One mosquito against the world
New Delhi: What is the common thread between the Zika outbreak that is causing babies to be born with abnormal heads in several South American countries, the urban yellow fever outbreak in a few African nations and the annual spurt of dengue cases in India and several other Asian countries?
The answer—the Aedes mosquito.
Forty-seven countries are grappling with a Zika virus outbreak which began in 2015, and is now linked to microcephaly or abnormally small heads in newborns and other neurological complications such as Guillan Barre syndrome. Given that Brazil is one of the worst affected countries, this also puts a shadow on the upcoming Olympic Games in Rio.
The World Health Organization (WHO) declared the cluster of microcephaly cases linked to Zika a “public health emergency of international concern” in February. Till now, Brazil has reported 1,616 cases and other countries 39.
Meanwhile, Angola and Democratic Republic of Congo are facing an outbreak of yellow fever with 4,400 suspected cases and 422 reported cases. A few other African and South American countries are facing a separate yellow fever outbreak.
All these outbreaks can be traced back to the same mosquito.
Tracing the Aedes journey
The Aedes genus of mosquitoes, or more specifically Aedes aegypti and Aedes albopictus, has established its presence in all continents except Antarctica over the years. The global spread of the vector, facilitated by trade, has been accompanied by deadly outbreaks and epidemics.
The Zika virus was first isolated in 1947 from a monkey in the Zika forest of Uganda. For decades, the disease, transmitted by the Aedes genus of mosquito, affected mainly monkeys. In humans, Zika sporadically caused a mild disease that aroused little concern. That is until the latest outbreak in South America.
John McNeill, author of Mosquito Empires, explains that genetic analysis has confirmed that Aedes aegypti came to the Americas from Africa. “The likeliest hypothesis is that it crossed the Atlantic with one or several of the ships of the slave trade in the 16th or early 17th century,” McNeill wrote in an email.
McNeill, who is also a professor at Georgetown University, explained that the role of Aedes aegypti in yellow fever was hypothesized by a Cuban doctor, Carlos Finlay, in the 1880s and confirmed by American medical researchers in 1900 (in Cuba).
A study published last year in peer-reviewed journal eLife, looking at the global distribution of the mosquito, said that Aedes aegypti originated in Africa, but due to harsh conditions and the beginning of slave trade, got introduced into the New World, from where it subsequently spread globally to tropical and sub-tropical regions of the world.
Meanwhile, Aedes albopictus, originally a zoophilic forest species from Asia, spread to islands in the Indian and Pacific oceans during the 1980s; it rapidly expanded its range to Europe, the US and Brazil. Around 1986 it was clear that the mosquito was widely present in the US and Brazil, and it was concluded that it had been introduced to both countries in shipments of used tyres.
The US Aedes albopictus originated in Japan and reached the country through used tyres.
While the Aedes aegypti or the yellow fever mosquito, is known as a sneaky biter which mostly occupies urban areas, Aedes albopictus or the Asian tiger mosquito is a more aggressive biter that is associated with forests and thickets.
But both mosquitoes have a preference for feeding on human blood and hence are efficient vectors of diseases.
“Albopictus is much more capable of getting into wider areas as it can breed even in lower or sub-tropical temperatures. So initially these were just present in areas with tropical climates, but in subtropics also these mosquitoes can grow. In most parts of India we have aegypti, but in coastal regions we have albopictus,” said G. Arunkumar, professor and head of the virus research department at Manipal University.
Aedes and India
A study led by the University of Oxford in April noted that large parts of India have an ideal environment for a Zika virus outbreak. Global tropical and subtropical regions inhabited by over 2.7 billion people are at high risk of Zika.
One of the reasons is that both Aedes aegypti and Aedes albopictus are found in India and diseases such as dengue, which are caused by the Aedes aegypti mosquitoes, emerge every year in the country. This is why scientists are trying to understand how Zika has not yet been detected in India.
“We have all conditions conducive for the transmission, yet the virus is not here. This is a major outbreak but not a lot of people are travelling from South America to India. Also, an outbreak will not immediately start; it will take time for the virus to be introduced,” said Arunkumar.
At the end of February, in addition to the two diagnostic facilities at the National Centre for Disease Control and the National Institute of Virology, Pune, 10 new facilities became operational, according to the health ministry.
“We have to increase our surveillance. We have labs set up so that people coming from affected countries to airports are tested if they have fever or other symptoms. But with Zika, there won’t always be symptoms, so just screening the airports is not enough. If the virus has not yet been detected in India, it does not mean it will not be found here,” said Arunkumar.