Helminthiasis

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This article is about the infection. For the organisms, see Helminths.
Helminthiasis
Classification and external resources
Collage of various helminth eggs.png
ICD-10 B83.9
ICD-9 128.9
DiseasesDB 28826
MeSH D006373

Helminthiasis /ˌhɛlminˈθəsis/ (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.[1]

Soil-transmitted helminthiasis (SHT) and schistosomiasis are the most important group of helminthiases, collectively belonging to the "neglected tropical diseases".[2] Soil-transmitted helminthiases are responsible for parasitic infections in a quarter of the total human population.[3] As of 2014, schistosomiasis is the most prevalent of all parasitic infections in humans.[4]

A naming convention exists whereby the ending "-asis" (or in veterinary science more commonly the ending "-osis") at the end of the name of the worm is added to signify the infection with that particular worm, e.g. Ascaris is the name of a particular helminth, and Ascariasis is the name of the infectious disease caused by this helminth.

Signs and symptoms[edit]

Example clinical photo: Guinea worm infection (dracunculiasis), worm coming out of the foot of an infected person.

In most cases helminthiasis is asymptomatic. However, 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, diarrhoea, loss of appetite, loss of blood, fatigue, and listlessness. In humans, under chronic infections, such as those in schistosomiasis, extreme morbidity is the common symptom.[5] Morbidity is accompanied by persistent poverty, decreased productivity, poor birth outcomes, poor school and work performance, and poor socioeconomic development.[6] 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.[7]

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.[8] Systemic changes such as eosinophilia, edema, and joint pain reflect local allergic responses to parasites.[9] 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.[10][11] Coinfection cases have become serious medical problems particularly in African countries.[12] Heavy infection reduces HIV progression and viral load, most likely by improving helminth-induced immune suppression.[13]

Malnutrition[edit]

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.[14] This relationship is particularly alarming because it is gradual and often relatively asymptomatic.[15] 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.[16][17][18] In pigs and humans, Ascaris has been tied to temporarily induced lactose intolerance and vitamin A, amino acid, and fat malabsorption.[14] 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.[19] 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.[16][18] In some cases diarrhea due to worms can cause speed “transit time” through the intestinal system, reducing absorption of nutrient.[14]

Malnutrition due to worms can give rise to anorexia.[17] A study of 459 children in Zanzibar revealed spontaneous increases in appetite after deworming.[20] This could be due to a side effect of body’s immune response to the worm and the stress of combating infection.[18] Specifically, some of the cytokines released in the immune response have been linked to anorexic reactions in animals.[16]

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.[14] One scholar estimated that “the blood loss caused by hookworm was equivalent to the daily exsanguination of 1.5 million people”.[18] Whipworm is also attributed to anemia in the small intestine.[14][18]

Cognitive ability[edit]

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.[16] 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”.[14] 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.[21] 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.[22] Studies in the Philippines and Indonesia found significant negative impacts of helminthic infection on memory and fluency,[23] 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.[24]

School performance[edit]

Helminthiasis is attributed to absenteeism, under-enrollment, and attrition in school children.”[16] Deworming programs improve school attendance by 25%.[25] This may lead to a long-term increase in income, and higher literacy rates.[26]

Causes[edit]

Ascaris life cycle: Adult worms in the lumen of the small intestine (1). The female produces eggs (approximately 200,000 per day) that are excreted with the feces (2). Unfertilized eggs are harmless, but fertilized ones are infective after 18 days to several weeks (3). Infective eggs are ingested (4), enter the gut (5), develop into larvae in the intestine, and penetrate the blood vessel to enter lungs, where they develop further (6), after 10 to 14 days, penetrate the alveolar walls, ascend the bronchial tree to the throat, and are re-swallowed (7). Upon reaching the small intestine, they develop into adult worms (8). It takes 2 to 3 months for one complete cycle. Adult worms can live 1 to 2 years.
Collage of various helminth eggs, from left to right: Trichosomoides egg, Ascaris lumbricoides with larva hatching, sample of adult roundworms, Hymenolepis nana, Schistosoma mansoni and Toxocara canis with larva hatching

Types[edit]

Helminthiases are classified as follows (the disease names end with "-sis" and the causative worms are in brackets):

Roundworm infection (nematodiasis)
Tapeworm infection (cestodiasis)
Trematode infection (trematodiasis)
Acanthocephala infection

Neglected tropical diseases[edit]

Among helminthiases, the followings are classified under neglected tropical diseases:[2][27]

  1. All soil-transmitted helminthiases
  2. Roundworm infections such as lymphatic filariasis, dracunculiasis and onchocerciasis
  3. Trematode infections such as schistosomiasis and food-borne trematodiases (including fascioliasis, clonorchiasis, opisthorchiasis, and paragonimiasis)
  4. Tapeworm infections such as cysticercosis, taeniasis, and echinococcosis

Transmission[edit]

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.[5] In the developing world contaminated water is the major risk factor of infection.[28]

Most common[edit]

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 faeces, faecal sludge, wastewater and sewage sludge are:[29] 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.

Mechanism[edit]

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[edit]

Identification and quantification of helminth eggs at UNAM university in Mexico City, Mexico

Diagnosis is the mainstay in the control of helminthiases. For basic diagnosis, specific helminths can be generally identified from the faeces, and their eggs microscopically examined and enumerated using the fecal egg count method. This is particularly useful in veterinary investigations.[30] 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.[31] Sophisticated tests such as serological assays, antigen tests, and molecular diagnosis are also available;[30][32] however, they are time-consuming, expensive and not always reliable.[33]

Prevention[edit]

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 faeces as fertilizer.[3] 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.[34]

Treatment[edit]

Main article: Anthelmintic

Medications[edit]

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.[35][36] Artemisinins and derivatives are proving to be candidates as drugs of choice for trematodiasis.[37]

Mass deworming[edit]

In regions were the disease is common, mass treatments are performed, particularly among school-age children, who are the high-risk group.[38][39] Most initiatives are undertaken by the World Health Organization with visible success in many regions.[40][41]

Successful deworming was achieved by the Essential Health Care Program implemented by the Philippine Department of Education in the Philippines. The UNICEF noted it as an "outstanding example of at scale action to promote children’s health and education".[42] 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.[43]

Criticism on outcome[edit]

Although mass dewormings definitely improve the health of an individual, the alleged outcomes such as improved cognitive ability, nutritional benefits, physical growth and performance, and learning are in question.[44] A randomised controlled trials in more than 15,000 children showed no special benefits on weight gain and cognitive performance.[45] A 2007 Cochrane analysis found no significant improvements in physical growth and cognitive performance among routinely dewormed children.[46] An independent report noted no benefit on cognition and school performance, with an inconclusive effect on weight gain.[47] A 2012 Cochrane review found no benefit on weight, blood improvement (haemoglobin content), cognition and school performance.[48]

Epidemiology[edit]

The most affected regions are tropical and subtropical areas. Highest incidences are in subsaharan Africa, central and east Asia, and the Americas.

Infection estimates[edit]

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.[30][32] Schistosomiasis alone is the second most prevalent parasitic disease of all times in humans, next only to malaria.[49]

As of 2014, the World Health Organization estimates that over 1.5 billion people (a quarter of the total population) are with soil-transmitted helminthiases,[3] 249 million with schistosomiasis[4] (which may have even surpassed malaria at ~207 million cases in 2013),[50] 56 million people with food-borne trematodiasis (i.e. other than schistosomiasis),[51] 120 million with filariasis,[52] 50 million people with cysticercosis,[53] at least 15 million people with onchocerciasis,[54] and 148 people with dracunculiasis.[55] 1 million people with echinococcosis,[56] (Total infection of 2 billion people.) Further, 4.5 billion people are at constant risk of STH infection."[57] Because of their high mobility and lower standards of hygiene, school-age children are particularly vulnerable to these parasites.[58] A child in a low-economy country is estimated to harbour at least one helminth, and multi-species infections are very common.[59]

Deaths[edit]

Estimates for for the annual death toll which is directly due to soil transmitted helminthiasis is as high as 135,000.[14][42][60] The death toll due to the malnutrition link is likely to be much higher.

Worldwide in 2010 for schistosomiasis the Global Burden of Disease estimated 12,000 direct deaths[61] while the World Health Organization (WHO) estimates more than 200,000 people die from causes that are related to schistosomiasis yearly.[3][62] Another 20 million have severe consequences from the disease.[63] It is the most deadly of the neglected tropical diseases.[64]

Helminth genera Common name Infections (million per year) Direct deaths per year Indirect deaths per year Regions where common
Soil transmitted helminthiasis (STH) (classified as neglected tropical disease):
Ascaris lumbricoides Roundworm 1000 to 1450 20,000 Many regions of South-east Asia, Africa, and Central and South America[42][65][66][67][68][69]
Trichuris trichiura Whipworm 500, 600-800[citation needed] In moist, warm, tropical regions of Asia, Africa, Central and South America, and the Caribbean islands.[66][67][68][69][70]
Ancylostoma duodenale Hookworm 900 to 1300 In tropical and subtropical countries (Sub-Saharan Africa)[67][70]
Necator americanus
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[67][68]
All STH together 1500 to 2000[3] No estimates Not quantified but due to nutritional impairment Tropical and subtropical areas, in particular sub-Saharan Africa, the Americas, China and east Asia.[3]
Not transmitted via soil but classified as neglected tropical disease:
Schistosoma mansoni Blood fluke All types of Schistosoma together: 160 to 200

(210 "affected"[71])

12,000[61] 200,000[62] In tropical and subtropical regions[66][67][68][69][70]
Schistosoma haematobium 112 (in Sub-Saharan Africa alone)[72] 150,000 deaths from renal failure[72]
Echinococcus granulosus 3[73] 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[42][65][66][67][68][69]
Taenia solium Pork tapeworm 50 South America, Southeast Asia, West Africa and East Africa[66][67][68][69]
Taenia saginata Beef tapeworm 50

(all types of Taenia: 40 to 60[74])

Hymenolepis nana Dwarf tapeworm 100
Hymenolepis diminuta Rat tapeworm
Fasciola hepatica,
Fascioloides magna
Liver fluke 50 Largely in southern and eastern Asia but also in central and eastern Europe[67][68]
Fasciolopsis buski Giant intestinal fluke
Dracunculus medinensis Guinea worm Nowadays negligible thanks to eradication program[75] Formerly widespread in India, west Africa and southern Sudan[67][68]
Trichostrongylus orientalis Roundworm 1-3 ("several") Rural communities in Asia[67][68]
Other 100 Worldwide[67][68]
Total (number of infections) Approx. 3.5 billion Worldwide

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