Helminths Flashcards
Name three helminths.
Nematodes (roundworms), Trematodes (flatworms) and Cestodes (tapeworms).
What are helminths?
Eukaryotic multi-cellular parasitic worms with a complex life cycle.
What is the phylum for nematodes (roundworm)?
Nemata or Nematoda.
Nematodes are only free living. True or False?
False. Some are free living and some are parasitic.
Why is the structure of nematodes described as ‘a tube within a tube’?
Because it’s digestive system is like a tube with openings at both ends. They are large in size and are cylindrical with unsegmented bodies.
What are the usual sites of infection?
Gut, tissue and blood.
How are gut nematodes diagnosed?
By analysing eggs or larva in stool.
Give four examples of gut nematodes.
Roundworm, Pinworm, Hookworms and Whipworm.
How are blood and tissue nematodes transmitted?
Via insect bites.
Give three examples of diseases caused by blood and tissue nematodes.
Filaria, river blindness and Calabar swellings.
What is the name of the large intestinal roundworm?
Ascaris lumbricoides
What disease does it cause?
Ascariasis.
What is the common age of infection and why?
5-9 years due to contaminated soil, food and toys. Also contracted by the use of human faeces as fertiliser.
Where is the incidence highest?
In warm climates with poor sanitation.
Explain the life cycle.
Food contaminated with infective eggs are ingested. They hatch in the upper small intestine.
The larvae penetrates the intestinal wall and ethers the venules or lymphatics where they pass through the liver, heart, and lung to reach alveoli. In 1 to 7 days they grow up to 1.5 cm.
They then move to the small intestine where they mature in 2 to 3 months.
A female grows to 20-35cm and can live in the intestine for 12 to 18 months with the ability to produce 25 million eggs.
The eggs are excreted in faeces and infective larvae are formed within the egg.
The eggs are resistant to chemical disinfectant and survive for months in sewage but are killed by heat (40 degrees C for 15 hours).
List some symptoms of the worm burden.
Vague abdominal pain, vomiting, fever and distension. In severe cases, weight loss and loose stool may occur.
What are the means of asymptomatic carriers?
10-20 worms may go unnoticed except in a routine stool examination.
Name the drug used for treatment of ascariasis.
Mebenazole.
What are the three filariae and what do they cause?
Wuchereria bancrofti: Bancroftian Filariasis.
Brugia malayi: Brugian lymphatic filariasis of humans.
Brugia timori: a human filarial parasite from Indonesia.
Many species of mosquitos including Anopheles are responsible for the transmission of lymphatic filariasis. True or False?
True.
How many people are threatened by LF?
1.4 billion people.
When is it usually contracted?
Before the age of 5.
What signs are produced by LF?
Swelling of the limbs and breasts (lymphoedema) and genitals (hydrocele), or swollen limbs with dramatically thickened hard, rough and fissured skin (elephantiasis).
What problem does this propose?
It prevents afflicted individuals from experiencing a normal working and social life, furthering the cycle of poverty.
What is the epidemiology?
Mostly in the tropical and subtropical regions, amongst the poorest countries such as central Africa and in many parts of Asia.
How is LF contracted?
Through poor sanitation and housing quality.
How do adult worms damage the lymphatic system and how long do they survive?
By causing fluid to collect and cause swelling, leaving patients permanently disabled and disfigured. They can persist for as long as 10 years.
When do symptoms appear in children?
During puberty because of the long latency period of the disease. Of all LF sufferers, children will benefit most from the elimination programme.
Explain the life cycle of Wuchereria bancrofti.
The female worms release large numbers of very small worm larvae which circulate the bloodstream. When a human is bitten by a mosquito, the mosquito ingests the larvae. The larvae develop in the mosquito into an infective stage and are then spread to other people via mosquito bites. After a bite, the larvae pass through the skin, travel to the lymphatic vessels and develop into adult worms.
What is the periodicity of microfilariae?
During the day they are present in the deep veins and during the night they migrate to the peripheral circulation.
How is the asymptomatic form of infection characterised?
By the presence of thousands or millions of larval parasites (microfilariae) in the blood and adult worms lodged in the lymphatic system (maintains the fluid balance between tissues and blood, essential for immune defence system).
Acute episodes of local inflammation involving skin, lymph nodes and lymphatic vessels often accompany the chronic lymphoedema or elephantiasis. True or False?
True.
How do some of these come about?
By the body’s immune response to the parasite, but most are the result of bacterial infection of skin where normal defences have been partially lost due to underlying lymphatic damage.
How can the damage be slowed down or even reversed?
By careful cleansing which is helpful in healing the infected surface areas.
Chronic and acute manifestations of filariasis tend to develop more often and sooner in who?
Refugees or newcomers rather than in local populations who are continually exposed to the infection.
LF affects men more often than women with worse symptoms. True or False?
True. 10-50% of men suffer from hydrocele (fluid-filled balloon like enlargement of the sacs around the testes) and elephantiasis of the penis and scrotum.
Where can lymphoedema occur?
Arms, legs, breasts, scrotum and penis. Others who are infected show no signs of clinical disease, but have damaged lymphatic vessels and are at risk of developing lymphoedema.
Why does the skin become hard and thick?
Because patients suffer from many acute attacks of infection over time and the disease progresses from the early stages of lymphoedema to elephantiasis (thickening and hypertrophy of tissues infected with the worms may lead to the enlargement of tissues, especially the extremities).
What are the methods used for diagnosis?
Blood test during 10pm-2/4am to view parasites in peripheral blood during their ‘nocturnal periodicity’. Urine specimens and Antigen detection kits called ICT can detect infection within minutes and can be carried out at any time of day.
The strategy for interrupting transmission is an annual single co-administration of two drugs for at least five years. Name these drugs.
Single doses of albendazole and mectizan or DEC.
What is DEC?
An inexpensive and effective anti-filarial drug which is used to treat LF in many countries. Available in tablet form for daily intake at meal times. Can not be used in most of Africa because severe side reactions can occur when other infections, such as river blindness are present.
How does A. lumbricoides affect the respiratory system?
After infection with many larvae, migration of worms to the lungs can produce pneumonitis resembling an asthmatic attack.
How are the eggs in stool identified?
Knobby-coated, bile stained, fertilised and unfertilised oval eggs with thick walled outer shell.
What causes inflammation seen for W. bancrofti infection?
Caused by the presence of molting adolescent worms and dead or dying adults within the lymphatic vessels.
How are the worms identified in terms of structure?
W. bancrofti, B. malayi and Loa loa demonstrate a sheath on their microfilariae. Further identification is based on study of head and tail structures. Exact species identification is not necessary because treatment is the same.
There are no animal reservoirs for W. bancrofti unlike B. malayi which sees infection in cats and monkeys. True or False?
True.
How is LF prevented?
Education regarding filarial infections, mosquito control, use of protective clothing and insect repellents.
What is the phylum for Trematodes?
Platyhelminthes
Describe their structure.
Flat, leaf-shaped worms. Hermaphroditic. 2 muscular suckers; oral (incomplete digestive system) and ventral (attachment).
How many hosts are required?
A definitive host and two intermediate hosts. Intermediate hosts (in which the larval asexual phase occurs): molluscs in all cases (snails and clams), some flukes need other intermediary hosts, vectors. Final host (in which the adult sexual phase occurs).
What disease does blood flukes such as schistosoma haematobium (urinary form), S. mansoni (intestinal) and S. japonicum (intestinal) cause?
Bilharzias.
What are the infective forms for schistosomes?
Skin penetrating cercariae released from snails.
What is the epidemiology?
Major parasitic infection of tropical areas.
How are people put at risk?
Lack of hygiene, play habits, agricultural domestic and recreational activities which expose them to infested water.
What is the distribution of S. mansoni? And is there any reservoirs?
Found in Africa, South America (Brazil, Suriname, Venezuela), the Caribbean (Puerto Rico, St. Lucia), parts of the Middle East. No.
What is the distribution of S. haematobium? And are there any reservoirs?
Found in large parts of Africa, Arabia, the Middle East, Mauritius. No.
What is the distribution of S. japonicum? And are there any reservoirs?
Found in Far East (china and Philippines) and few places in indonesia. Water buffalo is a reservoir.
Describe the schistosome life cycle.
Infection is initiated by ciliated, free-swimming cercaria in fresh water that penetrate intact skin, enter the circulation and develop in the intrahepatic portal circulation (S. mansoni and S. japonicum) or in the vesical, prostatic, uterine plexuses and veins (S. haematobium).
How do they escape the host immune system?
Adult worms coat themselves with self-antigens. Minimal host inflammatory response to adult worms. This leads to chronic infection.
Schistosomes’ eggs elicit strong inflammatory response from the host. True or False?
True.
Production of enzymes by the larvae inside the eggs causes what?
Tissue destruction and allow the eggs to pass through the mucosa and into the lumen of the bowel and bladder, where they are passed to the external environment in the faeces and urine.
What does the 1) penetration of cercariae 2) onset of laying eggs 3) presence of eggs in tissues all lead to?
1) skin rash. 2) katayama syndrome. 3) chronic schistosomiasis, granulomas, fibrosis.
Asymptomatic allows for brown hematin pigment present in macrophages and neutrophils so what does symptomatic schistosomiasis, bilharziasis show?
The first sign is inflammation at the site of penetration of cercariae.
