Climate, air travel maps identify countries in Africa, Asia at greatest risk of Zika virus

The Lancet notes that in this study, the research team, which included scientists from the London School of Hygiene & Tropical Medicine, Oxford University, and the University of Toronto, established the ecological niche for Zika virus in the Americas (where Zika virus transmission has been reported or where conditions are suitable). At the time of the analysis, local transmission of Zika virus had been confirmed in forty countries in Central and South America and the Caribbean (at the time, no cases of local transmission had been reported in the United States). The researchers then gathered data on airline ticket sales from all 689 cities with one or more airports in the region travelling to Africa or Asia-Pacific over a whole year (December 2014 to November 2015).

The research team then modelled three different scenarios of seasonal suitability for mosquito-borne transmission of Zika virus. The first, which modelled monthly suitability for dengue virus transmission, produced the most conservative geographic region of risk. The second also included areas with Aedes aeqypti occurrence and the third included both A. aeqypti and A. albopictus occurrence — these scenarios each increased the size of the region at risk.

In addition, the team mapped the monthly volume of travelers arriving into Africa and Asia-Pacific in order to identify countries at greatest risk of Zika virus importation across seasons. Health expenditure per capita was used as a proxy of a country’s capacity to detect and effectively respond to a possible Zika virus outbreak.

Countries with large volumes of travelers arriving from Zika virus-affected areas of the Americas and large populations at risk include India (67,422 travelers arriving per year; 1.2 billion residents in potential Zika transmission areas), China (238,415 travelers; 242 million residents), Indonesia (13,865 travelers; 197 million residents), the Philippines (35,635 travelers; 70 million residents), and Thailand (29,241 travelers; 59 million residents).

Of the countries with the largest at risk populations, the authors suggest that India, the Philippines, Indonesia, Nigeria, Vietnam, Pakistan, and Bangladesh might be most vulnerable to impact because of their limited per capita health resources.

While the analysis emphasizes the potential for human infection via mosquitos, sexual transmission of Zika virus infection is now well documented. The authors say that travelers returning from affected areas would benefit from health education to prevent sexual transmission.

The authors note a number of limitations and assumptions. Firstly, understanding about the efficiency of A. albopictus to transmit Zika virus is still evolving and the study did not include other Aedes species such as A. africanus or A. Hensilli, which could prove capable vectors in particular regions. Importantly, the health consequences of imported Zika virus will depend on local ability to diagnose and respond to a possible outbreak, but will also depend on possible underlying levels of immunity to Zika virus. Although sporadic cases of Zika virus have been reported in both Africa and Asia-Pacific, the breadth and extent of previous infection with Zika virus remains unknown, and it is not known whether the current Asian strain of the virus (seen in the Americas) will affect individuals differently if they have previously been infected with the African strain.

Writing in a linked Comment, Dr. Abraham Goorhuis and Professor Martin P Grobusch, of the Center of Tropical Medicine and Travel Medicine at the University of Amsterdam, Netherlands, say: “An important prerequisite for the occurrence of large-scale epidemics is the presence of an immunologically naive human population. This is a big unknown in the high-risk countries identified, because urgently needed reliable seroprevalence studies are in short supply. Bogoch and colleagues assume low levels of pre-existing immunity, which in their study translates into a worst-case scenario. This assumption might be true and therefore, vigilance is needed. A best-case scenario would be the existence of previously unknown widespread Zika virus immunity in Asia and Africa. Such immunity could potentially exist if Zika virus infections in the past were not diagnosed, either because they were asymptomatic or because symptomatic infections were misdiagnosed due to their clinical similarity to other (arboviral) tropical infectious diseases. The development of protective herd immunity at a young age would also protect pregnant women against foetal complications. Under such a scenario, the risk of large Zika virus epidemics in Asia and Africa would be low, despite the introduction of the virus by infected travelers.”

— Read more in Isaac I Bogoch et al., “Potential for Zika virus introduction and transmission in resource-limited countries in Africa and the Asia-Pacific region: a modelling study,” The Lancet Infectious Diseases (1 September 2016); and Abraham Goorhuis and Martin P Grobusch, “Zika virus: who’s next?” The Lancet Infectious Diseses (1 September 2016)