FEATURE: Epidemics and wild chimpanzee study groups

Editor's note: As noted in several articles in this issue both chimpanzees and bonobos in the wild are threatened by habitat destruction, hunting and the pet trade. However, the possibility of infection by dangerous pathogens also poses a serious threat to chimpanzee welfare. In particular, those groups well-habituated for scientific research or tourism face the most serious risk of infectious disease transmission by humans.
An outbreak of a paralytic disease at Gombe, probably poliomyelitis, was reported to have presumably killed six chimps and crippled six others in 1966 (for the details, see Dr. Jane Goodall's book In the Shadow of Man). Dr. Goodall suspected that the chimps had been afflicted through an epidemic in the local human population. According to Dr. Craig Stanford, the second tragedy occurred in the wet season of 1987 when a pneumonia epidemic broke out. Up to ten chimps in Kasakela community died including two of the prime adult males, Satan and Jomeo. At the same time two adult males of the neighboring Mitumba community were found dead in the early stages of the habituation project there. Although it was not certain where the pathogens came from, it is very likely that the pneumonia epidemic affected the chimpanzee population across a wide area.

We have also heard disturbing news from Tai forest, Cote d'Ivoire th at a number of chimps were infected with the Ebola virus, notorious for its high fatality rate. The following article about these events was contributed by Dr. Christophe Boesch and Dr. Hedwige Boesch-Achermann who have led the chimpanzee research project since 1979. (KH)

Tai chimpanzees confronted with a fatal Ebola virus

In November 1994, a new strain of Ebola virus presumably killed 12 chimpanzees of the Tai study community. One student fell seriously ill from the same virus after participating in the autopsy but recovered (Lancet vol. 345 May 1995; Science vol. 268 May 1995). It took four months to find out what had happened in November and the analyses were done very thoroughly because it was the second time we had been faced with this kind of epidemic. We know for sure what illness the student had and what caused the death of one infant chimpanzee female. We suspect, because of the rapid, epidemic-like spread and sudden deaths, that the probability is very high that all other chimpanzees died of the same virus. Now the main queries are to what extent the whole chimpanzee population of Ta??has been s truck by this virus, whether other animal species are involved, what is the natural host of this virus, and how it is transmitted to the victims. The possibility that all chimpanzees as well as other mammals in Tai forest migh t fall victim to this virus is extremely worrying as Ta??is by far the large st remaining area of tropical rainforest in the whole of West Africa.

What can be learned by other field workers from our experience?

The students working at Tai during this period had been informed by the director of the Swiss Center for Scientific Research that it was of high risk to undertake autopsies. They decided, however, that they would do all that was needed to find out what killed the chimpanzees. Therefore, when they found the fresh body of an infant female they conducted an autopsy, wearing gloves and being as careful as possible. It was this courageous decision which later allowed the identification of the virus. One is never optimally prepared in the field to correctly handle all possible emergencies. Many mistakes can happen due to all sorts of circumstances. It is also not an easy task to dissect an individual you have been following for weeks or months. The students certainly did the best they could. No one had veterinary training, but one student assisted by the other two managed to take samples from most vital organs.

While the analyses of material from the dead chimpanzees went on Ch. Boesch tried to take blood from some of the survivors and became aware of the dangers of taking such actions without sufficient preparation. Although he managed to correctly dart a chimpanzee with a blowpipe and to insert the needle into the vein, it was for unknown reasons very difficult to extract blood. He obtained enough for the necessary analyses but this was due to luck and not professional skill. The blood-taking was later repeated with the help of a trained technician and blood from another adult male and one adult female was collected. The surprisingly unaggressive reaction of other group members towards this intervention must have partly been due to the fact that they did not totally realise what was happening. None had been darted before and the anaesthetized chimpanzee and humans were covered with a blanket to hide the whole scene from any approaching chimpanzees. Blood was centrifuged within an hour after collection and at least two observers remained with the chimpanzees until they recovered, which always took at least five hours. The chimpanzees were all fully active the following morning. They were darted with a mixture of Tiletamine 250 mg and Zolazepam 250 mg (Zoletil 100), 4 mg per kilogramme from a distan ce of 6-8 meters, however our experience shows that this dose is far too big and that even with three quarters of a dose they still needed the same time to fully recover. Aside from that the chimpanzees showed a great tolerance to this anaesthetic and recovered without any apparent complications. This sort of action is against all our principles of non-interference with the chimpanzees, but at the same time it was a "now or never" situation for finding out what was happening.

It is important to be aware of the following points:

1 - It is only because we have a perfect knowledge of the community size and of all its individual members that we were able to demonstrate an abnormally high mortality rate for some periods of the year. As there are only restricted numbers of communities of various species known to a comparable degree on earth, it cannot be excluded that similar epidemics have gone unnoticed in many other natural reserves.
2 - Symptoms of illness are difficult to observe because individuals that feel weak tend to leave the group very quickly to avoid the aggression of other members. This is especially true for adults within a fission-fusion social structure like the chimpanzees. Therefore, the absence of syptoms before disappearance is not a very reliable criteria for differentiating between death due to illness or disappearance due to emigration. A clear inference of illness is possible when chimpanzee bodies discovered shortly after death show no wounds of any kind.
3 - A pathogen can be identified only if sample tissues from dead bodies are kept in good condition. Muscles including the heart are not ideal material for identifying pathogens. Liver, spleen, guts, kidneys and blood are all preferred body parts. For some illnesses, the brain is a necessity, but opening the skull is anything but an easy task. Sample tissues should be kept under two different conditions: first in formaline at 10% for histological studies and second at low temperature (around 4 degree C) for bacterial and virological studies. Some viruses and bacteria survive in very unfavourable conditions and dead bodies that are already partly decayed could still be useful for identification.
4 - Manipulating wild animals (dead or alive) is risky and requires precise knowledge. We suggest this should be done by professionals, whenever possible. In emergencies, rubber gloves and masks are an absolute necessity for such manipulation. Bringing the dead body of an infected animal to a city for processing can be extremely risky considering that it could contain a highly contagious virus. Thus, we have decided to leave such conspicuously infected bodies in the forest and attempt to bring in a specialist as quickly as possible.
5 - The virulence of a pathogen is the function of many different factors and our knowledge of it is often very rudimentary. Pathogens known not to be dangerous to humans can kill other species.
6 - It has been known for years that humans can transmit fatal diseases to chimpanzees. At Gombe, the study community suffered many losses from poliomyelites and pneumonia brought into the site by people. The only security measure we can take against such incidents is that as a rule no one who is ill should go to the forest and follow chimpanzees.

Christophe Boesch and Hedwige Boesch-Achermann
University of Basel

Editor's note: The next article concerns an epidemic in Mahale Mountains National Park. At the end of September 1993 a flu-like illness struck almost all M group chimpanzees with serious consequences. The contributor is Kazuhiko Hosaka, a Kyoto University graduate student who did research on M group from March, 1993 - February, 1994.

Mahale: A single flu epidemic killed at least 11 chimps

Events began at the end of September, 1993. Pulin, an approximately 23 year old female chimpanzee, was found dead in the dry Kasiha riverbed by Dr. K. Kawanaka as he returned to Kansyana Research Station in the late evening of October 7. Although many chimps were suffering from a flu-like illness at the time, we had not considered it extraordinary since we knew such an outbreak occurred at least once every year. However, some members of the study group were missing and those members all usually followed the main party including the alpha male. The longer they remained missing the stronger became our conviction that they had succumbed to some terrible illness.

We think at least 11 chimps fell victim to this epidemic. Eight of them (three adult females, two adolescent males, three infants) died during the epidemic itself. Three (an old male, an old female, a juvenile female) died a few months later after becoming emaciated.

The deaths of the victims had vital meaning for the lives of others. For example, many young chimpanzees were orphaned. Pippi, Pulin's three years and five months-old daughter, passed at least two helpless nights in a fig tree just above her mother's corpse. When we finished collecting Pulin's body in a flour sack and began to move, Pippi, who had sometimes coughed weakly while eating fig fruits, started to whimper, quickly climbed down and finally ran into the bush crying loudly. Far away in the direction she headed, we heard many pant-hoots and we hoped she would reach others. Fortunately, nine days later we saw her walking with a large group. Although she has not been adopted by any non-relative female she has survived and looks healthy at present.

On the other hand, for Brahim, a small infant aged two years and five months, it was too early to lose his mother. When I saw him being carried on the back of a non-kin adolescent male I could not help suspecting that his mother was already dead. That hunch became stronger when the decomposed corpse of an adult female chimp was discovered not far from the area five days later. Brahim had no time to feel depressed; all he could do was try to survive. He searched for any chimp that would carry him on his/her back. Most chimps ignored him although no one was cruel to him. Once he whimpered toward a nearby chimp, that individual could not ignore him and allowed him to ride on the back. At least two adult males, two adult females and three adolescent males ranged with Brahim. However, we saw him with others for only three days and he has not reappeared since. Obviously the death of the mother chimp is of serious significance to her offspring's survival.

It is not certain where they caught this fatal disease. The virus might have been brought in by another chimpanzee group. Another fearful posibility was that an ill human was the origin since chimps are susceptible to human diseases. In some instances of flu epidemics at Mahale, both chimps and humans (researchers, and local inhabitants) suffered simultaneously and so it is plausible that either chimp or human was the initial carrier of the virus. However, in this case there was no flu epidemic among the human residents. I did however observe that some tourists to Mahale came to see chimps even though they had caught a bad cold during the long, chilly night journey on Lake Tanganyika.

Whatever the truth we now needed to consider how to protect the chimps from catching human diseases. We were truly apprehensive for the future of M group, which had decreased from quite a large unit-group of 101 chimps to 75 chimps by December 1994 (This unit-group also decreased in size in 1989-1990 when a lion was on the prowl in the area) and so I made the following proposals to the Park Warden of the Mahale Mts. National Park.

1. No one who is ill should not be permitted to go into the forest. Even healthy persons must not go closer than 5 meters to chimps.
2. There should be no smoking, eating or drinking in the forest and no feeding chimps.
3. No person should defecate in the forest. If necessary, you have to either make or look for suitable holes in which to defecate and cover them completely.
4. The people living inside M group's range must not scatter garbage around their houses but must make a large hole in which to dispose of, burn and cover the remains of garbage.

The administration of MMNP was quite cooperative. They issued two notices in Kasiha village (the only residential area inside M group's range). One was to give directions about garbage disposal. The other to declare strict action against those who violated park regulations and that only researchers, field assistants, park staff and their immediate families can live inside the park. This improved conditions because the population in Kasiha had increased since the construction of the tourist lodge and our research station proved attractive to the unemployed. In the beginning of 1993 we went through a horrifying outbreak of cholera and amoebic dysentry in Kasiha that had invaded from the northern villages outside MMNP and killed some of my own acquaintances.

Some problems still needed to be resolved. Although we know it is likely that chimps can be infected by pathogens brought to the forest by humans, park guides cannot control some tourists' apparently thoughtless behavior not only because most of the guides do not speak English and the tourists do not speak Swahili but they also feel intimidated by their "important" guests.

On the part of the researchers, since we owe our long-term accomplishments to the cooperation of the local Tongwe people and the staff of MMNP we have a responsibility to further their education about the invaluable wildlife treasure contained in the park, to teach them to conserve it, and to make every effort to reward their precious contribution by the improvement of their standard of living.

Kazuhiko Hosaka
Department of Zoology
Kyoto University

Back to Pan Africa News Vol.2. No.2

Back to Pan Africa News Home Page