Parasitic Invertebrates

Question 0

What are the similarities between the lifecycles of flukes, tapeworms and roundworms?

Answer 0

Similarities:
Adult parasites exhibit sexual reproduction.
Eggs may be released into water.
Larval stages are involved in transfer between hosts.
Larval stages usually enter the denitive host (i.e. final host) in food (although penetration by larvae may occur in some flukes and roundworms).

Question 1

What are Helminths? What are the two classes of Helminths?

Answer 1

Worms

Round worms - Nematoda
Flat worms - Platyhelminthes

Question 2

What are the differences between the lifecycles of flukes, tapeworms and roundworms?

Answer 2

Differences:
In flukes and tapeworms, an intermediate host is always involved in the life cycle.
Roundworm larvae do not usually use an intermediate host although, in a few groups, a vector is involved. The vector usually effects transfer actively, e.g. by biting, whereas if the larval stages are free-living, transfer to the human host is passive, e.g. by being eaten.
Asexual reproductive stages occur in all flukes and some tapeworms but only sexual reproduction occurs in roundworms.

Question 3

What are direct and indirect lifecycles?

Answer 3

When an intermediate host or a vector is involved, this is said to be an indirect life cycle.

In all other cases, it is a direct life cycle.

Question 4

Describe the lifecycle of a fluke?

Answer 4

A tiny ciliated larvae hatches (miracidium) from an egg and searches for a mollusc (intermediate host) which it is chemically attracted to
If it finds one before it depletes it’s energy stores it enters the soft tissue of the mollusc
It’s forms a sac like sporocyst larva

In some species of fluke the eggs do not hatch until they have been ingested by a mollusc where they then hatch in the gut and for sporocysts.

Sporocysts produce more sporocysts

Within some types of the sporocysts a third type of larvae called redia larvae develop

In other types redia larvae are not produced instead cercaria are produced. Both redia and cecaria are produced asexually within the sporocyst. They are then released from the mollusc
They are sperm like and can swim

If the cercaria find a second specific intermediate host they penetrate, lose their tails and encyst to form metacercariae
If the host is eaten the metacercariae excyst and develop into adult flukes

Adult flukes have two suckers which they use to adhere to their host
Can penetrate a variety of tissues depending on the species.

Question 5

Describe the lifecycle of a tapeworm?

Answer 5

A hexancth larva (six hooked larva) develops inside the egg
Once ingested by the intermediate host the oncosphere burrows into he but tissue and forms a cyst stage called cysticercus or bladderworm. This contains a tapeworm head.

If the intermediate host is eaten by a definitive host then the head pops out and attaches to the gut wall

In some species asexual reproduction occurs in the cyst stage and many cyst buds containing hundreds/thousands of tapeworm heads can be produced
These are called hydated cysts

Question 6

Describe the lifecycle of roundworm?

Answer 6

Straight forward life cycle
Eggs are shed from hosts faeces
Eggs hatch and undergo several sheddings of their outer layers
Larva reach their infective stage
They are then either ingested or penetrate host tissue and mature into sexually mature adults
No asexual stage

Eggs are incredibly resistant and can survive for many years in the environment

Question 7

What are flukes?

Answer 7

Trematode parasitic flat worms

Question 8

What are blood flukes?

Answer 8

Parasitic flat worms which cause the disease schistosomiasis
There are three species of the schistosoma parasite which care pathogenic to humans
Schistosoma mansoni - intestinal schistosomiasis
S. haematobium - urinary schistosomiasis
S. japonicum - katayama fever

The blood flukes exit the blood vessels using their sharp spines to tear through the blood vessels into the bladder or intestines (first two examples)

Question 9

What are the two main strains of parasitic blood flukes in humans?

Answer 9

S. haematobium - blood vessels of the bladder

S. mansoni - blood vessels of the intestines

Question 10

What are the requirements for the schistosome lifecycle?

Answer 10

Water, molluscs and a vertebrate host (such as humans)

Question 11

How do schistosome eggs leave the human blood vessels where they are produced?

Answer 11

They have sharp spines which tear the blood vessels and allow entry into the bladder or intestines (depending on the species) where they can then exit the body

Question 12

Once released from the human host what happens to the eggs of the blood fluke?

Answer 12

If they enter fresh water a miracidium is released which swims around until it finds the intermediate host (snail/mollusc)

S. haematobium
A prolific asexual stage occurs releasing thousands of cercariae which are released into the water system where they can be ingested by the human hosts again

S. mansoni
Penetrate the human host directly through skin

In both their are separate sexes of worm which pair up and migrate together to either the bladder or intestines where they begin to produce eggs.
Adult worms can live for 5-7 years and produce between 300-3000 eggs or day

Question 13

Define concomitant immunity?

Answer 13

An immune response which prevent infection of new invading cercariae even though there are adult worms already living in the blood of the host
This prevents further infection

Question 14

How do adult blood flukes (schistosomes) avoid the hosts immune response?

Answer 14

Coat themselves with the hosts own macromolecules

They may also suppress their own antigens and release substances which suppress the hosts Immune response

Question 15

What is the immune response to schistosomes?

Answer 15

Eggs are swept away in the blood and lodge in various organs such as the liver, lungs and spleen
The host immune cells attack the eggs and form granulomatous masses around them lodging them in the tissue
This causes the organs to enlarge
Eventually leads to death and has been linked to cancers

Question 16

How is a schistosome infection treated?

Answer 16

Three types of antihelminitic drug are used against ukes: ivermectin, oxamnaquine and praziquantel (PZQ);

Question 17

What are cestodes?

Answer 17

Cestodes are the other class of parasitic atworms within the phylum Platyhelminthes. 
They inhabit the intestines of vertebrate hosts

Question 18

Describe the lifecycle of the hookworm?

Answer 18

Eggs are passed in faeces to develop and hatch in moist, warm soil to a first stage larva.
After about 10 days in favourable conditions and two moults the third stage infective larva is reached. These larvae can survive in the environment for about 4 weeks, but on contact with human skin they penetrate into a blood
vessel and are carried via the heart to the lungs, where they emerge into the pulmonary cavity. They ascend the bronchi by the mucociliary escalator and are swallowed to eventually end up in the intestine.
Two moults take place once the larvae reach the hosts intestine, resulting in adult male and female worms. Adults usually survive for 12 years inside
the intestine, the female passing thousands of eggs in the faeces every day

Question 19

Describe the lifecycle of Ascaris lumbricoides?

Answer 19

The larva then undergoes a migration akin to that of the hookworms
The larva burrows through the intestinal wall and into the bloodstream after which it is carried first to the liver, where it moults to a third stage, and then
to the heart and lungs. It then migrates up the trachea, is swallowed by the host, and returns to the intestine where it started about 21 days previously!
In the small intestine the larvae moult, mature to adult stage, copulate and begin passing eggs roughly nine weeks after ingestion.

Question 20

What is Filariasis?

Answer 20

Filariasis is caused by roundworms that inhabit the lymphatic vessels and subcutaneous tissues.

Four species are responsible for much of the morbidity due to the disease and cause:
Elephantiasis
River blindness
Loiasis - eye worm

Question 21

What are microlariae?

Answer 21

First larval stage of the parasite which causes elephantiasis
They are sheathed in a soft white egg like shell
Shed by the species W. bancrofti and B. malayi which are filarial worms and cause lymphatic filariasis

Question 22

What is the life cycle of W. bancrofti and B. malayi which cause lymphatic filariasis?

Answer 22

The parasites spend 10-14 days in the insect vector developing from the microfarial stage to the infective third larval stage.

In the human host it takes from 6-12 months to moult twice more to reach the adult stage. Unlike
other nematode species, larial females do not lay eggs. Instead they shed tiny microlariae which are essentially the first larval stage sheathed in the soft membranous egg shell. Up to 10 000 microlariae can be shed every day, with the adult worms living 15-20 years.

Question 23

What are the two nematode responsible for Cutaneous filariasis?

Answer 23

Onchocerca volvulus and loa loa

Question 24

Describe the life cycle of Onchocerca volvulus?

Answer 24

Adult worms can survive for up to 9 years in a host, during which time they produce high numbers of micro
lariae. The micro
lariae migrate out of the nodules into the skin and blood capillaries, from where they can be ingested by biting blackflys with the blood meal. The blackflys a true intermediate host as well as a vector, because the larvae undergo a migration out of the fly gut into the haemocoel and enter thoracic muscle cells where they develop into third stage infective larvae. The process takes about 2 weeks, and it is this infective stage of the parasite that migrates to the flys proboscis and is released into the next host during a blood meal.

Question 25

Describe the lifecycle of loa loa?

Answer 25

This condition is characterised by an allergic type reaction to dead worms or to their metabolic byproducts. Adult worms move freely in the subcutaneous tissues of hosts and can often be seen migrating across the cornea, giving rise to the common name of eye worm. Their life cycle is very similar to that of Onchocerca, with the difference being that Tabanid flies are the vectors, rather than blackflys. Micro
lariae of both Onchocerca and Loa loa show periodicity, but the latter show a diurnal pattern with peaks of circulating larvae in the peripheral blood around midday, which once again corresponds to the feeding habits of their vectors.

Question 26

Describe the lifecycle of Trichinella spiralis?

Answer 26

Upon ingestion, larvae are released from the meat by digestive juices in the stomach. They then locate to the small intestine and penetrate the epithelial cells lining the intestinal villi. Here they develop very quickly moulting four times in a 30 hour period to reach the adult stage. The smaller female adults (which are around 3 mm long) begin producing larvae, not eggs, only
5 days after mating. These larvae are equipped with a sharp sword-like stylet in their mouth, which they use to penetrate the epithelial basement membrane and gain access to either the lymphatic system or the bloodstream. The larvae migrate around the body entering a variety of cell types, but become resident in skeletal muscle cells. Most other cells types that larvae enter die as a result of the invasion, and it is this that causes most of the pathology of the infection. Once inside skeletal muscle cells the larvae initiate a remarkable series of changes to the host muscle cell known as nurse cell formation. Over a period of two weeks the fully differentiated muscle cell becomes totally transformed into a cell that is no longer contractile but is instead a support system for the developing larva. This nurse cell or parasite complex can remain viable for the life of the host, but most become calci
ed after a few months.

Question 27

Briey explain the difference between the terms de
finative host and intermediate host in the life cycle of parasitic worms.

Answer 27

The de
nitive host harbours the sexually mature stage of the parasite. The intermediate host harbours a series of different larval stages, which often undergo asexual reproduction, so increasing the chance of transmission to the de
nitive host.

Question 28

Define redia?

Answer 28

(Plural, rediae.) A larval form with an oral sucker, it will produce either more rediae, or cercariae.

Question 29

Define metacercariae?

Answer 29

(Singular, metacercaria.) Cercaria larvae encysted and resting, prior to the parasites being transferred to a de
nitive host.

Question 30

Define miracidium?

Answer 30

Free-living motile form, covered with cilia, which penetrates or is ingested by the intermediate host (mollusc) to become a sporocyst.

Question 31

Define schistosomiasis?

Answer 31

An important human parasitic disease caused by infection by blood flukes of a number of species of Schistosoma (schistosomes). The disease was originally known as bilharzia.

Question 32

Define sporocyst?

Answer 32

An elongated sac that produces a form of parasitic larva (rediae) or, in some cases, more sporocysts.

Question 33

Define vector?

Answer 33

An agent such as a bacterial plasmid or disabled virus that is used to carry DNA into cells. The same term is applied to an organism that carries a parasite or pathogen from one host to another.

Question 34

Define oncosphere?

Answer 34

Egg sac containing the six-hooked (hexacanth) larva of a tapeworm

Question 35

Describe the recombinant subunit vaccine?

Answer 35

Preparation for injection or ingestion containing fragments (subunits) of an infectious agent obtained from the expression of certain pathogen genes, which have been cloned (copied many times) and inserted into the genome of a harmless strain of bacteria or yeast (the expression vector). This vector then manufactures the vaccine subunits in commercially useful quantities

Question 36

Define cercaria?

Answer 36

(Plural, cercariae.) The larval form of the parasite, developed within the germinal cells of the sporocyst or redia. A cercaria has a tapering head with large penetration glands. It may or may not have a long swimming tail, depending on the species. The motile cercaria
nds and settles in a second intermediate host where it will become either an adult or a metacercaria, according to species.

Question 37

Define concomitant immunity?

Answer 37

A condition in which hosts appear to be able to resist further parasitic infections, even though adult forms survive in their blood vessels. The immune response acts against new invading parasites.

Question 38

Define cystericus?

Answer 38

Larval stage of a tapeworm, consisting of a single invaginated scolex enclosed in a fluid-
filled cyst.