Medicine for Africa - Medical Information Service
Parasitic worms – helminthes – are eukaryotic (one-cell) parasites that live inside the intestinal tract of their (human) host. They are organisms that live off the living host’s nutritional requirements, receiving their nutritional needs from nutrients within the host’s physiological system, thus causing weakness and disease. Worms that live inside the intestinal tract (small and/or large intestines) are called intestinal parasites.
Diseases caused by helminthes in humans include ascariasis (Ascaris lumbricoides – giant roundworm), dracunculiasis, elephantiasis, lymphatic filariasis, oncherciasis, schistosomiasis and trichuriasis.
Helminthes are divided into three major groups – cestodes, nematodes and trematodes.
Cestodes – are tapeworms or ‘specialized’ flatworms that resemble a narrow piece of adhesive tape. Tapeworms are among the oldest and largest intestinal parasites in humans and animals alike. Through a so-called ‘scolex’ segment, they can absorb predigested food for their own survival requirements. This scolex securely attaches to the intestinal wall of their host via suckers or hooks. The cestode’s body usually consists of hundreds or thousands of proglottids (individual body segments), each of which is a sexually complete unit that can produce an almost innumerable number of offspring without outside ‘help’. Cestodes can reach lengths of more than ten meters (30 feet) and life 30 years and more within their host, unless treatment has been instigated.
While most tapeworms require a human and animal host for their development, Echinococcus (E.) granulosis and E. multilocularis larvae produce slowly growing cysts in humans (while adult worms are infecting animal hosts), called ‘echinococcosis’ or ‘hydatid disease’, which requires surgical intervention, in order to remove the cysts that are often located within the liver. Echinococcosis usually results from eating insufficiently cooked and infected meat, especially beef (taenia saginata), pork (taenia solium) and fish (diphyllobothrium latum). Fleas of both dogs and cats can also transmit tapeworm larvae.
Nematodes – are the most numerous multi-cellular worms on earth. They are simple organisms, consisting of about 1,000 units (segments) and hundreds of reproductive cells. Just a handful of ‘infected’ soil can contain thousands of worms – there are close to 20,000 different species of nematodes described in scientific literature.
The nematodes that invade the human organism, i.e. the digestive tract, can range from as little as 0.3 mm to over 8 meters (more than 24 feet) in length. Nematodes are complete organisms, possessing a digestive, nervous, excretory and reproductive system; they lack a cardio-respiratory system.
Hookworm infestation is a leading cause of maternal and childhood morbidity in developing countries of the (sub-) tropics. Maternal hookworm infection can lead to intrauterine growth retardation, prematurity and low birth weight of the newborn; while the infection in young children can result in intellectual, cognitive and growth retardation.
Trematodes – the most important trematodes for humans consist of the blood flukes, Schistosoma (S.) mansoni, S. hematobium and S. japonicum, causing Schistosomiasis (Bilharziosis). Other significant trematodes include Clonorchis sinensis, the liver-invading fluke, the lung fluke Paragonimus westermani, and the intestinal fluke, Fasciolopsis buski.
Ascariasis – a parasitic roundworm, Ascaris lumbricoides, infects as many as one quarter of the world’s population. The infection is particularly prevalent in tropical regions and in areas of poor hygiene. Infection occurs via the ingestion of food that is contaminated with Ascaris eggs. The larvae hatch in the intestinal tract, then migrate to the lungs and up the respiratory tract, until they are being re-swallowed and mature again in the intestine, reaching about 30 cm in length (12 inches), while anchoring themselves tightly to the intestinal wall.
Dracunculiasis – also called Guinea worm disease, because it was first described by Europeans on the Guinean coast of West Africa. Once very prevalent in many countries in Asia and Africa, the disease is currently endemic in only five countries of sub-Saharan Africa – Ghana, Mali, Niger, Nigeria and Sudan as of June 2008.
Elephantiasis – also called lymphatic filariasis, is caused by parasitic worms such as Wucheria bancrofti and others, all of which are transmitted by mosquitoes, which are most prevalent in tropical regions, especially on the African continent. The worms can cause an extreme increase of regional lymph nodes, resulting in sometimes disconfiguring masses of convoluted lymph node regions (the ‘Elephant man’).
Onchocerciasis – also called River Blindness, is caused by the nematode Onchocerca volvulus, and considered to be the second most frequent cause of blindness. The worm is transmitted by the bite of a black fly and can live up to 15 years within the human body. Infection can cause intense itching, a strong reaction of the immune system with subsequent destruction of local tissue, such as the eye.
Schistosomiasis – also called bilharziosis or snail fever, is caused by several species of flukes of the genus Schistosoma. Freshwater snails, found in Africa, South America and Asia, are the carriers of the parasite and transmit the worm to bathing people in infested waters. Schistosomisasis is a chronic illness that can cause damage to internal organs, to one’s cognitive development, and even cause cancer of the bladder.
Trichuriasis – an infection with the whipworm Trichuris trichiura. A worldwide present parasitic worm, especially in warm and humid regions, where the whipworm infests the large intestine, in particular of young children and adults, who have become contaminated via contact with infected soil or vegetables (insufficient hygienic measures).
In general, parasitic worms such as hookworms produce fertile eggs by joining sperm and eggs, or by breaking off worm segments that contain both male and female sex organs, and are thus capable of producing fertile eggs.
All worm offspring are usually passed by way of poorly-cooked meat (especially pork), fish and beef, as well as via contaminated water (with feces), via mosquitoes, or by way of generally poor hygienic standards.
The worms, larvae or eggs usually enter the human body through the mouth, nose, skin or anus, commonly attaching to the mucosa of the intestinal tract. The digestive enzymes within the intestines dissolve the egg shells, allowing the release of a new worm, which is shelled from the host’s digestive enzymes by producing a protective keratin layer.
Symptoms related to parasitic infections vary depending on the specific organisms that are causative to the infection; however, most parasitic infections have the following symptoms more or less in common:
A first physical examination may reveal urticaria (reactive rash) and, at times, an extended abdomen, especially with hydatid cysts in echinococcosis, or in young children with extensive intestinal worm infestation.
The most definite and decisive diagnosis of any parasitic worm infestation of the intestinal tract can be obtained by examining one (or at times two timely separated) sample of the patient’s feces under the microscope – this will usually allow to identify either the worm itself or its characteristic eggs. This will pave the way for the most effective therapy, provided adequate treatment drugs are available. If a fecal specimen does not result in an appropriate diagnosis, an endoscopy including an optional biopsy can be performed to secure the diagnosis in dubious cases, or certain others, such as onchocerciasis.
Furthermore, blood tests may reveal a considerable (non-specific) increase in eosinophils, a type of white blood cells which respond to allergy-causing attacks by foreign bodies and organisms (such as parasites) to the body’s system.
Hemoglobin levels may decrease and a more or less significant iron deficiency anemia may develop, depending on the offending parasite. In addition to an iron deficiency anemia, a pernicious anemia, caused by a severe lack of Vitamin B12, can also develop after an extended period of a parasitic infection without appropriate treatment (especially in infections with the fish tapeworm Diphyllobothrium latum).
The primary goal of any treatment for helminthes is to remove both the adult worms and, if present, eggs, as thoroughly as possible from the host’s organism.
Medical treatment consists of so-called anthelmintic drugs which attack the parasite, eggs or larvae. Typical anthelmintic drugs and their indications include:
Usually one single dose, or sometimes, two doses spaced over a short period of time, is sufficient to treat most helminthic infestations. However, at times surgical intervention may also be required in cases such as:
Ascariasis – in acute intestinal obstruction due to the extensive number of worms present, surgical intervention is required.
Dracunculiasis – treatment of choice: direct removal of worms from tissues without breaking or tearing the worm.
Elephantiasis – in a long-standing infection, surgical intervention in order to increase lymphatic drainage may be required.
The most important preventive measurement is an appropriate hygienic environment and personal behavior. Thus, defecation should be confined to latrines and toilets, and proper hand washing after defecation is adamant. Furthermore, adequate footwear to prevent contact of bare feet with potentially infected soil should always be worn.
Regarding pork, beef and fish tapeworms – adequate cooking or pickling of the meat and fish, usually kills off most of the worms and/or eggs.
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