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Zika Virus Risk Assessment in the WHO African region - A Technical Report

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BACKGROUND

Zika viral disease is transmitted through the bite of an infected mosquito, primarily Aedes aegypti. The virus was first identified in 19471 in rhesus monkeys in the Zika forest of Uganda, and human disease was first identified in 1952 in Uganda and the United Republic of Tanzania. Zika virus disease outbreaks were reported for the first time from the Pacific in 2007 and 2013 in Yap and French Polynesia, respectively. The geographical spread of Zika virus has since been steadily increasing. Zika virus disease has similar clinical presentation as chikungunya and dengue, although it generally causes a milder illness. Symptoms of Zika virus disease include fever, skin rashes, conjunctivitis, muscle and joint pain, malaise, and headache, which normally last for 2 to 7 days. There is no specific treatment for Zika virus disease, but symptoms are normally mild and can be treated with common pain and fever medicines, rest and drinking plenty of water.

Neurological complications have been reported in Polynesia and in Brazil in 2014 and 2015 respectively. More recently increased number of microcephaly cases has been reported in Brazil since October 2015. Although these microcephaly cases are spatially and temporally associated with the Zika outbreak, more robust investigation and research is needed to better understand a causal link. Other countries with current outbreaks such as Colombia, El Salvador, Cape Verde and Panama have not reported an increase in microcephaly.

For many years, despite lack of systematic surveillance mechanism for Zika virus disease, sporadic human cases were detected in Africa. Since 2007 the spread of the virus has been confirmed in 8 Pacific islands, 25 countries and territories of the Americas, and a few Asian countries. In the African region, Cape Verde has reported outbreak with over 7000 cases from October 2015 to January 2016. However, the number of cases has been on the decline since December 2015 according to available data.

ASSESSING THE POTENTIAL RISK OF ZIKA VIRUS OUTBREAK IN THE WHO AFRICAN REGION

To assess the risk of a Zika outbreak in the countries of the WHO African region, consideration must be given to a number of ecological, epidemiological, structural and system factors that contribute to the likelihood and magnitude of an outbreak.

There is limited data on the epidemiology and transmission cycle of the Zika virus. The main transmitting vector for Zika virus is Aedes aegypti mosquito, although Ae. Albopictus and other mosquitoes of the Aedes genus are thought to have adapted to the virus and in some cases have been shown to transmit it. Ae aegypti is also the main mosquito that transmits Yellow fever, chikungunya and dengue viruses. Considering the wide distribution of the vector and its efficiency in transmitting several arboviruses on the continent, all the countries in the African Region are at risk of Zika virus transmission.

Historically, the sylvatic (forest) form of the virus was the main one reported in the few studies in Africa. As urbanization increased, however, the Ae aegypti mosquito has adapted to and flourished in the urban environment breeding in open water containers and other collections of stagnant water. In many African cities, there is also a high proportion of the population who reside in slum areas where shelter, water storage, drainage and overall sanitation is poor, potentially increasing the availability of breeding sites for the urban mosquitoes.

Over time, African countries have become more connected with each other through land, water and air transportation increasing the risk of disease spread. They have also increasingly become connected with other countries outside Africa, mainly through air transportation but also through shipping. The potential risk of importation of infections such as Zika virus from other countries is therefore high.

In the event that Zika transmission starts, variations in access to health care and their use for the treatment of acute conditions also contribute the early detection, management and eventual prevention of the spread of infections. The effectiveness of other government systems to respond to a potential outbreak is critical.

Countries with strong health systems, efficient Integrated Disease Surveillance and Response (IDSR) and are implementing the International Health Regulations (IHR 2005) requirements are likely to cope better with a Zika outbreak. Countries in the WHO African region are therefore advised to strengthen: i) vector surveillance and control, ii) disease surveillance and laboratory detection, iii) monitoring the occurrence of neurological complications, and iv) increasing public awareness.