Helminths Flashcards
What is a helminth and how are they categorised
Parasitic worms that are multicellular organisms with differentiated tissues.
Many types, widespread across the globe. Can colonise a wide variety of tissue niches (alimentary tract, cavities, organs, circulatory system, skin). Infections normally chronic and last years/decades.
Categorised on the basis of morphology and biology: Roundworms - nematodes / Flatworms - cestodes (tapeworms) and trematodes (flukes)
Nematode overview and lifecycle
Direct lifecyle using a single host to complete their development. Tend not to replicate inside the host, rather enter as a single worm and remain this way throughout lifespan. This has direct repercussions on pathology - inoculum determines worm burden and severity of symptoms. Eggs are very resilient and can remain infective in the environment for a very long time.
Hookworms
a) types
b) lifecycle
c) pathology
a) Ancylostoma and Necator - different species infect humans and animals, most commonly in humans and dogs
b) eggs hatch in soil releasing larvae. Larvae penetrate skin, access bloodstream, migrate to lungs. Migrate/get coughed up trachea and swallowed. Access GI tract and develop to adults in intestine. Use teeth to latch to intestinal wall where they remain for their lifespan, mating and releasing eggs into environment through faecal route.
c) Worm secrete anticoagulant to feed on nutrient in blood, can cause anaemia and malnutrition (interfere with normal absorption in intestine). Cause distended belly, lack of growth and cognitive developmental delays. Canine hookworm larvae can infect humans, but don’t enter bloodstream, rather migrate in subcutaneous tissue causing creeping eruptions
Whipworm
a) lifecycle
b) pathology
Trichuris
a) transmitted through oral-faecal route. Once ingested, can take up to 3 months to develop to adults in intestine and cause symptoms. Adults can remain attached for up to 5 years, eggs can remain infectious for up to a year in the environment
b) Thin end of the worm threads through mucosal lining and feeds on cellular secretions. Although no bleeding, often leads to anaemia. Pore-forming toxin secreted by worm contributes to generalised bone marrow failure. Colonises lower intestine, causing rectal swelling and false urge to strain, can cause rectal prolapse.
Ascarids (giant roundworm)
a) lifecycle
b) pathology
Ascaris lumbricoides
a) passes through lungs, migrates up trachea and swallowed to access GI tract. Worm feeds on liquid contents of intestine so don’t disrupt mucosal lining
b) severity of symptoms depends on size of worm burden. Single worm unlikely to cause pathology. Heavy worm burden can lead to allergic reaction to worm waste, blockages and malnutrition. Worm secretes anti-trypsin factor that interferes with nutrient absorption.
Pinworm overview
Enterobius vermicularis. Most common worm infection in UK (especially children). Evolved behaviour to crawl out of rectum to peri-anal region to lay eggs at night. Eggs spread by scratching and touching bed clothes, furniture etc. Once eggs are in the environment, become infective within hours (atypically fast) and survive for weeks. Outbreaks difficult to contatin and common in schools/nurseries.
Cestodes (tapeworms)
a) development
b) pathology of pork/beef tapeworms when human ingests
c) pathology of dog tapeworms when human ingests
Slightly more complex than nematodes, require two hosts for development (definitive and intermediate)
a) Taenia solium (pork tapeworm) and Taenia saginata (beef tapeworm) larvae form cysts (cysticerci) in animal muscle. If human eats undercooked meat, larvae develop into adult worm in intestine. Adult worm can be up to 10m long and segmented with each proglottid developing into either male or female, capable or cross-fertilisation. Most mature segment full of eggs will detach and pass into environment with faeces.
b) Infection generally asymptomatic, severe cases show anaemia and emaciation. Adult worm sheds gravid proglottids (up to 50,000 embryolated eggs) that are excreted by human host. Proglottid/eggs ingested by intermediate host (eg pig) where eggs hatch and larvae migrate to a variety of tissues (muscle, brain) to settle as cysticerci. Severe disease in intermediate host (esp humans) when eggs ingested and cysticerci grow and cause cysticercosis
c) If humans ingest Echinococcus granulosus (dog tapeworm that normallt cycles between sheep and dogs) cysticerci can grow into an intermediate form (20cm) and obstruct organ function. Doesn’t happen in livestock as dont live long enough to suffer serious disease. In brain of human, can cause epilepsy, seizures and blindness. Regions with poor sanitaion, cysticercosis is major cause of epilepsy. As cysticerci are fluid filled bladders filled with worm protein, can cause a fatal massive inflammatory response upon accidental rupture.
Trematodes (flukes)
a) examples and disease they cause
b) lifecycle
c) pathology
a) All are Schistosoma. S. mansoni, S. japonicum, S. makongi, S. intercalatum cause intestinal disease. S. haematobium causes urogenital disease
b)S cercariae in water come into contact with swimmers/waders. Penetrate skin, larval schistosomula develops to adult. Adult male and femal pair migrate and attach to mesenteric venules around liver or venous plexus around bladder, surviving 5-10 years. Deposit 300-3000 eggs a day. Eggs move into intestine or bladder, and pass into environment via faeces or urine. Once in water, they hatch, release miracidia and invade snail host
c) Schistosomiasis can be intestinal or bladder disease. Severe intestinal includes hepato-splenomegaly, varices, fibrosis and liver calcification. Severe bladder includes bladder fibrosis, calcification and cancer, with urogenital sandy patches on cervix. Morbidity due to egg deposition, when a fraction reflux with blood flow back into liver or bladder and cause granuloma. Persistence of wound healing cytokine milieu leads to tissue fibrosis and calcification. Heavy worm burden (so egg burden) and granulomas contribue to severe disease.
