Zoonotic Diseases Shared by Gorillas and Humans: A Veterinary Perspective

This paper provides a brief introduction into zoonotic diseases that affect gorillas and humans, and briefly outlines the ways in which the risk of disease transmission between gorillas and humans can be reduced or prevented. Of course, we cannot list all of the approximately 150 theoretically transmissible diseases between non-human primates (and therefore also between gorillas and humans). The section further reading lists detailed literature.
The World Health Organisation defines zoonotic diseases as naturally transmissible diseases between vertebrates and humans. The transmission of an infectious disease can be either direct through immediate contact, or indirect by exposure through inanimate objects (e.g. food, soil, leaves) or living organisms (e.g. ticks, mosquitoes, rodents). Zoonotic diseases are caused by infectious agents such as viruses, bacteria, fungi, parasites and presumably also through prions.

Infectious Diseases: an Existent Danger for Decimated, Small Populations
In the past, the threat of infectious diseases to wildlife was often underestimated; today, infectious diseases are recognised as a significant danger to wildlife species that have been decimated or artificially manipulated at their population level, habitat or geographic range. Acknowledging this role of infectious diseases and reducing the transmission risk is, therefore, of essential importance in wildlife conservation.
For a few years now, scientists have talked of pathogen pollution, the contamination of the environment with disease causing infectious agents. Although many infectious agents are species-specific, a number of pathogenic organisms can cross the species barrier and cause severe clinical diseases in the new host. The movement of humans into wild habitats leads to the occurrence of new infectious diseases which can be transmitted bidirectionally between wildlife and humans. Carriers of these infectious agents do not always have to be obviously sick.
As a result of these new diseases, the so-called emerging diseases, a number of wild animals have been pushed further towards extinction. Pathogen pollution is a global problem, which threatens all wildlife and also all humans. Historically, habitat destruction and chemical pollution were considered as major threats to biodiversity; today, in fact, pathogen pollution is the largest danger for threatened animal species and hence for our planet's biodiversity. If members of a decimated population were infected with a pathogenic agent, for which this population was immunologically naive, then this population could decrease and eventually become extinct. Particularly infectious agents of humans can be lethal for rare animal species, especially for primates. Consequences would be fatal not only for the suffering individual, but also for the entire species. This is why infectious agents are such a real danger for small, threatened populations.

Zoonotic Diseases Shared by Gorillas and Humans
Because of its close genetic relationship with humans, the gorilla is, like all apes, susceptible to infectious diseases of humans and vice versa. The pathogenicity of an infectious disease depends on host and pathogen evolutionary mechanisms; for example, a disease that may be highly pathogenic to gorillas may be harmless to humans and vice versa. As the immune system of gorillas is naive to infectious agents of humans, transmissible human diseases can cause more severe clinical illness in the gorilla than in its original host, humans. The knowledge of zoonotic diseases between gorilla and man comes mostly from zoological institutions, but also from work on wild ranging mountain gorillas.
Gorillas can fall ill with viral diseases, such as the viral human childhood illnesses. In general, people and gorillas are susceptible for a number of bacterial infections; any bacterial disease in the gorilla as well as in humans should be considered zoonotic with a risk of mutual transmission. Parasitic diseases of zoonotic importance are those with protozoic (e.g. amoebiasis), helminthic (e.g. strongyloidosis) as well as arthropodic aetiology (e.g. scabies). In comparison to other infectious germs, fungi seem to play a lesser role as zoonotic agents.
Gorillas and people can be asymptomatic carriers for certain infectious agents. Such individuals then serve as the often unrecognized source of infection (e.g. human infected with the Herpes simplex virus, gorilla infected with Strongyloides stercoralis).

Gorillas Living in the Wild: Danger and Protection from Human Diseases
Beside habitat destruction and hunting, human infectious diseases are another major threat to wild ranging gorillas. These diseases cannot only be transmitted through tourists, gamekeepers, veterinarians and scientists, but also by the local people. Vaccinating wildlife as a preventive measure is a controversial, often disputed issue. During an epidemic with fatalities in the 1980s in wild mountain gorillas which had shown heavy respiratory symptoms, antibodies for measles were detected. Suspecting the disease, some mountain gorillas were vaccinated against measles to prevent future disease. Scabies and yaws are other zoonotic diseases with severe clinical symptoms described for wild gorillas.
Today there are rules for tourists visiting habituated gorilla groups to protect the health of the gorillas: only a healthy person is allowed to go, a maximum number of participants is permitted, only one visit per gorilla group per day is allowed, minimum distance and minimum age are stipulated, and human faeces must be buried. But remember: one does not have to be diseased with tuberculosis to put the gorillas' health in danger, and a widely travelled tourist could be in the incubation period for flu and might already have spread infectious agents; this - to us - simple flu could then have severe consequences when transmitted to the gorilla group. Also scientists, veterinarians and gamekeepers have to act according to the rules of infectious disease prophylaxis when encountering gorillas.
There are good preventive measures for certain zoonotic diseases, such as vaccinations, deworming and treating skin parasites. We have the moral obligation to provide advantages of modern health care to people in so-called third-world countries including free examinations and, if necessary, treatment of the local people, which would also be a step in protecting gorillas from potential zoonotic agents.

And what about Zoos?
Humans can also transmit diseases to gorillas in captivity. Regularly, gorillas are infected by keepers with colds, particularly during spring and autumn, but this commonly does not threaten the lives of the gorillas. The zoo gorilla population of today is considered sustainable and its existence is not threatened by infectious diseases unlike their wild counterparts. Severe infectious diseases are extremely rare in zoos and they do not threaten the survival of the entire zoo population.
Today, the zoo gorilla is protected from contact with visitors and so from their infectious germs through sheets of glass or wide moats. This did not use to be that way: before introducing these protective measures, tuberculosis in particular was a fear in primate holdings. Today, visitors are not at all or only in exceptional cases allowed to go behind scenes. Access for children to behind scenes is prohibited completely in most zoos as they bear the risk of transmitting childhood illnesses to the gorillas.
Gorillas, their keepers and veterinarians are protected through the rules of the hygienic outline plan of each zoo. These rules together with those of good animal keeping and management reduce the danger of disease transmission between gorillas and personnel. Hygiene, pest control, regular health checks and vaccination of the personnel, and working clothes are all important, and employees are banned from work with apes if any suffers respiratory or gastrointestinal infections, or skin or mucous membrane lesions; the disease prevention programme will include quarantine, parasite control, TB-testing and vaccinations of the gorillas.

And what about Humans?
It should not be forgotten that humans, in the wild as well as in zoos, can in turn contract infectious diseases from gorillas. There will always be a certain risk of infection through direct or indirect contact for people who work with gorillas. Many agents are transmitted by oro-faecal contamination or by aerosol. Certain parasites such as Strongyloides can find the way into their host through skin penetration. It is advisable to avoid contact with the gorillas' body fluids (blood, faeces, urine, nasal discharge) or, in the case of a vet, to take care not to hurt oneself with contaminated needles.
In today's zoos, there is only a minimal risk for the personnel of contracting zoonotic diseases from zoo gorillas. Overall, only a few individual cases of zoonotic diseases of humans through work with gorillas in captivity are published (e.g. amoebiasis, strongyloidosis, mycoplasmatic arthritis); as the experience of more than 100 years of gorilla keeping shows, there is no reason to be overly concerned in the zoo. Still, everyone who works with apes must be aware of a residual occupational risk, as some infectious agents may remain undiscovered for some time. Because of this, certain hygienic norms must be fulfilled to protect the personnel as much as the gorillas from each other's infectious agents.
In the wild, there is a risk of infection for tourists, rangers, veterinarians, scientists as well as for the local people. According to a recent publication a few local hunters in Central Africa were infected and then died of the Ebola virus which they contracted through the consumption of infected meat of dead gorillas. In the literature, infection of humans with the malaria protozoon Plasmodium gorillae has been described. In theory, humans could contract an infection with some gorilla-specific agents which were only recently discovered, especially the gorilla Herpes virus or the gorilla Spumavirus, or with gorilla-unspecific viruses, such as the Simian T-lymphotrophic virus 1 (STLV-1). The rate of dissemination of these viruses in gorillas so far remains unknown. To the best of our knowledge no known case of a gorilla infecting a human with such viruses has been reported.
To reduce the risk of contracting a zoonotic disease, in the wild as well as in the zoo, certain hygienic rules have to be followed. This is of course human self-protection, but primarily protection of the gorillas from humans and their pathogens.

K. Alexandra Dörnath Aguirre Alvarez and Jürg Völlm

Zoonotic diseases of gorillas mentioned in the literature (examples)

Further Reading
Ashford, R. W. et al. (1990) The intestinal faunas of man and mountain gorillas in a shared habitat. Ann. Trop. Med. Parasitol. 84(4): 337-340
Bach, C. H. & Bach, F. (1988) Besteht ein Zoonoserisiko durch Zootiere? Tierärztliche Umschau 43(2): 95-101
Baumeister, B. M. & Ramsay, E. C. (1986) Tularemia in a lowland gorilla. Proc. Am. Ass. Zoo Vet. Illinois: 11-18
Brack, M. (1987) Agents transmissible from simians to man. Heidelberg: Springer Verlag
Butynski, T. M. & Kalina, J. (1998) Gorilla tourism: a critical look. In: Milner-Gulland, E. J. & Mace, R. (eds.). Conservation of biological resources. Oxford: Blackwell Science.
Cousins, D. (1972) Diseases and injuries in wild and captive gorillas. Int. Zoo Yb. 12:211-218
Cousins, D. (1990) The magnificent gorilla. Lewes: The Book Guild Ltd., 153-173
Cranfield, M. et al. (2002) The mountain gorilla and conservation medicine. In: Aguirre, A. A. et al. (eds.). Conservation medicine. Oxford: Oxford University Press, pp. 282-296
Fiennes, R. (1967) Zoonoses of primates. The epidemiology and ecology of simian diseases in relation to man. Ithaca, New York: Cornell University Press.
Graczyk, T. K. et al. (1999) Capillaria hepatica (Nematoda) infections in human-habituated mountain gorillas (Gorilla gorilla beringei) of the Parc National de Volcans, Rwanda. J. Parasitol. 85(6): 1168-1170
Graczyk, T. K. et al. (2001) Cryptosporidium parvum genotype 2 infections in free-ranging mountain gorillas (Gorilla gorilla beringei) of the Bwindi Impenetrable National Park, Uganda. Parasitol. Res. 87(5): 368-370
Graczyk, T. K. et al. (2001) Hyperkeratotic mange caused by Sarcoptes scabiei (Acariformes: Sarcoptidae) in juvenile human-habituated mountain gorillas (Gorilla gorilla beringei). Parasitol. Res. 87: 1024-1028
Graczyk, T. K. et al. (2002) Anthropozoonotic Giardia duodenalis genotype (assemblage) A infections in habitats of free-ranging human-habituated gorillas, Uganda. J. Parasitol. 88(5): 905-909
Graczyk, T. K. et al. (2002) A single genotype of Encephalitozoon intestinalis infects free-ranging gorillas and people sharing their habitats in Uganda. Parasitol. Res. 88: 926-931
Guerrera, W. et al. (2003) Medical survey of the local human population to determine possible health risks to the mountain gorillas (Gorilla gorilla beringei) of Bwindi Impenetrable Forest National Park, Uganda. Int. J. Primatol. 24(1): 197-207
Haberle, A. J. (1974) Human exposure to primate tuberculosis. J. Zoo Anim. Med. 5(4): 31
Heckel, J.-O. & Rietschel, W. (2001) Zoonosen - potentielle Gefahren für Mensch und Tier in zoologischen Gärten. Zookunft 2001 Gelsenkirchen.
Heckel, J.-O. et al. (2000) Experiences with the preparation of a zoonoses prevention concept for Zoo Heidelberg. Proc. Europ. Ass. Zoo Wildl. Vet. 3: 37-41
Heldstab, A. et al. (1981) Spontaneous generalized Herpesvirus hominis infection of a lowland gorilla (Gorilla gorilla gorilla). J. Med. Primatol. 10(2/3): 129-135
Heuschele, W. P. & Heuschele, C. B. (1988) Zoonotic diseases - reducing the risks. Proc. Am. Ass. Zool. Parks Aqu. 591-598
Homsy, J. Ape tourism and human diseases: how close should we get? A critical review of the rules and regulations governing park management and tourism for the wild mountain gorilla, Gorilla gorilla beringei
Jones, T. C. et al. (eds.) (1993) Nonhuman Primates I. Berlin: Springer Verlag.
Kaandorp, J. (ed.). (2004) Transmissible disease handbook. Bern: European Association of Zoo and Wildlife Veterinarians/Infectious Disease Working Group
Kalema, G. (1999) Mountain gorilla veterinary interventions - conservation versus welfare? http://www.Anthro.Ucdavis.Edu/Features/Gcn/G13vet.htm
Kalema, G. et al. (1998) An outbreak of sarcoptic mange in free-ranging mountain gorillas (Gorilla gorilla beringei) in Bwindi Impenetrable National Park, South Western Uganda. Proc. Joint Conf. Am. Ass. Zoo Vet./ Am. Ass. Wildl. Vet. 438
Lacoste, V. et al. (2000) Virology: KSHV-like herpesviruses in chimps and gorillas. Nature. 407(6801): 151-152
Loomis, M. R. (1991) Nonviral zoonoses of nohuman primates. Proc. Am. Ass. Zoo Vet. 221-223
Meehan, T. & Zdziarski, J. (1997) Zoonotic diseases. In: Ogden, J. & Wharton, D. (eds.). Management of gorillas in captivity: Husbandry manual. Gorilla SSP and the Atlanta/Fulton County Zoo, Inc., Atlanta, Georgia, pp. 191-196
Montali, R. J. (1999) Important aspects of zoonotic diseases in zoo and wildlife species. Verh. Ber. Erkrg. Zoo- und Wildtiere 39: 149-155
Mudakikwa, A. B. et al. (1998) An indicator of human impact: gastrointestinal parasites of mountain gorillas (Gorilla gorilla beringei) from the Virunga Volcanoes Region, Central Africa. Proc. Joint Conf. Am. Ass. Zoo Vet./Am. Ass. Wildl. Vet. 1998: 436-437
Nizeyi, J. B. et al. (2002) Cryptosporidiosis in people sharing habitats with free-ranging mountain gorillas (Gorilla gorilla beringei), Uganda. Am. J. Trop. Med. Hyg. 66(4): 442-444
Nizeyi, J. B. et al. (2002) Cattle near the Bwindi Impenetrable National Park, Uganda, as a reservoir of Cryptosporidium parvum and Giardia duodenalis for local community and free-ranging gorillas. Parasitol. Res. 88: 380-385
Nizeyi, J. B. et al. (2001) Campylobacteriosis, salmonellosis, and shigellosis in free-ranging human-habituated mountain gorillas of Uganda. J. Wildl. Dis. 37(2): 239-244
Pernikoff, D. S. & Orkin, J. (1991) Bacterial meningitis syndrome: an overall review of the disease complex and considerations of cross infectivity between great apes and man. Proc. Am. Ass. Zoo Vet. 1991: 235-241
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Sleeman, J. et al. (2002) Medical survey of the local human population to determine possible health risks to the mountain gorillas (Gorilla gorilla beringei) of Bwindi Impenetrable Forest National Park, Uganda. Proc. Am. Ass. Zoo Vet. 2002: 244-245
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Sleeman, J. M. et al. (2002) Medical survey of the local human population to determine possible health risks to the Mountain gorillas (Gorilla gorilla beringei) of Bwindi Impenetrable Forest National Park, Uganda. Proc. Am. Ass. Zoo Vet. 2002: 244-245
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K. Alexandra Dörnath Aguirre Alvarez, a veterinarian, worked in Great Britain and Germany as well as in several wildlife management projects. Presently she finishes her doctoral thesis on zoo gorillas in Europe.
Dr. Jürg Völlm has been working in Basle Zoo as a veterinarian regularly since 1976. The famous Basle gorillas are among his patients.

Gorillas in general - overview

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