Parasitology

Parasitology & Intestinal Protozoans
Chapter 34 & 35
Microbiology for Surgical Technologists
Hosts
There are three types of hosts for parasites
Reservoir
Intermediate
Final
Reservoir Hosts
The parasite resides on this host waiting to be transmitted to humans or other animal
Example – beavers serve as a reservoir of Giardia lamblia, they defecate in the mountain water and cysts can be ingested by humans who drink it
Intermediate Host
The host in which the immature parasites resides
Some parasites can only mature in a particular environment the the intermediate host provides
The parasite may be able to multiply in the intermediate host
A parasitic life cycle may include one or more intermediate hosts
Final Host
This is the host in which the parasite lives it’s adult lifecycle, including sexual maturity and reproduction
Parasitology
The study of invertebrates that are human pathogens is called parasitology
There are two categories of parasitic human pathogens:
Unicellular protozoans
Cause disease
Multicellular protozoans
Cause infestation
Protozoans
Protozoans belong in the kingdom protista and are classified into phyla
There are four phyla that are pathogenic parasites in humans
Apicomplexa
Ciliophora
Microspora
Sarcomastigophora
Apicomplexa
Better known as sporozoans, are unicellular
Nonmotile
Have alternating sexual and asexual life cycles
Example: Cryptosporidium
Transmitted through contaminated water
Responsible for outbreaks of acute gastroenteritis

Ciliophora
Are unicellular
Have cilia for movement
Reproduce by binary fission or sexual conjugation followed by binary fission
A common species is Balantidium coli that is an intestinal parasite of humans
Microspora
Are unicellular and are small intracellular parasites
Can be contracted by an intermediate host or person to person contact
A member that is known to have an affinity for causing enteritis in AIDS pts is Enterocytozoön beineusi
Sarcomatigophora
Are unicellular amebas, which are motile by means of pseudopods and flagellates
Reproduce by binary fission
Entamoeba histolytica is a well known ameba that causes dysentery in humans who ingest cysts that are contained in feces contaminated water or food
Trichomonas vaginalis is a flagellate that is the cause of a common STD
Helminths
The group helminths belong in the category metazoans
They are multicellular protozoans
Helminths are known as the worms and the human parasitic members belong to two phyla:
Aschelminthes – roundworms
Platyhelminthes – flat worms
Aschelminthes
Aschelminthes (round worms)
Are in the class of nematoda (nematodes)
Ascaris lumbricoides are a parasite of the GI tract and can cause bowel obstruction
The condition called ascariasis is diagnosed when the worm emerges from the anus
They have small teeth and can chew through the intestinal wall and enter that abdominal cavity
The worm can grow to be as long a 1 foot
Platyhelminthes
Platyhelminthes (flat worms) include two classes
Cestoides (tapeworms)
The species Taenia are ingested by humans eating undercooked or raw contaminated pork or beef
Trematodas (flukes)
Schistosoma mansoni is a well known blood fluke that uses snails as an intermediate host
Transmission of Parasitic Disease
Contaminated food and water are two of the most common methods of transmission
Primarily with feces contaminated water
Many underdeveloped countries use human feces to fertilize their crops, which then contaminates the water supply
Parasitic disease can be contracted by direct contact
It often occurs in areas of overcrowding
It can also happen in prisons, factories and businesses, and daycare centers
Direct transmission primarily involves the oral-fecal route due to poor hygiene
People forget to wash their hands
Transplacental infection is uncommon
But pregnant women are warned not to be exposed to cat litter, or eat undercooked pork or beef
They contain oocysts that will cross the barrier and infect the nervous system of the fetus and cause death
Transmission of Parasitic Disease
(continued)
Respiratory infection can occur when parasitic eggs are airborne
Humans develop infection when the eggs are inhaled
Protozoans survive harsh environments, such as the outside world and the acids of the GI tract, by forming cysts
The cyst has a thick cell wall for protection
It has a low metabolic rate so it has less nutritional requirements
Helminths survive by producing a large amount of eggs or larvae
This increases the odds that the next generation will survive
Asexual reproduction at the larval stage also increases survival
Parasitic Reproduction
Asexual
There are three methods of asexual reproduction
Mitotic fission
Schizogony
Budding
The first two are most common
Mitotic Fission
Commonly seen in protozoans
Produces two daughter cells
Trichomonas vaginalis reproduces with mitotic fission
Schizogony
Refers to multiple fission
The nucleus is divided several times by mitotic fission and the cytoplasm is distributed among the daughter cells
Budding
A few helminths are able to reproduce by budding
It enables the number of worms to increase in a host that may have only consumed a single egg
Tapeworms in the larval stage reproduce by budding
Sexual Reproduction
Some protozoans reproduce by conjugation, producing gametocytes
The majority of helminths reproduce by cross-fertilization and self-fertilization
Sexual reproduction is more efficient than asexual
Provides the ability to genetically adapt to changing environments, increasing the chance of survival
Conjugation
Two parasites fuse and exchange genetic material
The organisms separate and continue the cycle by conjugation with others
The majority of trematodes and cestodes are hermaphrodites
They contain both female and male reproductive organs and can either self-fertilize or cross-fertilize
Parasitic Life Cycle
There are two types of life cycles
Complex
Simple
Both types occur in species of protozoans and helminths

Complex Life Cycle

Parasites that rely on one or more intermediate hosts and a final host are said to have a complex lifecycle
It is more complicated for the organism to survive and multiply
Simple Life Cycle
Parasites with a single host that are transmitted by either direct contact or ingestion of cysts are considered to have a simple lifecycle
These are transmitted by the oral-fecal route
Unicellular Protozoa
Are eukaryotic cells
Are responsible for causing serious human diseases such as malaria and African sleeping sickness
More people are affected by malaria worldwide than by any type of bacterial disease
They can be just as complex as multicellular organisms
Some have cilia, food vacuoles for digestion, openings called cytoprocts to expel waste products, and contractile vacuoles to expel water so they won’t lyse
The two primary groups of unicellular protozoa that infect humans are
Amebas
Flagellates
They are transmitted by the oral-fecal route
Ameba
The lifecycle of the ameba is divided into two stages
Trophozoite - the metabolically active, motile, reproducing stage
Cyst – the dormant, infective stage
Reproduction is by binary fission, in which the trophozoite splits into two daughter organisms
Ameba
When conditions become harsh the cyst form develops in order to survive
Motility is achieved by the extension of a pseudopod (false foot)
Entamoeba histolytica
The most significant amebic intestinal human pathogen
It is found worldwide and is estimated to infect 50 million with 40,000 deaths annually
Causes the disease amebiasis, also known as amebic dysentery
Entamoeba histolytica
(continued)
The trophozoites produce a cytotoxin that damages the intestine causing lestions, inflammation, hemorrhage, and the ability of opportunistic organisms to cause secondary infections
Symptomes include abdominal cramps, pain, and bloody mucus diarrhea; accompanied by weight loss
Secondary infection occurs when E. coli enters the bloodstream or lymphatic system invading organs
This is referred to as extraintestinal amebiasis
The most common infected organ is the liver, but infections of the pulmonary, CV, and CNS can occur
Flagellates
The organism achieves motility by means of flagella
The flagella are surrounded by a semirigid, thin film called a pellicle the helps to define it’s shape
Giardia lamblia
A flagellate found worldwide
Has eight flagella to propel it in a rapid, twisting motion
Contracted by drinking contaminated water, or person to person by the fecal oral route
It is prevalent in daycare centers
The disease it causes is giardiasis
The trophozoites attach to the villi of the intestine by a ventrally located sucking disk
This prevents it from being removed by the peristaltic action of the small intestine
Symptoms are sudden with foul smelling, watery diarrhea, steatorrhea (excess fat in feces), abdominal pain and cramps, and flatulence
Trichomonas vaginalis
The only flagellate that is a parasite of the GU system
Motility is achieved by four flagella and an undulating membrane
It does not form cysts and only lives as a trophozoite
Primarily transmitted by sexual intercourse
Rarely transmitted by fomites such as clothing
Most cases are asymptomatic
Women will present with burning, itching, dysuria, and discharge
Men will present with urethritis and prostatitis
Surgical Implications
The most important factor for the ST concerns the pt with E. histolytica that is scheduled for endoscopy
Decontamination of the instrumentation should be thorough with care taken in handling the instruments to prevent the spread of infection
Homework
Words of Wisdom
Self doubt is your worst enemy. Believe in yourself, believe you can, and you will!
Tracey Carpenter
2003