Week 2 - The Helminths Flashcards
What does the word ‘helminth’ mean?
‘Helminth’ is a general term meaning worm, prefixes are used to determine the type of worm.
What type of worm are platy-helminths?
Platy-helminths are flat worms.
What type of worm are nemat-helminths?
Nemat-helminths are round worms.
How many major groups of parasitic helminths are there?
There are three major assemblages of parasitic helminths:
Nematodes, Cestodes and Trematodes
What characteristics do all helminths share?
All helminths are multicellular eukaryotic invertebrates with tube-like or flattened bodies and exhibit bilateral symmetry.
Do flatworms have body cavities?
No, the flatworms are a coelomate, they do not have body cavities.
Do the roundworms have body cavities?
Yes, the roundworms (nematodes) are pseudocoelomate, that is they have body cavities which are not enclosed by mesoderm.
Describe the nematodes anatomy.
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.
They have a tough outer layer (cuticle), that offers environmental resistance in free-living relatives. The nematodes use longitudinal muscles to provide a sideways thrashing motion.
How do nematodes typically mate?
Nematodes are ‘dioecious’, they generally mate in the host. Adult worms form separate sexes with well developed reproductive systems.
Do nematodes have any free-living lineages?
Yes, there are many free living nematode lineages. These lineages show parasitism with vertebrate, invertebrate and plant hosts.
Nematodes are primarily parasites of which body area?
Nematodes are primarily intestinal parasites.
What is the typical transmission method for nematodes?
Most nematodes are transmitted via DIRECT routes - Ingestion or Skin Penetration.
Describe the disease ‘Filariasis’.
Filariasis is a parasitic disease caused by a nematode infection. It is spread by black flies and mosquitos when they take a blood meal. A symptom is elephantiasis, rashes, arthritis and abdominal pain may also be present.
How many filarial nematodes use humans as their definitive hosts?
Eight known filarial nematodes.
How are the filarial nematodes that use humans as their definitive hosts grouped?
They are divided according to the niche within the body they occupy into 3 groups.
Lymphatic filariasis
Subcutaneous filariasis
Serious cavity filariasis
How do Soil-Transmitted Helminths (STH) reach a human host?
STH infect via eggs which are ingested from contaminated soil (Ascaris, Trichuris) or via skin penetration (hookworm).
Name some soil-transmitted helminths.
Ascaris, Trichuris (whipworm), hookworm and pinworm.
What climate do STH require?
Soil-transmitted helminths typically require warm and wet climates.
Why are STH a greater burden in poorer countries?
Poor sanitation conditions and poverty increase the burden caused by STH. This leads to stunted growth, anaemia and reduced school performance due to years of lost education due to infection.
Provide details of ‘Trichuris trichiura’.
Trichuris trichiura
A nematode parasite also known as ‘whipworm’. It is transmitted through egg ingestions and present in human hosts.
The worms are ~4cm and infect 470M and kill 10000 a year. Whipworm causes trichuriasis, a neglected tropical disease, that infects the human large intestine
Describe the disease Trichuriasis
Trichuriasis is also known as whipworm infection. It an infection by the parasitic worm Trichuris trichiura. A few worms cause no symptoms, however a large infection by many worms results in abdominal pain, tiredness and diarrhoea, sometimes containing blood. Low RBC levels may occur due to blood loss.
Describe the lifecycle of Trichuris.
Trichuris is a direct route parasite. The eggs are ingested and hatch in the human small intestine. Here they exploit the microflora and this stimulates hatching. The infective larvae penetrate the villi and continue to develop in the S intestine.
The young worms move to the cecum and penetrate the mucosa, they then develop into adult worms in the large intestine. It takes approx. 3 months to go from ingestion of eggs to mature worms.
Worms can live up to 5 years, during this time females can lay up to 20,000 eggs a day.
How are people typically infected by Trichuris?
Trichuris infection typically occurs through the ingestions of eggs and is more common in warmer areas. Whipworm eggs are passed in the feces of infected persons, and if an infected person defecates outside, eggs are deposited onto the soil where they can mature into an infective stage.
Ingestion of these eggs can occur through eating unwashed vegetables or not cleaning contaminated hands.
Provide details of ‘Ascaris lumbricoides’.
Ascaris lumbricoides
Causes the disease Ascariasis. It is a nematode also known as ‘giant roundworm’. It exists in human hosts and can grow to >50cm. 900M are infected and 60,000 die each year.
Describe the lifecycle of ‘Ascaris’.
- Embryo within shell passes in feces.
- The Egg develops into a first-stage juvenile.
- Two jolts occur and the Egg then contains a third-stage juvenile.
- The J3 egg is then ingested on raw fruit or vegetable.
- Egg hatches in the small intestine and the juvenile penetrates the intestinal wall and enters venues of hepatic portal system.
- The juveniles undergo further development during migration.
- J4 emerges into the alveoli and migrates in bronchioles to trachea and then up the trachea. They are then swallowed down the oesophagus.
- The worms reach the small intestine again and develop into adults.
- The fertilised and unfertilised eggs pass out in feces.
What symptoms does Ascariasis cause?
Larvae - eosinophilia and pneumonitas.
Adults - malnutrition, small bowel obstruction, wandering ascaris (biliary tract obstruction), liver abscess.
How many eggs can a female Ascaris lay a day?
200,000 eggs a day.
Provide details about the parasite ‘Necator americanus’.
Necator americanus Causes the disease Necatoriasis. Is a nematode. Known as 'New World Hookworm' Human hosts, penetrates through skin. 1cm long. Infects 660million, 125,000 deaths
How do Hookworms gain access to the host?
They burrow directly through the skin.
Where do hookworms occupy?
The adults live in the small intestine. They attach to the intestinal wall and take blood.
How much blood do hookworms take a day?
30-200microlitres per day, per hookworm.
This can lead to intestinal blood loss and therefore iron deficiency anaemia.
Describe the lifecycle of the Hookworm.
- The embryonated egg is passed in feces.
- The first-stage juvenile (rhabditiform) hatches.
- Two jolts ensue and the infective third-stage juvenile (filariform) enters developmental arrest until it reaches a new host.
- Filariform juveniles ‘quest’ on blades of grass and penetrate the skin of humans.
- Juveniles migrate through the circulatory system to the lungs.
- Juveniles break out of the circulatory system into alveoli where they molt to the fourth stage.
- Juveniles then migrate to small intestine via the trachea where they are swallowed.
- Fourth stage juveniles molt to adults in the small intestine, mate, and produce eggs.
