In the past, we have covered how pathogens like Ebola virus and Anthrax bacteria are causing thousands of deaths among great ape populations in Africa. In this post I will introduce another pathogen, one that causes Yaws disease, that is having severe affect on the reproductive success of gorillas, as reported by Florence Levréro et al. in the American Journal of Physical Anthropology. The paper is titled, “Yaws disease in a wild gorilla population and its impact on the reproductive status of males.”

A brief side-note, Florence was a co-author on the gorilla susceptibility to Ebola paper I reported on last July… so to say she knows a thing or two about wild gorilla populations and current pathogens affecting them is an understatement. And, before I get into the details of the paper, let me preface with some information on Yaws. Yaws is a tropical infection of the skin, bones and joints caused by the spirochete bacterium Treponema pertenue. This disease is similar to sphyllis. Symptoms of Yaws include:

  • The disease is transmitted by skin contact with infected individuals or eye gnats, the spirochete entering through an existing cut or similar damage.
  • Within ninety days (but usually less than a month) of infection a painless but distinctive ‘mother yaw’ ulcer appears.
  • These tracts heal with keloid formation which can cause deformities, disabilities and limb contractures.
  • The bone lesions caused are periostitis, osteitis, and osteomyelitis, damage to the tibia can lead to a condition known as sabre shins.
  • In a very few cases a condition known as goundou is caused where growths on the nasal maxillae can result in extensive and severe damage to the nose and palate.
  • Examination of ancient remains has led to the suggestion that yaws has affected hominids for the last 1.5 million years.

In the 1950s, Yaws was the center of attention of worldwide treatment program which sought to eradicate the disease. The program was successful among humans. So successful that the program managed to reduced the an estimated 50 million Yaws patients to nearly zero. But the disease is on the rebound as reported by the World Health Organization, just last month. According to Wikipedia, where I paraphrased all this information from, “Yaws is on the rise again, with roughly a half a million sufferers, mostly in poor, rural areas.”

Yaws’ growth among human populations, in Africa, is being transmitted to great ape populations, like the gorillas studied here, through flies. Flies are often found in places were there is overpopulation, caused by human development and deforrestation. Ultimately, as both Florence and I argued before, Yaws, Ebola, and Anthrax are products of human encroachment on gorilla territories. The acquired immune systems of these great apes did not have previous exposure to these pathogens, and made them ultimately more susceptible to these introduced communicable diseases.

Okay that being said, the authors studied how Yaws is affecting gorillas in the wild, specifically among a gorilla population in the Republic of Congo. The scope of the paper was limited to studying the reproductive success of gorillas with Yaws, and any conclusions drawn from that could indicate whether gorillas decided on mating or not mating with an individual infected with Yaws.

From what I can tell, and I am not completely sure about this, Yaws isn’t fatal… so they didn’t study the mortality. The bacteria seems to just manifest itself as lesions and causes deformities… but I don’t know if those lesions can get infected and compromise the immune system of the individual with Yaws.

Of the 377 gorillas, in population that was studied, about 64 of them had Yaws. That’s over 17%! As reported in the abstract of the paper, the trends in sex and age of the gorillas are as follows:

“Lesions were more prevalent among males than females above 8 years old. This sex-bias prevalence could result from the behavioral characteristics of males through a greater exposure to wounds. Lesions were also more prevalent in unmated adult males (either solitaries or those living in nonbreeding groups) than in males leading breeding groups.”

Here is an image, from the paper, of three infected gorillas with Yaws. Click the image for a higher resolution photo. In my opinion, this disease renders gorillas to look like ghastly gouls.

Gorillas with Yaws Disease

As I skimmed the paper, before I read it all, I wondered if there outward appearance affected their sociability?

In my mind the authors answered that question. They provided several examples that sealed the deal for me, but they decided on reporting their observations and conclusions modestly,

“Proportionally more unmated adult males presented lesions than did males leading breeding groups. In addition, breeding groups included no silverbacks at an advanced stage of the disease. This suggests that yaws is related to the competitiveness and vigor of males. Two complementary hypotheses can be formulated. Either affected males exclude themselves from reproduction because they do not have enough strength to compete with other males to gain and/or retain females, or they are not or are less chosen by females. Sexual selection by females could influence their decision to leave a group and their choice of group to integrate…

Since breeding groups led by affected and nonaffected silverbacks had similar numbers of immatures under 6 years old, these results suggest that the disease could have an impact on the reproductive success of males only at advanced stages….

For example, we can report the case of a skeletal adult female with a completely destroyed nose who became solitary after residing in a breeding group for at least one and a half years. During her seven subsequent visits to the clearing before disappearing, solitary males or adolescent males from groups that she approached rejected her and behaved agonistically towards her. Both a solitary life for a female gorilla and the rejection of an adult female by males are very uncommon. Given the cognitive abilities of primates, the question of social discrimination based on how gorillas perceive their partners with skin lesions warrants further investigation in the future. “

This is a double whammy for gorillas, actually make that a quadruple whammy. Not only do we have to worry about the regular three headaches such as deforestation, poaching and fatal pathogens like Ebola; but now conservation efforts are impacted because this new pathogen is affecting reproduction and gorilla social group cohesivity.

Although Yaws can be treated with several doses of antibiotics, how can people deliver that to wild gorillas? Instead, the authors advocate, as was done in the Ebola paper, that the pharmaceutical industry needs to roll up their sleeves and start working on vaccines for these diseases. There’s a lot at stake here.