"The widespread mortality may be a result of the communal scavenging or 'cannibalism' of carcasses of anthrax-killed hippos by other hippos," suggests Joseph Dudley, a biosecurity and agriculture analyst, based in Washington DC, US. Writing to ProMed Mail - an infectious diseases mailing list - he says this theory may be supported by the absence of deaths among buffalo or other anthrax host species in the area until late in the outbreak. Hippos are usually vegetarian, wading out of the rivers and ponds where they bathe for much of the day to eat grasses. But hippos are not the gentle giants their placid appearance might suggest, and they cause many human deaths in Africa every year. "I knew hippos were nasty, but I didn't know they went around eating each other," says anthrax expert Martin Hugh-Jones at Louisiana State University in Baton Rouge, US.
Dudley published the first scientific report of hippos turning carnivorous to scavenge meat off dead impala carcasses in 1996. And since then, cases of hippo cannibalism have also been documented. But the latest outbreak of anthrax in hippos is most likely to be related to overcrowding, with cannibalism playing just one role, says Roy Bengis, state veterinarian at Kruger National Park in South Africa.
Overpopulation can deplete resources, increase competition and conflict between the creatures. "Hippos can be extremely aggressive, and fights leave numerous lacerations, and may be fatal," Bengis writes to ProMed. These gaping wounds can also increase the risk of anthrax by opening up a potential route of infection. Cannibalism by hippos is rare, and may itself be induced by the stress of overcrowding, or "possible nutritional deficiencies or needs", he says. But this could further spread anthrax if hippos are eating infected hippo meat.
However, grazing on grasses contaminated with anthrax spores or ingesting contaminated water are also likely to have been important on the spread of the infection in overcrowded conditions, says Bengis. What he says makes sense," Hugh-Jones told New Scientist. "You always get an increase in infectious disease when you have an increased density - and there's a better chance of getting it if resistance is down because of stress." He adds that "no disease depends on just one route of spread".
He notes that where anthrax outbreaks in hippos were once "incidental events", over the last decade or so outbreaks have become "massive". The first such large outbreak in eastern Zambia was put down to huge numbers of hippos. The overcrowding was so severe that the animals were forced to walk 30 kilometres just to feed.
NewScientist
January 4, 2005
Original web page at NewScientist
