Endoparasites in Gorillas and Humans in the Same Habitat

It has been shown that almost all free-ranging animals have parasites. As sick individuals are actually observed only rarely, the parasites' impact on the host animal had been underestimated for a long time. It is now well known, however, that parasites introduced by humans and those that appear after restriction of the habitat can have a very severe, although perhaps temporary, impact on free-ranging animals.
Primate species are increasingly threatened by the destruction of their tropical habitats. They are forced to retreat to protected areas which are continually decreasing in size. Studies have shown that parasite infestations increase in "stressed" ecosystems or, alternatively, that habitat destruction leads to the disappearance of certain parasite species. This means that parasites can serve as sensitive indicators of changes in the ecosystem.

Parasites of Gorillas

Studies in American zoos and on free-ranging mountain gorillas have shown that in captivity parasites accounted for most deaths in gorillas between 2 and 12 years of age. In free-ranging mountain gorillas, they were the third most frequent cause of death. With the growing risk of decreasing genetic variety in threatened populations, the survival of the individual gains special significance. This is particularly true for the gorilla, as all gorilla subspecies are included in the IUCN Red List of Threatened Species. Prophylactic measures for the prevention of new infections and for restricting the spread of existing parasite species may gain considerable importance for free-ranging grorillas in the future.
My Ph.D. study, in which the parasites of a free-ranging population of eastern lowland gorillas (Gorilla gorilla graueri) were determined for the first time, was funded by the DAAD (German Academic Exchange Service) and also supported by the GTZ (German Technical Aid). During the study I also collected data on gorilla behaviour and ecology which helped to explain the occurrence and distribution of parasites within and between gorilla groups, as well as between humans and gorillas. As part of the study, the human population living around the park borders was examined, too. Park staff were also recruited from this population.
The Kahuzi-Biega National Park is the only protected area in which tourists may visit eastern lowland gorillas. Helminths (worms) were found in 67% of gorilla dung samples, and protozoans (single-celled organisms) in 12%. One tapeworm specific for gorillas which is transmitted by oribatid mites, 5 genera of roundworms and two protozoan genera could be detected. None of the gorillas showed signs of illness during the entire duration of the study. However, when certain parasites (tapeworms, "small" eggs) were passed, diarrhoea occurred more frequently.
All identified genera of helminths appeared to be the same as those of the other gorilla subspecies. The proportion of eastern lowland gorillas passing parasites lies between that of mountain and western lowland gorillas. The difference in parasite infestations among subspecies may be influenced by ecological and social factors. This has already been indicated by studies on other primate species. With respect to ecological and social factors, the position of the eastern lowland gorillas in the montainous section of the Kahuzi-Biega National Park lies also between the other subspecies.
In this context it is interesting to note that liver flukes, which are transmitted by ants, do not occur in eastern gorilla populations living at greater altitudes. Similarly, tapeworms transmitted by oribatid mites do not occur in the habitat of the western lowland gorilla. This suggests that either the transmitting species are missing in those areas or the gorillas do not ingest them.

Environmental Effects

Results of the study indicate that in the tropical rain forest, the conditions for the survival and the spread of parasites are favourable the whole year round. Seasonal fluctuations were found mainly in parasite species transmitted by intermediate hosts. For example, in eastern lowland gorillas there was a significant difference in the tapeworm infestation during and after the dry season, as well as during and after the time in which the gorillas ingested mainly fruits and bamboo. Similarly, in a species of roundworm (the transmitting host is not yet known) there were significant differences between the dry season and the rainy season. The reproduction of the intermediate hosts and the ingestion by the final host (i.e. gorillas) is probably influenced by rainfall, temperature and the switch between preferred food plants (Myrianthus, bamboo, lianas).
A comparison of the four gorilla groups which I studied showed that the infestation with parasites increased with group size but not with the number of animals. The home ranges of the four groups overlapped to different extents and the groups' day ranges differed considerably in length. The group whose home range overlapped most extensively with those of other groups and who had the shortest daily ranges, was most infested with parasites. Conversely, the group with the longest day range and least overlapping home range was the one with the least parasite infestation. This may indicate that repeated utilization of an area in short intervals results in higher parasite infestation.
If individual gorillas are considered, there is a clear correlation between parasite infestation and the age of the host. However, direct physical contact, such as between mother and dependent offspring, did not seem to have an impact on the transmission of parasites. All parasite species increased from infant (up to 3 years) to juveniles; 2 out of 6 parasite genera increased until maturity. This might indicate the development of an immune reaction to certain parasite genera, but this has not yet been investigated in gorillas.
There were no statistically significant differences in parasite infestations between male and female adults. Females, especially lactating females, tended to pass parasites somewhat more frequently than silverbacks. This might indicate the influence of intrinsic (internal physiological) factors such as hormones. Alternatively, changes in feeding behaviour caused by gestation and lactation, such as increase of food quantity and foraging in more accessible places, may affect the parasite infestation.

Parasites in the Human Population

54% of the human population in the surroundings of the Kahuzi-Biega National Park passed parasites. Although most genera were different from those of the gorillas, all parasites identified in humans may potentially be infectious for primates. Particularly alarming was the high number of potentially infectious protozoans. Three times as many humans were infected with protozoans (Giardia and Entamoeba) than gorillas. Gorillas are not considered natural hosts for Entamoeba. Studies in captivity have shown that these parasites can cause severe symptoms and frequently even death, especially in young animals up to 2 years of age. This is exactly the age group that attracts most attention from tourist groups and park staff. Protozoans can be transmitted from humans to gorillas more easily than helminths directly via smear infection from the faeces to the mouth. Giardia is found ten times as frequently in humans than in gorillas. The possibility of transmission of these parasites from humans to gorillas cannot be excluded, because of the gorillas' close contact to people.

Conclusions and Consequences

• The studied gorillas are living in balance with their parasites. A reduction of the protected area, which would result in a more intensive utilization of home ranges by the gorillas, might increase the infestation with parasites and disturb the balance.
• In the rain forest, the occurrence of parasites transmitted by intermediate hosts is clearly influenced by ecological factors. Changes in the frequency of these parasites might indicate changes in the ecological conditions.
• The human population close to the park is infected with parasites to a high degree. So far, the human parasites have overlapped only slightly with those of the gorillas. Both populations have parasites which are transmittable to the other population, however, and the risk of infection is considerable.

Practical measures should include:

• Constant monitoring of the parasites of gorillas and park staff. Contact between gorillas and people other than rangers should be prevented. This requires a more careful habituation; the gorillas should always retain a certain shyness of people and their settlements.
• The gorilla habitat should be conserved in its existing size and structure, especially in regard to the variety of food plants.

Tourism

If the concept of tourists visiting gorillas is to be maintained, the following measures should be taken in order to protect the gorillas from parasites and other infectious diseases:

• The minimal distance between gorillas and humans during visits should be increased.
• Park staff and tourists should be informed about the risks of infections and the importance of keeping a certain distance from the gorillas.
• It is urgent that toilets be constructed for tourists and the population in the park's surroundings.
• Park staff have to be paid a sufficient salary which encourages them to inform tourists of the regulations and to ensure that they follow these rules.
• It should be a requirement of visiting permits that visitors are obliged to follow the instructions given by park staff.
• Tour operators should pass on the rules and regulations to their customers even before they leave for their trip and inform them about the restrictions that they will have to respect in order to preserve the health of the animals.

Tourism conducted according to these guidelines could support the conservation of gorillas without exposing the animals to considerable health risk.

Ute Eilenberger

Dr. Ute Eilenberger worked as a veterinarian and studied guenons and mangabeys as well as eastern lowland gorillas in 1992 and 1993 in Zaire. Then she studied the health status of gorillas and people in Kahuzi-Biega. After various jobs in Germany, she led, with her counterpart Antoine Mudakikwa, the Mountain Gorilla Veterinary Center in Rwanda from 1999 to 2001.

Kahuzi-Biega overview

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