A few months ago I had a conversation with someone who had just canceled a long-planned trip to see mountain gorillas in Uganda. It wasn't an easy decision, but she had just gotten over a bad case of the flu. She knew that many human diseases have made their way into gorilla populations and didn't want to risk being a carrier.

It's a sad fact of modern conservation that as more and more great apes come into contact with humans—through ecotourism, rescue operations or simply close proximity in the wild—more and more diseases are jumping ship from human populations and finding their way into the ape populations. In recent years apes have been observed with colds or other respiratory diseases. Worse, many have been killed by anthrax and Ebola.

What can be done? One approach would be to vaccinate apes. But vaccination is expensive, difficult to manage and unproved. Even worse, if the wrong individuals are vaccinated, viruses could still spread rampantly.

A new paper published December 27 in PLoS One examines this conundrum. The researchers—from London's University of Roehampton and the Orangutan Tropical Peatland Project, along with other organizations and institutions—looked at two wild ape populations to see if they could identify "superspreaders"—the members of the society disproportionately likely to pass diseases on to other members of their population.

The first group consisted of 46 Bornean orangutans (Pongo pygmaeus wurmbii), which were studied from 2003 to 2011 in Indonesia. The researchers studied how the orangutans interacted with one another and found that three females in the group had a much higher centrality—they interacted with a greater number of other apes in their social networks. Most of the orangutans, however, were effectively loners and didn't spend a lot of time with a wide number of group members. The researchers plotted out their social interactions in this sociogram:

The researchers also studied a group of 55 eastern chimpanzees (Pan troglodytes schweinfurthii) in Uganda. Characterized by the researchers as much more "gregarious" than the orangutans, the chimpanzees had much denser social networks:

According to the paper, this difference in social structure suggests "that disease transmission between individual orangutans is likely to be limited, in contrast to chimpanzees that are all interconnected through a range of pathways, allowing for potentially very rapid disease spread. Contagious diseases may thus represent a lesser threat to orangutans than chimpanzees, which may be a cause of the generally lower mortality seen in this species compared to chimpanzees."

So where does that leave the idea of vaccination? The authors wrote that their results "suggest that targeted vaccinations are a potentially valuable preventative measure for orangutans" who might be facing the spread of a deadly disease, especially if potential superspreaders can be identified. Chimpanzees, however, with their stronger social bonds, would be less likely to benefit from vaccinations. Their "severe risk from human diseases" means that "alternative preventative measures need to be prioritized" in case human diseases make their way into chimpanzee communities.

Of course this doesn't mean that vaccinations should go to the top of the toolbox. Mike Cranfield, co-director of Gorilla Doctors, who was not affiliated with the new study, says delivering vaccines to apes is fairly common in captivity but much harder to accomplish in the wild. "With unhabituated great apes the only realistic delivery system is oral to get numbers of animals vaccinated to cause herd immunity. Since gorillas are basically vegetarians and are surrounded by food items and have little preference for one food item over another, this system would not be practical." There might also be a risk to any other animals that could accidentally ingest the vaccines intended for the apes.

There is definitely a need to manage the potential for making the apes ill. Cranfield says about 28,000 people visit mountain gorillas in Rwanda alone each year, so "the risks of contracting a disease appears to be high," especially because some governments do not require tourists to wear face masks that could slow disease transmission.

Although Cranfield points out that captive and rescued gorillas—especially young orphans whose parents have been killed by poachers—are routinely vaccinated, some experts feel doing so with wild populations would be unethical. "Right now the policy is to not vaccinate except as a clinical tool in the middle of an outbreak," he says. There's also the question of proving that the vaccines would work. "Many vaccines have not gone through clinical trials in gorillas, and so it would be unethical again to use them in the wild, unless as a last-ditch effort, without a strong clinical trial."

This is obviously early research and we can hope that it won't be necessary, but as the authors wrote in their paper, "increased deforestation and forest fragmentation...combined with the rise of ecotourism, will increase contacts between humans and wildlife and lead to a much higher risk of interspecific disease transmission." That's something to consider as future conservation models are developed.

(A note: Apes aren't monkeys, but I couldn't resist the headline wordplay.)

Previously in Extinction Countdown:

Orangutan photo by Marina Poblacion via Flickr. Used under Creative Commons license