Helminths either live as parasites, or free of a host, in aquatic and terrestrial environments. There are several types; the most common worldwide are intestinal nematodes or soil-transmitted helminths STH , schistosomes parasites of schistosomiasis and filarial worms, which cause lymphatic filariasis LF and onchocerciasis. Of the 5. Schistosomiasis is endemic in 70 countries with over million people infected; most living in poor communities without safe drinking water or adequate sanitation.
Schistosomiasis is estimated to cause 3. LF is endemic in 83 countries with an estimated million cases, including 25 million with hydrocele and 15 million with lymphoedema. LF is estimated to cause 2.
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Helminth infestation causes morbidity and mortality: it compromises nutritional status, affects cognitive processes, induces tissue reactions and causes intestinal obstruction or rectal prolapse. For reasons not well understood, school-aged children including adolescents and pre-school children tend to harbour the greatest numbers of intestinal worms and schistosomes and, as a result, experience stunted growth and diminished physical fitness, as well as impaired memory and cognition that leads to educational deficits .
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Helminthiasis control requires combined drug treatment, improved sanitation and health education. Helminths are invertebrates characterised by elongated, flat or round bodies. Flatworms platyhelminths include flukes trematodes , tapeworms cestodes and roundworms nematodes. Further subdivision is designated by the residing host organ e. Flukes, or trematodes , are leaf-shaped, and vary in length from a few millimetres to 8cm.
Excluding blood flukes, trematodes are hermaphroditic, having both male and female reproductive organs.
Both self-fertilisation and cross-fertilisation occur. Within the definitive human host, male and female worms inhabit the lumen of blood vessels and are found in close physical association. Flukes go through several larval stages before reaching adulthood. Eggs are passed in the faeces, urine, or sputum of humans and, on reaching an aquatic environment, the eggs hatch, releasing ciliated larvae, which either penetrate or are eaten by snail intermediate hosts.
A sporocyst develops from a miracidium within the tissues of the snail, producing cercariae that migrate to the external, usually aquatic, environment. Cercariae penetrate the definitive host and transform directly into adults. Tapeworms, or cestodes , are flat, hermaphroditic, parasitic worms that colonise the human gastrointestinal tract. Some are primarily human pathogens, others are animal pathogens that also infect humans. Adult tapeworms are elongated, segmented, flatworms that inhabit the intestinal lumen. Larval forms, which are cystic or solid, inhabit extra-intestinal tissues and include Taenia saginata, Taenia solium, Diphyllobothrium latum, Hymenolepis nana and Echinococcus species.
Segments proglottids are hermaphroditic and vary in length from 2mm—10m, with three to several thousand segments per adult tapeworm. Eggs are released when tapeworms shed gravid proglottids into the intestine, which are then shed in stools. All eggs are embryos that develop into different larval stages in both the immediate host crustacean and intermediate host vertebrate.
Larvae develop into adults in the definitive human host . Roundworms, or nematodes , are cylindrical in structure and usually bisexual; copulation between the male and female is important in fertilisation. Most nematodes that are parasitic in humans lay eggs that, when voided, contain either a zygote or a completely formed larva. Some nematodes, such as the filariae and Trichinella spiralis , produce larvae that are deposited in host tissues. The developmental process involves egg, larval and adult stages . Classification is via infection mode: direct, modified direct, or skin penetration.
Direct infection occurs when eggs are transmitted from anus to mouth without reaching soil e. Ascaris lumbricoides [roundworms]. Skin penetration is used by hookworms Ancylostoma duodenale and Necator americanus . Auto-infection can occur by scratching the perianal area and transferring infective eggs to the mouth with contaminated hands. Person-to-person transmission occurs by contaminating surfaces or food items with infective eggs.
Most infections are asymptomatic but pruritus ani is common. Abdominal pain, nausea and vomiting are experienced in patients with high worm burden. Investigation to confirm enterobiasis involves the pinworm paddle or tape test, where tape is applied to the perianal areas sticky part of the tape down mainly at night. The tape can then be examined microscopically. Treatment of enterobiasis is focused on the whole family on the first occasion of infection only . Mebendazole mg immediate dose is the treatment of choice for enterobiasis infection.
If re-infection occurs, the dose can be repeated in affected individuals after two weeks children older than six months and adults . Mebendazole is contraindicated in pregnancy and breastfeeding. Re-infection of threadworm from fingers and contaminated objects is easy and patients should be advised to avoid scratching and ensure to scrub under nails.
Bedding and clothing should be boil-washed or ironed with a hot iron to kill the eggs.
Worms may form a bolus in heavy infections causing intestinal obstruction, volvulus or perforation. The most common clinical presentation is Ascaris pneumonitis. Fever, cough, dyspnoea and urticaria may occur in a proportion of those infected on account of migration of larvae through the lungs.
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Symptoms usually resolve spontaneously after ten days as a result of migration of the worm through the lungs. A number of investigations can confirm clinical diagnosis of A. Larvae and eosinophils may be found in sputum. A chest X-ray can confirm the presence of larvae and stool microscopy is adequate in established infections may be negative if all worms are male.
Treatment of adults aged over 18 years after positive diagnosis of A. In children aged 1—2 years, mebendazole mg twice daily should be given for three days. In children aged 2—18 years, mebendazole should be given mg twice daily for three days, or mebendazole mg immediate dose . Toxocara canis and T.
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Human infection occurs after ingestion of eggs in sand or soil contaminated by dog or cat faeces. The United States has a seroprevalence of Toxocara eggs are not infective until two weeks post-shedding and ingestion of fresh dog or cat faeces is not a route of transmission. If old faeces are ingested, serological testing ELISA could be performed immediately and repeated three months later. In infections with T. Ocular larva migrans OLM may occur in lighter infections caused by larvae migrating to the eye producing a granulomatous reaction in the retina.
This may present as visual disturbance or blindness, which is diagnosed via ophthalmoscopy. Patients with suspected T. Eosinophilia and anaemia are indicative of active infection.
Positive serology tests using ELISA suggest the production of antibodies against antigens produced by the presence of worms and this is confirmed by Western blot. For the treatment for VLM and confirmed non-ocular cases, patients aged over 18 years should be given albendazole unlicensed mg twice daily for seven days .
Patients with T.