September 02, 2016 142
"Zika: Two billion at risk in Africa and Asia," BBC News reports. A new modelling study suggests the virus could spread, via air travel, to Asia and Africa.
Zika virus disease is mainly spread by mosquitoes. For most people it is a mild infection and isn't harmful. However, it may be more serious for pregnant women, as there's evidence it causes birth defects ? in particular, abnormally small heads (microcephaly).
The study estimates that areas of Africa and south and east Asia, where mosquito-spread diseases are already prevalent (such as dengue and malaria), could encourage the spread of infection. Countries which have a "perfect storm" combination of high population, high mosquito activity and underfunded health services may be particularly vulnerable. These include India, the Philippines, Indonesia, Nigeria, Vietnam, Pakistan and Bangladesh.
The researchers hope their findings will offer valuable information for public health decision makers at both national and international levels, helping them to help plan ahead.
As with any modelling study, the results were based on assumptions and data available. It's important to bear in mind the uncertainty that arises when predicting potential trends of any disease.
The study does highlight the fact that, due to air travel, viruses once confined to remote corners of the globe (Zika is thought to have originated in a Ugandan forest), now have the potential to spread quickly. And if you are travelling to a part of the world where any mosquito borne disease is prevalent it is important to take the correct precautions.
Zika does not naturally occur in the UK.
Where did the story come from?
The study was carried out by researchers from a range of institutions, including the London School of Hygiene and Tropical Medicine, the University of Oxford and the University of Toronto. It was funded by the Canadian Institutes of Health Research and the US Centres for Disease Control and Prevention.
Some of the authors are employees of BlueDot, a company that offers analysis of potential global disease threats.
The study was published in the peer-reviewed scientific journal, The Lancet Infectious Diseases. It is available on an open-access basis so it is free to read online.
BBC News' reporting of the study was accurate and provides some useful commentary from independent experts.
What kind of research was this?
This was an ecologic modelling study which aimed to identify regions in Africa and the Asia-Pacific and times of the year when the potential health, economic and social effect from Zika virus is the greatest.
Studies like this are useful for informing policymakers about what the potential impact of a disease might be, to help plan ahead and formulate recommendations for disease control.
What did the research involve?
The modelling study considered three different aspects to identify the areas most susceptible to the virus: air transportation, mosquito occurrences and Zika transmission. To do this, they used data from the International Air Transport Association (IATA), observations made of the main mosquito species that carry Zika, and climate conditions using WorldClim (a free collection of data sets on past, current and predicted climate trends).
IATA data such as airline ticket sales and destinations was analysed from December 2014 ? November 2015 to identify the monthly flow of passengers arriving from areas of the Americas (with conditions suitable for year-round Zika transmission) to Africa and the Asia-Pacific region.
As little is known about the incubation period of Zika virus in the mosquito, the researchers assumed the same model as that used for dengue, applying this as a proxy for Zika. They aimed to see whether the temperature in any geographical region would enable a mosquito newly infected with the virus to become infective. Seasonal mosquito occurrence data was then paired with data on temperatures in the region throughout the year to predict the number of days in which local transmission of Zika would be possible.
Finally, health expenditure per capita was used as a proxy to estimate the country's capability to detect and respond to Zika virus importation and transmission.
What were the basic results?
Worldwide 2.6 billion people live in regions of Africa and Asia-Pacific where mosquito prevalence and suitable climate conditions could support the transmission of Zika.
Based on the volume of air travel and health expenditure per capita, the populations at the highest risk of Zika virus exposure during the months of the mosquito's greatest activity was India (1.2 billion people), followed by the Philippines, Indonesia, Nigeria, Vietnam, Pakistan and Bangladesh.
China receives the largest number of travellers out of the entire Africa and Asia-Pacific region, and has a large population already living in geographical areas where dengue virus is prevalent. However, it spends more on healthcare than the majority of high risk countries.
On the other hand, countries such as Tanzania, Ethiopia, Mozambique, and the Democratic Republic of Congo have a smaller population at risk of Zika transmission, receive a moderately high volume of travellers but also have low healthcare expenditure per capita.
How did the researchers interpret the results?
The researchers conclude: "Many countries across Africa and the Asia-Pacific region are vulnerable to Zika virus. Strategic use of available health and human resources is essential to prevent or mitigate the health, economic, and social consequences of Zika virus, especially in resource-limited countries."
This study aimed to identify regions of Africa and the Asia-Pacific and times of the year when the potential health, economic and social impact of Zika virus would be the greatest.
From the predictions based on the volume of air travel and health expenditure per capita, it found that the populations at the highest risk of Zika virus exposure during the months of its greatest activity was India, followed by the Philippines, Indonesia, Nigeria, Vietnam, Pakistan and Bangladesh.
The above may be of particular interest to public health bodies in the UK, as many UK nationals may travel to these parts of the world to visit friends and family.
As with any modelling study, predictions are based on assumptions and data available and these may not necessarily be representative of the potential future Zika transmission in Africa and the Asia-Pacific. Diseases are difficult to predict as there is a great deal of uncertainty that comes with them. As noted, the incubation period of Zika in the mosquito is unknown, and so the incubation period for dengue was used here as a proxy. However, they may not be identical which may affect infectivity rates.
Additionally, this study used healthcare expenditure per capita as a proxy for the countries' capacity to respond to a potential Zika outbreak. However, this doesn't take into account other preventative interventions that may already be in place for mosquito control or population behaviour. It could also be the case that some populations have evolved an immunity to Zika-type infections.
The best place for up-to-date travel advice on the Zika virus, especially for those who are pregnant or are planning a pregnancy, is Public Health England's Zika virus guidance page.
Links To The Headlines
Zika: Two billion at risk in Africa and Asia, study says. BBC News, September 2 2016
Links To Science
Bogoch II, Brady OJ, Kraemer MUG, 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. Published online September 1 2016
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