Transcript Ch. 7, The Amebas - University of Evansville Faculty Web sites

Chapter 7 - The Amebas
Taxonomy
P. Sarcodina
S.P. Amoebozoa (Rhizopoda)
C. Lobosea
O. Amoebida
O. Schizopyrenida
P. Sarcodina
• Characterized by cytoplasmic extensions of the body called
pseudopodia; flagella, when present, are restricted to specific
developmental stages
• Internal organs are not fixed in position within the cell, but rather
they move freely within the cytoplasm
• Most members of phylum are free living; a few are parasitic and
commensalistic
O. Amoebida
• Usually uninucleate; no flagellate stage
Family Endamoebidae
• Parasites or commensals of the digestive systems of
arthropods and vertebrates
• Genera and species are primarily distinguished on the basis
of their nuclear structure
• Type example: Entamoeba
Genus Entamoeba
Morphology
• Ectoplasm is usually distinguishable from the endoplasm
• Nucleus is characterized by a a peripheral layer of chromatin
forming a beaded ring
• Cytoplasm of trophozoites contains numerous helical bodies with
ribonuceoproteins - chromatoidal bodies
• Chromatoidal bodies develop in
the motile stages of the amebas
and mature in the cyst, but
disappear as the cyst ages
• Golgi bodies and mitochondria
are completely absent in these
organisms
Entamoeba histolytica
• Best known species of ameba that parasitizes humans
• Causative agent of amoebiasis/amoebic dysentery
• Global in its distribution; the incidence of infection exceeds 400
million cases!
• First discovered in Russia in 1873 by Losch
Life Cycle: Overview
Life Cycle (detail)
• The uninucleate trophozoite inhabits
the colon, rectum and, at times, the
lower end of the small intestine of
humans and other primates
• It is typically monopodial, producing
one large fingerlike pseudopodium
• Trophozoites proliferate mitotically
(binary fission) in the host gut
• Under certain adverse environmental
and/or physiological circumstances the
trophozoites assume precystic
characteristics by becoming more
spherical and as food vacuoles are
extruded, shrinking in size
Life Cycle cont.
• Encystation begins with the secretion
by the precyst trophozoite of a thin,
surrounding hyaline membrane to form
a cyst
• Nucleus of the cyst then undergoes 2
mitotic divisions to produce 4 vesicular
nuclei in the mature cyst
• Cysts represent the infective form and
pass out of the host feces
• When food or water contaminated with
cysts is ingested by a host, cysts pass
through the stomach to the ileum where
excystment occurs
• Upon excystation, the tetranucleate
organism undergoes mitosis, giving rise
to 8 small, uninuceate metacystic
trophozoites, which pass down to the
large intestine where they feed, grow and
reproduce
Note:
• Main source of infection is the cyst-passing, asymptomatic carrier
or chronic patient - luminal amoebiasis
• Acutely ill patients, those with invasive amoebiasis, are not
significant transmitters since they pass the noninfective trophozoite
• Flies and roaches have been implicated as vectors in the spread of
this ameba, since cysts can survive for long periods in their digestive
tracts, later to be regurgitated or passed out in feces on food
Pathology
• In humans living in temperate zones, the organism often produces the
nonpathogenic luminal form of the disease - luminal amoebiasis
• In the tropics and subtropics, the invasive form of the disease is more common
• In the infected individual who develops dysentery, the mucosal ulceration may
penetrate deeper into the intestinal tissue, causing vast areas of tissue to be
destroyed; the overlying mucosal epithelium then may be sloughed off, exposing
these necrotic areas
• The destructive process is
usually followed by a
regenerative period,
resulting in thickening of the
intestinal wall as a result of
the deposition of fibrous
connective tissue
Human colon showing amoebic ulcer
Pathology cont.
•Trophozoites may also be carried to the liver via the hepatic portal system
causing hepatic amoebiasis
• The first sign of hepatic involvement is the formation of an early hepatic
abcess containing a matrix of necrosed hepatic cells, which eventually
become liquified
Liver abcess
Symptoms
Invasive amoebiasis may manifest itself in a number of ways:
1. Acute amoebiasis - severe diarrhea (blood and mucus in liquid)
develops and a fever
2. Chronic amoebiasis - continuous attacks of diarrhea with other
mild intestinal problems
3. Hepatic amoebiasis - abscesses may rupture the abdominal wall
or extend through the diaphragm into the lungs; any of these
manifestations can be fatal
Prevention
• Avoid food and water contaminated with feces containing the
cysts are the most common vehicles for transmission
• Boil or iodination of drinking water in endemic areas
• Westerners travelling to developing countries should drink
bottled water, avoid ice cubes, salads and those fruits not peeled
by the person consuming them
Entamoeba coli
• An intestinal ameba, generally considered to be nonpathogenic in
humans
• Does not ingest or invade host tissue; usually feeds on bacteria,
yeast and fragments of intestinal debris
• Life cycle is similar to the of E. histolytica, including precystic,
metacystic and trophozoite stages, with infection of the host initiated
by ingestion of cysts
Entomoeba gingivalis
• Cosmopolitan in distribution
• Commonly found in the tartar and debris associated with the
gingivital tissues in the mouth
• Food vacuoles may contain epithelial cells, WBCs and
occasionally RBCs
• No indication that it is
pathogenic
• No cyst is formed; it is
transmitted either directly
(kissing) or indirectly via
trophozoite-contaminated food,
chewing gum, etc.
Order Schizopyrenida
• Members of this order possess both a flagellated and amoeboid
form
• During the amoeboid form that reproduction (asexual) occurs
• The amebas occur in soil and water and mostly consume
bacteria
Family Vahlkamphidae
• Although members of this family typically live in the soil and
water and feed on bacteria, they can become facultative parasites
of animals, including humans
• Example: Naegleria fowleri
Naegleria fowleri
• Responsible for primary amebic meningoencephalitis (PAM)
• Most victims have a history of recent exposure to warm, fresh or brackish
water, such as swimming pools, ponds, lakes and streams
Life Cycle
• Includes flagellated and amoebic trophozoites and cysts; rapid
transformation from one form to the other
• Flagellated trophozoites are
capable of rapid movement through
the water, and transmission to
humans likely occurs when the
nasopharyngeal mucosa is invaded
by these forms
• Amoebic trophozoite form
migrates through the nervous
system to the brain, where
inflammation occurs followed by
death
• No cyst stage occurs in the human
host
Family Hartmannellidae
• Contains a single genus, Acanthamoeba
• Worldwide in distribution; isolated from freshwater, brackish water, hot
tubs, sewers, etc.
• Like Naegleria, it’s a facultative parasite of humans
• Other than the absence of the flagellated trophozoite stage, it has a life
cycle similar to N. fowleri
• Responsible for acute PAM, Acanthomoeba causes chronic PAM
Acanthamoeba
Family Hartmannellidae cont.
• Acanthamoeba have been implicated in skin infections
• Recently gained public attention because of the devastating effects of its
invasion to the human eye (= keratitis), corneal inflammation and corneal
perforation often followed by blindness)
Keratitis. Note the ring-shaped infiltrate of the cornea