Parasitic Amoebae

 

v In general characteristics amoebae:

·       Possess shapeless mass of cytoplasm - (Ectoplasm & endoplasm).

·       Move and ingest by means of pseudopodium and reproduce by simple binary fission.

·       Respiration is performed by simple absorption of dissolved oxygen from the liquid environment. and Excretion of gases and wastes is performed by diffusion out of the organism through the cell membrane.

·       Liquid regulation inside the body is controlled by contractile vacuoles special for fresh water ameba.

·       Metacyst is the matured and infective stage

v Most amoebae species have two life cycle stages:

a.      Trophozoite is a living stage of protozoa when they can move, take food and reproduce. (It is usually the pathogenic stage.)

b.     Cyst is the resting stage of a protozoon with a protective wall. It is usually the infective stage. Its functions are protection and transmission

Ø  Major amoebae species :-

                                i.     Pathogenic amoebae

Ø  Entamoeba histolytica

                           ii.      Non-pathogenic (Commensal) amoebae

Ø  Entamoeba coli,Entamoeba dispar, E. Gingivalis,E. Hartmanni, Endolimax nana and Iodamoeba butschlii

                          iii.     Pathogenic free-living amoebae

Ø  Naegleria fowleri , Acanthamoeba spp. And Balamuthia spp.

i.       Pathogenic amoebae

o    Entamoeba histolytica

Ø  Causes amoebiasis - (Amoebic dysentery & liver abscess).

v  Geographical distribution  of Entamoeba histolytica (Epidemiology):

§  E. histolytic is world-wide in prevalence - in tropical & subtropical regions-high with areas with overcrowding & poor sanitation occur.

§  Estimated global prevalence of human are about 400 millions .

§  about 90% of the infections  and 50,000 – 100,000 deaths /year

v  Morphology: E. histolytica occurs in two forms – the trophozoite and cystic stage.

v Trophozoite

·       The trophozoite or the vegetative form and is the growing or feeding stage

·       It is irregular in shape and has one nucleus and pseudopodium.

·       Motility is rapid, progressive and unidirectional, through pseudopodia.

·       Cytoplasm of trophozoite- finely granular with few ingested bacteria or tissue debris in vacuoles.

·       Invasive strains possess engulfed RBCs.

·       Trophozoites are delicate organisms and are killed by drying, heat and chemical disinfectants.

v Cystic stage

Ø  The early (immature) cyst contains one or two nuclei & two other structures – a mass of glycogen .The mature cyst possesses four nuclei – quadrinucleate.Which is the infective stage

v Transmission of ameba

        infections occur through the ingestion of a quadrinucleate infective cyst found in contaminated food or water with faeces and also by hand to mouth contact.

v Life cycle

·       After ingestion, the cyst ,in small intestine l ileum , excystation takes place.

·       Trophozoites being actively motile invade the tissues and ultimately lodge in the sub mucous layer of the large bowel. Here they grow and multiply by binary fission.

·       Invasion of blood vessels may lead to secondary extra-intestinal amoebiasis.

·       Then trophozoites -Encystation occurs in the caecum & colon.

·       Finally, mature quadrinucleate cysts form which is the infective forms.

·       The infective cysts are excreted in the faeces.

v Pathogenesis and Clinical features

§  Lysis of target cells by release of toxins and introduction of membrane channels.

§  Phagocytosis of target cells epithelial immune cells and RBC

§  Inflammatory response-

§  Non-invasive trophozoite

·       Amoebae on mucosa surface that never  penetrated that mucosal layer

·        Their mostly in asymptomatic cysts

·       Non-dysenteric diarrhea, cramps, abdominal discomfort

§  Invasive trophozoite

·        Necrosis of mucosa → ulcer or lesion

·       Hematophagous trophozoites-engulf RBC

·       Ulcer enlargement –FLASK SHAPE ULCERS

o   Cause acute amoebic diarrhea  è abdominal pain dysentery with blood & mucus.

o   .

§  Extra intestinal amoebiasis

v Invasion into the deeper mucosa with extension into the peritoneal cavity may occur.

v This can lead to secondary involvement of other organs, primarily the liver but also the lungs, brain and heart.

v Clinical signs associated with the extra intestinal amoebiasis

§  Pain, tenderness in the region of the liver, lung

§  Wasting – loss of body weight 

§  Fever associated with chills & night sweating

v Laboratory diagnosis

         1.            Microscopy diagnosis

Ø  Examination of a fresh dysenteric faecal specimen for trophozoite stage.

v Eg. - direct wet mount examination /saline/

  - Concentration methods -Floatation and Sedimentation – formalin-ether

        2. Immunodiagnostic methods-Antigen detection in stools

        3. Molecular detection methods -DNA probes  and PCR

4.         Imaging- Ultra sound,-Amoebic liver abscess

v Prevention & control

o   Keeping Personal hygiene, environmental hygiene Hand washing after defecation & before eating.

o   Health education about the routes of transmission is very important.

B.  Non-pathogenic amoebae of man

Ø  Other amoebae inhabiting the alimentary canal.

Ø  Most of these amoebae are commensal that can parasitize the human gastrointestinal tract.

·       Entamoeba hartmanni. Entamoeba dispar, E. coli., E. polecki, Endolimax nana Iodamoebabutschlii, Entamoba gingivalis and Blastocystis hominis

C. Pathogenic free- living amoebae (Opportunistic)

General characteristics

o   They are facultative parasitic amoeba.

o   Usually free living but rarely infect humans when an opportunity exists to enter.

o   Acquired by soil or water contact.

o   Live mainly in stagnant water, sewage system and polluted soil.

o   The nuclei of the opportunistic amoebae possess a large central nucleolus or karyosome, and a nuclear membrane without chromatin granules.

·       Member of the genera:

Ø  Naegleria fowleri, Acanthamoeba spp., and Balamuthia mandrillaris

a.     Naegleria fowleri

·        Causes an acute, usually lethal, central nervous system (CNS) disease called primary amoebic meningoencephalitis (PAM) or Naegleriasis in humans and other hosts as well.

·        Humans have been the hosts of greatest concern, but it seems likely that these amoebae have little host specificity.

·        It is the only species of Naegleria genus that is pathogenic to humans.

v Geographical distribution

• It is found worldwide in soil, warm fresh water bodies such as lakes, ponds, rivers and hot springs as well as swimming pools.

v Morphology

o   Naegleria fowleri has three stages in its life cycle.

1. Amoeboid trophozoite

§  It is the main invasive and infective stage. And It is the feeding, growing and replicating form.

§  Amoeboid stage is actively motile with blunt pseudopodia èlobopodia.

§  Trophozoites can turn into temporary flagellated forms which usually revert back to the trophozoite stage.

§  Contains single nucleus and large central karyosome.

§  It has distinctive phagocytic structures called amoebostomes and reproduces by simple binary fission.

2. Flagellate form

§  Transformation from amoeboid form è to flagellate form and at  flagella stage it can swim rapidly.-It contains two flagella and

§  It is non-feeding and non-dividing stage which after a time reverts back to amoeboid form.

3. Cysts stage

o   It doesn’t form in the hosts. And it is dormant and don’t divide.

o   Non-motile and non-feeding stage. And Possess single nucleus, spherical in shape.

o   Has thick resistant and smooth double wall.

v Mode of transmission

§  Source of infection: contaminated swimming pools, stagnant ponds, fresh water lakes, stream and thermal (hot) springs as well as contaminated dust.

v Life cycle

§  The amoeboid trophozoite forms of N. fowleri enter through the nose during inhalation of contaminated dust or water during swimming in fresh water lakes, ponds or swimming pools.

§  After entering to the nose, the amoeboid trophozoites invade the nasal mucosa, migrate along the and followed to olfactory nerve branches into the meninges and brain to initiate an acute meningitis and encephalitis called primary amoebic meningoencephalitis (PAM).

v Pathogenesis & clinical symptoms

o   fatal illness of the brain and meninges develop and  in death within 72 hours

v The main symptoms:

·        Upper respiratory pain with headache and lethargy.

·         Initially may notice change in taste or smell, Fever, nausea and vomiting 

·        Sore throat, blocked or discharging nose and Meningeal irritation

     Neck stiffness è mental confusion è coma è death

v Diagnosis

Ø  Diagnosis of PAM can be made by microscopic examination of specimens from nasal discharge and CSF.

        è Motile amoebae can be seen with phase contrast microscope.

v Prevention and control

Ø  Adequate chlorination of swimming pools may effective.

Ø  Plug your nose or wear nose clips when diving into fresh water.

¨    Acanthamoeba species

v Geographical distribution

Ø  Free-living trophozoites and cysts occur in both the soil and freshwater.

¨     Morphology

·        Similar to Naegleria, (except: no flagellate form, cyst is formed in host tissue).

·        The trophozoite has an irregular appearance with spike-like structures known as acanthopodia; hence the name, Acanthamoeba (acanth = spine or thorn).

·        Both the trophozoites & cysts are infective.

                 è  Both the cyst and torphozoite have a single nucleus.

v Life cycle and Pathogenesis  of Acanthamoeba species

¨      Portal of entry the protozoa includes: Broken or ulcerated skin.; Eyes & genitourinary tract.

      and noses -by inhalation of aerosol or dust containing trophozoites & cysts.

¨      After entering, they invade CNS through the blood system.

¨     Infection occurs in: Lower respiratory tract, ulcerated skin or mucosa è blood stream  è CNS.

v Clinical features  

§  GAE is usually of gradual onset and takes a prolonged chronic course  and Characterized by focal lesions.

§  Acanthamoeba may cause an ulceration of the eyes called Acanthamoeba Keratitis.

§  This may lead to loss of vision or blindness

§  This is in sharp contrast to N. fowleri infection, which is acute.

 

v Laboratory diagnosis

    

·        Wet mount smear from discharges.. Scrapings from lesions and Nasal discharges

and Stained preparation. CSF for microscopy

·        Serology test.