Ebola die-off: gorilla losses tallied in central Africa.

Between 2001 and 2005, Ebola virus ravaged the gorilla population in a remote section of equatorial Africa. A new analysis suggests that this outbreak, which killed 254 people, also claimed more than 5,500 western-lowland gorillas.

Genetic characteristics of the virus that emerged in the Republic of the Congo--the smaller of the two nations called Congo--indicate that it arose from an earlier outbreak that killed people in adjacent Gabon, says Peter D. Walsh of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. However, the carrier of the Ebola virus is unknown.

Ebola virus is highly contagious and causes hemorrhagic fever that almost always results in death. The virus infects people, gorillas, chimpanzees, and small antelope called duikers.

Although counting dead animals in the African bush is a practical impossibility, researchers in the Congo were well positioned to quantify the recent outbreak. In 1995, Magdalena Bermejo of the University of Barcelona and other researchers began monitoring gorilla groups in a game preserve just south of the Congo's Odzala National Park. That monitoring familiarized the gorillas with the presence of people, so the scientists had access to the animals.

Ebola infected people along the Congo-Gabon border in 2001, and a rash of gorilla carcasses turned up in and around the preserve starting in mid-2002. Tests indicated that most had died of Ebola. By January 2003,130 of 143 gorillas that Bermejo and her team were monitoring had died.

To extrapolate that toll to gorillas elsewhere in the area, the scientists counted the nests that each gorilla makes every night. Bermejo's team walked transects--straight lines through the bush--and tallied fresh gorilla nests encountered in infected and uninfected areas in and near the preserve.

The scientists estimated conservatively that before the outbreak, there were 2.2 gorillas per square kilometer. When they applied the death rate of 90 percent seen earlier in the monitored groups, they calculated a death toll of 5,500 gorillas in the infected areas in and around the preserve. Their report appears in the Dec. 8 Science.

"This analysis is very thorough," says William B. Karesh, a veterinarian at the Wildlife Conservation Society in New York City. "It demonstrates what a lot of people had been saying, but nobody had sat down and really crunched the numbers."

The die-off could be even more severe, says Walsh, a study coauthor. Odzala Park is far larger than the studied preserve and is home to 20 to 30 percent of the world's 100,000 lowland gorillas. Ebola's carnage there is still largely undocumented.

The devastation from Ebola in the Republic of the Congo arose in part because the lush habitat in the preserve and in Odzala Park supports a dense population of apes.

Vaccines against Ebola have been tested in monkeys (SN: 7/16/05, p. 45). Walsh predicts that researchers will someday vaccinate apes in the wild to stall outbreaks in gorillas and limit Ebola's spread from apes to people.