Causes of Death and Diseases of Gorillas in the Wild

Captive gorillas are frequently infected with pathogens transmitted by other animals or humans. A drastic change in diet, environment or persistent psychological stress can cause pathogens and parasites which live in their body to multiply and thereby cause illness, although they are not dangerous under normal conditions. Free-ranging gorillas are also susceptible to human pathogens. The first incidence which became known of a free-ranging gorilla that was infected with a pathogen most probably coming from humans occurred in Rwanda in 1988. The autopsy revealed that the animal had suffered from viral pneumonia. As a precaution against further infection, all gorilla groups which had regular contact with humans were successfully vaccinated using blowpipes to administer the vaccine.

Diseases of the Respiratory Tract

The mountainous climate of the Virunga Volcanoes is chilly and damp, and the gorillas seem not to be optimally adapted to these conditions although they have been living there since many thousand years. Due to the moist climate, gorillas frequently succumb to diseases of the respiratory tract, notably pneumonia, which are the most common cause of death in the mountain gorillas of the Virunga Volcanoes. Wild gorillas may suffer from colds, coughs, rhinitis, sinusitis and inflammations of the laryngeal sac, especially in times of heavy precipitation. If the weather is fine, mountain gorillas can often be seen lying in the sun, while gorillas in the lowland generally seek the coolness of the shade.

Complaints Due to Old Age

In comparison to the dentition of other great apes, the teeth of gorillas show only slight attrition. Whereas western lowland gorillas sometimes get caries, the mountain gorillas' diet is low in sugar, and caries is therefore rare in these animals. However, their teeth are commonly coloured blackishly, and tartar often leads to periodontal disease and gingivitis, reabsorption of the jaw bone and eventually toothloss. As a consequence of tooth decay the sinuses of the upper jaw can becomme inflammated, especially in males. Frequently in adult gorillas the teeth of the upper jaw may be so affected that the dental roots are exposed. It is possible that older animals actually starve to death because they are unable to chew their food. Lowland gorillas, on the other hand, deposit considerably less tartar and rarely have periodontal disease.
Animals aged 35 years and older frequently suffer from arthritis, particularly affecting the joints of their hands and feet. Afflicted animals therefore require more time to forage, feed and move than non-affected group members. However, healthy group members usually adjust their activities to meet the needs of slower family members.

Ectoparasites and Skin Diseases

The jigger Tunga penetrans, the tick Rhipicephalus appendiculatus and the mite Sarcoptes scabiei, all common human ectoparasites, are also occasionally found on gorillas. The gorilla louse Phthirus gorillae belongs to the same genus as the human louse Phthirus pubis.
Lowland gorillas are occasionally affected by yaws, a chronic infection caused by the bacterium Treponema pertenue and spread by body contact. Yaws can cause necrosis of the tissue down to the bone, similar to leprosy.

Endoparasites

The digestive tract of gorillas may contain a variety of parasites (e.g. Balantidium coli, Entamoeba histolytica and Strongyloides fuelleborni) which also infect humans and mammals in various classes. Some other endoparasites are known in humans and African apes, including the hookworm Ancylostoma duodenale, various Necator species, and Onchocerca volvulus. The pinworm Enterobius lerouxi has only been found in the lowland gorilla, while the tapeworm Anoplocephala gorillae has only been found in the mountain gorilla. Various African primates are also hosts to the threadworm Oesophagostomum stephanostomum, the most prevalent intestinal parasite of the lowland gorilla. Besides, many species are parasites in the stomach and small intestine of gorillas, Probstmayria gorillae, Murshidia devians and several species of trichostrongylides, worms of the intestinal tract. A strong infection with those parasites may result in gastritis. Protozoa, e.g. of the genus Troglodytella, are very common; however, it is not yet clear whether they are intestinal parasites or actually aid digestion.
In the connective tissue of the abdominal cavity and in the musculature of the gorilla occasionally the filaria Loa loa, also found in humans, is living, in the connective tissue the filaria Mansonella perstans and in the skin various species of Dipetalonema. The threadworm Tetrapetalonema vanhoofi may be found in the lymphatic and the blood system of gorillas (also in the gall bladder of chimpanzees). The lungs of gorillas and other old world monkeys, but not of humans, may be infected with mites of the genus Pneumonyssus.
Various forms of malaria have been diagnosed in western lowland gorillas. The species Plasmodium rodhaini, which infects both gorillas and chimpanzees, is most likely identical to Plasmodium malariae, which also affects humans. In fact, it is the only malaria pathogen which affects both African apes and humans under natural conditions. The other types of human malaria do not appear to threaten gorillas, just as the gorilla malaria species P. reichenowi and P. schwetzi, which are also found in chimpanzees, obviously are not dangerous for humans under natural conditions.

Injuries and Fatalities through Conspecifics

Most probably humans are responsible for most killings of gorillas in the wild. However, many individuals die during interactions with conspecifics. At least 37% of all mountain gorillas in Rwanda who die within the first 3 years of life succumb to injuries incurred by conspecifics - to infanticide. Infants are known to die most frequently when their mothers transfer to another group, or when the leading silverback dies and a new silverback assumes leadership of the group. In general, silverbacks are responsible for these infanticides, only rarely blackback males, and according to the observations which were recorded so far, females never kill infants.
Infanticide by a new group leader has been documented in other primate species as well, for example in hanuman langurs, baboons and chimpanzees. Since nursing naturally suppresses the resumption of cycling in lactating females, infants in their first year of life who still have to be nursed for at least another 2 years are particularly vulnerable to infanticide. The oldest infant killed in this context was nearly 3 years old.
Adult gorillas also attack other adults and may receive and inflict wounds during intergroup encounters. It is estimated that 62% of all injuries to mountain gorillas of the Virunga Volcanoes are the result of intergroup confrontations. Usually, the wounds are healing quickly, but they may also lead to death. During fights between silverback males, their canines may break off and sometimes even get stuck in the skull of the opponent. Injuries incurred during such fights have been fatal in several cases which were recorded in the Virunga Volcanoes.

Angela Meder

Dr. Angela Meder observed the behaviour and development of captive lowland gorillas for 10 years. A major aim of her various studies was the improvement of the management, rearing and housing of gorillas in zoos. Today she works as a book editor. Since 1992 she has been part of the Board of Directors of Berggorilla & Regenwald Direkthilfe.

Diseases overview

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