Mountain Gorilla – Credit:

In the late 1970’s and early ‘80’s I worked at a veterinary hospital that specialized in treating exotic animals.  Besides the thousands of dogs and cats that passed through our cages over the years, we treated a menagerie of unusual pets and rescues: raccoons, skunks, parrots, ducks, turtles, wolves, foxes, goldfish and even a pet chicken.

And we saw the occasional monkey.  There were a couple of little capuchins that were regulars at the clinic. I also clearly remember a pet spider monkey, grabbing at me through the cage bars with hands, feet, and an alarmingly strong prehensile tail –all at the same time. I was terrified of him (probably no more than he was of us).

I also clearly remember my boss, a veterinarian and former air force colonel, vividly relating the horrors of “Green Monkey Disease” (more correctly Marburg Hemorrhagic Fever), and how a careless person could contract it from a monkey.  His description of how the disease would turn a person’s brain into mush was morbid and disturbing. (That I remember the name of the disease after all these years speaks to the impact of that conversation.)

In those days, and even until recently, science and communicable disease experts focused on understanding and controlling zoonoses – that is, diseases that humans can contract from other animals.  The list of zoonotic diseases and animal carriers is long and, as human populations grow and crowd animals (both wild and domestic) and viruses jump species, the list has grown even longer.

Just this week, for example, a team of researchers from UK, Netherlands and Vietnam published findings that show that slaughterhouse pigs in Vietnam are a major reservoir of Streptococcus suis.  S. Suis is an emerging zoonotic agent in Asia and in Vietnam is the leading cause of bacterial meningitis in adult humans.[i]

But what about diseases that spread in the other direction – from humans to non-human animals?  When I worked at the veterinary clinic, we wore gloves and face masks, not to protect the animals, but to protect ourselves. Only in the last decade have researchers begun to report on the destructive effect of human diseases on wild animal populations – especially on non-human primates.

The most shocking example is that of Ebola haemorrhagic fever and its effect on gorillas and chimpanzees.  While it has been known that hunting and the bushmeat trade have drastically reduced ape populations, the devastating effect of Ebola virus took researchers by surprise. One study found that in areas where there is little or no hunting, 90% of the ape population has died of Ebola since 1991.[ii]

Ebola is currently spreading through Western gorilla (Gorilla gorilla) and chimpanzee (Pan troglodytes) populations in Gabon and Congo. Between 1994 and 2003 Gabon experienced four human Ebola epidemics. Ape carcasses were found in the forests near three of those sites and in two cases apes tested positive for Ebola. [iii]

In just one study area in the northwest Republic of Congo, researchers confirmed that the Zaire strain of Ebola virus (ZEBOV) had a gorilla mortality rate of 90-95% resulting in the death of more than 5,000 gorillas in 2002 and 2003.[iv] Ebola now rivals hunting as a threat to apes.

This week there is even more troubling news, this time from Rwanda. The CDC, in its journal Emerging Infectious Diseases,  reports that for the first time a virus that causes respiratory disease in humans – metapneumovirus — has been linked to the deaths of two wild mountain gorillas (Gorilla beringei beringei). [v]

In 2008 and 2009 a respiratory disease infected a group of twelve gorillas known as the Hirwa group.  All of the animals, except one, became ill, developing coughs, nasal discharge, and lethargy. Researchers were able to administer antimicrobial drugs to most of the animals, but two untreated animals, an adult female and an infant male died.  Post mortem analysis confirmed the presence of human metapneumovirus (HMPV) in both animals.

The death of two gorillas doesn’t seem like a reason to worry. But there are only 786 wild mountain gorillas left in the world, and they all reside in just two parks in a tiny corner where Rwanda, Uganda, and the Democratic Republic of the Congo meet. The Hirwa outbreak described in the study was just one of five outbreaks of respiratory disease in mountain gorillas that occurred in 2008 and 2009.  The number of outbreaks is on the rise as is the severity of the disease.  Today communicable respiratory disease is the number-two cause of mountain gorilla deaths.

The sad irony is that this risk to the gorillas is brought on, in no small part, by the people who love them most –researchers and tourists. The regions where the gorillas live rely heavily on eco-tourism, bringing in visitors from all over the world to view and have direct and indirect contact with human-habituated gorillas. Contact is greatest in Virunga National Park where 75% of the 200 mountain gorillas there are habituated to the presence of humans.

Unfortunately, gorillas not only share our susceptibility to many diseases, but they don’t have the resistance that humans have.  In humans, metapneumovirus is the most common cause of lower respiratory disease, causing a mild respiratory infection.  It is has been shown that, by age five, nearly every child has been exposed to the virus. But to gorillas the infection can be debilitating and, as we have now seen, can be deadly.

Recognizing this danger, Virunga Park officials request that tourists and researchers only visit the gorillas when they are healthy. They also require that humans maintain at least a 25 foot distance between themselves and the animals.

But tourists and researchers aren’t the only problem.  The war in Eastern Congo has brought more than 300,000 refugees to Virunga National Park’s southern border. With no place else to turn, these refugees must enter mountain gorilla habitat for fuel and food.  Inevitably, communicable disease outbreaks will occur in this desperate population and when they do, the pathogen is likely to spread to neighbouring gorillas.

Yes, this recent outbreak took “only” two gorillas. But human encroachment into animal habitat, lack of sanitation, poor farming and feedstock practices, and overcrowding have put both humans and non-human animals at increased risk of sharing disease.  It is estimated that up to 75% of emerging pathogens may be of zoonotic origin. To protect ourselves and wild animals such as gorillas, we are going to have to find ways to balance our need to be among these captivating creatures with our need to preserve them. Otherwise, we will very soon be looking at the end of the mountain gorilla.

[i] Hoa NT, Chieu TTB, Nga TTT, Dung NV, Campbell J, et al. (2011) Slaughterhouse Pigs Are a Major Reservoir of Streptococcus suis Serotype 2 Capable of Causing Human Infection in Southern Vietnam. PLoS ONE 6(3): e17943. doi:10.1371/journal.pone.0017943

[ii] Whitfield, John. “Ape populations decimated by hunting and Ebola virus.” Nature 422, no. 6932 (April 10, 2003): 551.

[iii] Walsh, Peter D., Kate A. Abernethy, Magdalena Bermejo, Rene Beyers, Pauwel De Wachter, Marc Ella Akou, Bas Huijbregts, et al. “Catastrophic ape decline in western equatorial Africa.” Nature 422, no. 6932 (April 10, 2003): 611-614.

[iv] Bermejo, Magdalena, José Domingo Rodríguez-Teijeiro, Germán Illera, Alex Barroso, Carles Vilà, and Peter D. Walsh. “Ebola Outbreak Killed 5000 Gorillas.” Science 314, no. 5805 (December 8, 2006): 1564.

[v] Palacios G, Lowenstine LJ, Cranfield MR, Gilardi KVK, Spelman L, Lukasik-Braum M, et al. Human metapneumovirus infection in wild mountain gorillas, Rwanda. Emerg Infect Dis [serial on the Internet]. 2011 Apr [cited Mar 28, 2011]. DOI: 10.3201/eid1704.100883