Zika virus: its effects, how it is spread, and the possible threat to pregnant women

The disease is believed to have infected 1.5 million people since its outbreak in Brazil and dramatic spread around the world

Scientists are warning people living in tropical climates, and especially pregnant women, about a new and alarming virus that once only affected subtropical forests in Africa. Called Zika, public health officials in the US believe it could be an emerging health threat.

What is Zika?

Until recently, Zika was a rare tropical disease associated with mild symptoms. It takes its name from the Zika forest in Uganda where it was first identified by scientists in the 1940s. It is spread by mosquitoes and so is called an “arbovirus”, in the same family as West Nile and dengue fever.

Only a handful of Zika cases had ever been documented before 2013. But scientists began sounding the alarm after multiple outbreaks were discovered in Pacific islands and south-east Asia. Since then, Zika has spread to Brazil, where scientists estimate as many as 1.5 million people could be infected. 

The large-scale outbreak in Brazil led virologists to link the virus to a birth defect called microcephaly, the medical term for abnormally small heads, for the first time. The World Health Organization said no deaths attributable to Zika have been reported. Still, little is known about the virus, which has spread across the world in just over a couple of years. There is little published research on the disease, and it is clinically difficult to diagnose. 

What is microcephaly?

Microcephaly is a serious and uncommon birth defect that doctors believe is caused by below-normal brain development in Utero.  Microcephaly causes lifelong physical and developmental problems for babies born with it. Problems can be wide-ranging, from seizures, to difficulty walking and learning, to hearing loss and vision difficulties. 

In the United States, microcephaly affects between two and 12 babies for every 10,000 live births. It is usually associated with women who have certain infections while pregnant, such as toxoplasmosis, or use alcohol during pregnancy.

How did microcephaly come to be linked with Zika?

In Brazil, as thousands became infected with Zika, the rate of microcephaly increased exponentially. In 2013 and 2014, the country documented 167 and 147 cases of microcephaly respectively. But by 2015 as Zika spread, Brazilian officials registered 2,782 cases before the end of the year, according to the New York Times. That’s a 1,792% increase year on year. Virologists who studied Zika in Brazil said they have “a lot of indirect evidence” that connects the virus to microcephaly. 

The US Centers for Disease Control and Prevention has said there is no direct link yet between the virus and the birth defect. Nevertheless, the CDC warned pregnant women to avoid travel to countries where Zika is present, and unofficially some Brazilian health authorities have warned women against getting pregnant.

How do you get Zika?

Zika is spread by the Aedes genus of mosquito, which transmits the disease when it bites a person infected with Zika, then bites another host. The two most common mosquitoes to transmit the disease are the Aedes aegypti and Aedes albopictus species. 

Both are invasive species with worldwide distributions, including in the American south and south-west. It is believed that Aedes albopictus, also called the Asian tiger mosquito, was imported to the United States in the mid-1980s in used tires from Asia.

What are Zika’s symptoms?

Usually, Zika’s symptoms include fever, red and bumpy rash, joint pain and pink eye. Hospitalization rates are low and the World Health Organization said no deaths have been reported from Zika. 

However, a “high rate” of patients with Zika have no symptoms at all, according to the WHO. This is concerning to virologists, who believe that Zika could easily establish a foothold in the southern United States and other areas with tropical climates because people may not know they are infected.

Is Zika in the United States?

Yes. Most recently, officials in Harris County, Texas, found a middle-aged woman contracted the disease in El Salvador before returning to Texas. Three more cases were reported in Florida, two in Illinois and one in Hawaii. 

In Puerto Rico, an American territory, locally transmitted cases have been reported, meaning that at least some mosquitoes on the island became infected with the virus.

Why is it spreading so quickly?

Virologists studying the spread of subtropical diseases say it’s unclear why the virus, which was recently rare, spread around the world in just a couple of years. Most believe increased international travel is at least partly responsible, but others theorize that increasingly warm global temperatures resulting from climate change could play a role.