Buruli ulcer is a skin infection whose transmission method has been a long-standing scientific puzzle. Recognized by the World Health Organization as a neglected tropical skin disease, its presence is not confined to the tropics, with a notable emergence in other regions. This has prompted research into how the infection spreads, with investigations pointing towards an unexpected carrier: common insects.
The Nature of Buruli Ulcer
Buruli ulcer is caused by the bacterium Mycobacterium ulcerans. This organism belongs to the same family as the bacteria that cause tuberculosis and leprosy. The infection begins when M. ulcerans gets into the skin, often without an obvious break or injury. It typically starts as a painless, raised nodule or a broader area of swelling that can resemble an insect bite.
Over weeks or months, the bacteria release a toxin called mycolactone. This substance destroys skin cells, fat, and small blood vessels, leading to a large ulcer. The toxin also has local immunosuppressive properties, which is why the lesion is often painless and not accompanied by fever. This lack of pain can delay medical treatment, allowing the ulcer to become more extensive.
Investigating the Mosquito Link
For many years, the exact way humans contract Buruli ulcer was unclear, though it was known to be associated with stagnant or slow-moving water. Research, particularly from Victoria, Australia, has provided strong evidence implicating mosquitoes in its spread. Scientists have detected M. ulcerans DNA in and on mosquitoes collected in areas where people have been diagnosed with the disease. This discovery was a significant step in understanding the transmission pathway.
Further studies have shown a direct link between infected possums, mosquitoes, and human cases. In Australia, native possums can carry the bacteria and excrete it in their feces. Mosquitoes are thought to pick up the bacteria when they feed on these possums or come into contact with their fecal matter in the environment. One particular species, Aedes notoscriptus, a common backyard mosquito, has been identified as a primary vector in these regions.
The transmission is believed to be mechanical. This means the mosquito acts like a contaminated needle, physically carrying the bacteria on its mouthparts from an infected source to a human. The bacterium does not reproduce or undergo a life cycle inside the mosquito. This mode of transfer is supported by experimental studies where mosquitoes were able to transmit the pathogen to mice, and by genomic analysis showing that the bacterial strains found in humans, possums, and local mosquitoes are indistinguishable.
Global Hotspots and Environmental Triggers
Buruli ulcer is primarily found in tropical and subtropical regions, with West and Central Africa reporting the majority of cases across 33 countries. Specific coastal areas of Australia, particularly in Victoria, have become well-known hotspots in recent decades. The common thread linking these geographically distant locations is their environment. The disease is consistently associated with wetlands, swamps, and slow-moving or stagnant bodies of water.
These aquatic environments are thought to provide a favorable habitat for Mycobacterium ulcerans to thrive. The bacteria have been detected in various components of these ecosystems, including water, soil, and aquatic insects. Environmental changes, both natural and human-made, can trigger outbreaks. Events like floods, which can displace soil and alter water bodies, may contribute to the spread of the bacteria.
Human activities such as construction, agriculture, and deforestation can also disturb the environment and potentially increase exposure risk. These disturbances can create new breeding grounds for mosquitoes or alter the habitats of animal reservoirs. The connection between environmental disruption and disease emergence highlights the complex interplay between human land use and public health.
Prevention and Treatment Approaches
Preventing Buruli ulcer centers on reducing the risk of mosquito bites in endemic areas. Health advisories recommend several actions:
- Using personal insect repellents that contain DEET or picaridin.
- Wearing long, loose-fitting, and light-colored clothing to provide a physical barrier.
- Avoiding mosquito-prone areas, particularly around dusk and dawn when they are most active.
- Eliminating mosquito breeding sites around homes by removing standing water from pot plant saucers, buckets, or old tires.
- Promptly cleaning any cuts or abrasions and covering them with a dressing to prevent bacteria from entering the skin.
Once diagnosed, Buruli ulcer is treatable with a course of antibiotics, typically lasting six to eight weeks. Early diagnosis and treatment are important to minimize skin and tissue damage and prevent disability. In addition to antibiotics, wound care is a necessary part of management. For more advanced cases, surgery may be required to remove necrotic tissue and aid the healing process.