Since antiquity Dracunculus medinensis, commonly known as the Guinea worm, has been documented by scholars in a wide range of countries throughout Europe, Asia and Northern Africa, including Greece, Egypt, Iraq, and India. Dead female worms have been found in the mummified remains of 3000-year-old Egyptians. Currently, the parasite is found in 13 sub-Saharan African countries: Mauritania, Mali, Niger, Cote d'Ivoire, Ghana, Togo, Benin, Nigeria, Burkina, Sudan, Central African Republic, Ethiopia, and Uganda. Of these 13 countries, Sudan, Ghana, and Nigeria account for 93% of all cases worldwide. Other species of the genus Dracunculus can also be found in North and South America. (Cairncross, et al., 2002; Greenaway, 2004; Hopkins, et al., 2008)
Dracunculus medinensis favors freshwater habitats, mainly stagnant waters such as ponds, cisterns, pools in dried up riverbeds, and temporary hand-dug wells. These habitats are reservoirs for copepods, the vector of the Guinea worm disease. Known cases of dracunculiasis, or the Guinea worm disease, are in sub-Saharan Africa where the climate is dry at least part of the year, creating the perfect conditions for warm, stagnant, shallow waters like the ones where D. medinensis larvae are commonly found. (Greenaway, 2004)
The environment within the D. medinensis copepod host varies. In some hosts the Dracunculus larvae remain relatively unharmed. They survive in the gut of the crustacean and live off of its host's resources without major harm to the copepod. In other cases, the copepod will attempt to digest its occupant. Dracunculus medinensis larvae typically target intermediate hosts that are less likely to reject their occupancy or die due to the added burden of sharing its resources. (Bimi, 2007)
The female D. medinensis worm is a long white worm that can reach over a meter in length and a thickness between 1-2 mm. The male is significantly smaller, only a few centimeters in length, and dies soon after copulation. During the mating phase, both the male and female are approximate in size. Larval Dracundulus medinensis are free living, microscopic in size, and resembles free living nematodes. The female worm is covered in three layers of epicuticle and one thick layer of cuticle. Adult female worms have intestines but the organs are thought to be nonfunctional. In the larval worm, the anterior portion of the intestines can store food. In adult females, the majority of the body cavity is occupied by the uterus. (Greenaway, 2004; Lamah, et al., 1989; Watts, 1998)
Copepods containing D. medinensis larvae enter the human digestive system and are broken down, releasing the infective worm larva. The freed larva move into the small intestine, penetrates the intestinal wall and burrows its way into connective tissues. Around sixty to one hundred days after infection the larva begins to mature and mate. Maturation occurs through a series of molts, characteristic of the nematode phylum. It is unsure how sex is determined in D. medinensis but scientists believe it is similar to other nematodes via sex chromosomes. Dracunculus medinensis can continue to grow through molting for an indeterminate number of molts. Approximately a year after mating, mature females with fertilized eggs move toward the surface of its host’s skin. This causes burning and painful blisters to the host. When the blisters are broken, the worm is exposed to the outside environment. Mature worms typically grow to an average length of one meter. To ease pain the afflicted host will often rinse his or her ailing body part into water. The sudden change in temperature will prompt the female worm to release her stage one larvae into the water. Newly released larvae are eaten by a copepod and within their new host the larvae mature to the infective third stage. The maturation process usually takes up to fourteen days. If a new host drinks the water and ingests a copepod the infection process continues. (Bimi, 2007; Greenaway, 2004; Ruiz-Tiben and Hopkins, 2006; Tayeh, et al., 1993)
The most painful period of the life cycle of D. medinensis and the one which causes the most damage is when the mature female emerges to the surface of the skin. Before emerging there are no symptoms of infection and for a year up until the moment after emerging a victim could be completely unaware that he or she is infected. (Cairncross, et al., 2002; Greenaway, 2004; Knopp, et al., 2008)
Once the adult female worm is ready to emerge she usually chooses the lower extremities such as the feet or lower legs to exit but worms can and will emerge from any part of the body. The emergence of D. medinensis causes the formation of blisters. Once these blisters occur the victim will feel a painful burning sensation and is often bedridden for the duration of the illness. In addition, the Guinea worm is unique in that there is little to no acquired immunity against it. As a result, it is likely that people can develop the disease more than once in their lifetimes and as often as once each year. (Chippaux, 1991; Greenaway, 2004; Ruiz-Tiben and Hopkins, 2006)
Dracunculus males and females mate within the walls of the abdomen and thorax. They reach this location before sexual maturity but after two molts they are ready to mate. During this stage, they are approximate in size. After mating, the female grows to a size significantly greater than her mate. It is not widely understood how the worms find or attract one another but once they find each other, they mate one male to one female. This is due to the short lifespan of the male. Two months after mating with the female, the male is encapsulated and dies. Because population density of Dracunculus medinensis inside the host is low, there is no social structure among the parasites and there is no special mating behavior required to attract a mate. In general, once a male and female find one another, they reproduce immediately. (Cairncross, et al., 2002; Lamah, et al., 1989)
Dracunculus medinensis can reproduce at any time of the year. Once they have reached physical maturity (60 to 100 days after entering the host), and have found a mate of the opposite sex, the worm can reproduce within the walls of the abdomen and thorax. After mating, it takes between 10 to 14 months for the female to mature and emerge from the host ready to release mature larvae into the water. The female carries 3 million embryos but releases only hundreds of thousands of stage one larva into the water. Once released into the water, the larvae can immediately begin searching for a copepod intermediate host. (Bimi, 2007; Greenaway, 2004; Hunter, 1996; Lamah, et al., 1989)
Once fertilized, the embryos of D. medinensis develop inside the mother until they have matured into stage one larvae. The worms live off the mother's resources, which in turn are the host's resources, until a change in temperature (due to sudden exposure to water) triggers the release of the larvae from the exposed mother into the surrounding aquatic environment. Once released there is no parental involvement. The larvae can survive for a few days on their own but must find their intermediate host, the copepod or cyclops (a microscopic crustacean), within that time. For male worms, there is even less parental involvement. After mating with the female, the male remains inside the walls of the abdomen and thorax where he is eventually encapsulated in tissue and dies two months later. (Greenaway, 2004; Lamah, et al., 1989)
Dracunculus medinensis larvae survive for about three days in the water before entering their first intermediate host: the copepod. When it is in its human host, it undergoes its normal life cycle inside the host (see Development) and emerges about a year later. The worm continues to affect its host months after emerging. This leads to an general life span of around 1-2 years. (Cairncross, et al., 2002; Greenaway, 2004)
Dracunculus medinensis is a solitary species. Once inside the host, individual worms independently burrow to the site of reproduction. After mating, paired worms go their separate ways. The male remains in the walls of the thorax and abdomen while the female burrows to other tissue inside the host, usually in the lower extremities. Both the male and female have multiple sites of occupation within their hosts during their life span. As larvae, they are able to live independently for a few days in water and are adapted to swimming. Within that time they are taken up by their intermediate host, the microscopic crustacean called the copepod or cyclops. During this time they live off of the nutrients of their host, sometimes greatly decreasing the fitness of the copepod. Then when ingested by the definitive host, humans, the larva arrives first in the stomach, then migrates through the intestinal wall to to the abdomen and thorax where it molts and reproduces. After mating, the mature female worm burrows into the tissue of some other part in the host's body to prepare for offspring dispersal. There is no seasonality to the movement of the worm, though release of eggs is dependent on the availability of water. Only when the female burrows through the tissue and is exposed to water does she release her larvae. The difference in temperate when immersed in water signals to the female worm that she must release her larvae. (Bimi, 2007; Greenaway, 2004; Muller, 1979)
Dracunculus medinensis has no visual or audio sense organs. The larva has developed a morphology similar to the prey of their intermediate hosts, copepods, which increases their probability of being eaten by the correct hosts. Once inside its definitive host (the host in which the worm reproduces), D. medinensis uses physical means to borrow through the intestinal tract to the abdominal cavity where females and males mate. It is not well understood how the mates find each other. When the worm is ready to release its eggs, it excretes an acid substance which aids it in surfacing from the skin of its host. This behavior causes an intense burning sensation which forces the host to place the ailing body part in water. The difference in temperatures triggers the release of larvae from the exposed worm into the surrounding water. (Adeyeba and Kale, 1991; Belcher, et al., 1975)
Dracunculus medinensis is a parasite that feeds off of the nutrients of its hosts. In its larval stage, the worm feeds off of the internal tissue of its host, a copepod. When inside the human host, D. medinensis feeds on the surrounding tissue of its host, either through fluids in the intestines or muscles in the lower extremities. (Greenaway, 2004)
Dracunculus medinensis has two hosts, copepods and humans. The human is the Guinea worm's only definitive host and the copepod is its most common intermediate host. Copepod populations fluctuate according to the seasons and because copepods are the primary intermediate host for D. medinensis, the parasite populations also fluctuate accordingly. (Cairncross, et al., 2002; Greenaway, 2004; Hunter, 1996)
Abiotic factors also play a role in the prevalence of D. medinensis. Heavy rains correlate with fewer cases of the disease while dry seasons have a greater prevalence of the disease. Heavy rain dilutes the density of copepods and decreases the probability that a human will ingest a D. medinensis worm, but dry seasons increase the density of copepods and the probability of contracting the disease. Global warming may result in greater variability in both precipitation and temperatures. Such changes in climate will certainly have an affect on the dynamics of the Guinea worm disease and the strategies needed to eradicate the disease. (Hunter, 1996; Salinger, 2005)
Thus far, there are no visible benefits to humans from the Guinea worm disease. Due to its parasite nature, it almost always causes harm to its human hosts. (Cairncross, et al., 2002; Greenaway, 2004; Tayeh and Cairncross, 1998)
Dracunculus medinensis can only reproduce in humans. Physically, it causes problems for its host including fever, bacterial infections from the open wound it creates to release its eggs, severe and often paralyzing pain, and temporary (though sometimes permanent due to secondary infections) paralysis due to the emerging worm. Because of the debilitating nature of D. medinensis during its active stages, it can cause significant damage to the economy of its endemic region. People become bedridden and cannot work for as long as 3 months, resulting in a decline in productivity. Land isn't farmed and so agricultural output decreases which results in less sales, food shortages, and an overall financial loss of millions of dollars. For example, Nigeria lost $20 million in 2008 due to the Guinea worm disease. Even in areas where occurrences of GWD are fairly small, temporary disability could result in a five percent reduction in productivity. (Adeyeba and Kale, 1991; Audibert, 1993; Muller, 1979; Njepuome, et al., 2009)
In addition to the economic impacts, children are affected when their parents become disabled and are unable to work during the busy sowing and harvesting seasons. In order to substitute for their ailing family members, children must miss school, thus impacting their education and future prospects. As a result, schools must often close for one month during the dry season simply to accommodate the absent students who must step in for their parents when dracunculiasis cases reach its peak. (Cairncross, et al., 2002)
Dracunculus medinensis has been a target for eradication for many years. Once widespread in Asia and Africa, this parasite has been funneled down to contained cases in a handful of countries within Northern Africa. Emphasis on a region’s water supplies, its seasonality, and local habits are needed for effective eradication. Neighboring villages and programs followed through to completion have helped eradicate this parasite. (Cairncross, et al., 2002; Greenaway, 2004; Hopkins, et al., 2008; Molyneux, et al., 2004)
Early detection methods on humans need to be established. If a method was developed to detect the disease months before the worm begins to emerge patients can be found and contained sooner so that they are unable to spread the disease into public drinking waters. (Greenaway, 2004; Hunter, 1996)
One study found D. medinensis infections corresponded with an overall cytokine release depression as well as specific IgG1 and IgG4 antibody isotypes. Cytokine release depression, which went beyond parasite-specific immune responses, can help to explain why there are no discomfort or other symptoms associated with the early stages of dracunculiasis. The compromised immune system may be unable to initiate an inflammatory response against the intruder. Thus in the human host, D. medinensis is relatively less bothered by the immune system in comparison to its other macroparasitic counterparts. (Knopp, et al., 2008)
Lian Liu (author), University of Michigan-Ann Arbor, Heidi Liere (editor), University of Michigan-Ann Arbor, John Marino (editor), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
union of egg and spermatozoan
mainly lives in water that is not salty.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
Animals with indeterminate growth continue to grow throughout their lives.
(as keyword in perception channel section) This animal has a special ability to detect heat from other organisms in its environment.
fertilization takes place within the female's body
Having one mate at a time.
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
found in the oriental region of the world. In other words, India and southeast Asia.
an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
young are relatively well-developed when born
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