Zebras attracted to lethal anthrax grass

Ecologist Wendy Turner of the University of Oslo has come across a fatal attraction for zebras in Namibia. Areas of tempting turf containing anthrax bacteria lure zebras and wildebeest to their peril.

When visiting a spot where a zebra had died the previous year of anthrax infection she saw that it was greener than the adjacent vegetation. Lush grass sprouted up from the ground around the remains of the carcass. Zebras were taking advantage of these spots and preferring them for grazing. But anthrax bacteria from the carcasses also survive in the soil and vegetation at such spots, so scientists fear such greener spots can increase the spread the infection. The anthrax bacterium, Bacillus anthracis, is primarily an animal disease.

Turner and her team of researchers have monitored the grazing activities of zebras in hope of predicting future outbreaks of anthrax, writes LiveScience.

Biological weapon

The dreaded anthrax bacterium can cause symptoms ranging from itchy sores on the skin to pulmonary problems and fever. It can often be fatal. About 95 percent of known anthrax infections in humans are passed to them through skin contact with livestock.

Anthrax spores have also been used as a biological weapon by terrorists. Several years ago American mail and parcel delivery workers died after coming in contact with post containing anthrax.

Grazing herbivores, however, are infected by ingesting the microbe. Anthrax bacteria are so widespread in Namibia that the microbe is considered part of the Etosha National Park’s natural ecosystem. Most of the anthrax cases in the park are found among zebras. The disease can kill grazing herbivores within a few days after the unfortunate animal consumes a deadly dose. 

Grass is always greener…

The spots where dead zebra remains have been found to support greener grass. When Wendy Turner visited spots in 2007 where she had noted that zebras had died of anthrax the previous year, she saw extra lush grass growing up in what otherwise was a sea of short, dry grasses, she reported.

Soil that had been fertilised by carcasses is richer in nutrients such as phosphorus and nitrogen. The researchers found that the grass growing in the immediate vicinity of such remains was also richer in nutrients.

Significantly, they found anthrax spores on the grass up to two years after an infected zebra had died.

This got Turner wondering whether such spots where the animals succumbed to the pathogens would attract new grazing animals, or whether the beasts would be instinctively frightened off in some way by the bones of their kindred species.

The researchers analysed 35 spots which tested positive for anthrax and eight that did not. They mounted motion-triggered cameras at 13 of the sites with anthrax carcasses and ditto at 13 spots devoid of such skeletal remains.

Turner stresses that she and her team were diligent about using protective gear when conducting their research, both in the field and in the laboratory.

Herbivores feast in infected spots

The zebras and springboks (a small type of gazelle) grazed up to four times as often at potentially contagious sites where zebras had died the previous year than they did in areas where no such deaths had occurred.

Anthrax prevents blood from clotting and prior to death blood spouts from the victim’s orifices. So the researchers suspect that because of this flow, the anthrax-ridden carcasses might release more nutrients into the soil than the remnants of animals that died in other ways. This would make these spots more mouth-watering to herbivores.

The researchers are now trying to ascertain the factors that impact the outbreaks of anthrax infections and their intensity. They would like to know if new outbreaks could be predictable.

Two years ago a Danish drug addict died from injecting anthrax-contaminated heroin. Norwegian heroin addicts are more prone to injecting the drug rather than smoking it than their Danish counterparts, and the that tragedy led to cautionary warnings to addicts from Norwegian authorities.

“In Namibia the farmers let their livestock share grazing land with wild animals, so there is much less separation between tame and wild animals than in other countries,” says Wendy Turner.

Despite that, anthrax is not a common problem among Africans in the region. When it does infect humans, it is usually because they consumed animals that died of the disease, she explains.

Where they wander

“Up to now, my research has been narrowly focused on the carcass sites because no data had been obtained on how these places could contribute to the spread of anthrax with the passage of time,” says Turner.

The next step will be to scale up from these spots where animals died to study more of the landscape, to see the migration patterns of the animals, long-term trends in weather conditions and anthrax mortality in these areas.

On the long term, Turner also wants to initiate parallel studies in the USA to see if North American herbivores are also attracted to such spots where animals died.

“This will increase our understanding of the transmission of pathogens which can survive in the environment for extensive periods,” she explains.

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Read the Norwegian version of this article at forskning.no

Translated by: Glenn Ostling

External links

• Wendy C. Turner et. al.: Fatal attraction: vegetation responses to nutrient inputs attract herbivores to infectious anthrax carcass sites (2014) Proceedings of the Royal Society B, doi: 10.1098/rspb.2014.1785 (abstract)