25 - Helminth Parasites Flashcards
Helminth parasite
- Complex, multicellular
- Extracellular
- Large genomes, very immunogenic
- Can survive for decades
- Drugs are toxic
Categories of helminths
- Nematodes round worms
Types of Platyhelminths flat worms
- Cestodes
- Trematodes
Caenorhabditis elegans
- Most important nematodes for medicine
- Transparent, emit blue/green fluorescence when they die
- Subject of three Nobel Prizes
- Only worm in space
Nematodes (Nematoda)
- Second largest phylum in animal kingdom
- Half are parasitic
- Cylindrical, male & female, have teeth
Gastrointestinal nematodes
- Commonest worm infections
- No intermediate host, not zoonotic
- Eggs or larvae infect humans
- Ingested larvae burrow into skin
Trichinella spiralis
Only intracellular helminth
Toxocara canis
Worm often targets the eye
Cestodes (tapeworms)
- Haemaphrodite (each worm has male and female reproductive organs)
- Body composed of segments or proglottids
- Adults attach to gut by scolex
- Live for decades
- Intestinal infections mainly asymptomatic, tissue stages cause pathology
Intestinal tapeworms
Exists as worms in intestine
Tissue tapeworms
Form cysts in the tissues
Trematodes (flukes)
- Haemaphrodite
- Complex life cycles (primary vertebrate host, intermediate invertebrate host)
- Obligate parasites
- Live for decades
Blood Flukes (Schistosomes)
Pathology caused by eggs
Tissue (liver) flukes
Live in organs (liver, lung)
Why are there so few drugs and why no recent drug development?
- Helminths are eukaryotes (similar metabolic pathways to humans, drugs are toxic)
- More prevalent in the tropics, countries that
have limited resources (drug companies cannot make a profit)
Ascaris
- Found worldwide
- Largest nematode parasites
- Adults live in upper part of small intestine
- Pathology caused by large amounts of worms
Ascaris lumbricoides (Ascariasis) pathology
- 85% asymptomatic
- Worms cause intestinal obstruction
- Can move around the body inadvertently blocking narrow organs like the bile duct
- Can cause sensitising phenomena
- Malnutrition especially in children
- Migrate away from gut to avoid drugs
Trichuris trichura
- Whipworm
- Human pathogen
- Adult worms are 3-5 cms
- Eggs very distinctive,
barrel shaped with mucus plugs
Trichuris trichura life cycle
Eggs hatch in intestine, larvae then migrate to the villus crypts for protection, later migrating to large intestine where they attach and mature in ~ 3 months,
living for about 1 year.
Trichuris trichura transmission
- Eggs are shed in human feces, become mature through embryonation and infective to humans
- Disease contracted from contaminated food or water
Trichuris pathology
- Most infections are asymptomatic
- Cause some anemia and eosinophilia
- In heavier infections the cecal mucosa is damaged, can also cause appendicitis
- Lead to nutritional deficiencies and stunted growth
Filariasis
- Caused by Wuchereria bancrofti and Brugia malayi
- Vectors are various genera of mosquitoes
- Adult worms block the lymphatics and
cause immune reaction
Lymphatic filariasis transmission
Mosquitoes that transmit the infective filarial worm larvae from one human to another
Lymphatic filiariasis clinical features
- Asymptomatic with microfilaraemia
- Acute: inflammation of lymph nodes, mild fever
- Chronic: filarial fever, lymphadenitis, Swelling of
peripheral limbs, Eosinophilia
Filariasis treatment
- No treatment for chronic filiariasis
- Acute infection can be treated with doxycycline (does not target the worm but an obligate bacterial symbiont)
Echinococcus tissue tapeworm (Hydatid disease)
- Tissue dwelling tapeworms
- Adult tapeworms in dog intestines
- Humans are dead end hosts
- Larvae hatch in gut, migrate around the
body into tissues and organs and create large slow growing cysts - The cysts contain the daughter parasites which turn into adults worms when eaten by dogs.
Echinococcus tissue tapeworm (Hydatid disease) treatment
Requires surgery and aggressive treatment with albendazole
Schistosomiasis (Bilharzia)
- Adult worms are mesenteric (intestinal) or vesicular (bladder)
- Contracted by bathing or washing in water contaminated with motile form of the parasite that burrow into the skin.
