Describe general features of nematodes.
- Nematodes are the most abundant multicellular species with over 236 species.
- They are bilaterally symmetrical with distinct sexes that are separate in most species.
- Vary greatly in size. However size has no bearing on pathogenicity, e.g. trichinella is very small and causes great damage.
- Include the most common parasites – ascaris, hookworms and they also include the most unpleasant parasite infections – river blindness and elephantiasis. They are the most important veterinary parasite.
- Simple uniform life cycles (unlike in Platyhelminthes) = Egg –> 4 juvenile stages (that resemble adult form) –> Adult
- they vary in their life styles –> parasites can be free-living (usually in freshwater or soil), plant-parasitic, animal parasitic.
What is the cuticle?
Cuticle –> outermost covering on the nematode’s body wall and is covered in a glycoprotein coat.
- It is protective (maintains the nematode form) and strong yet highly flexible and is used in locomotion.
- It is important in host-parasite interactions, such as evasion of immune response – the cuticle is resistant to host digestive enzymes.
Explain the molting process.
o Molting –> the process by which the cuticle is replaced with a new underlying one – the newly hatched larvae will grow until constrained by its cuticle, futher growth will only be possible if the larvae grows a new flexible cuticle and removes the old one.
– It involves two steps –
Synthesis of a new cuticle. At this stage the larva, is completely enclosed by its old cuticle.
Exsheathment - the old cuticle is loosened and ruptured followed by the larva wriggling out of the casing of the old cuticle.
** Molting occurs four times each life cycle – at the end of each larvae stage including transition to adult.
What are differences between nematode direct and indirect life cycles?
- Direct – one host – i.e. all pre-parasitic stages are found free-living in the environment and their development to infective stage may take place either inside the egg or after hatching. E.g. ascaris, trichuris, hookworms.
- –> In nematodes where the first stage larvae hatch from their eggs, subsequent development takes place in the environment and the third stage larva is the infective stage. Examples of this type of direct life cycle occur among members of the family Trichostrongylidae
- –> In nematodes where eggs do not hatch, preparasitic larvae develop inside their eggs so that the infective stage is an egg containing an infective larva. Hatching will take place after these eggs are eaten by another host and the infective larva escapes. Example: Ascaris suum, the roundworm of pigs. - Indirect –> two hosts – i.e. the larvae will develop to the infective stage inside an intermediate host. E.g. trichinella, drancunulus, onchocerca. There are two possible methods of transmission of infective stage to definitive host
a. The intermediate host is ingested by the definitive host and infective larvae are released by digestion in the alimentary tract. Example: Trichinella spiralis
b. The intermediate host is a biting or sucking arthropod. In these cases transmission of infective nematode larvae occurs during feeding on the definitive host.
List the three nematodes with direct life cycle.
Passive infection
- Trichostrongyles.
- Ascaris lumbricoides
Active infection
1. Hookworm (Acylostoma duedonale & Necator americanus)
List the two nematodes with an indirect life cycle.
Passive
- Trichinella spiralis
- Dracunculus medinensis ‘Guinea worm’
Active - via vector.
1. Onchocerca
Describe general features of the trichostrongyles.
- They are major veterinary parasites of cattle, pigs and horses.
- Mainly parasites of small intestine or stomach and cause great economic loss in domestic animals.
- Some of species show arrested development in winters – they enter dormant phase. This aids survival. The larvae eventually resume their development in the start of the next grazing season.
- Approx. 10 species have been reported in humans.
- Treatment – anthelmintics such as flubendazole are highly effective. Cooking vegetables will prevent infection in humans.
Describe the life cycle of the trichostrongyles.
Direct passive life cycle –>
- Life cycle in all species – eggs will hatch in soil or water and develop through the larvae stages to J3.
- 3rd stage juvenile (J3) is infective stage –> when the 2nd stage moults to form the 3rd stage it will retain its cuticle so that the 3rd stage has an extra body wall, called a sheath. The sheath will reduce the vulnerability for it to dry out and increase chance of transmission to host.
- When they are eaten by host (method of transmission), they trigger the sheath to be lost so they can develop into adult form. The biggest trigger is the presence of CO2, which is in the ‘correct hosts’.
- The J3 will burrow into small intestine wall and molt. They will return to lumen as adult form and begin egg production.
- Eggs are then passed in faeces so they hatch in soil/water to complete the cycle.
Describe the general features of ascaris lumbricoides.
- Arguably the world’s most common parasite.
- Largest white intestinal roundworm – up to 45cm long (females being slightly larger).
- Parasites of small intestine.
- They increase their chances of transmission by producing high numbers of eggs –> an adult female can produce up to 200,000 eggs per day.
- The eggs are very persistent and can survive for up to 10 years –> they have thick outer shell wall which means they are resistant to chemicals and dessication. This contributes to success of the parasite.
Describe the life cycle of ascaris lumbricoides.
Passive direct life cycle –>
- Eggs are passed into faeces containing the L1 larval worm.
- If moisture, oxygen and temperature under 30 degrees is available, the fertile egg will differentiate into the L2 which is infective to humans.
- Direct infection occurs when unhatched eggs are ingested in contaminated raw food.
- The eggs will hatch in the small intestine to release the L2 worm, which will penetrate the intestinal wall, enter the blood stream – while travelling they molt twice to form the L4.
- The L4 worm returns to the small intestine where it will mature to the adult form.
- Here it lives in the lumen of small intestine and will feed on semi digested contents of the gut. The female form will lay an estimated 200,000 eggs per day which are passed out with the faeces.
Describe the pathology of ascaris lumbricoides.
–> Most infections asymptomatic.
– Symptoms include diarrhoea, fever and abdominal pain.
– other symptoms: pneumonia, difficulties breathing – hypersensitivity (asthma) due to large numbers of larvae migrating through the lungs – highly allergenic.
– Also can cause digestive disorders due to blockage of alimentary canals, can also cause malnutrition esp in children.
The worm can travel to other organs and cause damage –> rupture of the peritoneum, suffocation if they enter trachea, bile duct obstruction – causing jaundice.
— Causes ~100,000 deaths per year. Drugs are very effective but they are not always available in the poorest areas.
Describe the general features of the hookworm.
Caused by two species –>Acylostoma duedonale & Necator americanus.
- Ancylostoma predominates in Middle East, N. Africa and S. Europe while Necator is found in South America, Australia and Pacific islands.
- Very important human pathogens –> 1200 million cases annually.
- The adult parasites are small worms (only 1mm long) and has two rows of small teeth.
Describe the life cycle of the hookworm parasites.
Direct active life cycle:-
- Eggs are passed in the faeces and once exposed to air and moisture they will mature and hatch to liberate the L1 larvae.
- The L1 worms will feed on nutrients in the soil and molt into L2 and then L3 – which is more slender and is the infective form of the parasite.
- Infection takes place when the L3 actively penetrates the skin (usually of foot) where it enters the bloodstream. – L3 is carried around the body, until they reach the gut where they burrow into the intestinal villi.
- Here they will molt into the L4 form which will have buccal capsules which allow them to adhere to the gut wall. They will then molt again into adult form (which is much smaller than larvae) and begin egg production.
Describe the pathology of hookworm infection.
- The hookworms can extract 0.2ml blood daily from the blood vessels in intestinal wall (potentially causing anemia in heavy infections)
- Cause ground itch between the toes after they penetrate the skin.
- Other symptoms – gastric pain, abdominal peristalsis, anemia
- In children it can retard mental and physical development. After de-worming school children however, these effects were found to be reversible.
Describe methods of diagnosis and treatment for hookworm.
- Diagnosis – usually by analysing stool samples to look for hookworm eggs – however it’s hard to distinguish from other helminth eggs or determine hookworm species.
- Treatment – albendazole and mebendazole are drugs of choice. They are effective with few side effects. Vaccines are currently in trial for two species.
Describe the general features of Trichinella spiralis.
– Not usually a parasite of humans – zoonosis. Humans are considered as an accidental host as the parasite reaches a dead end here – causes trichinosis disease.
– It reaches a dead end since to complete a life cycle, the flesh of the host containing infective larvae must be ingested by another host.
– The major source is pork sausages that are undercooked.
– Prevalent in Mexico, parts of southern Africa, southern America. It has an extremely broad host range –> nearly every mammal species can become infected.
The normal life cycle is rodent –> pig.
– Once common in the USA –> trichinosis is very easy to control, once people learnt to freeze meat the prevalence fell below <1% in the US.
– There are periodic minor epidemics – e.g. recently in Thailand.
–There are nine genotypes, which vary in their temperature sensitivity – e.g. Trichinella spiralis is more heat-tolerant than T.nativa.
Describe the life cycle of Trichinella spiralis. And what are nurse cells?
Indirect passive life cycle –>
- 1st stage larvae are ingested in raw and undercooked meats which is released when exposed to gastric acids.
- The L1 worm will molt four times and travel all over the body until they take up residence in small intestine where they will mature into dioecious adults.
- The adult female will penetrate the mucosal wall of intestine and lay larvae into lymph vessels – approx. 1,500 over 4-16 weeks.
- The larvae can then be carried throughout bloodstream via mesenteric veins and enter skeletal muscle cells – where it causes them to differentiate into nurse cell – providing perfect environment for the parasite.
- The nurse cell is thought to nourish the parasite and conceals it from the immune response. It is morphologically distinct from any other cell type and exact mechanisms the parasite employs to promote differentiation is unknown.
- Thus a larvae-nurse cell complex in muscles which will become calcified due to the host’s immune response. Can survive for up to 30 years in human.
- The larvae can also encyst in heart muscle (can lead to heart failure) or CNS – neuronal dysfunction.
What are the pathological symptoms of infection with T. spiralis? What are treatment options?
- -> Symptoms – nausea, diarrhoea, fatigue, fever, vomiting – can be frequently mistaken for other common illnesses. Heavy infections (with encysted larvae) can cause CNS damage and heart problems – occasionally fatal.
- -> Treatment - Mebendazole and Thiabendazole are effective against intestinal stage.
Describe the general features of Dracunculus medinensis .
Causes dracunculiasis, also known as guinea worm disease.
- The largest tissue parasite that affects humans.
- Prevalence is very high in belt in western Africa. Also found in India, Pakistan and Yemen.
- The guinea worm inhabits freshwater lakes and rivers.
- Historically infected 10 million people per year but has dramatically reduced due to an eradication program – hope of eradication.
- The parasite has caused economic loss, school absences and wasting valuable resources.
- The parasite has several hosts – humans, dogs, cows, wolfs, monkeys and baboons.
- No preventive or curative drugs are available but several control programs are in place.
Describe the life cycle of Dracunculus medinensis..
Indirect passive life cycle.
- Two host life cycles – If in water, the female worm releases hatched live juveniles which can live for up to 3 weeks in muddy water.
- If the juveniles are eaten by fresh water crustaceans (Cyclops species) which actively chase them they will be develop into the 3rd stage juvenile.
- The crustaceans are not visible to naked eye, so that they won’t be seen by humans.
- If humans drink contaminated water they will be infected with the 3rd stage larvae.
- The larvae will penetrate the stomach and then mature into adult form and mate – the male then dies and is absorbed into the larger female.
- When the embryos reach maturity, the female worm will migrate to areas of body in contact with water and penetrate the skin and discharges larvae into the water and dies.
Describe pathology of guinea worm disease.
Pathology –>If the worm is ruptured accidentally then there is serious bacterial infection
- also when female dies, an abscess forms which can become secondarily infected.
- Heavy infestations in the joints can cause arthritis.
