It is generally recognised that naturally occurring treponematosis is found in some African nonhuman primates. In addition, bone lesions of treponematosis have been identified in wild chimpanzees and gorillas (Lovell et al. 2000), while yaws has been documented and photographed in gorillas in Republic of Congo (Levréro et al. 2007). It is not yet known which pathogenic agent is responsible in these cases, but it is thought to be the spirochaete Treponema pertenue, which causes the disease commonly known as yaws (or framboesia). This is a social disease that usually affects children and adolescents; it is endemic to hot, moist, humid areas in the tropics, and is particularly persistent in African Pygmies and some tropical Asian populations.

Spirochaetes are a form of bacteria, distinguished by their spiral shape. They include the types of bacteria which cause diseases such as relapsing fever and leptospirosis, and the treponemes, which cause yaws, syphilis and pinta (known as "treponematoses").
While there is a wealth of literature documenting endemic yaws amongst people in the former Belgian Congo (now the Democratic Republic of the Congo), it has never been recorded in wild great apes in that country. On the other side of the Congo River, in the Republic of Congo, however, there are areas where yaws has been reported to be rife in both humans and apes. It is worth noting in this respect that many African nonhuman primates, including gorillas, have tested serologically positive for treponematosis without showing any obvious sign of disease; in humans, latent yaws may occur when secondary lesions heal spontaneously and the disease enters the non-infectious, clinically-negative latent period that can last the sufferer's entire life.

Primary and Secondary Yaws
Yaws progresses in three distinct stages - primary, secondary and tertiary. It begins with the development of the primary skin lesion at the site of exposure to the pathogen, usually an abrasion or cut. Incubation period is 9-90 days (on av. 21 days) and the disease normally lasts 3-6 months, but it may heal spontaneously before the appearance of secondary papillomas (small outgrowths). If not, it appears as a red patch, called an erythemous macule, which enlarges and may ulcer-ate. About 3 weeks after the primary (mother yaw) lesion appears, the serology becomes positive and within weeks proceeds to the secondary stage.
Treponemes spread systematically via the circulation and, 6-16 weeks after the primary lesion, papules (dome-shaped swellings) break out around the mother yaw; these become generalised and spread over the body surface. These lesions are common around the nose and mouth, and they exude fluids rich in treponemes that are highly contagious, particularly during the rainy seasons, and are easily transmitted to unaffected individuals via direct skin contact; there is also some evidence to suggest that spirochaetes may be transported by insects feeding on open sores, and even by contact with treponemes occurring on non-living surfaces. During its early life the disease can be treated with antibiotics, and treponemes are in fact very sensitive to even mild antiseptic chemicals. If left untreated, however, the affliction may be characterised by periods of remission and relapse that can persist for decades. Nevertheless, recurring infection can create bone deformities, soft tissue sloughing, weakness and possible reproductive inefficiency.

Gangosa
Gangosa (Spanish for "muffled voice") is a destructive ulcerative rhinopharyngitis (inflammation of the nose and throat) that may occur during the second phase of a yaws infection. The condition normally starts in the nose, on the junction of the mucous membrane with the skin close to the septum, which ulcerates and spreads inwards, destroying the whole of the nose, palate and pharynx until a large funnel-shaped cavity appears in the centre of the face (Trenouth 1975). Saddle-shaped nasal deformity due to ulceration of the nasal septum may be present. In extreme cases the ulceration may spread through the nose to involve the eyelids, and lead to scarring and consequent ulceration of the cornea.
G. W. Harley (in Trenouth 1975) selected 284 patients with nose and throat lesions in from 5597 cases diagnosed as yaws. He observed 41 cases of goundou (see below), of which 19 were associated with lesions of active gangosa. He concluded that goundou was related to early gangosa.

Tertiary Yaws
Tertiary yaws develops in less than 10% of affected humans and is probably equally uncommon in nonhuman primates. It occurs several years after the primary infection, leading to severely deformed bones and joint lesions which consist of new bone formation, destruction of bone by soft tumours, and osteomyelitis (bony inflammation). This late stage of the disease is non-contagious. Tertiary lesions are mostly restricted to the postcranial skeleton, but when cranial lesions occur they most commonly appear on the face, in contrast to the cranial lesions of venereal syphilis (Treponema pallidum), which are difficult to distinguish from those of yaws but are more likely to occur on the cranial vault. Gorillas appear to be more prone to progression to tertiary yaws than chimpanzees.

Periostitis
This is a reaction to inflammation of the periosteum, the thin layer of tissue that adheres to the outer surfaces of the bones. Because the periosteum contains bone-producing cells, one way it responds to inflammation is to lay down new bone tissue, producing a layer that has the general appearance of a roughened plaque-like deposit, punctuated by small channels and small holes caused by blood vessels (Lovell et al. 2000). In time, if the infection does not persist, the bone will heal and remodel itself; but persistent infection may initiate vigorous bone replacement, leading to extreme overgrowth.
Periostitis is most commonly localised and is frequently indicative of inflammation due to injury, but it may also occur as a response to a number of infectious diseases, including yaws. In such cases, periostitis may be generalised throughout the skeleton. If the infection invades the bone marrow it produces osteomyelitis (bony inflammation), which results in the destruction of bone tissue.

Goundou
Goundou (also called "big nose") is a rare manifestation of tertiary yaws, produced by bilateral bony swelling of the maxilla or upper jaw bone (Trenouth 1975). The lesions are caused by deposition of new bone under the periosteum. Rarely, unilateral cases occur, and as the extosis (bony outgrowth) becomes larger, the nasal passages are obstructed and later on the line of vision is interfered with. Indeed, the expansion may displace the eye and destroy it. Although there were earlier reports of the disease, Maclaud (1895) was the first to document the term "goundou", the name used locally in the upper region of Niger. He described the malady as consisting of nasal tumours. Interestingly, he related that some species of monkey were susceptible to goundou, and he observed a captive young chimpanzee with double nasal tumours, although he did not examine it.

Goundou in Gorillas?
Cranial deformities have been observed in a number of gorilla specimens from museum collections and from photographs of animals killed in the wild. These skulls exhibit robust overgrowth of bone, principally in the malar (cheek) region, both bilaterally and unilaterally, presumably due to persistent periostitis. Encroachment of the orbital cavities is so common as to be almost obligatory. Some of the protuberances are so grotesque that it is difficult to visualise the appearance of the animals in life. These deformities are quite unlike those described for chimpanzees in which goundou is suspected, which are much closer in appearance to the human disease.
Specimen A: Incomplete skull of adult male (sagittal crest) in the collection of the Paris Natural History Museum. The cranium exhibits extreme periostitis manifested in globular bone formations that are well-calcified. Pettit (1909), who examined the specimen, gave a height of 13.5 cm for the right growth and 11 cm for the left. Bilateral bone overgrowth in the malar, and with involvement of the orbital cavity close to the cheek, compares favourably with the deformities of the present series of skulls for which goundou is suggested.
Specimen B (in Petit 1920): Complete skull of an adult male that formed part of the private collection of M, Rouppert, a Parisian osteologist, and is now in the Paris museum; it was obtained in the interior of the Republic of Congo. The specimen displays a large glob of bone on the right cheekbone, with a similar deformity beginning to form on the left side. It was described simply as "exostosis" (bony outgrowth).
Specimen C: Adult male, one of two, obtained from Bossango in the former French colony of Oubangui-Chari (now the Central African Republic) and presented to the Paris Natural History Museum; they were examined by A. Lévri. The cranium displays multiple bone overgrowth on the cheekbones bilaterally, two protuberances on the right and one on the left. The bone of the two huge masses on the right side were described as porous and spongy, which may be suggestive of osteomyelitis, while the protuberance in the left was likened to a "handlebar" and measured 6 cm. The double outgrowths were compared to "champagne corks", measuring 3.5 cm, with a diameter of 5 cm, and had deformed the right orbital cavity. The condition was diagnosed as "goundou" (Seques 1929).

The second cranium (Specimen D) is remarkable for its astonishingly high sagittal crest, a "fortress of bone", that Seques thought was probably the result of goundou, he said was prevalent in apes of that particular region. However, there are no obvious lesions or other abnormalities of the skull, and it may simply be a genetic anomaly.
Specimens E and F are of wild-killed gorillas photographed by Armand Denis in the area around Ewo in what is now the Republic of Congo in 1942, and reproduced in Schultz (1950). Specimen E shows a condition closely resembling that of gangosa in humans (which, as described above, is a disease believed to be allied to goundou), while specimen F exhibits unilateral swelling of the zygomatic or cheek bone, the site of infection apparently being the right eye.
Specimen G (pers. comm. Ulrich Roeder, many years ago): Perfect skull of adult male killed by Pygmies at Lolodorf, southwest Cameroon, in 1979. The animal was part of a group in which it was the dominant male. It illustrates the usual accumulation of reconstituted bone in the cheek region bilaterally, with the distinctive "handlebar" deformity. The right orbital cavity has suffered trauma on the rim of the cheek.

Discussion
If these specimens do indeed represent goundou in gorillas then it is problematic. Whereas soft tissue lesions are similar in both gorillas and humans suffering from yaws, manifestations of bone hypertrophy that typify goundou are very different in the two species. In humans the disease is expressed on the nose and the upper jaw (maxilla), and is manifested in symmetrical oval-shaped swellings in the maxilla on both sides, with the long axis pointed downwards and outwards; if the infection is durable these swellings can grow to the size of an orange, or even larger. In the gorilla, however, bone hypertrophy is related to the malar (cheekbone), bilaterally or unilaterally, often invading the orbits (eye sockets), and bone expansion can be extreme and invasive. Of the 5 skulls described only one is free of encroachment or deformity of the orbit, while in 3 of the remaining 4 specimens the right eye has been traumatised. This suggests that the original site of injury or infection may have occurred in the eye region.
The exaggerated and sometimes grotesque bone formations in gorillas may have several explanations: the greater volume and density of gorilla bone tissue, particularly in the male, with this tissue being more robust in the cheekbone (zygoma) than in the maxilla; a more aggressive periosteal response to infection or injury in an extremely sensitive area (i.e. the eyes), resulting in vigorous, defensive bone replacement; and the age and virulence of the infection. Another factor could be that the pathogens involved are actually slightly distinct in humans and gorillas. The most compelling argument relating the two is that reports of goundou in gorillas appear to originate from those regions of Africa where yaws is endemic in human populations. A convenient explanation might be that human goundou represents tertiary yaws in the maxilla, and gorilla goundou represents tertiary yaws in the malar.
Levréro et al. (2007) describe a female gorilla in Republic of Congo that was afflicted with a severe yaws infection. The unfortunate animal was so ravaged by the disease as to be reduced to a skeletal condition. She had lived with a breeding group for at least 18 months, and during 7 subsequent visits to a forest clearing before disappearing, solitary males or adolescent males from groups that she approached rejected her and behaved aggressively towards her. Both a solitary life for a female gorilla and the rejection of an adult female by males is practically unheard of.

Don Cousins

References
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Lovell, N. C. et al. (2000): Skeletal evidence of probable treponemal infection in free-ranging African apes. Primates, 41, 275-290
Maclaud, J. (1895): Notes sur une affection désignée la boucle du Niger et le pays de Kong sous les noms de goundou et anakhre. Arch. Med. Navale 63, 25-32
Petit, L. (1920): Notes sur le gorille. Bull. Soc. Zool, FR. (Paris) 45, 308-313
Pettit, A. (1909): Lesions osseuses chez deux singes (Cebus Fatuellus L. et Gorilla Wymann). Bull. Soc. Path. Exot. 2, 220-223
Schultz, A. H. (1950): Morphological observations on gorillas. In: Gregory. W. K. (ed.): The anatomy of the gorilla, pp. 227-251
Seques, F. (1929): Un cas diagnostique "goundou" chez le gorille. Rev. Méd. D'Hyg. Trop. 21, 50-53
Trenouth, M. J. (1975): Goundou-tertiary yaws in the maxilla. Brit. J. Oral. Surg. 13, 166-171}