But a new study has found potential early markers of diseases similar to Ebola, suggesting it may be possible to test for these viruses much earlier.
In the study, researchers infected monkeys with either Lassa virus or Marburg virus, which is a "cousin" of Ebola virus. Like Ebola, both Lassa and Marburg can cause hemorrhagic fevers, meaning fever accompanied by damage to the blood vessels, which can result in bleeding.
To find the early signs of infection, the researchers looked for clues that the body's immune cells were responding to the virus — rather than searching for the viruses themselves. Specifically, the scientists looked for certain patterns of gene expression (whether certain genes are "turned on" or not).
They found gene expression "signatures" that were distinct enough to distinguish Marburg virus infection from Lassa virus infection, before the animals showed any symptoms of either disease.
"It looks like there are some very early and distinct ways in which the immune system is responding to different diseases," said study researcher John Connor, an associate professor of microbiology at Boston University School of Medicine. "That could be an important way of trying to find people that are infected before they show overt symptoms," Connor told Live Science.
However, the researchers cautioned that the findings are preliminary, and more work is needed to see if the method can be translated into a practical test for use in people, Connor said. Additional work is also needed to look specifically for gene expression signatures of immune cells that are reacting to an Ebola infection.
One challenge in developing a screening test for Marburg, Lassa or Ebola is that the test will need to be very specific, meaning it will need to have a low number of false positives.
"These are the types of diagnoses that you don't want to get wrong," Connor said. "If you think somebody is infected with one of these diseases, there are fairly significant consequences" for them, such as being quarantined, Connor said.
Current tests for Ebola, Marburg and Lassa look for the viruses in the blood. But these viruses first infect internal organs, such as the spleen, so the blood tests do not detect the viruses until after they spread to the bloodstream, which takes some time. For Ebola, it can take between two and 21 days before a person infected with the virus shows symptoms.
"We hope that our study will help in the development of better diagnostics, especially during the early stages of disease, when treatments have a greater chance of being effective," study researcher Nacho Caballero, a doctoral candidate at Boston University School of Medicine, said in a statement.
The study was published Nov. 6 in the journal BMC Genomics.