Ascaris worms (one type of helminth) in the small bowel of an infected person in South Africa (X-ray image with barium as contrast medium)
|Classification and external resources|
Helminthiasis // (alternatively spelled helminthosis; plural helminthiases), also known as helminth infection or worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms (helminths). These parasites are broadly classified into tapeworms, flukes, and roundworms. They often live in the gastrointestinal tract of their hosts, but may also burrow into other organs, where they induce physiological damage. They remain the major cause of wildlife diseases, economic crises in the livestock industry, and human socio-economic problems in developing countries.
Some types of helminthiases are among the neglected tropical diseases targeted under the joint action of the world's leading pharmaceutical companies and non-governmental organizations through an ambitious project called "London Declaration on Neglected Tropical Diseases" which was launched on 30 January 2012. It aims to control or eradicate those particular diseases by 2020, by ensuring necessary supply of drugs and other intervention, and promoting sanitation and health education.
Soil-transmitted helminthiasis (SHT) and schistosomiasis are the most important group of helminthiases, collectively belonging to the "neglected tropical diseases". Soil-transmitted helminthiases are responsible for parasitic infections in a quarter of the total human population. One well-known example of soil-transmitted helminthiases is ascariasis. As of 2014, schistosomiasis is the most prevalent of all parasitic infections in humans.
In cases of light infections there can be no symptoms.
Heavy infections directly damage tissues as the parasites can block internal organs or exert immense pressure in the gut. Infections are predominantly found in alimentary tract and sometimes in circulatory system, as the parasites inhabit these organs. General symptoms are stomachache, fever, vomiting, diarrhea, loss of appetite, loss of blood, fatigue, and listlessness.
Antimesenteric splitting of the outer layers of the bowel wall can occur due to large amount of helminths, such as ascaris: The bowel can be so tightly packed with ascaris that it becomes semi-rigid and inflexible and the outer layers of the bowel wall, which are the muscle layers, split under the tension.
Abdominal distension and abdominal pain might be the symptoms together with vomiting, sometimes vomiting of worms. Patients might present with peritonitis or a gangrenous bowel. This often requires emergency surgery. Volvulus complicating ascariasis still carries a high mortality and morbidity rate. Early detection and early operative intervention is very important.
In humans, under chronic infections, such as those in schistosomiasis, extreme morbidity is the common symptom. Morbidity is accompanied by persistent poverty, decreased productivity, poor birth outcomes, poor school and work performance, and poor socioeconomic development. A severe case of taeniasis can occur when the brain is infected by accidental ingestion of cysts, a clinical condition called neurocysticercosis, which is the leading cause of acquired epilepsy.
Indirect effects also associate with the disease. As pathogens, helminths induce immune reactions. Immune-mediated inflammatory changes occur in the skin, lung, liver, intestine, CNS, and eyes as they invade these tissues.The migration of Ascaris larvae through the respiratory passageways can also lead to temporary asthma and other respiratory symptoms. Systemic changes such as eosinophilia, edema, and joint pain reflect local allergic responses to parasites. In many cases, they can induce hypersensitivity leading to an acute allergy reaction called anaphylaxis. These immune responses can lead to increased susceptibility to other infections such as tuberculosis, HIV and malaria. Coinfection cases have become serious medical problems particularly in African countries. Heavy infection reduces HIV progression and viral load, most likely by improving helminth-induced immune suppression.
Helminthiasis is associated with nutritional problems such as vitamin deficiencies, stunting, anemia, and protein-energy malnutrition, which in turn affect cognitive ability and intellectual development. This relationship is particularly alarming because it is gradual and often relatively asymptomatic. Worms most probably compete directly with their hosts for nutrients; but the magnitude of this effect is likely to be minimal as the nutritional requirements of worms is relatively small. In pigs and humans, Ascaris has been tied to temporarily induced lactose intolerance and vitamin A, amino acid, and fat malabsorption. Impaired nutrient uptake may result from direct damage to the intestines' mucosal walls as a result of the worms’ presence, but it may also be a consequence of more nuanced changes, such as chemical imbalances caused by the body’s reaction to the helminths. Alternatively, the worms’ release of protease inhibitors to defend against the body’s digestive process may impair the breakdown of other nutritious substances, as well. In some cases diarrhea due to worms can cause speed “transit time” through the intestinal system, reducing absorption of nutrient.
Malnutrition due to worms can give rise to anorexia. A study of 459 children in Zanzibar revealed spontaneous increases in appetite after deworming. This could be due to a side effect of body’s immune response to the worm and the stress of combating infection. Specifically, some of the cytokines released in the immune response have been linked to anorexic reactions in animals.
Helminths may also cause iron-deficiency anemia. This is most severe in heavy hookworm infections, as N. americanus and A. duodenale feed directly on the blood of their hosts. Although the daily consumption of an individual worm (0.02-0.07 ml and 0.14-0.26 ml respectively) is quite low, the collective consumption under heavy infection can be significantly high. One scholar estimated that “the blood loss caused by hookworm was equivalent to the daily exsanguination of 1.5 million people”. Whipworm is also attributed to anemia in the small intestine.
Malnutrition due to helminths may directly affect cognition. This includeds low educational performances, decreased ability to focus, difficulty with abstract cognitive tasks, and “lower scores...on tests of mental and motor development...[as well as] increased fearfulness, inattentiveness, and decreased social responsiveness” among very young children. Anemia has also been associated with reduced stamina for physical labor, a decline in the ability to learn new information, and “apathy, irritability, and fatigue”. Study on deworming of 47 students from the Democratic Republic of the Congo, using iron supplements, shiowed that it produced better effects on mental cognition. Among 159 Jamaican schoolchildren, deworming led to better “auditory short-term memory” and “scanning and retrieval of long-term memory;” which they achieved in nine-week period. Studies in the Philippines and Indonesia found significant negative impacts of helminthic infection on memory and fluency, and between worm infection and intellectual performance, particularly because their findings were significant in aspects of intellect that went beyond mere cognition and reaction time.
Helminthiasis is attributed to absenteeism, under-enrollment, and attrition in school children.”
Helminthiases are classified as follows (the disease names end with "-sis" and the causative worms are in brackets):
Among all helminthiases, the following helminth infections are classified under neglected tropical diseases:
Helminths are transmitted to the final host in several ways. The most common infection is through ingestion of contaminated vegetables, drinking water and raw or undercooked meat. Contaminated food may contain eggs of nematodes such as Ascaris, Enterobius, and Trichuris; cestodes such as Taenia, Hymenolepis, and Echinococcus; and treamtodes such as Fasciola. Raw or undercooked meats are the major sources of Taenia (pork, beef and venison), Trichinella (pork and bear), Diphyllobothrium (fish), Clonorchis (fish), and Paragonimus (crustaceans). Schistosomes and nematodes such as hookworms (Ancylostoma and Necator) and Strongyloides can directly penetrate the skin. Finally, Wuchereria, Onchocerca, and Dracunculus are transmitted by mosquitoes and flies. In the developing world contaminated water is the major risk factor of infection.
Infection can also take place by mistake when people eat soil on purpose - a practice called geophagy which is not uncommon in sub-Saharan Africa. The soil is eaten for example by pregnant women to counteract a real or perceived deficiency of minerals in the diet.
Of all the known helminth species, the most important helminths with respect to understanding their transmission pathways, their control, inactivation and enumeration in samples of human excreta origin, namely dried feces, faecal sludge, wastewater and sewage sludge are: soil-transmitted helminths (including Ascaris lumbricoides, which is the most common worldwide, Trichuris trichiura, Necator americanus, Strongyloides stercoralis and Ancylostoma duodenale), Hymenolepis nana, Taenia saginata, Enterobius, Fasciola hepatica, Schistosoma mansoni, Toxocara canis and Toxocara cati.
Response to worm infection in humans is a Th2 response in the majority of cases. Inflammation of the gut may also occur, resulting in cyst-like structures forming around the egg deposits throughout the body. The host's lymphatic system is also increasingly taxed the longer helminths propagate, as they excrete toxins after feeding. These toxins are released into the intestines to be absorbed by the host's bloodstream. This phenomenon makes the host susceptible to more common diseases, such as viral and bacterial infections.
Diagnosis is the mainstay in the control of helminthiases. For basic diagnosis, specific helminths can be generally identified from the feces, and their eggs microscopically examined and enumerated using the fecal egg count method. This is particularly useful in veterinary investigations. But it fails to identify mixed infections, and on clinical practice, the technique is highly inaccurate and unreliable, such as those for schistosomes and soil-transmitted helmiths. Sophisticated tests such as serological assays, antigen tests, and molecular diagnosis are also available; however, they are time-consuming, expensive and not always reliable.
Prevention and control measures include: 1) use of clean water for personal and domestic uses; 2) sanitation and health education such as by promoting use of latrines; 3) awareness on personal hygiene such as hand washing and washing of food; 4) avoiding the use of uncomposted human feces as fertilizer. In epidemic areas, mass deworming programs of school children is an important preventive method. Simple measures can be effective including constant wearing of shoes, soaking vegetables with bleach, adequate cooking of foods, handwashing at critical times (before contact with food and after use of the toilet), reducing open defecation, deworming of pet and proper disposal of their feces.
Broad-spectrum benzimidazoles (such as albendazole and mebendazole) are recommended for treatment of intestinal roundworm and tapeworm infections; while macrocyclic lactones (such as ivermectin) are effective against adult and migrating larval stages of nematode; and praziquantel is the drug of choice for schistosomiasis, taeniasis, and most types of food-borne trematodiases. Oxamniquine is also widely used in mass deworming programmes. Pyrantel is commonly used for veterinary nematodiasis. Artemisinins and derivatives are proving to be candidates as drugs of choice for trematodiasis.
Surgery - even emergency surgery - might be required to remove parts of the intestine if the worm infestation has reached such a high number of worms that the worms block that piece of intestines. Patients who are heading for surgery, for example to extract worms from the biliary tree, can be pre-treated with albendazole to kill worms prior to the surgery. In the case of intestinal obstruction it is however an emergency and there is no time to do this so surgeons have no alternative than manually pulling the worms out.
In regions where the disease is common, mass deworming treatments may be performed, particularly among school-age children, who are the high-risk group. Most of these initiatives are undertaken by the World Health Organization with positive outcomes in many regions.
Successful deworming and positive health outcomes were also achieved by the Essential Health Care Program implemented by the Philippine Department of Education in the Philippines. UNICEF has noted it as an "outstanding example of at scale action to promote children’s health and education". Deworming twice a year, supplemented with washing hands daily with soap, brushing teeth daily with fluoride, is at the core of this national program. It has also been successfully implemented in Indonesia.
Deworming programs can improve school attendance by 25%. This may lead to a long-term increase in income, and higher literacy rates.
When deworming drugs were randomly phased into schools in Kenya, rather than to individual pupils, improved school attendance and health was observed at those schools where treatment occurred. This outcome was not dependent on whether or not the specific student had received treatment, pointing towards externality benefits of school deworming programs.
Although mass dewormings improves the health of an individual, outcomes such as improved cognitive ability, nutritional benefits, physical growth and performance, and learning are still in question. For example:
The most affected regions are tropical and subtropical areas. Highest incidences are in subsaharan Africa, central and east Asia, and the Americas.
The soil-transmitted helminths (A. lumbricoides, T. trichiura, N. americanus, A. duodenale), schistosomes, and filarial worms collectively infect more than a quarter of human population at any one time, far surpassing HIV/AIDS and malaria taken together. Schistosomiasis alone is the second most prevalent parasitic disease of all times in humans, next only to malaria.
As of 2014, the World Health Organization estimates that over 1.5 billion people (a quarter of the total population) are infected with soil-transmitted helminthiases, 249 million with schistosomiasis (which may have even surpassed malaria at ~207 million cases in 2013), 56 million people with food-borne trematodiasis (i.e. other than schistosomiasis), 120 million with filariasis, 50 million people with cysticercosis, at least 15 million people with onchocerciasis, and 1 million people with echinococcosis. This adds up to between 1.5 to 2 billion people worldwide infected with one or more types of helminths - some could have multiple infections. Another source estimates a much higher figure of 3.5 billion infected with one or more soil-transmitted helminths. Also, it is estimated that 4.5 billion people are at constant risk of STH infection.
In 2014, only 148 people were estimated to have dracunculiasis thanks to a successful eradication campaign for that particular helminth, which is easier to eradicate than other helminths as it is only transmitted by drinking contaminated water.
Because of their high mobility and lower standards of hygiene, school-age children are particularly vulnerable to these parasites. A child in a low-economy country is estimated to harbour at least one helminth, and multi-species infections are very common.
It is estimated that intestinal nematode infections cause 5 million disability-adjusted life years (DALYS) to be lost, of which hookworm infections account for more than 3 million DALYS and ascaris infections more than 1 million. There are also signs of progress: The Global Burden of Disease study published in 2015 estimates a 46% (59% when age standardised) reduction in years lived with disability (YLD) for the 13-year time period from 1990 to 2013 for all intestinal/nematode infections, and even a 74% (80% when age standardised) reduction in YLD from ascariasis.
Estimates for the annual death toll which is directly due to soil transmitted helminthiasis is as high as 135,000. The death toll due to the malnutrition link is likely to be much higher.
The 1990-2013 Global Burden of Disease study estimated 5,500 direct deaths due to schistosomiasis, whilst more than 200,000 people are estimated to die yearly from causes that are related to schistosomiasis. Another 20 million have severe consequences from the disease. It is the most deadly of the neglected tropical diseases.
|Helminth genera||Common name||Infections (million per year)||Direct deaths per year||Regions where common|
|Soil transmitted helminthiasis (STH) (classified as neglected tropical disease):|
|Ascaris lumbricoides||Roundworm||1000 to 1450
807 to 1,121
|20,000||Many regions of South-east Asia, Africa, and Central and South America|
|In moist, warm, tropical regions of Asia, Africa, Central and South America, and the Caribbean islands.|
|Ancylostoma duodenale||Hookworm||900 to 1300
576-740 (hookworm in general)
|In tropical and subtropical countries (Sub-Saharan Africa)|
|Strongyloides stercoralis||Hookworm, pinworm||50 to 100||Thousands||In moist rainy areas of the tropics and subtropics, in some areas of southern and eastern Europe and of the United States of America|
|All STH together||1500 to 2000||135,000||Tropical and subtropical areas, in particular sub-Saharan Africa, the Americas, China and east Asia.|
|Not transmitted via soil but classified as neglected tropical disease:|
|Schistosoma mansoni||Blood fluke||All types of Schistosoma together: 160 to 200
|12,000 150,000 deaths from renal failure
200,000 indirect deaths from "causes related to" Schistosomiasis
|In tropical and subtropical regions|
|Schistosoma haematobium||112 (in Sub-Saharan Africa alone)|
|Echinococcus granulosus||3||Developing countries|
|Not transmitted via soil and not classified as neglected tropical disease:|
|Toxocara canis||Dog roundworm||50||Many regions of South-east Asia, Africa, and Central and South America|
|Taenia solium||Pork tapeworm||50||South America, Southeast Asia, West Africa and East Africa|
|Taenia saginata||Beef tapeworm||50
(all types of Taenia: 40 to 60)
|Hymenolepis nana||Dwarf tapeworm||100|
|Hymenolepis diminuta||Rat tapeworm|
|Liver fluke||50||Largely in southern and eastern Asia but also in central and eastern Europe|
|Fasciolopsis buski||Giant intestinal fluke|
|Dracunculus medinensis||Guinea worm||Nowadays negligible thanks to eradication program||Formerly widespread in India, west Africa and southern Sudan|
|Trichostrongylus orientalis||Roundworm||1-3 ("several")||Rural communities in Asia|
|Total (number of infections)||Approx. 3.5 billion||Worldwide|
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