SCHISTOSOMIASIS, HOOKWORM AND WORM-RELATED DISEASES
hookworms in the intestines Worms like hookworm and schistosomiasis infect 1.3 billion people a year. Less than one percent lead to death but infection and reinfection lead to long period of discomfort, missed days from school and work, and stunted growth among children.
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An estimated one in five people in the world have hookworms, sharp-toothed parasites that attach themselves to intestinal walls and suck blood. Hookworms crawl up from the ground, penetrate the skin of their hosts and settle in the intestines.
Hookworms eggs hatch in shady area. The worms themselves live in the soil. They enter the body through the skin, often bare feet, and travel through the blood to the lungs and reach the stomach where they are coughed up and swallowed. From the stomach they move to the intestines, where make their home and can reach lengths of four inches (most are less than a half inch). People can be infected by a thousand worms that can live four or five years and can collectively suck up to a cup of blood a day.
There are treatments that can kill the worms. Hookworm is treatable with one or two pills that cost about $1 but people who live in areas where it is found tend to get it repeatedly. The parasites favor damp, cool environments and are particularly common in cotton, rapeseed and tobacco fields. Hookworm can be avoided by the sanitary disposal of human wastes and by wearing shoes.
Affect of Hookworms
Hookworms can stunt growth and cause anemia and lethargy. Hookworm-related ills include increased child mortality, stunted learning capacity and reduced economic production.
Hookworms take hold of certain villages or areas through a cycle of reinfection. Many of those who get the parasites are too poor to afford even flip-flops, and the parasites infect them when they walk barefoot in fields or bend down and touch the soil. Even when the worms are killed with medicines the victims often get reinfected again. Because most houses in infected areas don’t have toilets the worms are constantly re-introduced to the ground, where they can infect others.
Hookworms sap it victims of energy. Describing a village in Brazil where 80 percent of the inhabitants have hookworms a nurse that lives there told the Washington Post, “The people seem tired all the time, and they never eat. They don’t know what’s wrong with them.” In villages where hookworms have taken hold malnutrition is a problem because victims are too tired to tend their fields and their appetites are low.
Efforts to Get Rid of Hookworm
In the early 1900s, John D. Rockefeller, one of the richest men in the world at that time, tried to eradicate hookworm. He was successful in launching a campaign that wiped it out in the United States, where the parasite was eliminated after World War II, but not globally.
Efforts to develop a vaccine have been slowed by the fact that most victims are poor and drug companies have little incentive to spend the money necessary to develop one. As it stands now vaccines have been developed that successfully fight bacteria and viruses but none have been conceived that prevent parasitic diseases.
Trials for a hookworm vaccine have been conducted near Belo Horizonte in Gerais state in Brazil. The project has been made possible by a $53 million grant from the Bill and Melinda Gates Foundation and other donations from the Rockefeller Foundation and Doctors Without Borders. So far the result have been promising, Renata Diniz, a coordinator of the project, told the Washington Post, “Even after the first treatments, people started telling us they were feeling better, and that got them planting more crops.”
hookworm life cycle
Tapeworms are kind of fluke (parasitic relatives of flatworms) that have no sense organs or even a digestive system and nourish themselves by absorbing digestive fluids directly through their skin from their hosts.
Tapeworms are found in almost all living creatures. In many cases they cause little damages other than robbing their hosts of food but in some cases can seriously damage their hosts' health by physically injuring tissues or producing waste products that poison them.
Of the 1,500 kinds of tapeworms, three are common in man. One is picked by eating "measly meat." It has a head with suckers that clings to the intestines. Another, which reaches a lengths of 35 feet, is found in infected fish.
Tapeworms are secure as long as their host stays alive. The main problem they have is making sure their young make their way to another host. Many do this by depositing their eggs in their host's excrement, which in turn finds it way into the prey of another animal.
The eggs hatch and grow in this animal which is eaten by another animal, providing a safe home for a new generation. Since the chances of a host animal making their way into the food of another animal, and this animal being consumed by the animal is extremely unlikely, tapeworms often lay millions of eggs.
Tapeworm that live in the human digestive system need to get their larvae into a pig in order to infect another pork-eating human. The chances of this happening are so slim that the tapeworm may lay a million eggs a day and 7 billion eggs in a lifetime.
Flatworms are regarded as the simplest and most basic creature found in the sea. There are 3,000 species of them. Most but not all live the sea. Many are found in reefs, clinging under rocks and hidden in crevasses. Some of those found in coral reefs are quite colorful. Some flatworms cause serious illnesses in humans. Tapeworms and flukes are parasitic flatworms.
Like jellyfish, flatworms have a single opening to their gut which is used to take in food and excrete waste but unlike jellyfish they have a solid body. Flatworms have no gills and breath directly through their skin. Their undersides are covered by cilia, which beat and allow them to move slowly over surfaces. They have a network of nerve fibers but nothing that would qualify as a brain and they don’t have a circulatory system.
Despite their simplicity, flatworms have amazing powers. Some have been taught to negotiate their way through a maze. Not only that if they are killed and their flesh is feed to another flatworm they too can negotiate the maze.
schistosomiasis worms and snails
Schistosomiasis (bilharziasis or bilharzia) is caused by a blood fluke (a tiny worm also called a flatworm) that goes through a complicated life cycle utilizing a species of freshwater snail. After maturing inside a human host, adult flukes pair for life and produce thousands of eggs that damage organs and are discharged in urine and feces. The larvae that hatch from the eggs work their way into the snails that in turn produce large number of larvae capable of penetrating human skin. The flukes lives in the veins, bladder and large intestine of their human hosts and borrow molecules form their hosts to wear on their surfaces so the hosts’ immune system can't recognize them as alien.
Schistosomiasis is found in rural and suburban areas in 71 countries in Asia, Africa and Latin America (where it first arrived in the bodies of slaves). Globally, 200 million , or one in about 30 people, have the schistosomiasis parasites in their bodies. The WHO estimates that a quarter of million people died from it every year in sub-Saharan Africa alone.
Schistosomiasis was known to the ancient Egyptians. Schistosome eggs have been found in royal mummies thousands of years old. X-ray examinations of mummies have revealed pathological calcification associated with schistosomiasis and schistosomiasis worm tissues have been found in rehydrated ancient tissue. The name bilharzia is derived from Theaodor Bilharz, a German physician who discovered the parasites in Cairo in 1851. [Source: Patrick Skelly, Natural History, June 2008]
Schistosomiasis is painful. Infection often occurs two to three weeks after exposure to the contaminated water. Symptoms include fever, lack of appetite, weight loss, abdominal pain, weakness, headaches, joint and muscle pain, diarrhea, nausea and coughing. Rarely, the central nervous system is involved. Chronic infections can cause disease of the lungs, liver, intestines and/or bladder and enlargements of the spleen and liver and bloody diarrhea and urine.
Schistosomiasis Prevention and Treatment
Since there is no practical way to distinguish infested water from non-infested water, fresh water swimming should be avoided except in chlorinated swimming pools. The snails are usually more common in shallow, still or slow-moving water near shorelines and places with reeds, grasses or where human excrement has been discharged. The disease can be carried by insects and fish and can be contacted by drinking contaminated water (purification with iodine or chlorine kills the flukes) There is no chance of acquiring schistosomiasis in salt water.
The schistosomiasis can be treated with effective oral drugs (usually Nirdazole). Treatment involves one or two pills that cost about $1. Routine checks for the disease are advised if you have been some place where you make have contacted the disease. Diagnosis of infection is usually confirmed by microscope examinations of stools and urine.
In the 1950s and 60s schistosomiasis was one of the diseases targeted in China by the legions of "barefoot doctors" were sent out into the countryside by Mao Tse-tung. One of their most successful herbal medicines, pumpkin seeds, was used to rid victims of worms. Today the treatment is also used in Africa to combat schistosomiasis.
An algae fund off the Caribbean island of Curacao contains a poison that kills the snail that caused schistosomiasis.
schistosomiasis life cycle
The worms that cause the disease are a kind of flatworm called schistosome. There are 21 known species of schistosome. Five infect humans. Others attack antelope and zebra. One Schistosome hippopotami specializes in hippos.
Schistosomes live for years maybe even decades in the host’s body. A German medical journal described a woman who picked up the parasites while on a trip to Mwanza, Tanzania and had no idea they were in her body until she sought help from a doctor for stomach pain 15 year later.
Flatworms are regarded as the most primitive organism with a head. Under the “head” of the schistosome is large suction cup which the worm uses to anchor himself against the blood flow and shimmy through the veins.
Most of what is known about schistosomes is the result of research by Vaughan R. Southage of the Natural History Museum in London and the Louis-Albert Techum Tchuente of the Center of Schistosomiasis and Parasitology at the University of Yaounde in Cameroon.
Schistosomiasis Worm Sex
Most flatworms are hermaphroditic. Schistosomes are not. There are males and females, and they look quite different from one another. Males weigh three to four times more than the longer, slimmer females. Females don’t mature until they are paired with males; unmated females are only half as long as mated ones; and mated females lose their large size and stop laying eggs if their lose their partners. Males can be cut in half and even quarters and each piece will try to mate with a female. The urge to mate is so strong that if no females are present males will try to mate with other males. [Source: Patrick Skelly, Natural History, June 2008]
Males and female schistosomes locate each other in the blood system and mate to continue the cycle. When schistosomes of the opposite sex meet, the male holds the female in a groove under his undersides in way that has been compared to a “hot dog in a bun.” The grove which the female attaches to makes the male look like it has been cut open like a fileted fish. The grove gives the parasite its name, schistosome, or “split body.” [Ibid]
It was long thought that males and females paired for life but work Patrick Skelly of Tufts University found that paired females do latch onto new partners if new males are introduced. Instances of a “tug of war” involving one female and two males have been observed in petri dish. [Ibid]
The initial rendevous between a male and female takes place in the liver. After that the couple make their way to the blood vessels around the intestines and bladder. There the female begins laying eggs, which degrade the intestinal walls and move through the gut. Many of the eggs are discharged in urine and feces but many remain in the host and circulate through the body. It is the eggs that make a person sick. [Ibid]
Schistosomes in the Body
The schistosomiasis cycle begins when schistosomes eggs are expelled into water in the urine and feces of their human hosts. The eggs hatch into free-swimming microscopic miracidia , which penetrate the tissues of certain freshwater snails and reproduce into microscopic forms called cercariae that are capable of infecting human hosts.
The cercariae latch onto the skin of their hosts, burrow inside and take up residence in the body. They have powerful tails but no teeth or hooks. To get a good grip they vomit up sticky secretions from their mouths which they use to glue themselves on to their hosts. This makes them hard to wash off and give them time to probe the surface of the skin looking for places where the scales of skin overlap and they can burrow inside. Enzymes help them degrade the barrier and it takes only a couple minutes for them to get under the skin.
Patrick Skelly wrote in Natural History magazine, “Once inside the subdermal tissues, the parasites jettison their tails, and further outfit themselves for a new environment. No longer in freshwater, they alter their entire physiology and biochemistry. They shed their old surface coats and synthesize new ones, slipping into something more comfortable, more suited to the indoors. After a few days just under the skin the baby invaders push on to hit a blood vessel, that will carry them to the lungs and, after several days of further maturation there to the blood vessels of the liver, where males and females mature and mate.”
Image Source: Centers for Disease Control and Prevention cdc.gov/DiseasesConditions
Text Sources: CDC Health Information for International Travel, U.S. Department of Health and Human Services; National Geographic, New York Times, Washington Post, Los Angeles Times, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.
© 2009 Jeffrey Hays
Last updated March 2011