzibar and has inquired whether other countries have experienced something similar. So far, 20 countries have responded, of which 13 countries report having seen at least one case of malaria imported from Zanzibar and/or Tanzania in 2023/2024.

Additionally, local news media on Zanzibar have reported that in the fall of 2023, there was a resurgence of malaria in several places on the island in relation to unusually heavy rainfall. The incidence of malaria in Zanzibar had otherwise been declining in recent years, thanks to an effective national malaria control program. However, there remains a certain permanent risk of malaria, and occasional local outbreaks can occur, significantly increasing the risk of malaria, even for travelers.

SSI continues to recommend that all travelers to Zanzibar protect themselves against malaria by using both preventive malaria medication and by avoiding mosquito bites through the use of mosquito repellents and bed nets. Additionally, travelers should be aware of malaria symptoms and seek medical attention if they fall ill, mentioning that they have traveled in a malaria area.

Read more about malaria and malaria prevention on the SSI website.

(P.H. Andersen, L.S. Vestergaard, Department of Infectious Epidemiology and Prevention)