1- Life cycle parasites
Schistosomiasis (Schistosoma)
1.
I. What is the route of infection of the human host: the name of the developmental
stage of the parasite, the point of entry and the initial destination?
The cercariae that originate from freshwater snails penetrate the human skin. The cercariae
lose their tails during penetration and become schistosomula. These schistosomula end up in
the bloodstream and settle in the liver. Here, they mature and when they have matured, they
migrate to the mesenteric venules of the rectum and the venous plexus of the bladder. Here,
they lay eggs into the blood that then migrate into the bladder and intestines.
II. Where and how does multiplication of the parasite take place?
The asexual multiplication of schistosomiasis happens in the snail though mother and
daughter sporocyst stages, in several rounds.
Furthermore, the worm pairs lay eggs into the bloodstream.
III. What is the process of transmission from host to vector and from vector to host?
Host to vector: Eggs from urine and feces enter the water and release miracidia. These
penetrate the snail tissue.
Vector to host: The cercariae penetrate the human skin.
IV. What is the pathology that can be caused by the parasite?
Pathology of schistosomiasis includes hematuria, scarring, calcification, squamous cell
carcinoma, and occasional embolic egg granulomas in the brain or spinal cord.
V. Which stage(s) of development of the parasite is(are) responsible for the
pathology?
The eggs of the mature worms are responsible for the pathology.
VI. Through which mechanisms does pathology occur?
The eggs induce an immune-mediated granulomatous response, when they are not excreted
and become trapped in the human body. The granuloma formation causes inflammation in
those tissues
2. Retrieve the life cycle diagrams from Blackboard and mark in the diagrams the
events you have described in i, ii, iii & v
,Malaria (Plasmodium)
1.
I. What is the route of infection of the human host: the name of the developmental
stage of the parasite, the point of entry and the initial destination?
Enter the blood from the mosquito salivary glands as sporozoites. It invades the liver cells →
cell division → merozoites. The merozoites are released in the bloodstream and start
invading erythrocytes. In the erythrocytes, the parasites develop into ring, trophozoite and
schizont stage parasites. Trophozoite-infected erythrocytes express parasite-derived
adhesion surface molecules so they can adhere to blood vessels, to avoid being destroyed
by the spleen. Then they develop into merozoites again → released in the bloodstream again
to invade other erythrocytes. Apart from this cycle, some of the trophozoites develop into
male and female gametocytes, which can be picked up again by a mosquito. In the mosquito
gut, the gametocytes will develop into gametes, a zygote, an ookinete, and an oocyst
respectively, and sporozoites will be released again into the salivary gland. If this mosquito
bites another host, the sporozoites will go through this cycle again.
II. Where and how does multiplication of the parasite take place?
The multiplication takes place in hepatocytes, erythrocytes and mosquito gut.
Hepatocytes: asexual reproduction of sporozoites
Erythrocytes: asexual reproduction of merozoites/trophozoites (when the erythrocytes burst,
they are merozoites)
Mosquito gut: sexual reproduction by fusion of gametocytes → develop into an oocyst and
release sporozoites.
III. What is the process of transmission from host to vector and from vector to host?
Vector to host: The parasite is transmitted when a mosquito takes a blood meal: sporozoites
from mosquito’s salivary glands into the bloodstream.
Host to vector: Gametocytes are consumed by the mosquito when he takes a blood meal.
Those will end up in the stomach and fuse into gametes.
IV. What is the pathology that can be caused by the parasite?
The pathologies that can be caused are: severe anemia, Cerebral Malaria, organ failure and
placental malaria
,It doesn't give symptoms while staying in the liver, because the parasite doesn’t harm the
hepatocytes
V. Which stage(s) of development of the parasite is(are) responsible for the
pathology?
When the parasite has replicated in the red blood cells, the cells are lysed so that the
daughter parasites can enter the bloodstream. This is called hemolysis of the red blood cells
and causes anemia.
When the infected erythrocytes enter the brain and adhere to the blood vessels there and
block them.
VI. Through which mechanisms does pathology occur?
Pathology occurs by the rupturing of erythrocytes (RBCs) and the release of toxins.
2. Retrieve the life cycle diagrams from Blackboard and mark in the diagrams the
events you have described in i, ii, iii & v
, 3. With these concepts in mind and based upon your knowledge of the different life
cycles indicate the points in the life cycle of Plasmodium and Schistosoma that are
most susceptible to control.
Plasmodium (Malaria):
The multiplication of the cells in the liver.
Mosquito combatting, by preventing mosquito bites and controlling mosquito
population through:
o Insecticides
o Preventing the formation of stagnant water
o Larva-eating fish
Block the molecules the parasite uses to enter the erythrocytes.
Adherence of infected erythrocytes to the blood vessels. Erythrocytes would be
destroyed in the spleen, but if this causes less hemolysis, the parasite would be
eliminated.
Chemoprophylaxis
Schistosoma (Schistosomiasis):
