Helminth Parasites The word ‘helminth’ is a general term meaning ‘worm’, but there are many different types of worms. Prefixes are therefore used to designate types: platy-helminths for flat-worms and nemat-helminths for round-worms. All helminths are multicellular eukaryotic invertebrates with tube-like or flattened bodies exhibiting bilateral symmetry. They are triploblastic (with endo-, meso- and ecto-dermal tissues) but the flatworms are acoelomate (do not have body cavities) while the roundworms are pseudocoelomate (with body cavities not enclosed by mesoderm). In contrast, segmented annelids (such as earthworms) are coelomate (with body cavities enclosed by mesoderm). Many helminths are free-living organisms in aquatic and terrestrial environments whereas others occur as parasites in most animals and some plants. Parasitic helminths are an almost universal feature of vertebrate animals; most species have worms in them somewhere.



Biodiversity

Three major assemblages of parasitic helminths are recognized: the Nemathelminthes (nematodes) and the Platyhelminthes (flatworms), the latter being subdivided into the Cestoda (tapeworms) and the Trematoda (flukes): nematode cestode trematode > Nematodes (roundworms) have long thin unsegmented tube-like bodies with anterior mouths and longitudinal digestive tracts. They have a fluid-filled internal body cavity (pseudocoelum) which acts as a hydrostatic skeleton providing rigidity (so-called ‘tubes under pressure’). Worms use longitudinal muscles to produce a sideways thrashing motion. Adult worms form separate sexes with well-developed reproductive systems. > Cestodes (tapeworms) have long flat ribbon-like bodies with a single anterior holdfast organ (scolex) and numerous segments. They do not have a gut and all nutrients are taken up through the tegument. They do not have a body cavity (acoelomate) and are flattened to facilitate perfusion to all tissues. Segments exhibit slow body flexion produced by longitudinal and transverse muscles. All tapeworms are hermaphroditic and each segment contains both male and female organs. > Trematodes (flukes) have small flat leaf-like bodies with oral and ventral suckers and a blind sac-like gut. They do not have a body cavity (acoelomate) and are dorsoventrally flattened with bilateral symmetry. They exhibit elaborate gliding or creeping motion over substrates using compact 3-D arrays of muscles. Most species are hermaphroditic (individuals with male and female reproductive systems) although some blood flukes form separate male and female adults. Unlike other pathogens (viruses, bacteria, protozoa and fungi), helminths do not proliferate within their hosts. Worms grow, moult, mature and then produce offspring which are voided from the host to infect new hosts. Worm burdens in individual hosts (and often the severity of infection) are therefore dependent on intake (number of infective stages taken up). Worms develop slowly compared to other infectious pathogens so any resultant diseases are slow in onset and chronic in nature. Although most helminth infections are well tolerated by their hosts and are often asymptomatic, subclinical infections have been associated with significant loss of condition in infected hosts. Other helminths cause serious clinical diseases characterized by high morbidity and mortality. Clinical signs of infection vary considerably depending on the site and duration of infection. Larval and adult nematodes lodge, migrate or encyst within tissues resulting in obstruction, inflammation, oedema, anaemia, lesions and granuloma formation. Infections by adult cestodes are generally benign as they are not invasive, but the larval stages penetrate and encyst within tissues leading to inflammation, space-occupying lesions and organ malfunction. Adult flukes usually cause obstruction, inflammation and fibrosis in tubular organs, but the eggs of blood flukes can lodge in tissues causing extensive granulomatous reactions and hypertension. Life-cycles

Helminths form three main life-cycle stages: eggs, larvae and adults. Adult worms infect definitive hosts (those in which sexual development occurs) whereas larval stages may be free-living or parasitize invertebrate vectors, intermediate or paratenic hosts. Nematodes produce eggs that embryonate in utero or outside the host. The emergent larvae undergo 4 metamorphoses (moults) before they mature as adult male or female worms. Cestode eggs released from gravid segments embryonate to produce 6-hooked embryos (hexacanth oncospheres) which are ingested by intermediate hosts. The oncospheres penetrate host tissues and become metacestodes (encysted larvae). When eaten by definitive hosts, they excyst and form adult tapeworms. Trematodes have more complex life-cycles where ‘larval’ stages undergo asexual amplification in snail intermediate hosts. Eggs hatch to release free-swimming miracidia which actively infect snails and multiply in sac-like sporocysts to produce numerous rediae. These stages mature to cercariae which are released from the snails and either actively infect new definitive hosts or form encysted metacercariae on aquatic vegetation which is eaten by definitive hosts. nematode cycle

egg - larvae (L1-L4) - adult cestode cycle

egg - metacestode - adult trematode cycle

egg-miracidium-sporocyst-redia-cercaria-(metacercaria)-adult Helminth eggs have tough resistant walls to protect the embryo while it develops. Mature eggs hatch to release larvae either within a host or into the external environment. The four main modes of transmission by which the larvae infect new hosts are faecal-oral, transdermal, vector-borne and predator-prey transmission: faecal-oral



trasdermal



vector-borne



predator-prey



> f aecal-oral transmission of eggs or larvae passed in the faeces of one host and ingested with food/water by another (e.g. ingestion of Trichuris eggs leads directly to gut infections in humans, while the ingestion of Ascaris eggs and Strongyloides larvae leads to a pulmonary migration phase before gut infection in humans). > transdermal transmission of infective larvae in the soil (geo-helminths) actively penetrating the skin and migrating through the tissues to the gut where adults develop and produce eggs that are voided in host faeces (e.g. larval hookworms penetrating the skin, undergoing pulmonary migration and infecting the gut where they feed on blood causing iron-deficient anaemia in humans). > vector-borne transmission of larval stages taken up by blood-sucking arthropods or undergoing amplification in aquatic molluscs (e.g. Onchocerca microfilariae ingested by blackflies and injected into new human hosts, Schistosoma eggs release miracidia to infect snails where they multiply and form cercariae which are released to infect new hosts). > predator-prey transmission of encysted larvae within prey animals (vertebrate or invertebrate) being eaten by predators where adult worms develop and produce eggs (e.g. Dracunculus larvae in copepods ingested by humans leading to guinea worm infection, Taenia cysticerci in beef and pork being eaten by humans, Echinococcus hydatid cysts in offal being eaten by dogs). Taxonomic overview

Two classes of nematodes are recognized on the basis of the presence or absence of special chemoreceptors known as phasmids: Secernentea (Phasmidea) and Adenophorea (Aphasmidea). While many different orders are recognized within these classes, the main parasitic assemblages infecting humans and domestic animals include one aphasmid order (Trichocephalida) and 6 phasmid orders (Oxyurida, Ascaridida, Strongylida, Rhabditida, Camallanida, and Spirurida). > trichocephalid ‘whip-worms’ have long thin anterior ends which they embed in the intestinal mucosa of their hosts. They have simple life-cycles where infections are acquired by the ingestion of eggs and emergent larvae moult and mature to adults in the gut. Trichuris infections in humans may cause inflammation, tenesmus, straining and rectal prolapse. > oxyurid ‘pin-worms’ have small thin bodies with blunt anterior ends. They have simple life-cycles, but with an unusual modification. Female worms emerge from the anus of their hosts at night and attach eggs to the skin. This causes peri-anal itching and eggs are transferred by hand to mouth. Infections by Enterobius cause irritability and sleeplessness in humans, especially children. > ascarid ‘roundworms’ have large bodies with 3 prominent anterior lips. Their life-cycles involve a stage of pulmonary migration where larvae released from ingested eggs invade the tissues and migrate through the lungs before returning to the gut to mature as adults. Ascaris infections in humans cause gastroenteritis, protein depletion and malnutrition and heavy infections can cause gut obstruction. > strongyle ‘hookworms’ have dorsally curved mouths armed with ventral cutting plates or teeth which they embed in host tissues to feed on blood. They have complex life-cycles where larvae develop in the external environment (as ‘geo-helminths’) before infecting hosts by penetrating the skin. Once inside, they undergo pulmonary migration before settling in the gut to feed. Heavy infections by Ancylostoma and Necator cause severe iron-deficient anaemia in humans, especially children. > rhabditid ‘threadworms’ have tiny bodies which become embedded in the host mucosa. Their life-cycle includes parasitic parthenogenetic females producing eggs which may hatch internally (leading to auto-infection) or externally (leading to transmission of infection or formation of free-living male and female adults). Super-infections by Strongyloides may cause severe haemorrhagic enteritis in humans. > camallanid ‘guinea worms ’ infect host tissues where the large females cause painful blisters on the feet and legs. When hosts seek relief by immersion in water, the blisters rupture releasing live larvae which infect copepods that are subsequently ingested with contaminated drinking water. The ‘fiery serpents’ mentioned in historical texts are thought to refer to Dracunculus infections. > spirurid ‘filarial worms’ occur as long thread-like adults in blood vessels or connective tissues of their hosts. The large female worms release live larvae (microfilariae) into the blood or tissues which are taken up by blood-sucking mosquitoes or pool-feeding flies and transmitted to new hosts. Onchocerca infections cause nodules, skin lesions and blindness in humans, while those of Wuchereria cause elephantitis. whipworm pinworm hookworm threadworm



roundworm filarial worm guinea worm

Two subclasses of cestodes are differentiated on the basis of the numbers of larval hooks, the Cestodaria being decacanth (10 hooks) and the Eucestoda being hexacanth (6 hooks). Collectively, 14 orders of cestodes have been identified according to differences in parasite morphology and developmental cycles. Two orders have particular significance as parasites of medical and veterinary importance.

> Cyclophyllidean cestodes have terrestrial 2-host life-cycles where adult tapeworms develop in carnivores (scolex with 4 suckers and sometimes hooks) while larval metacestodes form bladder-like cysts in the tissues of herbivores. The larvae of Taenia spp. cause cysticercosis in cattle, pigs and humans, while those of Echinococcus cause hydatid disease in humans, domestic and wild animals. > Pseudophyllidean cestodes have aquatic 3-host life-cycles, involving the sequential formation of adult tapeworms in fish-eating animals (scolex with 2 longitudinal bothria), procercoid larval stages in aquatic invertebrates (copepods) and then plerocercoid (spargana) stages in fish e.g. Diphyllobothrium in humans, dogs and cats being transmitted through copepods and fish. Cyclophyllidea Pseudophyllidea

Two major groups of trematodes are recognized on the basis of their structure and development: monogenean trematodes with complex posterior adhesive organs and direct life-cycles involving larvae called oncomiracidia; and digenean trematodes with oral and posterior suckers and heteroxenous life-cycles where adult worms infect vertebrates and larval miracidia infect molluscs to proliferate and produce free-swimming cercariae. Monogenea are almost exclusively ectoparasites of fishes while Digenea are endoparasites in many vertebrate hosts and have snails as vectors. Some 10 digenean orders are recognized on the basis of morphologic and biologic differences, two orders are of particular medical and veterinary significance. > echinostomatid fasciolids (liver flukes) live as adults in hepatic bile ducts of mammals where they cause fibrotic ‘pipestem’ disease. The parasites proliferate in freshwater snails and mammals become infected by ingesting metacercariae attached to aquatic vegetation. Several Fasciola spp. cause hepatic disease in domestic ruminants and occasionally in humans. > strigeatid schistosomes (blood flukes) are unusual in that the adults are not hermaphroditic but form separate sexes which live conjoined in mesenteric veins in mammals. Female worms lay eggs which actively penetrate tissues to be excreted in urine/faeces or they become trapped in organs where they cause granuloma formation. Miracidia released from eggs infect aquatic snails and produce fork-tailed cerceriae which actively penetrate the skin of their hosts. Several Schistosoma spp. cause schistosomiasis/bilharzia in humans. liver fluke blood fluke < Back to top