Yaws, the rare bacterial infectious disease: All you need to know

• 1807: William Mariner described a disease affecting the arms, legs, and perineal regions of children living on islands in the South Pacific Ocean [1].
• 1905: The bacteria causing yaws was discovered by Aldo Castellani, a physician who specialized in neglected tropical diseases. He suggested that yaws was caused by a bacteria that he called spirochaeta pertunis (now known as Treponema pallidum pertenue or T.pertenue) [2]. While he had discovered the bacteria earlier, he did not connect the dots until T.pallidum, a similar bacteria that causes syphilis, was discovered later that year. Wellman confirmed it later that year [3,4].

The bacteria is genetically a close cousin to T. pallidum, which causes syphilis, bejel, and pinta [5].

The term ‘yaws’ may have originated from the indigenous Caribbean name for a sore (yaya). The disease is also known as framboesia tropica and, locally, is also called buba, pian, paru, and parangi [6,7].

• Yaws incidence is over 80,000 cases each year [8]. 
• Although the overall incidence of yaws has declined significantly worldwide in the last 70 years, it remains endemic in 15 countries in Africa and Southeast Asia.
• Between 2008 and 2012, more than 300,000 new cases were reported to the World Health Organization. However, cases may be underreported because yaws is difficult to diagnose and are often overlooked [9]. 
• In 2020 new cases continued to be reported, including in places such as the Philippines, where it had not been seen for nearly 50 years [10]. 

Who is at risk?

• Yaws affects only humans, so there are no animal hosts or vectors.
• Around 75% of cases occur in children under 15 years. The highest number of cases is seen in children aged 6-10 years [11].
• Males are more commonly affected by yaws. One theory is that males may be more likely to be outdoors and engage in physical activity that may give them bruises on the lower parts of their bodies [11].

Endemic regions

• Yaws is endemic to tropical regions with warm and humid climates and is considered a Neglected Tropical Disease (NTD) [9].
• Countries where the average yearly temperature is above 27°C (about 81°F) and the rainfall exceeds 1,300 mm are especially prone to yaws. This includes tropical areas of Africa, Southeast Asia, Latin America, and the Pacific Islands [8].
• Yaws cases usually increase during the rainy seasons, showing a clear seasonal pattern [9]. 
• According to WHO, as of 2020, the majority of yaws cases reported were from the Western Pacific Islands, like Papua New Guinea and the Solomon Islands [8].

Yaws eradication efforts

WHO first targeted yaws for eradication in the late 1940s, right after World War II. From 1952 to 1964, the Global Yaws Control Programme eradication efforts were very successful, with 50 million people treated and a 95% reduction in prevalence [12].

However, without sustained public health efforts worldwide, yaws reappeared in the seventies. Once again, WHO’s 2012 roadmap planned for the eradication of yaws by 2020. While cases have dropped considerably, the bacterial infection is still not eradicated completely.

There is some good news on this front; India and Ecuador successfully eradicated the disease by 2004 [8,9].

Now WHO plans to eradicate yaws globally by 2030. Could it be that the third time’s the charm?

Yaws spreads primarily through direct contact with the skin [11, 13]. 

The disease is transmitted when discharge from the skin lesions of an infected individual comes into contact with the broken skin on an uninfected person (cuts, scratches, or bites), typically on the lower legs [11, 13].

Yaws is not transmitted from a pregnant mother to her child. This is unlike syphilis, a disease caused by another bacteria in the same family (Treponema pallidum) [13].

Yaws causes disease in humans by beginning with a local lesion or papule at the site where the bacteria enter the skin, commonly referred to as the ‘mother yaw.’ This lesion can develop into a large, oozing bump or a crusty sore, typically appearing on the legs but capable of emerging on other parts of the body [13]. 

Notably, unlike syphilis, yaws seldom affects the genital areas. If untreated, these sores may heal on their own within 3 to 6 months, often leaving behind a dark mark [13].

About 1 to 2 months after the initial infection, and sometimes longer, the bacteria can disseminate through the bloodstream and lymphatic system. This spread results in additional symptoms of yaws, including further skin lesions and bone complications, along with symptoms like fatigue and swollen lymph nodes [13].

In its later stages, similar to syphilis, yaws can cause various skin problems such as warty growths, ulcerations, scaly patches, or thickened skin on the hands and feet. This thickened skin can become cracked and infected, leading to significant pain and difficulty in walking, a condition often called “crab yaws.” At this stage, yaws rarely affects the mouth and throat [13].

Besides skin manifestations, yaws can also impact the bones, particularly the fingers and long bones like the forearms and legs. This involvement can lead to painful bony swellings, with multiple bones often affected. For instance, in a study from Papua New Guinea, 75% of children with advanced yaws experienced joint pain. This comprehensive effect on both the skin and bones underscores the disease’s debilitating potential if not adequately treated [13].

The incubation period for yaws is estimated to be 9–90 days [14]. 

Typically, yaws has 3 stages:

• Primary: The initial skin lesion, also known as the ‘mother yaw,’ is usually painless and appears on the lower half of the body and then progresses to an ulcerated nodule. 
• Secondary: Secondary yaws may appear weeks, months, or even years after the primary yaws. Usually, the infected individual has pain, swelling in the long bones and fingers, and raised yellow skin ulcers.
• Tertiary: When yaws is left untreated for a while, it may progress to severe bone deformities, damaged skin and joints, and even bone loss. The palms of the hand and soles of the feet can become thick and painful. This stage is also known as ‘crab yaws’ [14].

What is the difference between yaws and syphilis?

• Syphilis is sexually transmitted; yaws is not [15].
• Syphilis is found around the world, while yaws is mostly in remote parts of Africa, Asia, and the Western Pacific islands [15].

Diagnosing Yaws is challenging because the bacteria responsible cannot be cultured outside the human body. Also, the disease is rare, and healthcare providers may often misdiagnose it. 

• In remote areas where advanced lab tests are not available, quick diagnostic tests, similar to those used for syphilis, are used to identify the disease. These tests cannot distinguish between Yaws and syphilis, so need further assessments. 
• Doctors can use a technology called dark field microscopy to see the bacteria directly, but this method is costly and requires specialized skills. 
• Another way to diagnose Yaws is by testing for antibodies in the blood, but this can sometimes be inaccurate because the bacteria look similar to others, causing different diseases [16]. 

Treponemal rapid tests, while easily available and inexpensive, cannot distinguish between prior and current infections. So, they are not as useful to help determine if transmission has been interrupted [16].

Since yaws spreads through direct contact, it can easily be transmitted to family members and others close to the infected person. Therefore, it’s important to test other household members for signs of the infection as well [16].

• Single dose of antibiotic: A single dose of azithromycin or benzathine penicillin is effective in treating yaws. Antibiotics are quite effective, and complete healing may be observed in 95% of cases. Patients should be monitored and checked four weeks after initial treatment.
• Follow-up treatment: Additional doses of antibiotics may be necessary if symptoms persist or recur.
• Community-wide treatment: In areas with high rates of infection, mass treatment of the community might be conducted to eliminate the disease [13].

Unfortunately, in 2018, Mitjà et al. published their research on azithromycin-resistant yaws, a setback in global yaws eradication efforts [17].

Linezolid, an affordable oxazolidinone (a class of antibiotics), shows promise both in lab studies and real-world use against T. pallidum, the bacteria causing yaws. Research is ongoing to determine if linezolid can effectively treat yaws that is resistant to azithromycin [18].

• Break the chain of transmission: Focus on stopping the spread of the disease in countries where Yaws is common.
• Treat early: Identify and treat cases of Yaws early, especially at the community level.
• Avoid direct contact: To reduce the risk of spreading the infection, prevent skin-to-skin contact, particularly among young children.
• Prophylactic treatment: Give a preventive dose of antibiotics like azithromycin or penicillin to those who have been in contact with a confirmed Yaws patient.
• No yaws vaccine available: Currently, there is no vaccine for Yaws, so prevention relies heavily on preventive therapy [5,16].

Yaws, a neglected tropical disease, is on the World Health Organization’s list for eradication by 2030. Caused by the bacterium Treponema pallidum pertenue, yaws mainly affects children in rural, tropical regions and primarily impacts the skin, bones, and joints. While there is no vaccine, the disease is treatable with a single dose of antibiotics like azithromycin or penicillin, highlighting the importance of accessible healthcare.

The WHO’s efforts to eradicate yaws began shortly after its formation in 1948, with significant progress made in the mid-20th century. However, challenges remain, including misdiagnosis and the disease’s persistence in remote areas. Increased awareness and community-level interventions are crucial for stopping transmission and achieving the goal of eradication.

Understanding yaws goes beyond just knowing its symptoms and how it spreads; it also means tackling the socio-economic factors that sustain it. 

To eradicate yaws, we need global cooperation and sustained public health efforts to prevent it from continuing to harm and disadvantage those affected. By emphasizing treatment, prevention, and broad access to healthcare, we can make a significant difference in combating this disease.

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