2.6 billion people in Africa, Asia-Pacific may be at risk for Zika: researchers

TORONTO — Highly populated countries in Africa and the Asia-Pacific region may be vulnerable to outbreaks of Zika due to global travel patterns and climatic conditions that support the presence of mosquitoes capable of transmitting the virus, researchers say.

In a study published online Thursday in the journal Lancet Infectious Diseases, the researchers suggest India, China, the Philippines, Indonesia, Nigeria, Vietnam, Pakistan, and Bangladesh may be at the greatest risk of local outbreaks of Zika, which has become endemic in most of South America, Central America and the Caribbean.

“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,” said senior author Dr. Kamran Khan, an infectious disease specialist and scientist at St. Michael’s Hospital in Toronto.

“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,” he said.

“The impact on populations will also depend heavily on the country’s ability to diagnose and respond to a possible outbreak.”

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The authors, which include researchers from the London School of Hygiene & Tropical Medicine, Oxford University, and the University of Toronto, say that identifying where and when populations would be most susceptible to local transmission of Zika could help inform decisions about the use of finite public health resources.

To conduct the modelling study, researchers gathered data on airline ticket sales from 689 cities with one or more airports in the Zika-endemic region travelling to Africa or Asia-Pacific from December 2014 to November 2015.

The research team then looked at seasonal suitability for mosquito-borne transmission of Zika and areas where the virus-carrying mosquito Aedes aeqypti is among native species, as well as those where a second potential mosquito vector — Aedes albopictus — occurs.

The team mapped the monthly volume of travellers arriving into Africa and Asia-Pacific in order to identify countries at greatest risk of Zika importation across seasons. Health expenditure per capita was used as marker for a country’s capacity to detect and respond to a possible outbreak, Khan said.

Countries with large volumes of travellers arriving from Zika-affected areas of the Americas and large populations include India, with 1.2 billion people living in potential Zika transmission areas; China, with 242 million at-risk residents; Indonesia, 197 million; the Philippines, 70 million; and Thailand, 59 million.

“Warmer temperatures in the northern hemisphere (when mosquitoes are more active) increase the risk of new outbreaks appearing outside of the Americas,” said Khan.

“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.”

Zika virus has been linked to a number of neurological conditions in infected adults, including Guillain-Barre syndrome, which can cause temporary paralysis.

But the most serious fallout from Zika is the potential for birth defects among infants born to women infected during pregnancy. About 3,000 babies in Brazil and other countries of the Americas have been born with microcephaly, in which the infant’s head is undersized and the brain underdeveloped.

In countries unprepared for a possible Zika outbreak, and with limited health-care resources, the results of local transmission among their populations could be catastrophic, leading to cases of children with birth defects that could require life-long care, Khan said Thursday in an interview, noting that the virus can be transmitted both via mosquito bites and sexually from an infected partner.

Compounding the issue is that about 80 per cent of those infected experience either no symptoms or mild symptoms, making it difficult to identify and keep track of cases in any given population.  

Study co-author Dr. Isaac Bogoch, an infectious disease specialist at Toronto General Hospital, said the health consequences of Zika in Africa and Asia-Pacific will depend not only on local ability to diagnose and respond to a possible outbreak, but also on levels of existing immunity to Zika virus.

Even though Zika virus was first identified in Africa in 1947, and sporadic cases have been reported in both Africa and Asia-Pacific, little is known about whether the Asian strain of the virus now circulating in the Americas will affect individuals differently if they have previously been infected with the African strain.

Khan said it’s not known whether previously infected residents of those regions would have developed immunity to Zika.

In an accompanying commentary, Dr. Abraham Goorhuis and Prof. Martin Grobusch of the Center of Tropical Medicine and Travel Medicine at the University of Amsterdam write that a key prerequisite for large-scale epidemics is the presence of an “immunologically naive human population.”

“This is a big unknown in the high-risk countries identified, because urgently needed reliable seroprevalence studies are in short supply. (The authors) assume low levels of pre-existing immunity, which in their study translates into a worst-case scenario. This assumption might be true and therefore, vigilance is needed.

“A best-case scenario would be the existence of previously unknown widespread Zika-virus immunity in Asia and Africa … The development of protective herd immunity at a young age would also protect pregnant women against fetal complications. Under such a scenario, the risk of large Zika virus epidemics in Asia and Africa would be low, despite the introduction of the virus by infected travellers.”

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Sheryl Ubelacker, The Canadian Press