The Yaws Disease and a Spirochete Named Treponema
Linda Crampton is a writer and teacher with a first class honors degree in biology. She often writes about the scientific basis of disease.
Yaws is a chronic disease that affects the skin, bones, cartilage, and joints. Though it’s rarely fatal, it may have a major effect on quality of life by causing disfigurement, disability, and pain. In 2012, a doctor discovered that a single dose of an inexpensive, orally-administered antibiotic could cure the disease. This has led to the hope that it can soon be eradicated.
The disease is caused by a spirochete bacterium calledTreponema pallidum pertenue. The spirochete (or spirochaete) belongs to the same species but a different subspecies of the bacterium that causes syphilis. Yaws is not a venereal disease, however. The effects of the two subspecies in our body are very different, but apart from this fact they are hard to distinguish from one another.
Skin lesions are a common symptom of yaws. Bones, cartilage, and joints may also be affected by the disease. The origin of the name "yaws" is unknown.
Features of Spirochetes
Spirochetes (or spirochaetes) are microscopic, one-celled organisms belonging to the phylum of the same name, Members of the phylum cause a variety of human and animal diseases, including Lyme disease, leptospirosis, syphilis, and yaws. The bacteria in the phylum often have a helical shape and move by means of flagella. Their flagella are unusual compared to those in many other cells, however.
In most bacteria that have the appendage, the flagellum is a long, whip-like structure that is attached to one end of the cell and extends into the liquid or semi-liquid medium surrounding it. The flagellum rotates and propels the cell through the liquid as it does so. The rotation is driven by a structure called the flagellar motor, which is located at the base of the flagellum.
The cell of a spirochete is enclosed by two membranes. These have a space between them called the periplasm, which contains a thin gel. The periplasm contains multiple flagella. The rotation of the flagella within the periplasmic space allows the cell to move through a wide variety of environments, which is helpful for the bacterium when it's travelling through our body. It's able to move through a viscous medium such as the one located in our connective tissue, for example. The movement of each flagellum is controlled by a motor, as in other bacterial cells. The details of how the flagellar movement propels the spirochete are still being explored.
Treponema survives by absorbing chemicals that it encounters in our body. It uses the chemicals as nutrients.
The Subspecies of Treponema pallidum
Three subspecies ofTreponema pallidumaffect humans: T. p. pallidum (whose name is also written as T. p. ssp. pallidum), T. p. pertenue, and T. p. endemicum. The first bacterium causes syphilis, which is generally venereal, or sexually transmitted. The bacterium is very invasive, produces a wide range of symptoms, and is sometimes dangerous. T. p. endemicum causes a disease called bejel or endemic syphilis. Despite the word "syphilis" in its alternate name, it's not a venereal disease. Like T. p. pertenue, it affects the skin and bones, though it causes different symptoms. It doesn't cause a deadly infection.
Although the three subspecies of T. pallidum cause different effects in our body, they look identical. They also produce the same results when serological tests are performed. A serological test is a blood test that looks for specific antibodies. Another similarity is that the three bacteria affect the body in stages (primary, secondary, and tertiary).
Treponema carateumcauses a skin disease called pinta. At the moment, it's classified in its own species. Although genetic tests haven't yet been done to prove that it's a subspecies of T. pallidum, researchers strongly suspect that it is. It produces the mildest disease of the four bacteria and affects only the skin.
Whole-genome sequencing has demonstrated that the genome of T. p ssp. pertenue differs by only 0.2% from that of T. p ssp. pallidum.— Marks et al, British Medical Bulletin
Possible Symptoms of Yaws
Yaws almost always occurs in tropical areas. There may be three stages in the untreated disease, as described below. Only a doctor can diagnose the cause of the symptoms that are listed, which may not be due to yaws. Skin ulcers can be caused by a variety of factors, for example.
A major problem with the yaws bacterium is that it can produce a latent infection. In this stage of the disease, the patient seems to be well. The bacterium is still alive in their body, although it's inactive. At some point in time it may become active and cause symptoms.
The first symptom of yaws is a papilloma (a benign tumour) in the skin. The papilloma is loaded with bacteria and sometimes releases an exudate. It's often known as the mother yaw. The legs are a common site for the mother yaw. The papilloma may eventually form an ulcer, or an open sore. It sometimes heals on its own, leaving a scar.
Other symptoms may appear while the mother yaw exists or weeks or even months after it has disappeared. They are caused by the spread of the spirochete via the blood and lymph. Skin symptoms are variable during this stage. They may include the formation of additional papillomas or ulcers and/or the appearance of yellow lesions. A lesion is a damaged area or one with an abnormal structure.
The patient may also experience swelling and pain in the long bones and the joints. Long bones are found in the arms, legs, hands, and feet. Tissue in the bones and joints isn't damaged during the secondary stage of the disease, however. Another symptom that a patient may experience is a condition known as malaise, which is the feeling of being generally unwell.
The third stage of the disease develops in up to ten percent of patients. It may begin around five years after the previous stage begins or even later. During this stage the skin, bones, and joints may be seriously damaged. Tissue in the nose, lower jaw, roof of the mouth, and throat may be destroyed.
Transmission and Extent of the Disease
Yaws is transmitted by skin-to-skin contact. People get the disease by touching an exuding papilloma or an open sore on someone's body. The chance of infection is increased if the recipient has a skin wound. The initial infection occurs in children and teenagers. People aged two to fifteen are most commonly affected. The effects may last through adulthood if the condition isn't treated in time.
It has recently been discovered that some wild primates (monkeys and apes) carry spirochetes that closely resemble the one that causes yaws. There is no evidence that the disease is passing from these animals to humans at the moment.
Yaws is currently present in at least thirteen countries. Up-to-date records are not available for some places. In addition, it's sometimes hard to distinguish yaws from other diseases and a diagnosis can't be confirmed. Another problem is that advanced tests to identify the presence of the spirochete are unavailable in many communities. The disease is known to be an important public health problem in parts of West Africa, Southeast Asia, and the Pacific, however.
Researchers have noticed that in a few places yaws symptoms are less severe than they once were in that area and the number of people who develop tertiary symptoms has decreased. The reason for these observations is unknown. Suggestions have included improvement in living standards, the wider availability of health checks, and treatment for other conditions with drugs that also help yaws.
According to WHO (the World Health Organization), in 1952 a huge effort began to eliminate yaws. Until the 1970s to 1990s, depending on the region, the effort seemed to have worked very well and the number of cases had dropped dramatically. Unfortunately, the incidence of the disease then began increasing again.
In 2012, a Spanish doctor and scientist working in Papua New Guinea made a very important discovery. Dr. Oriol Mitjà found that a single dose of an antibiotic named azithromycin could cure many cases of yaws. His discovery spurred interest in the disease again and renewed the idea that it could be eliminated. WHO's current goal is to eradicate the disease by 2020.
Azithromycin is the preferred treatment for yaws today because it's often effective and well tolerated, it's taken orally, and it's easy for caregivers to administer. It doesn't work for everyone with the disease, though. Fortunately, another treatment is available. A single dose of benzathine penicillin is very often an effective cure for the disease. It has some disadvantages, however. The medication must be given as an intramuscular injection by trained caregivers. In addition, administration by injection is often not well accepted by young patients. In some people the drug causes an allergic reaction.
Unfortunately, if treatment is started after serious tissue damage has occured, some of the effects of yaws may be irreversible. The treatment can destroy the bacteria and prevent further damage, however.
Eradicating the Disease
There may be some stumbling blocks to WHO's goal of eradicating yaws by 2020. Positive identification of the disease is important and its true distribution needs to be discovered. An effective drug must then be administered to everyone who needs it and the effects of the medication program monitored. These processes may take time.
In February, 2018, a significant report from Dr. Mitjà and other researchers appeared in The Lancet. The scientists administered a single dose of azithromycin in the mass treatment of a group of people in Papua New Guinea. Additional targeted treatment was also provided.
At first the treatment protocol appeared to have a high success rate. However, as community members who had been absent during the treatments returned, infections appeared in people who had been treated and the incidence of the disease rose. The researchers also found that a few individuals had genetic resistance to the effect of azithromycin. The event shows the importance of monitoring a treated community and of keeping careful records. It also indicates that eliminating the disease may not be easy. More than one mass treament in an area may be needed and more than one type of drug may need to be available.
Running from 1952 to 1964, the Global Yaws Control Programme was a big success, treating some 300 million people in 46 endemic countries and reducing prevalence by 95%. But the last mile was never walked. Remaining cases were not adequately traced down, latent infections overlooked, and surveillance measures not enforced, with yaws ultimately resurfacing in many areas.— Andrea Rinaldi, The Lancet Global Health Blog
A Worthy Goal
The goal of eradicating yaws is important. It's an unpleasant condition and in its worst form can seriously interfere with life. Eliminating the disease by 2020 would be wonderful, but the yaws researcher in the first video in this article thinks that reaching this goal is unlikely. Even if the goal isn't reached, I hope the effort to eliminate the disease continues and that it disappears soon.
The determination of some individuals to treat and monitor the disease is admirable. They need the support of local communities and authorities as well as international ones. The history of yaws treatment has shown that the spirochete will regain the upper hand if it's able to. Diligence is important in order to completely eliminate the disease. Money is needed, too. Azithromycin is being donated by its manufacturer, but many other expenses are involved in mapping and treating the disease.
The eradication of yaws in India has shown us that success is possible. A government-run program eliminated the disease in 2006 and WHO declared the country free of yaws in 2016. Hopefully the disease will soon be gone from the rest of the world.