The Ebola strain that ravaged West Africa last year is less virulent than the first one that appeared in 1976, researchers reported today.
The results of the test on monkeys by scientists at the National Institutes of Health are considered important because they suggest the virus that caused at least 11,000 deaths is not becoming more severe.
"In fact, the new study suggests the current virus has a decreased ability to cause disease in their animal model compared to the 1976 strain," the study said.
That one is known as the Mayinga strain, while the one that hit Liberia, Guinea and Sierra Leone last year is called the Makona strain.
In 1976, the Ebola outbreak was limited, killing 318 people in the Democratic Republic of Congo, the former Zaire. The deadliest case with this strain came in Uganda in 2000 with the death of 425 people.
These outbreaks had a weaker impact because they happened in rural, sparsely populated areas.
In the NIH experiment, two groups of monkeys were infected with one or the other of the Ebola strains.
In three days both were spreading the virus, and the ones with the Mayinga strain developed a rash on day four and became extremely ill on days five and six.
But the monkeys with the other strain did not get rashes before the sixth day, and severe symptoms did not appear until day seven.
Furthermore, liver damage, which is a typical result of Ebola infection, appeared in the Makona-strain-infected monkey two days later than the ones with the Mayinga strain.
And the immune systems of the animals with the weaker Makona strain produced around three times as much virus-fighting protein as those with the Mayinga strain did.
The virologists who conducted the study plan to do more research on how immune systems react to both strains of Ebola.
But they believe that at least seven days are needed after infection to mount an effective response.
This response does not seem to happen in monkeys infected with the Mayinga strain because the disease progresses too quickly.
Several phase 3 trials are underway in Africa with two vaccines against the Makona strain. They have shown promising results in terms of protection.
