micro sheet # 1 Dr. 7assan - Nada Qadiri

PARASITES
1st lecture of Dr. Hassan, Sunday 27/3/2011
The meaning of the word parasite is someone who eats at someone else’s table.
They infect/infest the body like any other microorganism (like bacteria, fungi and viruses). But of course, there are many differences between parasites and other microorganisms. These differences include the complex lifestyle of the parasites (a lot of morphological changes) and a different nomenclature.
The patient (i.e the human being) is the host.
The human is the primary host
Intermediate host: transmitis the parasite from one human to another (e.g mosquito in malaria). Intermediate hosts are usually animals (such as cows or pigs); if an insect acts as an intermediate vector then they are referred to as vectors.
Parasitic Disease
• Widespread in the world
• Cause mild diseases in many cases, but nonetheless some are very chronic and serious. They can even cause death.
• Prevalence is outstanding; 1 billion people in the world have intestinal nematodes!
• 2nd most common cause of blindness (the first being chlamydia)

General examples of parasitic diseases:
• Bilharzia affects 200 million people worldwide
• Filaria (Elephantiasis disease) affects 200-300 million people worldwide
• Malaria: serious and causes death (mostly affects children- 1 million deaths)
• Parasitic diseases  mostly developing world
There are treatments for parasitic diseases but NO vaccines.
Classification of Parasites





















I. Unicellular - Protozoa
• Each organism is made up of one cell
• Size varies from 0.3 micron to 80 micron. When trying to imagine theses sizes try to imagine the size of them compared to RBCs which are 7 microns
• So we can have intracellular (if small – like malaria in RBCs) or extracellular (if large – like amoeba 20 micron) infection.

• Can infect tissues or lumens of organs in the body.
Those that affect lumens  LUMINAL PARASITES (e.g amoeba)
Those that affect tissues  TISSUE PARASITES (e.g malaria)

For example, if a parasite infected the lumen of the GIT, the urethra or the vagina then they are open to the outside and are luminal.
The amoeba lives in the lumen of the large intestine and produces diarrhea and dysentry so it is known as a luminal parasite. But note that amoeba can also degrade tissues, go through the wall of the large intestine, reach the blood and form liver abcesses so amoeba are considered both tissue and luminal. Malaria parasites affect RBCs and these are inside the tissues so they are tissue parasites.

As you would expect, tissue parasites are more dangerous

• Protozoa multiply by binary fission. So protozoa numbers can increase in body and thus can produce aggressive diseases. Note that not all parasites have the capability of increasing their numbers in the body.



How do they transmit themselves from one host to another?

1. Trophozoites
• Vegetative form
• Active form
• Can eat, multiply and metabolize.
• It is the form that causes disease and divides by binary fission
• Only exist in tissues. They are very delicate and do not live for long outside the body as they are sensitive to changes in pH, temperature and humidity. So, trophozoites are not a very effective way of transmitting disease.
• Transmitted via intermediate host.
• Actually cause pathology
2. Cysts
• The Protozoa change their morphology into a more resistant form known as a cyst. If there is no intermediate host, then the protozoa will use this form of transmission
• Morphological form
• Inert form
• Doesn’t divide
• Well protected by a coat against environmental conditions
• Doesn’t divide
• Survives outside of the body for a considerable time until it is ingested or taken up by another host.
• Don’t actually cause pathology
• Amoeba use this method
Over here the doctor put a slide titled Intestinal and Luminal Protozoa…Fig 3… Systemic Representation of Entamoeba
 you can see the cyst of the amoeba and the stages it undergoes to produce the cysts.


 Cysts can live in faeces for a long time. If ingested, the cysts will be resistant to stomach acids. The cysts will release the nuclei under the effect of the hydrolytic enzymes in the intestine. The nuclei will become trophozoites and establish infection! Even if someone ate food contaminated with fresh faeces (traphezoites still being alive) he/she will not get infected because the stomach acids will kill the traphezoites!

II. Multicellular – Meltazoa (Helminths)
General Features:
• Their size ranges from 1 mm to 1m
• Their size does not allow them to esatablish intracellular infection so they produce extracellular infection (either luminal or tissue infections).

• How do they reproduce?
Their size is too large for binary fission! They produce eggs. These can reach up to 20,000 eggs every day. Fortunately, these eggs do NOT hatch inside the host - but are passed outside and from then onwards they are either:

a) Intermediate Host route. Eaten by an intermediate host (pig or cow). Then if you eat the meat of one of these animals an infection is established.
b) Fecal-oral route. Eaten immediately by the primary host and establish infection.

Note that there is another route of transmission that is tissue-ward for TISSUE parasites. ((a) and (b) were luminal… *I’m not completely sure about this) This route involves does not involve eggs and are occasionally transmitted by mosquitoes. (the doctor only mentioned this briefly)
Nematodes – Roundworms
The doctor put a slide containing a picture of 4 worms and then a cross sectional section on the bottom (sorry I don’t have the slides).
• Size ranges from 1 mm to 1m !
 Ascaris is 30 cm, for example

• They have an oral and anal opening, a GI tract, a rudimentary nervous system, a rudimentary excretory system but no cardiovascular system (they rely on diffusion).
• They have a reproductive system that is of utmost importance. They have both sexes. If you only had female worms in you GI tract, then only unfertilized eggs will be produced and these will NOT cause infection. The female worm is usually larger than the male worm. The male worm has testes and vas deferens as usual and the female worm has a uterus and ovaries as usual.


Done by: Nada Kadri
Good luck!