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Zika – a viral tale

Aedes aegypti or Yellow Fever mosquito

The Aedes aegypti or Yellow Fever mosquito is the sole vector for the transmission of chikungunya in the Americas at this time.

Zika has always been one of those overlooked viruses. In a large family, the Flaviviridae, its media savvy relatives, Yellow Fever, dengue and West Nile grabbed the headlines. There, in the background of all the family photos, was Zika.

People first noticed Zika at a monitoring station in the Zika Forest of Uganda in 1947. A rhesus monkey became ill, and testing for the usual viral illnesses proved negative. The doctors looked further and found Zika.

The Zika virus is carried from infected animal to infected animal by infected mosquitoes. The Aedes sisters, the female mosquitoes of a large group of related mosquito species, are to blame. The girls need blood for reproduction, and a Zika infection is a byproduct of their search for blood.

Zika attracted little attention in East Africa. A 2007 study in Gabon, West Africa, discovered that the strain of Zika in that country was distinct yet similar to the Ugandan virus. Two strains, now, East and West African.

About that same time, Zika began an extended visit to Asia and the Pacific Islands. A large outbreak on the island of Yap revealed a new strain, Asian Zika. That strain spread into Polynesia in 2014.

How did Zika arrive in Brazil? We don’t know. Most folks believe that the 2014 football World Cup brought someone with the illness to Brazil from Polynesia.

Let’s call him Poly, for short. He, or she, We don’t know. arrived in Brazil with the Zika virus circulating in their blood. That blood attracted the attention of more than one Aedes aegypti female mosquito. One lucky Lady Aedes bit Poly and ingested the infected blood.

Imported case of Zika

Viruses exist for one reason, to reproduce. They have no metabolism. Viruses like Zika don’t even have DNA. They are a strand of RNA, a manipulative strand of RNA just looking for some DNA to manipulate.

In Lady Aedes, Zika found cells to infect. It made its was through the cell wall We don’t know how, made its way through the contents of the cell to the nucleus We don’t know how and broke into that cellular structure where the DNA is kept We don’t know how.

In a very general way, RNA can program DNA. Zika took over that cell, and other mosquito cells, and programmed them to create more Zika. But Lady Aedes has an innate immunity that keeps viral illnesses like Zika at bay. Finally, all those Zika RNA viruses were in the mosquito’s salivary glands.

Lady Aedes likes her blood sipped. She drinks a little and moves on, often biting and drinking from several hosts before her reproductive cycle is over. It takes a few days for Zika to build up in the mosquito and then she can pass it on.

At some point, Lady Aedes bit a Brazilian. More than one, probably. And Zika found a new home.

Local transmission of zika

Let’s pick one of these Brazilians and call her Anna. Lady Aedes bit her and an unknown amount of Zika entered her blood. The hunt for cells to take over began, as Anna’s immune system identified the virus and began to hunt it down. It’s a complex interaction.

Based upon one study from the Zika outbreak on Yap in 2007, it is believed that about 75 percent of the island’s population was infected. Additionally, about 80 percent of those infected had no symptoms.

That is the only study on the spread of Zika in a large population, and it took place on an island. Do the same ratios hold true for a continental outbreak like we now have in the Americas? We don’t know.

Infected with no symptoms? Well, technically, if you have a virus in you, you are infected. The infection can resolve in several ways. Your immune system stomps the virus into little, bitty pieces and you never notice. Your immune system stomps the virus into little, bitty pieces and you experience some signs and symptoms of illness. Or, on occasion, the virus beats your immune system and you die.

Zika is the shy, retiring member of the Flaviviridae family. Tricky, like all its relatives, but if it produces any signs and symptoms of illness, they are mild. It won’t kill you under normal circumstances. It can, however, fight off the immune system long enough that an uninfected Lady Aedes can come along and bit you, and become infected.

Anna got infected. She was bitten by at least one Lady Aedes who got infected. That mosquito went on to bite and infect more humans. Pretty soon, May to October, there were lots of Zika viral infections in Brazil. How many? We don’t know.

Lady Aedes likes human blood. Some of her relatives will bite other animals, like that first rhesus monkey in Uganda. In Gabon, her pushy sister, Aedes albopictus, the Asian Tiger mosquito, was probably the vector for the illness. We don’t know which Aedes was the vector on Yap.

PAHO map showing countries with Zika mid February 2016

PAHO map showing countries reporting locally acquired Zika vira illnesses through mid February 2016

The Pan American Health Organization in monitoring the spread of Zika throughout the Americas. As of this writing, there have been 2,566 confirmed cases of Zika viral illness and 125,396 suspected cases. That doesn’t sound like many, but Zika is tricky.

Because this outbreak is the largest ever, public health authorities are discovering just how much we don’t know. Careful testing is needed to detect Zika and its sisters, the four dengue viruses, are often mistaken for it in false-positive tests.

Three Flaviviridae species are circulating in Brazil. The four dengue viruses, chikungunya and Zika are all carried and transmitted from person to person by Lady Aedes. Signs and symptoms can be similar. It is possible to have all three illnesses at the same time.

Zika’s tale is not done. The virus is still spreading. Lady Aedes is still biting. And what we don’t know remains much greater than what we do know.

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