10 April 2007
An overview of 3 parasitic infections: Chagas disease, schistosomiasis, and dracunculiasis
By Jessica Arabski, Mollie Hartung, Emily Iarocci, Akilah Jefferson, Cynthia Moreno and Elizabeth Morgan, Georgetown M.S. Graduate Program in Biohazardous Threat Agents and Emerging Infectious Diseases
Parasites depend on other organisms for survival and acquire resources at the expense of their hosts, which suffer health consequences as a result. Parasites such as protozoans and helminthes (worms) are a leading cause of disease in tropical, impoverished areas and enter the body through ingestion or penetration of the skin. The Centers for Disease Control and Infection (CDC) lists nearly 100 different parasitic infections affecting humans (www.cdc.gov). While many diseases exist, studying three prominent infections, Chagas disease, schistosomiasis and dracunculiasis, provides a broad overview of the parasites, hosts, symptoms, environmental factors, treatments and public health issues associated with parasitic infections.
Chagas disease, also known as American sleeping sickness, is caused by infection with the protozoan Trypanosoma cruzi. The illness is endemic in Mexico, Central America and South America, although cases have been documented as far north as the U.S. border. The parasite affects an estimated 11 million people annually, according to CDC. The Brazilian physician Carlos Chagas first discovered the infection in 1909.
Chagas disease is transmitted by certain species in the triatomine genus – part of the heteroptera, or “true bugs” order of insects – and survives on blood meals. The insects acquire the parasite after preying on an infected person or animal and then infect others during subsequent feedings. Triatomine are commonly found in mud and thatch houses, which provide spaces for the insects to hide during the day and are also prevalent in Central and South America, where most Chagas cases are isolated. Aside from direct contact with insect vectors, indirect infection can also occur through medical procedures such as blood transfusions and organ transplants, as well as consumption of food contaminated with insect feces. Congenital transmission is possible, too.
Chagas disease can assume both acute and chronic forms. The acute phase occurs directly after exposure and consists of mild non-specific symptoms such as fever, fatigue, muscle aches, diarrhea, vomiting, headaches, and localized swelling where the parasite enters the body, typically the facial area where the insect preferentially feeds. A trademark of acute Chagas disease is a symptom known as Romaña’s sign: swollen eyelids resulting from accidentally rubbing the feces the contaminated insect left behind into the eyes. Approximately five percent of patients – usually young children and the immunocompromised – develop a fatal infection of the heart or brain. Symptoms associated with the acute form of the illness usually last several weeks to months, although the untreated infection can persist and remain dormant for years. During the chronic phase, a third of those infected will develop cardiac and intestinal complications roughly 10 years after the original exposure. These symptoms can include enlargement of the heart, altered heart rate, heart failure and cardiac arrest, as well as enlargement of the esophagus or colon and complications related to eating or excretion.
Anti-parasitic treatments are effective against Chagas disease if administered early in the acute phase. Later delivery significantly diminishes effectiveness. Once the disease progresses to the chronic phase, treatment is restricted to alleviating symptoms.
While Chagas disease is a protozoan infection, schistosomiasis results from infection by flatworms, a type of helminth. The disease has a low mortality rate, but has high morbidity and causes incapacitating sequelae in millions of people each year. Formerly known as bilharzias after Theodor Bilharz, who identified the parasite in Egypt in 1851, schistosomiasis is acquired by humans through infection with Schistosoma haematobium, S. intercalatum, S. japonicum, S. mansoni and S. mekongi. A 2000 article in the tropical medicine journal Acta Tropica estimated that 85 percent of all cases occur in sub-Saharan Africa, although the parasite has also been isolated in southern Africa, Egypt’s Nile River valley, Brazil, Suriname, Venezuela, Antigua, the Dominican Republic, Guadeloupe, Martinique, Montserrat, Saint Lucia, Iran, Iraq, Saudi Arabia, the Syrian Arab Republic, Yemen, Southern China, the Philippines, Laos, Cambodia, Japan, central Indonesia, and Vietnam’s Mekong Delta.
Humans acquire schistosomiasis in freshwater bodies contaminated with infected human excretion. Feces and urine carry schistosome eggs that hatch once in water and seek a host in the form of specific snail species. The parasite multiplies asexually by the thousands and develops into a larval form that leaves the snail and is capable of infecting humans, who are exposed through ingestion, wading, bathing, washing laundry, or numerous economic activities involving fresh water. The flatworm penetrates the skin and grows inside the blood vessels, eventually producing eggs that migrate to the liver, intestines, or bladder and are passed in excretion. In some rare instances, eggs disseminate to the brain or spinal cord, resulting in seizures and paralysis. Schistosomiasis symptoms represent an immune response to parasite eggs rather than to the actual flatworm. Typical manifestations include a rash or itchy skin several days after infection, followed by non-specific symptoms such as fever, chills, coughing, and muscle aches roughly one to two months later. People repeatedly infected over the course of many years develop liver, intestinal, lung, and bladder damage. Schistosomiasis can also contribute to anemia and stunted growth.
Because schistosomiasis is acquired through contaminated water, the disease is often a symptom of poverty and represents lack of access to potable water. Globally, 200 million people are infected with the disease, according to the CDC. World Health Organization (WHO) statistics estimate the mortality at 11,000 deaths and the burden of disease representing 1.7 million Disability Adjusted Life Years annually. These figures, however, fail to consider sequelae and indirect health problems including liver disease, hydronephrosis, portal hypertension, haematemesis and other kidney problems, as well as cancer of the bladder.
A WHO Expert Committee on the Control of Schistosomiasis developed a morbidity control strategy based on chemotherapy in 1984. Previous control strategies revolved around snail elimination. In tandem with chemotherapy and snail control, other control measures include health education, adequate sanitation, and provision of potable water. According to the Special Programme for Research and Training in Tropical Diseases, a joint effort of WHO, the United Nations and World Bank, this control strategy is difficult to administer in communities with resource constraints. Many infected areas remain unidentified. The drug praziquantel, used in chemotherapy, is generally well-tolerated and effective.
Like schistosomiasis, dracunculiasis, also known as Guinea worm disease, is an illness found in impoverished areas with unclean water. Spread by the roundworm Dracunculus medinensis, the disease is endemic to the Sudan, which represents over 50 percent of all cases, according to CDC. The parasite is also considered endemic to Ghana and Nigeria, and has been isolated in Benin, Burkina Faso, Côte d'Ivoire, Ethiopia, Mali, Mauritania, Niger, Togo and Uganda. The latest information on the global eradication program against dracunculiasis can be found on the WHO website at: www.who.int/dracunculiasis/en/
Dracunculiasis typically results from ingestion of fresh water contaminated with microscopic crustaceans called copepods, or water fleas, carrying the roundworm Dracunculus medinensis. In their aquatic habitat, copepods consume D. medinensis larvae, which develop in the organism’s gut and assume an infectious form in 10 to 14 days. When a human ingests infected water, stomach acid dissolves the flea, but the larvae remains viable and, once freed, migrate to the small intestine. The larvae burrow through the intestinal wall and enter into body cavity, where the organism remains for about a year until developing into a mature, female roundworm two to three feet in length. A painful blister develops when the worm is ready to emerge and the area ruptures within 24 to 72 hours. Because the parasite grows vertically in hosts and approaches the lower limbs, approximately 90 percent of the worms emerge from the leg and foot area, according to CDC. The emerging worm can be pulled out centimeter by centimeter each day and wrapped around a small stick. A complete extraction can take weeks to months.
Dracunculiasis symptoms appear days to hours before the worm emerges from the skin, an event that, as noted, typically occurs a year after initial infection. Symptoms include fever, as well as swelling and intense burning in the area of emergence, which is designated by blistering and is key to perpetuating the cycle of infection. Regular activity, or potentially the desire to quell burning, can lead a person to accidentally or intentionally expose blisters to water. In response, the worm immediately produces larvae, which copepods will consume, thus allowing the parasite – and infection – to persist. Aside from transmission, the blisters associated with dracunculiasis have other implications, as they frequently become infected, leading to secondary bacterial infections. These infections extend the period of incapacitation and in some cases even lead to disabling complications such as locked joints and permanent crippling. When a worm emerges, a person often cannot work for three months, on average. This period typically coincides with planting or harvesting season. Because dracunculiasis occurs in areas that already face poverty, the economic ramifications of immobilization are profound.
Medication to end or prevent infection by D. medinensis larvae has not yet been developed. Surgical removal prior to blister formation is possible, however. Aspirin, ibuprofen and other analgesics can help reduce swelling and antibiotics can prevent secondary bacterial infections. Abstaining from standing pond water prevents infection. When in endemic areas, safe alternatives include using filtered water or water from underground sources. The provision of clean water is imperative to preventing dracunculiasis infections. Furthermore, political stability is key to eradication, as war clearly disrupts public health efforts.
A brief study of Chagas disease, schistosomiasis and dracunculiasis provide a broad overview of the diverse scientific and public health issues associated with parasitic infections. While parasitic infections come in many forms, what they all share in common is a detrimental relationship with a human host. Disrupting parasite-host interaction – whether through improving sanitation conditions, ensuring the availability of potable water, fostering political stability conducive to disease control efforts or a number of other measures – is an often daunting but pivotal prerequisite for halting infections.
1. Chitsulo L., et. al. The global status of schistosomiasis and its control. Acta Tropica, 2000, 77(1):41-51.
2. Fact sheet: Chagas disease. Centers for Disease Control and Prevention. Division of Parasitic Diseases.
3. Fact sheet: Dracunculiasis. Centers for Disease Control and Prevention. Division of Parasitic Diseases. http://www.cdc.gov/ncidod/dpd/parasites/dracunculiasis/factsht_dracunculiasis.htm.
4. Fact sheet: Schistosomiasis. Centers for Disease Control and Prevention. Division of Parasitic Diseases. http://www.cdc.gov/ncidod/dpd/parasites/schistosomiasis/factsht_schistosomiasis.htm.
5. Schistosomiasis. Special Programme for Research and Training in Tropical Diseases. World Health Organization. http://www.who.int/tdr/diseases/schisto/default.htm.
6. “Strategic direction for research: Schistosomiasis.” Special Programme for Research and Training in Tropical Diseases. World Health Organization. http://www.who.int/tdr/diseases/schisto/direction.htm.