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updated 2/13/2012 2:27:36 PM ET 2012-02-13T19:27:36

Diseases -- often transmitted by humans -- are decimating great ape populations, to the point that some are now calling for vaccinating gorillas and chimpanzees.

Over the last two decades, the Zaire strain of Ebola has killed roughly one-third of the world's gorilla population and only a slightly smaller proportion of the world's chimpanzees, according to a new study in the journal PLoS ONE.

That means that infectious disease is now a major threat to the survival of African great apes, along with poaching and habitat loss.

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"The situation is now getting so bad that if we don't take the gloves off and get more invasive, there is not much hope for long term ape survival," co-author Peter Walsh, a quantitative ecologist at the University of Cambridge, told Discovery News.

Walsh and colleague Sadie Ryan, an assistant professor of ecology at SUNY-ESF in Syracuse, analyzed how disease outbreaks have affected great ape populations over the years.

Using mathematical modeling, they found that the predicted recovery time for just one gorilla population from a single outbreak ranged from five years for a low mortality respiratory outbreak to 131 years for an Ebola epidemic that killed 96 percent of the population.


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Resilience of the population is key, since gorillas and chimps reproduce more slowly than most other animals, including humans.

Concern is mounting because many of the diseases are spilling over from humans. On the other hand, tourism is now essential for great ape conservation, so preventing human access to wilderness areas is not a viable option.

"It is important to remember that conservation funding is often largely derived from tourism, that tourism money pays for guards to prevent further hunting, and motivates the preservation of intact habitat by the countries' governments," Ryan said. "Without the funding, the great apes would be doomed."

One way to keep tourism, but also preserve the animals' safety from disease could involve establishing hygiene and behavior guidelines.

Fabian Leendertz, a wildlife disease epidemiologist at the Robert Koch Institute in Berlin, has held that "we need to be much more proactive about instituting strict hygiene precautions at all ape tourism and research sites."

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Ryan and Walsh agree, but believe these precautions can be difficult to enforce. They think the most effective solution is to vaccinate primates.

"Vaccines against all the major childhood respiratory diseases (such as measles, mumps and rubella) should be the first priorities," Walsh said. "Influenza is also likely a possibility. Multiple vaccines are already administered to human children in 'cocktails,' so why not gorillas?"

Walsh is the president of the organization VaccinApe, which has already done a pilot measles vaccination project using hypodermic darts. He has also helped to conduct an Ebola vaccine trial on captive chimps, but the results of those experiments have not yet been published.

While vaccinating the animals with a hypodermic dart remains a viable option, the researchers believe the best way of vaccine delivery is oral. For great apes, vaccine could be included in bait eaten by gorillas and chimps.

The process is not without risks.

Vaccines are capable of causing mild infections in animals. Also, the bait may be eaten by other species. Nevertheless, as Walsh pointed out, "literally hundreds of millions of oral rabies baits have been spread with virtually no spillover problems," with the benefits far outweighing the risks.

Funding for vaccinating great apes poses yet another challenge. Walsh and Ryan envision that funding will, at least at first, come from private sources.

Walsh said, "Once it becomes a widely accepted part of management, it will be built into park budgets and funded from governmental, multilateral and private sources."

© 2012 Discovery Channel

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