Investigating the emergence of the Zika virus in Angola

U of T-affiliated study sheds light on the transmission of the Asian genotype of the virus in the country
Subtype of Zika virus likely introduced to Angola from Brazil.
Subtype of Zika virus likely introduced to Angola from Brazil. DAVID GOODSELL/CC WIKIMEDIA

In 2016, the World Health Organization declared an international health emergency over the transmission of the Zika virus, which broke headlines for causing brain damage in infants.

Now, in 2019, a University of Toronto-affiliated study has investigated the outbreak of the Asian lineage of the Zika virus in Angola, with the goal of providing “the first cohesive insight into the introduction, circulation, and possible public health effects of Zika virus in Angola.”

The co-authors concluded that this subtype of Zika virus has been present in Angola, and that the transmission most likely originated from Brazil.

Why is the Zika virus so dangerous?

The virus has two distinct lineages: the African genotype and the Asian genotype. Researchers have detected the African genotype in Africa since the mid-twentieth century, according to the co-authors, but there is little data on the presence of the Asian genotype in Africa.

Until 2007, the Zika virus was only identified in 14 people in Africa and Asia. At the time, infection was believed to cause mild symptoms, such as a fever, headache, and rash. Since 2013, however, the Asian genotype of Zika virus has spread to locations in the Pacific Islands and the Americas, and has resulted in more than 800,000 suspected and confirmed cases of the disease.

The conditions in sub-Saharan Africa, where there is a mosquito population that can spread the disease and an appropriate climate for infection, means that the residents are especially susceptible to the disease. Research has also revealed that Zika virus is dangerous during pregnancy, as it can cause severe birth defects.

The investigation’s findings

The study’s findings suggest that either a single event introduced the Zika virus in Angola and continued until at least June 2017, or that there was a recurrent and later introduction of the virus belonging to a specific lineage present in Brazil. The co-authors believe that the virus “probably circulated in Angola for 17–28 months.” This implies that the outbreak was substantially larger than the small number of cases detected by the Angolan ministry.

In order to investigate the possible source of Zika virus in Angola, the co-authors analyzed the global incident of Zika virus infection and human mobility data. They considered two major factors as contributors to a high risk of exporting Zika virus to Angola, which were high local incidences of Zika virus and a high number of air passengers travelling to Angola.

To achieve this they determined the monthly number of passengers to Angola from countries who were reporting Zika virus outbreaks, based on the worldwide ticket sale of data from the International Air Transport Association, from 2015 to 2017. They also used surveillance data to estimate the average Zika virus incidence per person per week in each country.

What did the investigation reveal about emergence and spread of Zika virus?

Previous research has established the transmission of mosquito-borne viruses between Brazil and Angola through studies on the spread of chikungunya virus from Angola to Brazil in 2014. Angola and Cape Verde, the two African countries with confirmed Asian-lineage Zika virus, have regular air connectivity with Brazil.

Moreover, the co-authors found that Angola, out of all African countries, received the largest number of travellers from Zika virus-affected countries in the Americas.

The researchers concluded that Zika virus was most likely introduced to Angola from Brazil, according to data for human flight mobility and the global incidence of the disease. However, the possibility of spread to Angola from affected locations where genomic data are unavailable is not out of the question.

The study’s methods

In order to conduct their study, researchers undertook “a multi-component investigation into Zika virus and suspected microcephaly cases in Angola.”

They assessed surveillance data from the Ministry of Health in Angola in order to identify acute cases of Zika virus infection. The co-authors also screened samples from a separate 2017 study involving 349 people with HIV living in Luanda, Angola to expand the dataset. They sequenced Zika virus from three samples and performed an analysis to explain the origins and duration of the outbreak in Angola.

In addition, the researchers analyzed human air travel and data on the global outbreak of Zika to support findings about the geographical source of the introduced strain. They further assessed suspected cases of microcephaly, a birth defect characterized by abnormal brain development, notified to the Ministry of Health in Angola.

The co-authors also conducted an analysis of evolutionary relationships, called a phylogenetic analysis, that showed that the three Angolan Zika virus genomes that they analyzed had a common ancestor in June 2016, improving the understanding of the virus.

A better look at Zika outbreaks in Africa, noted the co-authors, is critical in order to safeguard the health of people living in the continent and across the globe.

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