‘Why Nigeria, India top list of countries at greatest risk of Zika virus

Nigeria has emerged as one of the countries at greater risk of Zika virus transmission. According to a new study published yesterday in The Lancet Infectious Diseases, 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 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 (Aedes aegyptica), suitable climatic conditions, large populations and limited health resources.

Earlier studies by the Nigerian Institute of Medical Research (NIMR) Yaba, Lagos, found that Aedes aegyptica mosquitoes, that transmit the Zika virus, are common in most parts of the country.

The authors said 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.

Author of the study, Dr. Kamran Khan of St Michael’s Hospital, Toronto, Canada, said: “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.

“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 high 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.”

In this study, the research team, which included scientists from the London School of Hygiene and 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 (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 Aedes aeqypti and 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.

According to the study, 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.