An international team of disease detectives is now racing to connect with the more than two dozen passengers who disembarked the MV Honius cruise ship on the Atlantic island of St. Helena before the hantavirus outbreak was identified.
These individuals have flown across the world, including to the United States.
The risk of further spread of this virus is low since it requires close and prolonged contact with an infected individual — and those infected seem to transmit the virus for only a brief period of time. But public health officials want to make sure the outbreak is contained.
Here's how authorities are using the practice of contact tracing to contain the outbreak and keep the hantavirus from spreading.
Contact tracing 101
The concept of modern contact tracing dates to the 1930s and was part of an effort to stop the spread of syphilis. It involves locating the close contacts of anyone who may have been infected. "By identifying people who are at risk of infection," says Preeti Malani, an infectious disease physician at the University of Michigan, "you try to get ahead when people don't have symptoms yet with the goal of preventing the infection from continuing to propagate."
This is a well-tested approach for containing an infectious disease. "It's the oldest tool in the epidemiologic toolbox," explains Malani. "We thought about this a lot early in the pandemic with COVID. But we also do contact tracing for sexually transmitted infections, for things like meningitis and even measles."
Malani likens contact tracing to monitoring ripples in a pond, "trying to prevent those outer rings from propagating by isolating individuals and by identifying individuals who might be at risk of infection."
The idea that "there's a time period where people don't have symptoms but could be harboring the virus, that's what contact tracing helps identify," says Malani.
It starts by pinpointing someone with an infection or suspected infection of the disease in question — in this case, hantavirus. Epidemiologists then look to see with whom they've recently had close contact since these individuals are more likely to have been infected.
This hunt for those with the greatest probability of infection is important. "Otherwise, it becomes an impossible web to contain because everyone is connected to everyone," says Boghuma Titanji, an infectious diseases doctor at Emory University. "So you have to stratify by high, intermediate and low-risk contacts."
The next step involves public health agencies ordering precautions for those who are infected or who may be infected but aren't showing symptoms yet. Such measures may include quarantine, so that an individual doesn't come into contact with even more people — who may then become infected.
One challenge that hantavirus presents is that its incubation period can last up to several weeks. In other words, "people take a long time to become symptomatic after they've been exposed," says Titanji. "Some of these primary contacts would have to be monitoring themselves for symptoms for up to 45 days to be at the tail end of that very long incubation period."
Aboard and ashore
The work isn't high-tech but it is painstaking, requiring officials to reconstruct the many interactions someone may have had over days or weeks.
Onboard the cruise ship, "you might have an individual who is a source of an infection," says Titanji, laying out a hypothetical example. "And then they were sitting at a dinner table with one individual who then goes back to their cabin and shares a bed with their partner who has a conversation with someone else on the deck."
Once someone disembarks the ship, the number of potential interactions can grow quite quickly. This is why officials were concerned when a KLM flight attendant fell ill after being aboard a flight with one of the infected cruise ship passengers. Fortunately, the flight attendant ultimately tested negative for hantavirus.
Titanji is heartened by what she's seen playing out so far. "It seems like the international collaborative effort has been really robust and the mechanisms for containment are in place and underway," she says.
Public health officials argue that contact tracing is a powerful approach that will reduce further spread. "We can break this chain of transmission," said Abdi Mahmoud, the director of the World Health Organization's health emergency alert and response efforts, at a press conference on Thursday.
He has good reason to be confident. Contact tracing was vital during the fight against COVID-19 and helped end the Ebola crisis in Liberia, containing the epidemic there more than a decade ago. Some of the contact tracing even involved hours-long hikes through the jungle to a remote village.
Authorities are hoping for similar success with this hantavirus outbreak.
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