One way to figure out how anthrax spreads in the wild is to
build electrified cages around zebras that have died in a Namibian
Anthrax is a nasty disease caused by the bacteria B.
anthracis. It infects animals as they graze, and they in
turn spread it as they are eaten either by predators or scavengers.
rate varies depending on how the bacterial spores are absorbed,
from 20 percent for contact with skin to as high as 75 percent if
B. anthracis can live in the soil for decades,
and it has historically caused hundreds of thousands of human
deaths. That number has dropped considerably over the last century
thanks to comprehensive vaccination programmes (and the discovery
of effective antiobiotics), but it still costs a lot of money each
year to keep B. anthracis out of the food chain.
In wild environments such as those found in national parks, a
lot of time and effort goes towards preventing scavengers from
eating animals that have died from anthrax. Most of those
scavengers -- such as jackals, hyenas and vultures -- have evolved
immunities to B. anthracis, so they don't get sick from
eating infected meat -- but the thinking is that, by ripping open
dead animals, the B. anthracis spores are given a chance
to escape back into the soil and infect even more animals that
might graze there later. Just one of many reasons scavengers get a
Stopping the scavengers this way is very time-consuming, though
-- just think of how big many parks can be, and how difficult it
can be to track down every dead animal, and test each one -- so a
team of ecologists from the
University of Texas at Austin wondered if it was an entirely
effective method for dealing with the problem. It turns out that
the ripped-open-animals-spreading-anthrax hypothesis isn't quite
The study's lead author, Steven Bellan, said: "The
hypothesis is that when a carcass is intact, the anthrax bacteria
are forced into a kind of death match with putrefying bacteria from
the gastrointestinal tract. But when the body is opened to the air,
either by a scavenger or the haemorrhaging from all
bodily orifices that occurs at death, the anthrax bacteria can
escape that competition and more successfully produce spores."
To test the hypothesis, the team found seven zebras (and one
wildebeest) dotted around Etosha National
Park in northern Namibia that had died from anthrax infection.
Cages (with electric fencing) were erected around four of them,
leaving them just as exposed to the elements but safe from being
nibbled by any scavengers. The unprotected animals were left to be
torn apart by scavenging beasts.
Regular samples taken were from the soils around each animal to
see if there was a bigger rise in the number of anthrax spores near
to the scavenged animals. The results showed that
"soil stained by terminally haemorrhaged blood and by
non-haemorrhagic fluids exhibited high levels
of B. anthracis spore
contamination (ranging from 10^3-10^8 spores per gram)
even in the absence of vertebrate scavengers" -- that means
"scavenging by vertebrates is not a critical factor in the
life cycle of B. anthracis and
that anthrax control measures relying on deterrence or exclusion of
vertebrate scavengers to prevent sporulation are unlikely to be
Bellan said: "It appears that the anthrax bacteria can
survive for some time in the carcass even though it may be
competing with other bacteria. It also appears that fluids can
escape from the carcass into the soil via mechanisms other than
scavenging or through haemorrhages occurring at the time of death."
Bloating (caused by gases building up), and maggots, are two of the
other things that can lead to skin leaking.
However, it's only a small study in one part of the world, so
its findings will have be further investigated before anthrax
control policies are altered. Maybe, though, we should ease up on
our general distaste for scavengers. They're not entirely
The study has been published in Applied and Environmental Microbiology.