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Yellow fever goes intercontinental

Aedes aegypti is present in many parts of South Asia, aiding the spread of yellow fever Aedes aegypti is present in many parts of South Asia, aiding the spread of yellow fever Torres Garcia
05 Jul
Increased air travel is transporting yellow fever to Asia, as the first ever cases are diagnosed in China

An outbreak of yellow fever, the acute, viral disease, has killed 277 people in Angola since December, the worst incidence in 30 years, and has also been linked to cases in Kenya and the Democratic Republic of Congo. However, it could jump even further afield to countries in South Asia via unprecedented numbers of travellers flying from Angola to China. At the time of writing, 11 cases have been diagnosed in China, the first in the region’s history.

So far, the virus is still ‘imported’ as all those diagnosed had returned from Angola, the workplace of around 100,000 Chinese expatriates. ‘Undoubtedly there are more that have not been detected,’ says Sean Wasserman, Doctor of Infectious Diseases at the University of Cape Town. ‘Mandatory vaccination before travel has not been enforced, and now there are thousands of Chinese workers who are returning home, some with the virus in their blood.’ It has so far not been transmitted within China.

China itself is not a particularly high risk case, but infected travellers pose a threat to more vulnerable countries in South Asia, such as India, Laos and Cambodia, which have ripe yellow fever conditions. They have the mosquito responsible for its spread (Aedes aegypti), plenty of forested areas with monkeys where the virus could be maintained, and opportunities for human-to-human transmission in urban, mosquito-prone regions.

If enough infected travellers bring the virus into a region with dense Aedes aegypti infestation, there is a real chance of yellow fever becoming established

‘If any infected travellers from China end up in one of these places – particularly during the upcoming rainy season when there are a lot more mosquitoes – it could set up a local yellow fever transmission,’ says Wasserman, ‘which would be a disaster.’

That yellow fever hasn’t already established itself in South Asian countries is still a mystery, and offers hope for the region’s resistance. The prevailing theory is that there have been few historical opportunities for the disease’s introduction, allowing the closely-related Dengue virus to dominate the ecological niche instead. It is hoped that high levels of exposure – and immunity – to Dengue will protect local populations from yellow fever.

Wasserman, however, is unconvinced by this, as yellow fever and Dengue are already known to coexist in both Africa and South America. For him it is a question of critical mass: ‘if enough infected travellers bring the virus into a region with dense Aedes aegypti infestation, there is a real chance of yellow fever becoming established. We don’t yet know what that critical mass is, but steps must be taken to minimise the risk.’

This was published in the July 2016 edition of Geographical magazine.

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