Zika data point to sexual transmission in women

Brazil couple
Brazil couple

Warren Goldswain / iStock

Women in the sexually active age-group are overwhelmingly more likely than men to be infected with Zika virus, with sexual transmission the most likely cause, according to a new report based on data from the city of Rio de Janeiro from Brazilian researchers.

In other developments, scientists from Emory University today revealed that Zika can infect two types of placental cells, which might explain how the virus passes from mothers to their babies.

Threat to pregnant women

The team set out to see if there were any age-related patterns for Zika infections and to gauge the role of sexual transmission in disease incidence using data from the city's health secretariat. For comparison, they also looked at dengue levels by age for the same Zika-active period in 2015 and 2016, and for 2013 before Zika virus was thought to have reached Rio. They published their findings yesterday in bioRxiv.

Even after correcting for bias due to systematic testing for Zika virus in pregnant women, they found 90% more registered Zika cases per 100,000 population in women ages 15 to 65, defined as the sexually active age-group, compared with men of the same age.

To tease out whether the higher levels in women are linked to visiting doctors more often, they compared the incidence of dengue in men and women in 2015 and 2013. For both years, dengue infections were 30% more likely to be reported in women.

Markedly higher Zika levels in women support a significant role of sexual transmission in the spread of the virus, though other factors could help explain the higher incidence, the authors concluded. Two such examples are that women might be more likely to stay home, where they are exposed to the mosquito or pregnant women with febrile illnesses might see a doctor as soon as they have symptoms because of a fear of complications.

The researchers said the consequence of the higher incidence of Zika infections in childbearing-age women may be higher-than-expected numbers of babies born with birth defects than if the disease were only transmitted by mosquitoes.

Virus can replicate in placental cells

In findings that provide new clues on how Zika virus passes from infected mothers to developing fetuses, researchers from Emory University identified two types of placental cells that might be involved. They published their report today in Cell Host and Microbe.

To determine the cell types, they infected different cell types from five donated full-term human placentas with the Zika strain currently circulating in the Caribbean.

They detected infection in Hofbauer cells, which have direct access to fetal blood vessels. Hofbauer cells are placental macrophages that originate from connective tissue stem cells of the developing fetus.

Senior author Mehul Suthar, PhD, assistant professor of pediatrics at Emory University School of Medicine, said in a Cell Press media release that another group had earlier detected viral antigen in Hofbauer cells in the placental tissue of a fetus who died from a Zika virus infection.

The Emory team also found infection in cytotrophoblasts—cells found in the middle layer of the placenta—but to a lesser extent. Their findings suggest that the virus can cross the outermost placental layer, made up of cells known as syncytiotrophoblasts, and target other cells, such as Hofbauer cells and cytotrophoblasts, where it can replicate.

Suthar said one interesting findings was that placental cells from the five different donors showed different viral replication levels over time, suggesting that host factors might play a role in immune response to Zika virus. "What our study suggests is not everyone is predisposed to having the virus replicate in the placenta, but the full meaning of this needs to be explored further," he said.

Totals grow in US travelers, pregnant women

In separate updates yesterday the US Centers for Disease Control and Prevention (CDC) reported more Zika infections in US travelers, residents of US territories where local transmission is occurring, and pregnant women.

Affected territories reported 103 more Zika illnesses, most of them in Puerto Rico, raising the total to 935 local infections. So far 5 related Guillain-Barre syndrome (GBS) cases have been reported.

On the mainland, US states reported 47 more Zika cases, lifting that total to 591. States have confirmed 11 sexually transmitted Zika cases, along with 1 GBS case.

Meanwhile, Zika pregnancy registries on the US mainland and Puerto Rico as of May 19 reported 31 more infections, raising the total to 310, the CDC said yesterday. Of the new total, 168 involve US women on the mainland and 142 from US territories.

Last week the CDC announced a change in the way it reports the number of pregnant women with Zika virus, which takes into account those with lab-confirmed asymptomatic infections. According to last week's total, the CDC was monitoring 279 cases.

Other developments

  • The only US distributor of traps to collect Aedes mosquitoes, which can carry Zika, has a backlog of 1,950 orders from governments and businesses, the Associated Press (AP) reported yesterday. But a representative of the US distributor, BioQuip Products, Inc., said she anticipates a shipment of 1,500 traps in the next few days that should ease much of the backlog of the German-made traps. Florida reported it has received only about 120 of the 310 traps it has ordered.

  • The Republican-led US House has moved to officially begin Zika-funding talks with the Senate but then immediately left Washington, D.C., for about a 2-week recess, according to a separate AP report today. The House passed a bill for $622 million after the Senate had passed one for $1.1 billion. Three months ago President Obama requested $1.9 billion.

  • Zika cases in Central and South America continue to decline, but are rising in most Caribbean countries and territories, the Pan American Health Organization (PAHO) said yesterday in its weekly update. Little has changed in terms of areas newly affected by the virus or its complications, but PAHO provided a glimpse of different epidemiologic patterns in different countries, noting that Brazil's Pernambuco state might see another microcephaly uptick in September and October, based on a Zika spike it experienced in the middle of February.

See also:

May 26 bioRxiv abstract

May 27 Cell Host Microbe abstract

May 28 Cell Press media release

May 25 CDC number of cases in the United States and territories

May 26 CDC number of Zika cases in pregnant women

This week's top reads