The gorilla Ebola toll

The title of the one-page paper by Magdalena Bermejo and colleagues tells most of the story: "Ebola oubreak killed 5000 gorillas."

Over the past decade, the Zaire strain of Ebola virus (ZEBOV) has emerged repeatedly in Gabon and Congo. During each human outbreak, carcasses of western gorillas (Gorilla gorilla) and chimpanzees (Pan troglodytes) have been found in neighboring forests (1). Opinions have differed as to the conservation implications. Were these isolated mortality events of limited impact (2)? Was ZEBOV even the cause (3)? Or, were they part of a massive die-off that threatens the very survival of these species (4)? Here, we report observations made at the Lossi Sanctuary in northwest Republic of Congo, where ZEBOV was the confirmed cause of ape die-offs in 2002 and 2003 (5). Our results strongly support the massive die-off scenario, with gorilla mortality rates of 90 to 95% indicated both by observations on 238 gorillas in known social groups and by nest surveys covering almost 5000 km2. ZEBOV killed about 5000 gorillas in our study area alone.

Gorillas aren't alone; the authors estimate that ca. 85 percent of chimpanzees also died, but don't have detailed observations on their initial numbers, so they can't give a census number.

The paper doesn't answer an important question -- how do these apes get Ebola? There is a Timesarticle that tries to address this:

Precisely how gorillas contract the disease is a mystery. Scientists assume they must catch it somehow from another animal that acts as a natural reservoir host and carries the virus without being harmed by it. Fruit-eating bats are suspected, but none has been confirmed as the reservoir, Dr. Nichol said.
Whatever the host, it could infect western gorillas by defecating on their food, which is mostly fruit. The gorillas could then infect one another, both inside their own social groups and between groups. The virus is spread by bodily fluids and by touching sick or dead animals. Dr. Walsh said that gorillas commonly eat one another's dung, which could also transmit the virus.
Scientists have debated about whether gorillas' infecting each other plays much of a role in spreading the disease, or whether the reservoir host is really the main culprit. The new report says the gorillas themselves do play an important part: it shows that the spread of the disease and the timing of outbreaks match the pattern that would occur if the animals were infecting one another, both within and between groups.

It's hard for me to imagine it isn't spreading directly from individual to individual. Still, it seems to be jumping between groups very quickly -- certainly faster than individuals transfer between social groups. Which suggests it may spread more easily between these apes than between humans. Which is scary, since it suggests it might evolve to spread more readily between people than it does now.



Bermajo M, Rodríguez-Teijeiro JD, Illera G, Barroso A, Vilà C, Walsh PD. 2006. Ebola oubreak killed 5000 gorillas. Science 314:1564. DOI link