The World Health Organization on Tuesday declared the official end of the latest outbreak of Ebola in the Democratic Republic of the Congo, which tallied 54 cases, including 33 deaths. But a study published this week in the Lancet Infectious Diseases highlights just how tenuous such declarations can be when it comes to Ebola.

The study, led by researchers at the Centers for Disease Control and Prevention, traced a puzzling cluster of Ebola cases in Liberia to a mother who appeared to have had a stealthy, undiagnosed infection for more than a year. The cluster came to light when the woman’s 15-year-old son arrived at a hospital in Monrovia on November 17, 2015—more than two months after the country had been declared free of the disease in the wake of a massive outbreak that began in 2013. The woman’s husband and another of their four sons, an eight-year-old, subsequently tested positive for the virus.

Based on extensive epidemiological tracking, blood work, and viral genetic data, the researchers concluded that the woman likely picked up an Ebola infection in July of 2014. At the time, she was pregnant and caring for her brother, a nurse’s aide at a local clinic, who had come down with an illness after helping to treat an undiagnosed patient with symptoms consistent with Ebola. Her brother’s supervisor, wife, and seven-year-old son also fell ill.

Though the woman’s brother died of his infection, he was never tested for Ebola. The other three (the brother’s supervisor, wife, and son), were tested, though, and were all positive. Luckily, they all survived, and their blood samples were conveniently stored for future reference.

Meanwhile, the woman fell very ill, too, and suffered a miscarriage in August of 2014. But she recovered, despite not receiving treatment. She was soon pregnant again and gave birth to a healthy baby boy in September 2015 with no complications.

In October, she started feeling unwell again, complaining of fatigue, swelling, and trouble breathing. She went to a local hospital and was treated for anemia and malaria—there was no suspicion of Ebola at that point, and the country had recently been declared free of the dreaded disease.

But in November, her 15-year-old son fell ill with Ebola, kicking off the cluster. He died from the disease on November 23. Her husband and her eight-year-old tested positive for the virus next. They survived. She and her new baby, meanwhile, tested negative and had plenty of protective antibodies against the virus, according to blood test results. Her remaining son, the five-year-old, escaped the virus.

Persistent plague

The CDC researchers scoured their contacts and history to figure out how the family had come in contact with Ebola. They homed in on the woman’s contact with her sick brother back in 2014. Reaching back into the stored blood samples, the researchers discovered that the genetic sequence of the Ebola virus that infected her brother’s supervisor in July of 2014 was a genetic match to the viral strain that infected her 15-year-old more than a year later.

The researchers speculate that the woman survived an initial Ebola infection after caring for her brother but that the virus merely went into some mysterious, latent stage afterward. Then, after the birth of her baby in 2015, the virus flared. This, they suggest, makes sense because pregnancy dampens maternal immune responses—to avoid attacking the baby—and the immune system reignites shortly after delivery. This burst in inflammatory responses may have sparked the dormant Ebola into a symptomatic disease, which then spread to her husband and children.

Such a scenario isn’t unheard of for Ebola. Earlier studies have found that the virus can lurk in men’s semen for up to 18 months after a victim recovers. It has also been spotted in the vaginal fluids and breast milk of asymptomatic women.

In this case, researchers couldn’t figure out where Ebola was hiding in the woman's body or how exactly it spread to her family. But they speculate that the antibodies she built up to fight the virus were transferred to her baby through her breast milk, protecting him.

While researchers say such poorly understood, stealthy cases are rare, they still “highlight the risk of Ebola virus disease flare-ups even after an outbreak is declared over.” They urge continued vigilance and surveillance.

The recommendation is particularly relevant, not just in the wake of the latest outbreak in the DRC, but also amid the recent boom in Ebola survivors. The 2013-2016 Ebola outbreak in west Africa (in which the woman was infected) was monstrous. More than 28,000 people were infected, leaving 11,000 dead—and more than 17,000 survivors. That number of survivors exceeds that of all other Ebola outbreaks combined since the disease was first discovered in 1976.

Health experts now face the challenge of monitoring survivors for flare ups, but also making sure not to add to any stigma around survivors, who are often shunned by their communities in fear that they’re still contagious.

The Lancet Infectious Diseases, 2018. DOI: 10.1016/S1473-3099(18)30417-1 (About DOIs).