Background

The outbreak of hantavirus was first reported in May on a cruise ship called the MV Hondius, which carried around 150 passengers that left Ushuaia in Argentina in April for the Canary Islands, making several stops along the way.

In total, 13 cases have been linked to the outbreak, three of which were fatal. However, World Health Organization (WHO) Director General Tedros Adhanom Ghebreyesus said last week that "The situation is stable for now. We continue to remain vigilant and in close contact with all relevant governments."

Meanwhile, an outbreak of Ebola was confirmed by health officials on May 15. So far, the outbreak has been linked to more than 900 cases and 220 deaths in the Democratic Republic of Congo (DRC), Uganda, and South Sudan. On May 16, WHO declared a public health emergency of international concern (PHEIC).

In its PHEIC, WHO noted there are several signs of "a potentially much larger outbreak than what is currently being detected and reported, with significant local and regional risk of spread."

Why these outbreaks are concerning experts

In both outbreaks, the respective viruses have been behaving out of the ordinary. Hantaviruses are generally carried by rodents and other animals and don't typically move person-to-person as they did on the MV Hondius.

"I don't know of any other cases reported on a cruise ship before," said Emily Abdoler, a doctor and clinical associate professor of medicine at the University of Michigan Medical School.

Meanwhile, the Ebola viruses involved in the most recent outbreak aren't as affected by the treatments and vaccines that scientists have developed for Ebola over the years.

Jens Kuhn, a virologist who serves on the International Committee on Taxonomy of Viruses, said the recent outbreaks show the major gaps in our understanding of what's called the virosphere, or the many millions of virus species that exist.

"These are case-use examples of why taxonomy is important," he said. "Is something the same, or is it different? Well, if it's different, then stuff we know about the other thing will not work on it."

Ebola viruses first got their name in 1976 from the location of one of the first documented outbreaks, the Ebola River, in what was then Zaire and is now the DRC. When scientists examined the blood of victims, they saw long, snakelike viruses distinct from any others previously known.

Then in 1976, another outbreak occurred hundreds of miles away in what was then Sudan, now South Sudan, that was also caused by snakelike viruses. But when scientists compared the viral genes, they found a significant number of differences. In later years, Ebola outbreaks occurred multiple times and in the majority of cases, the viruses either resembled the type first seen in Zaire or Sudan.

Eventually, Kuhn and his colleagues formally differentiated the two kinds of viruses: Orthoebolavirus zairense and Orthoebolavirus sudanense.

Then, in 2007, a new version of Ebola virus caused an outbreak in the Bundibugyo District of Uganda, infecting 149 people and killing 37. This species was over 30% different than the viruses isolated in Zaire and Sudan and was eventually named Orthoebolavirus bundibugyoense.

The outbreak on the MV Hondius was caused by a species called Orthohantavirus andesense, carried by rodents in South America. Specifically, the cruise ship outbreak was caused by the Andes virus, one of four strains of Orthohantavirus andesense.

Unlike the other three strains of the virus or the 37 other species of hantavirus, Andes virus can spread from person to person. "It seems like there are some mutations that under certain circumstances can make Andes virus person-to-person transmissible," Kuhn said, though no one knows what those mutations are.

Kuhn said he believes other strains related to Andes virus are lurking in rodents and share this same ability to spread from person to person. After the outbreak on the MV Hondius, Kuhn predicts that scientists in Argentina and Chile will "go into their freezers with all the samples and sequence the crap out of everything and figure out — what are all these things?"

As for Ebola, Kuhn said he expects more surprises, specifically calling out Orthoebolavirus taiense, also referred to as Taï Forest virus. This version of the virus has only ever been seen once — in 1994 when it infected a scientist who was dissecting a dead chimpanzee. The scientist developed Ebola symptoms but eventually recovered.

"I'm sure it's still out there, but nobody focuses on it because it caused only one case," Kuhn said. "I think that's a big mistake."

Going forward, Kuhn doesn't expect the public to remember the detailed taxonomy of every species of Ebola virus, but instead of referring to the current outbreak as the Ebola virus, he suggests calling it Bundibugyo virus.

"The moment you mix up Bundibugyo virus and Ebola virus, the impression will be, 'Oh, we've got something for that,'" he said. "But we don't."