Also see Humanopathogenic worms
Once again, I find myself writing about that grotesque but nonetheless compelling topic - parasitology! If for some reason you don't want to read about tiny little critters burrowing through your skin and growing up into worms that release thousands of eggs into your body, then turn back now. Otherwise, continue to learn about one of the world's most pressing public health issues.
The basics
Schistosomiasis, or bilharzia, is a disease caused by various flatworms of genus Schistosoma, contracted when tiny larvae burrow into your skin when you step into contaminated water. That's right, folks - it's not enough to avoid eating when in certain parts of the world. Horrible things can happen to you just as a result of bathing or fetching water from a river. Worldwide, there are around 200 million people infected; the World Health Organization and many countries are working to eradicate this disease. Nevertheless, it remains common throughout the tropics, in the Caribbean, eastern South America, Africa, the Middle East, and Southeast Asia - 74 countries in total. In fact, it's so common in certain communities in Africa that infection with it is treated as a rite of passage.
Schistosomes belong to phylum Platyhelminthes, class Trematoda. Trematodes, also called flukes, are rhomboid in shape and responsible for many parasitic infections in humans. Schistosomes, or blood flukes, cause a chronic disease that damages the urinary tract, bowel, and liver in infected individuals. The disease is rarely fatal but can be extremely painful. Five main species affect humans: S. mansoni, present in Africa, the Middle East, and Brazil; S. haematobium, in Africa and the Middle East; S. japonicum, mainly in China; and the slightly rarer S. mekongi (found around the Mekong River) and S. intercalatum (certain parts of West Africa).
The disease was known in ancient Egypt; it is named for Theodor Bilharz, the German pathologist who first discovered the worm responsible. It is most prevalent in women and children; children frequently play in the water and are thus more likely to be infected. Women catch the disease by cleaning and doing laundry in contaminated water sources. Fishermen, not surprisingly, are also frequently infected.
The disease has major social and economic effects in areas where it's endemic. Even if most people have only mild symptoms, it has a major overall effect on economic activities and school performance is hurt in children infected with the worms. The disease is extremely common in some areas - around Lake Volta in Ghana, over 90% of children are infected. Incidence is so high in Egypt that bladder cancer secondary to schistosomiasis infection is the leading cause of death in men between 20 and 44 - an age rarely affected by bladder cancer in the developed world.
Memoirs of a schistosome
Let's start at the beginning. Okay, so someone infected with the disease releases eggs in their feces or urine. In areas with inadequate sanitation (those who've been following my writeups on parasites will note that inadequate sanitation is a major theme here), human waste eventually makes its way into rivers or streams. The eggs, upon reaching water, hatch into little baby miracidia, which then penetrate the schistosome's intermediate host, a snail. (Many parasites have two hosts in their life cycle: a definitive host, where they reproduce, and an intermediate host, where they spend another portion of their life.)
Once inside the snail, the miracidium then moves on to the next stage of its life: it grows up into a sporocyst, which is a sac-like larval stage. After two sporocyst stages, it breaks up into thousands of cercariae, which emerge from the snail in a daily cycle. The cercariae are able to swim, and they respond to turbulence in the water and chemicals detected from human skin. They explore and then attach to any skin they manage to find, and secrete enzymes that break down the skin, permitting them to burrow in.
At this point, the cercariae shed their tails, becoming schistosomulae. There won't be a test; you are not required to memorize all these words. At any rate, they migrate through the circulatory system into the lung, and thence to the liver, where they develop into their adult forms and begin feeding on red blood cells. Adult schistosomes average about a centimeter in length, and they are dioecious - that is, they have two distinct sexes.
Female and male worms pair up, and unlike televangelists they are completely faithful to one another; in fact, they attach to each other and the larger female worms become completely dependent upon males for nutrition. They travel through the circulatory system to their permanent home, whereupon they attach to the walls of veins surrounding the intestines or, in the case of S. haematobium, the bladder.
The happy couple finally starts having babies - they lay eggs that work their way through the intestinal lining or into the ureters and bladder. Adult worm pairs live on average for five years, and they release hundreds or thousands of eggs per day depending on their species. The eggs are released when the human host goes potty, thereupon starting the next generation.
So what's it like to have schistosomiasis?
It's not great, let's face it. It doesn't start out too bad. An itchy rash occasionally occurs within a few days of infection; within a month or two, a cough, fever, chills, and muscle aches can happen. In severe infection, particularly by S. japonicum and S. mansoni, symptoms may be bad enough to severely sicken or kill the patient; this syndrome is called Katayama fever. In most patients, though, this stage is asymptomatic.
After awhile, though, it gets worse. Those eggs we talked about, that burrow through the lining of the intestines and bladder? Over years and years, that causes long term damage. Bleeding is a common symptom - bloody shit, or in the case of the bladder-dwelling S. haematobium, bloody piss, is common in affected people. The intestinal wall and bladder can become scarred as time passes. Occasional bouts of bloody diarrhea affect those with intestinal varieties, seriously impacting their success in the dating world. Abdominal pain, fever, and fatigue occur as well; colon damage can accumulate until a chronic inflammation develops and polyps form; patients are at increased risk of other bowel diseases due to the worms' damage. Bladder cancer is common in people with urinary schistosomiasis, as are other problems with the kidneys and bladder.
Not all the eggs make it. Many are instead swept up in the blood and they end up in the liver. The eggs themselves are harmless, but the body's immune response results in inflammation and eventual damage, as granulomas - fibrous lesions - form around the eggs. An enlarged, diseased liver and spleen are common after long-term infection. Rarely, eggs can make their way to the brain or spinal cord, which can lead to inflammation and seizures or paralysis. Occasionally the skin, lungs, adrenal glands, or eyes can be involved as well. Some varieties cause sores on the vulva that increase the likelihood of HIV transmission.
Even after the initial infection clears, which takes a number of years, reinfection is common, compounding damage to the bladder, intestine, and liver from the previous go-round.
So what can I do if I get it?
The disease is diagnosed by examination of the stool or urine for eggs, which may have to be repeated over time as release of eggs can be intermittent. As it happens, schistosomiasis is eminently treatable. A single dose of praziquantel, a common drug used to treat parasites, will clear up infection. It's a remarkably safe drug; before its invention, therapy was highly dangerous. However, where the disease occurs, reinfection is common and broader efforts are needed to control the spread of the parasite by interfering with other phases of its life cycle.
Well then, what are people doing about it?
The World Health Organization and various other international organizations and NGOs (non-governmental organizations) formed the Partners for Parasite Control in 2001 to help control parasitic infections. One of the organization's goals is to regularly treat 75% of school children infected with the disease by 2010, and to ensure that drugs are available for it in every country where it occurs. Targeting children in particular is reasonable because it's easiest to achieve widespread treatment by using school personnel in the process and because children are more frequently infected than adults. Sadly, the 20¢ average cost to treat a child for schistosomiasis is a serious barrier to adequate control of the disease.
The Partners' current strategy for treatment involves providing praziquantel to every child in regions where the disease is rampant; since reinfection is often inevitable, ensuring that people are and remain completely cured is less useful than simply working continuously to ensure that no child is severely infected. The number of worms present is directly related to the severity of damage; if few are present, the likelihood of symptoms developing is low.
Some areas use other strategies as well as drug treatment. Other steps to control the disease involve the more generally useful step of making sure adequate sanitation exists so that doody doesn't end up contaminating the water supply. Streams and lakes heavily populated with the snails that harbor the intermediate stages of the parasite are treated to kill the snails off, which can very significantly reduce the rate of infection.
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