Once inside a cell, the ebola virus replicates rapidly until the cell bursts, flooding the bloodstream with new viruses that seek out yet more cells. (It prefers cells that line the blood vessels, which accounts for the grotesque bleeding from nose, eyes and gums.) A normal healthy immune system could marshal resistance in perhaps three weeks, but most ebola victims don’t have that kind of time.
There may be a way of buying that time. Scientists have now succeeded in stimulating the immune system of macaque monkeys so that they develop an immunity to the disease. This vaccine, reported this week in the British scientific journal Nature, may with a bit of luck eventually work in humans. “We’ve learned that it’s possible to protect against ebola by vaccinating an animal that’s quite [similar] to humans,” says coauthor Gary J. Nabel, a virologist at the National Institute of Health in Bethesda, Maryland. “Does that tell us it will work in humans? No, not yet, but it tells us we’re on the right track.”
An ebola vaccine is an especially tricky thing to develop because the virus is extremely virulent. Researchers had to go down to the Centers for Disease Control and Prevention in Atlanta to use their containment lab, which was built (at a cost of more than $1,000 per square foot) especially to accommodate the most dangerous pathogens. To prevent transmission, air is continually sucked out of the room and filtered just in case a speck of virus wafts up from a test tube. And the common virologist’s trick of using a killed or weakened virus was out of the question for ebola. Instead, researchers resorted to clever engineering. They took a bit of DNA from a harmless bacterium and modified it to produce the same molecule that ebola releases into the bloodstream–a long protein connected to a bit of sugar, called a glycoprotein. To give an added boost, they also grafted the glycoproteins onto a cold virus that had been stripped of its ability to reproduce. They administered this cocktail to the monkeys in gradually increasing doses until they began producing a high-enough level of immune response. Then came the ultimate test: they exposed eight monkeys to a live ebola virus. Within a week the entire control group was dead; the vaccinated four were still alive six months later.
The next step is to administer the vaccine in gradually increasing doses to humans to make sure it is safe and does stimulate an immune response. More- ambitious tests would determine if the vaccine truly protects again ebola. Scientists would pick a high-risk area and inoculate residents–most likely nurses and other hospital workers in central Africa–and wait for another outbreak. If all goes well, a human vaccine could be readied in five years. Some researchers even talk about a “hyperimmune serum” that would deliver antibodies to combat the disease after infection occurs, much like a tetanus shot.
Much, however, could go wrong: the vaccine might not work in humans or at high exposure to ebola; it might have adverse side effects or even prove toxic. For this reason, scientists are still trying to figure out one of the disease’s central mysteries: where it comes from. Most suspect it’s an animal, but which one? Monkeys and other primates, a common source of human infections, die quickly from ebola, which rules them out. Some bats survive inoculation, but none has been found carrying the virus in the wild. One theory holds that the prime source may be a plant virus. “It seems like there’s some very unconventional carrier,” says the CDC’s James LeDuc.
For now, the best weapon is the Boy Scout’s motto: be prepared. In Uganda, scientists and doctors are training hospital staff to take precautions against infection. They’ve even asked Ugandans to abandon their custom of washing their dead at funerals: big social gatherings are ground zero for an ebola epidemic. For a disease of such terrifying power, it’s best not to count on finding a magic bullet.
