Zika virus outbreak threatens Asian nations

A new Canadian-led study states many countries across Africa and Asia-Pacific may be vulnerable to Zika virus outbreaks, with India, China, the Philippines, Indonesia, Nigeria, Vietnam, Pakistan, and Bangladesh expected to be at greatest risk of Zika virus transmission.

This is due to a combination of 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, according to a new modelling study published in The Lancet Infectious Diseases.

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

"An estimated 2.6 billion people live in areas of Africa and Asia-Pacific where the local mosquito species and suitable climatic conditions mean that local Zika virus transmission is theoretically possible. However, there are still many unknowns about the virus and how it spreads, including which local species of mosquito are most capable of transmitting the virus, and whether immunity exists in areas that have previously reported cases of Zika virus. The impact on populations will also depend heavily on the country's ability to diagnose and respond to a possible outbreak," says study author Dr Kamran Khan, St Michael's Hospital, Toronto, Canada.

"Warmer temperatures in the northern hemisphere (when mosquitos are more active) increase the risk of new outbreaks appearing outside of the Americas. The potential for epidemics to occur in parts of Africa and the Asia-Pacific region is particularly concerning given that the vast numbers of people who could be exposed to Zika virus are living in environments where health and human resources to prevent, detect, and respond to epidemics are limited. Our findings could offer valuable information to support time-sensitive public health decision-making at local, national, and international levels," he adds.

In this study, the research team, which included scientists from the London School of Hygiene & Tropical Medicine, Oxford University (UK) and the University of Toronto (Canada), 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 40 countries in Central and South America and the Caribbean (at the time, 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 over a whole year (Dec 2014 to Nov 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 travellers 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 travellers arriving from Zika virus-affected areas of the Americas and large populations at risk include India (67422 travellers arriving per year; 1.2 billion residents in potential Zika transmission areas), China (238415 travellers; 242 million residents), Indonesia (13865 travellers; 197 million residents), the Philippines (35635 travellers; 70 million residents), and Thailand (29241 travellers; 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 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.

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.

In the most conservative scenario, they found that the 10 countries with the greatest risks of exposure are:

India (1.2 billion at risk in peak month)

China (240 million at risk in peak month)

Indonesia (197 million at risk in peak month)

Nigeria (179 million at risk in peak month)

Pakistan (168 million at risk in peak month)

Bangladesh (163 million at risk in peak month)

Vietnam (83 million at risk in peak month)

Philippines (70 million at risk in peak month)

Thailand (59 million at risk in peak month

Burma (51 million at risk in peak month)

Their least conservative scenario is much the same, but with more than 1.1 billion people affected in China.

Dr. Khan told CTVNews.ca that his team chose to focus on Asia, Africa and the Pacific because they are the least technologically and financially prepared to deal with the virus.

He said India is particularly worrying because of its huge population living with climates favourable to the spread of the virus, not to mention the fact that there is plenty of standing water in which mosquitos can breed.

“Hopefully we will not see the same kind of situation as occurred in Brazil,” Dr. Khan said of India.

Dr. Khan said the results have implications for the international community, the countries involved and Canadian travellers.

Canadians need to know that they may be at risk if they travel to these countries, particularly if they are pregnant or planning to become pregnant. Zika virus has been linked to the birth defect microcephaly.

The findings are published in the September issue of The Lancet Infections Diseases.