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This week at Infection Landscapes I will cover dracunculiasis, more commonly known as Guinea worm or the "fiery serpent". Dracunculiasis is an ancient disease, so embedded in human experience that it is stylistically represented as the very symbol of medicine and health across much of the world. Probably. It is also likely to become only the second human infection ever eradicated through public health effort (smallpox was, of course, the first and only human infectious disease eradicated).
The Worm. Dracunculiasis is caused by Dracunculus medinensis, which is a nematode and an obligate helminth:
Let's examine the complex life cycle of this waterborne worm. D. medinensis infects both a definitive host, humans, and an intermediate host, copepods, to complete its life cycle. I'll begin this helminth's developmental story in medias res: a mature adult female occupying an infected human host releases fully motile infective larvae into a community water supply by way of a blister on the host's skin. This blister is formed, and burrowed into, by the female worm. As the larvae are released from the human host, they enter the freshwater source and await their intermediate hosts, the copepods. Copepods are microscopic crustaceans that are ubiquitous in bodies of freshwater throughout the world. There are roughly 2800 species of copepods that occupy freshwater habitats, but those of the genus Cyclops, which alone comprises about 400 species, are probably the most important for maintaining the life cycle of D. medinensis.
This week at Infection Landscapes I will cover dracunculiasis, more commonly known as Guinea worm or the "fiery serpent". Dracunculiasis is an ancient disease, so embedded in human experience that it is stylistically represented as the very symbol of medicine and health across much of the world. Probably. It is also likely to become only the second human infection ever eradicated through public health effort (smallpox was, of course, the first and only human infectious disease eradicated).
The Worm. Dracunculiasis is caused by Dracunculus medinensis, which is a nematode and an obligate helminth:
Dracunculus medinensis larvae
Let's examine the complex life cycle of this waterborne worm. D. medinensis infects both a definitive host, humans, and an intermediate host, copepods, to complete its life cycle. I'll begin this helminth's developmental story in medias res: a mature adult female occupying an infected human host releases fully motile infective larvae into a community water supply by way of a blister on the host's skin. This blister is formed, and burrowed into, by the female worm. As the larvae are released from the human host, they enter the freshwater source and await their intermediate hosts, the copepods. Copepods are microscopic crustaceans that are ubiquitous in bodies of freshwater throughout the world. There are roughly 2800 species of copepods that occupy freshwater habitats, but those of the genus Cyclops, which alone comprises about 400 species, are probably the most important for maintaining the life cycle of D. medinensis.
Cyclops copepod
These copepods ingest, and are subsequently infected by, the larvae that have been introduced into the body of freshwater by the infected human host. A further 2 to 3 weeks of larval development are then required in the copepod host before the larvae reach their 3rd stage, which is then infectious to new susceptible human hosts. The infectious larvae are transmitted to humans when people consume the same water that is contaminated with the D. medinensis-infected copepods. Thus, transmission is exclusively by way of the common vehicle, water. In fact, this helminth infection is the only worm we will cover in this series that is strictly waterborne. After consumption of the contaminated water, the copepods are digested and the D. medinensis larvae are released in the small intestine. The larvae then migrate out into the abdominal cavity where they begin their migration to and within connective tissue, mature to the adult stage, and mate. Males die after mating, but females continue their subcutaneous migration, usually, but not exclusively, moving distally toward peripheral structures in the lower limb, i.e. bottom parts of the leg or the foot. After approximately one year following the initial infection, the female adult worm, who now harbors the live 1st stage larvae, begins to form an induration on the surface of the host's skin. Underneath, a fluid-filled blister forms into which the tip of the worm protrudes. At this point, the blister causes a very painful, burning sensation that is typically relieved with cooling water. When the blister is submerged in water, it breaks and the larvae are released instantaneously into the body of freshwater from the protruding worm. Thus a new generation is introduced into the water and is capable of infecting new copepods and, thus, recontaminating the water. Here is a nice graphic developed by the Centers for Disease Control and Prevention (CDC) that nicely depicts the life cycle of D. medinensis:
The Disease. Dracunculiasis does not typically cause life-threatening illness, unless the worm is removed incorrectly and dies within the host leading to extensive secondary infection. Nevertheless, because of the pain that is almost always associated with mature infections, particularly in the extremities, the disability that attends dracunculiasis can be severe during the eruptive stage, lasting approximately 3 to 10 weeks.
In addition, chronic musculoskeletal dysfunction is not uncommon due to a hypersensitivity reaction, secondary infections, or if the worm fails to complete its migration and dies and calcifies in musculoskeletal tissue. When the migratory track of the worm intersects the articulation of bones, then joint problems can ensue:
Finally, secondary infection of the ulcer that forms at the site of the blister can be quite serious if this vulnerable tissue is not carefully managed. Such secondary infections can indeed cause fatal disease.
The Epidemiology and the Landscape. This is an ancient disease. At one time this infection was a scourge that disrupted the lives of many across a vast expanse extending from West Africa, across the Middle East, South Asia, and into Southeast Asia. It also occurred in parts of the Americas. Before the beginning of the global eradication campaign (discussed in the Control and Prevention section below) estimates had the global prevalence of disease anywhere between 3 and 4 million cases. It was likely substantively higher than this at various points in history. Today we may be down to the last couple thousand cases in a few localized parts of a couple of sub-Saharan African countries. Dracunculiasis may become only the second human infection to be eradicated. Time and human experience will tell.
Around the turn of the 21st century the global burden of dracunculiasis was roughly geographically distributed as depicted in the map below published in the Canadian Medical Association Journal (CMAJ February 17, 2004 vol. 170 no. 4 495-500):
By the close of 2007, a much reduced distribution was apparent as depicted in this map published in the American Journal of Tropical Medicine and Hygiene (Am J Trop Med Hyg October 2008 vol. 79 no. 4 474-479):
Official reporting has the number of incident cases identified in 2011 at close to 1100, most of which occurred in small pockets of South Sudan, but a small handful came from Mali, Chad and Ethiopia. These numbers are likely under-reported especially in the areas of conflict in South Sudan, where most of the current cases still occur. Nevertheless, while complete eradication still requires vigilance and is by no means inevitable, it certainly does seem that dracunculiasis is now within reach of genuine eradication. Let's explore the landscape epidemiology more closely to get a better sense of how this worm effectively occupies a shared ecology with humans, and how this can be targeted to block transmission.
As described above in the life cycle of D. medinensis, water is of the essence. Water is the shared ecology between this worm and humans. In fact, it is the way in which water occupies both the physical and social landscapes that is responsible for transmission of infection.
First, by adapting to intermediate copepod hosts, D. medinensis has located within specific bodies of freshwater. In the areas of the world where dracunculiasis is endemic, this fundamental landscape requirement, i.e. bodies of freshwater, frequently overlaps the human social landscape in that high concentrations of infected copepods are found in important water sources for human consumption. For example, stepwells were a critical source of infection in India before dracunculiasis was eliminated in that country at the turn of the 21st century. These sites are typically artesian aquifers, which essentially provide reliable sources of surface water from the groundwater under pressure due to its geologic confinement underground:
These are known as confined aquifers, but unconfined aquifers can also be important constant sources of water, and can sometimes provide larger sources of surface water because they delineate the water table:
The main point is that these water sources are constant over long periods of time, and they stem from those specific points in the landscape where the groundwater breaches the surface and establishes a constant (relative in geologic time) source of freshwater. Because of the geologic structure of these aquifers, they provide extremely reliable sources of water to communities, in contrast to those unpredictable water sources that are more dependent on seasonal precipitation. Furthermore, because these aquifers have typically been reliable across many human generations in the communities where they occur, they are also frequently centers of social gathering and exchange in addition to being fundamental sources of water for consumption. India provides some amazing examples of the extent to which these overlapping geologic/hydrologic systems can intersect with social systems to provide unique landscapes that ultimately provided ecologic niches favorable to D. medinensis. Here is a picture of what is probably the most impressive stepwell in India, the Chand Baori, which was built in Abhaneri in Rajasthan in the 9th century. This stepwell is a full 100 feet deep and its architecture reflects a design that is intended to allow for relaxation and recreation among community members who come to use the well:
First, by adapting to intermediate copepod hosts, D. medinensis has located within specific bodies of freshwater. In the areas of the world where dracunculiasis is endemic, this fundamental landscape requirement, i.e. bodies of freshwater, frequently overlaps the human social landscape in that high concentrations of infected copepods are found in important water sources for human consumption. For example, stepwells were a critical source of infection in India before dracunculiasis was eliminated in that country at the turn of the 21st century. These sites are typically artesian aquifers, which essentially provide reliable sources of surface water from the groundwater under pressure due to its geologic confinement underground:
These are known as confined aquifers, but unconfined aquifers can also be important constant sources of water, and can sometimes provide larger sources of surface water because they delineate the water table:
The main point is that these water sources are constant over long periods of time, and they stem from those specific points in the landscape where the groundwater breaches the surface and establishes a constant (relative in geologic time) source of freshwater. Because of the geologic structure of these aquifers, they provide extremely reliable sources of water to communities, in contrast to those unpredictable water sources that are more dependent on seasonal precipitation. Furthermore, because these aquifers have typically been reliable across many human generations in the communities where they occur, they are also frequently centers of social gathering and exchange in addition to being fundamental sources of water for consumption. India provides some amazing examples of the extent to which these overlapping geologic/hydrologic systems can intersect with social systems to provide unique landscapes that ultimately provided ecologic niches favorable to D. medinensis. Here is a picture of what is probably the most impressive stepwell in India, the Chand Baori, which was built in Abhaneri in Rajasthan in the 9th century. This stepwell is a full 100 feet deep and its architecture reflects a design that is intended to allow for relaxation and recreation among community members who come to use the well:
In the days before dracunculiasis was eliminated from India, these kinds of water sources, which were already places of social gathering, served as the prefect relief for blistering worms. Once infected individuals would seek relief in the water, the worms released their larvae, which readily infected copepods in the water and were subsequently consumed by the many people gathered at the well. This represents an incredible exploitation by this helminth of this unique landscape that represented the overlapping of geology, hydrology and society to form a very specialized ecologic niche. As mentioned, dracunculiasis is no longer endemic in India, so these stepwells are no longer sources of infection. However, in those parts of the world where dracunculiasis persists, similar hydrogeographic features in the landscape, representing constant water sources derived from aquifers, remain very relevant for continued transmission:
Control and Prevention. The primary approaches to preventing transmission of D. medinensis infection are comprised of 1) stopping the consumption of contaminated water sources, and 2) preventing contamination of water sources by implementing strictly controlled water submersion for infected individuals with blistering worms. In addition, the use of larvicidal agents to kill the larvae as they enter water sources from blistering worms, as well as the use of bore wells as primary water sources, can be important implementations that effectively block transmission. However, the latter two approaches are significantly more costly than the former two approaches.
Water filtering is a common way to prevent the consumption of contaminated water and can be achieved by using any fine mesh cloth (nylon is best) over the opening of an empty water vessel and pouring the potentially contaminated water through the mesh-cloth covering. This technique is very good at filtering out the larvae-infected copepods and blocks transmission by removing the intermediate host from the water. In some communities, filtering straws have been distributed that allow individuals to drink directly from the water source without ingesting the copepods (this is depicted in the photo above). Boiling is also an effective way to prevent infection as this kills the larvae before consumption.
Management of infected individuals is equally important in the control and prevention of dracunculiasis. These individuals must be prevented from contaminating the water sources used by the community. In order to break the life cycle of the worm and block transmission, infected individuals with blisters submerge their painful limbs in an isolated water container to allow the blister to burst and the adult female to release her larvae. The contaminated water is then sterilized and disposed of to prevent contamination of community water sources. The water can also be disposed of on dry ground as this will kill the copepods and the larvae they carry.
In addition, a combination of community education to prevent individuals from entering water sources, and ongoing rigorous field surveillance to detect any and all new cases of dracunculiasis are two important features of larger elimination programs. Indeed, the extraordinary effectiveness of water filtration and case management to prevent further infections, in concert with good field surveillance, epidemiology and community education are greatly responsible for the widespread successes of regional elimination programs in many parts of the world where dracunculiasis was previously endemic. Only four countries are still reporting cases: Mail, Chad, Ethiopia, and South Sudan, and most of these are coming from South Sudan. While there is still serious work to be done, we are on the verge of the global eradication of dracunculiasis. As mentioned before, global eradication of a human infectious disease is something that has been achieved only once before with smallpox.
The possibility of eradication in the case of dracunculiasis is borne of its epidemiology. First, this worm has no definitive host reservoir other than humans. While an intermediate host exists in copepods, and is required for the completion of the life cycle, without the human reservoir the worm cannot reach adulthood and reproduce. This makes transmission to humans essential for its survival. Pathogens that cause human disease, but which have animal reservoirs outside the human host are probably not eradicable because they don't require humans to replicate. Second, it is much easier to apply effective surveillance (an essential ingredient to any regional elimination program, and thus by extension to any global eradication program). Ongoing surveillance is required in order to monitor geographic and epidemiologic sources of current and new infections. Without this critical knowledge, it is impossible to direct control efforts, and without directed control efforts, you cannot eliminate a disease from a region. Dracunculiasis surveillance is made easier because cases are not easily missed. Infected individuals become clearly identifiable as the worm breaks through the skin, so case detection for this infection has much greater validity and reliability than many other infectious agents. Thus, with good case detection, it is possible to implement good surveillance. Third, there are no asymptomatic infectious individuals because the worm must penetrate the skin in order to release its larvae, without which no new infections can occur. As such, there is no possibility of missing infectious individuals who are capable transmitting infection subclinically. And, fourth, it is relatively simple and cheap to block transmission by 2 routes: 1) once identified, infectious individuals can be managed with relatively little cost to control the release of larvae from their worms and thus prevent further transmission, and 2) contaminated water can be easily decontaminated by filtering or boiling the water.
Treatment. Dracunculiasis requires a treatment that has transited many thousands of years. It is an ancient treatment for an ancient disease. Their is no modern drug that can be used, particularly because of the dangers to the host if the adult worm dies while occupying the musculoskeletal tissues. Some medications can be used to alleviate symptoms, but not to kill the worm. As such, the traditional approach to eliminating this pathogen, which transcends temporal, geographic, and cultural boundaries, is the stick. Yes, you read that correctly. As the worm begins to emerge from the surface of the infected individual's skin, the end of the worm is wound around a small stick. This is done very slowly, gradually winding only a centimeter or two per hour, or even per day, over the course of what can take days to weeks to complete:
The slow process is required because it is critical not to break the worm, which would kill it and present a far greater danger to the host than the mere presence of the worm.
This ancient treatment method for what was once an extraordinarily common disease across much of the world is likely the source of the symbolism in the Staff of Asclepius, who was the god of healing and medicine in Greek mythology:
And which today likely serves as the basis for the symbol of healing for many health and health care organizations around the world:
Control and Prevention. The primary approaches to preventing transmission of D. medinensis infection are comprised of 1) stopping the consumption of contaminated water sources, and 2) preventing contamination of water sources by implementing strictly controlled water submersion for infected individuals with blistering worms. In addition, the use of larvicidal agents to kill the larvae as they enter water sources from blistering worms, as well as the use of bore wells as primary water sources, can be important implementations that effectively block transmission. However, the latter two approaches are significantly more costly than the former two approaches.
Water filtering is a common way to prevent the consumption of contaminated water and can be achieved by using any fine mesh cloth (nylon is best) over the opening of an empty water vessel and pouring the potentially contaminated water through the mesh-cloth covering. This technique is very good at filtering out the larvae-infected copepods and blocks transmission by removing the intermediate host from the water. In some communities, filtering straws have been distributed that allow individuals to drink directly from the water source without ingesting the copepods (this is depicted in the photo above). Boiling is also an effective way to prevent infection as this kills the larvae before consumption.
Management of infected individuals is equally important in the control and prevention of dracunculiasis. These individuals must be prevented from contaminating the water sources used by the community. In order to break the life cycle of the worm and block transmission, infected individuals with blisters submerge their painful limbs in an isolated water container to allow the blister to burst and the adult female to release her larvae. The contaminated water is then sterilized and disposed of to prevent contamination of community water sources. The water can also be disposed of on dry ground as this will kill the copepods and the larvae they carry.
In addition, a combination of community education to prevent individuals from entering water sources, and ongoing rigorous field surveillance to detect any and all new cases of dracunculiasis are two important features of larger elimination programs. Indeed, the extraordinary effectiveness of water filtration and case management to prevent further infections, in concert with good field surveillance, epidemiology and community education are greatly responsible for the widespread successes of regional elimination programs in many parts of the world where dracunculiasis was previously endemic. Only four countries are still reporting cases: Mail, Chad, Ethiopia, and South Sudan, and most of these are coming from South Sudan. While there is still serious work to be done, we are on the verge of the global eradication of dracunculiasis. As mentioned before, global eradication of a human infectious disease is something that has been achieved only once before with smallpox.
The possibility of eradication in the case of dracunculiasis is borne of its epidemiology. First, this worm has no definitive host reservoir other than humans. While an intermediate host exists in copepods, and is required for the completion of the life cycle, without the human reservoir the worm cannot reach adulthood and reproduce. This makes transmission to humans essential for its survival. Pathogens that cause human disease, but which have animal reservoirs outside the human host are probably not eradicable because they don't require humans to replicate. Second, it is much easier to apply effective surveillance (an essential ingredient to any regional elimination program, and thus by extension to any global eradication program). Ongoing surveillance is required in order to monitor geographic and epidemiologic sources of current and new infections. Without this critical knowledge, it is impossible to direct control efforts, and without directed control efforts, you cannot eliminate a disease from a region. Dracunculiasis surveillance is made easier because cases are not easily missed. Infected individuals become clearly identifiable as the worm breaks through the skin, so case detection for this infection has much greater validity and reliability than many other infectious agents. Thus, with good case detection, it is possible to implement good surveillance. Third, there are no asymptomatic infectious individuals because the worm must penetrate the skin in order to release its larvae, without which no new infections can occur. As such, there is no possibility of missing infectious individuals who are capable transmitting infection subclinically. And, fourth, it is relatively simple and cheap to block transmission by 2 routes: 1) once identified, infectious individuals can be managed with relatively little cost to control the release of larvae from their worms and thus prevent further transmission, and 2) contaminated water can be easily decontaminated by filtering or boiling the water.
Treatment. Dracunculiasis requires a treatment that has transited many thousands of years. It is an ancient treatment for an ancient disease. Their is no modern drug that can be used, particularly because of the dangers to the host if the adult worm dies while occupying the musculoskeletal tissues. Some medications can be used to alleviate symptoms, but not to kill the worm. As such, the traditional approach to eliminating this pathogen, which transcends temporal, geographic, and cultural boundaries, is the stick. Yes, you read that correctly. As the worm begins to emerge from the surface of the infected individual's skin, the end of the worm is wound around a small stick. This is done very slowly, gradually winding only a centimeter or two per hour, or even per day, over the course of what can take days to weeks to complete:
The slow process is required because it is critical not to break the worm, which would kill it and present a far greater danger to the host than the mere presence of the worm.
This ancient treatment method for what was once an extraordinarily common disease across much of the world is likely the source of the symbolism in the Staff of Asclepius, who was the god of healing and medicine in Greek mythology:
The Staff of Asclepius
And which today likely serves as the basis for the symbol of healing for many health and health care organizations around the world:
The Star of Life: International and United States symbol for the emergency medical services
Flag of the World Health Organization