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Climate and air travel maps identify Africa & Asian countries at greatest risk of Zika

India, Indonesia and Nigeria are among the countries outside Latin America most at risk from the Zika virus, according to a new modelling study published in The Lancet Infectious Diseases.

The research, co-led by the London School of Hygiene & Tropical Medicine, Oxford University and the University of Toronto, found 2.6 billion people spanning countries across Africa and Asia-Pacific may be vulnerable to Zika virus outbreaks.

This is due to a combination of factors, including high travel volumes from Zika affected areas in the Americas, local presence of mosquitos capable of transmitting Zika virus, suitable climatic conditions, large populations and/or limited health resources. China, the Philippines, Vietnam Pakistan and Bangladesh were other countries found to be at greatest risk of Zika.

The authors say that identifying where and when populations would be most susceptible to local transmission of Zika could help inform public health decisions about the use of finite resources.

In this study, the research team established where Zika virus transmission has been reported or where conditions are suitable. At the time of the analysis, local transmission had been confirmed in 40 countries in Central and South America and the Caribbean (no cases of local transmission had been reported in the USA). 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 from December 2014 to November 2015.

They 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 Ae. aeqypti and Ae. albopictus occurrence - these scenarios each increased the size of the region at risk.

In addition, the team mapped the monthly volume of travellers arriving into Africa and Asia-Pacific in order to identify countries at greatest risk of Zika virus importation across the coming 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.

Study author Dr Oliver Brady, Research Fellow in Mathematical Modelling from the London School of Hygiene & Tropical Medicine, said: "As the current Zika epidemic unfolds throughout Latin America, there is an increasing concern that the virus may spread beyond the Americas and cause epidemics in other parts of the world. The risk is likely to be highest over the summer months when air traveller volume between America and other parts of the world peaks, and the hot temperatures mean the mosquitoes that transmit the virus can survive longer."

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.

Dr Brady said: "Countries such as India, Indonesia and Nigeria are predicted to be at highest risk of Zika introduction with up to 5,000 passengers a month arriving from Zika endemic areas. Should the virus be imported into these areas the impact on their health systems could be very severe.

"There remains a high degree of uncertainty about what degree of immunity people in Africa and Asia may already have to Zika. However, what is for sure is that billions of people live in areas outside Latin America that can sustain transmission of dengue, a very similar virus transmitted by the same mosquito as Zika. In theory, they are all at risk.

 "Many of these people live in some of the poorest countries of the world which lack the resources to detect or respond to a Zika epidemic until it's too late. The results of our study could help countries target funds more effectively. They also strongly suggest that countries in regions at risk add Zika testing to their current disease surveillance programmes and develop contingency plans should they start to detect cases."

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

The authors note a number of limitations and assumptions in the study. Understanding about the efficiency of Ae. albopictus to transmit Zika virus is still evolving and the study did not include other Aedes species such as Ae. africanus or Ae. hensilli which could prove capable vectors in particular regions. Also, the health consequences of imported Zika depend on the possible underlying levels of immunity to the virus, and 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.

Publication

  • Isaac I Bogoch, Oliver J Brady, Moritz U G Kraemer, Matthew German, Maria I Creatore, Shannon Brent, Alexander G Watts, Simon I Hay, Manisha A Kulkarni, John S Brownstein, Kamran Khan. Potential for Zika virus introduction and transmission in resource-limited countries in Africa and the Asia-Pacific region: a modelling study. Lancet Infectious Diseases. DOI: 10.1016/S1473-3099(16)30270-5

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