The Parasites

January 19th, 2010

Parasite biology

• Eukaryotic cells– Complex cell structure– Nucleus– Organelles– Mitochondria or similar structures

• Can be single celled or multicellular• Feed on bacteria and small nutrients

Parasite biology

• Complex life cycles• Distinct life stages that undergo structural

changes• Some stages are reproductive• Only certain stages are infectious• Different stages take place in different

places– Environment– Tissues of host– Tissues of vector (insect)

Example: Malaria life cycle

Example: Cryptosporidium life cycle

Parasite Classification

• The protozoa (protists)– Single celled

eukaryotes

• The helminths– Multicellular

eukaryotes

The Protozoa: Characteristics

• Single celled organisms• Environmental stages inhabit water or

soil• Most are aerobic• Consume food in several ways

– Transport across membrane– Taking food into cellular openings– Engulfing food– Digest food using specialized structures

inside cells (food vacuoles)

The Protozoa: Structure

Protozoa: life cycles

• Different from organism to organism, but some common elements

• Environmentally resistant life stages– The cyst

• Reproductive stages– Sexual and asexual

Protozoa: the cyst

• Environmentally resistant life stage

• Protective capsule forms around the cell

• Allows survival outside a living host– transmission from host to host

• Resistance to environmental stressors– Low moisture

– Lack of nutrients

– Lack of oxygen

– Unsuitable temperatures

Protozoa: Reproduction• Some have sexual reproductive

stages– gametocytes

• Asexual reproduction is common– Fission (like bacteria)– Budding– Schizogony

Protozoan taxonomy

• Protozoa are quite diverse• Divided into phyla• Currently done using ribosomal RNA

sequencing• As with bacteria, this is changing as

techniques advance and more organisms are sequenced

Protozoan taxonomy: the phyla

• Archaezoa• Microspora• Amoebozoa• Apicomplexa• Ciliophora• Euglenozoa

Archaezoa• Unique feature: lack mitochondria• Many live in intestinal tracts of animals• Flagellated• Species of public health importance:

Giardia lamblia

Microspora• Unique feature: lack mitochondria• Intracellular parasites (live inside host

cells)• Species of public health importance:

Enterocytozoon bieneusi

E. bieneusi cysts inside a eukaryotic cell E. bieneusi in stool smear

Amoebozoa

• The amoebas• Unique feature: use pseudopods for

motility and to get food• Intestinal parasites• Species of public health importance:

Entamoeba histolytica

CystTrophozoite

Apicomplexa• Unique feature: have organelles that

release enzymes to aid in penetration of host tissues

• Intracellular parasites (live inside host cells)

• Complex life cycles with multiple hosts• Genus of public health importance:

Plasmodium

Ciliophora

• Unique feature: cilia– Small “hairs” on the cell that propel the cell

and move food toward it

• Intestinal parasite• Genus of public health importance:

Balantidium coli

CystTrophozoite

Euglenozoa

• Unique feature: the hemoflagellates travel in the circulatory system of an infected host

• Long, slender cells move by undulation• Genus of public health importance:

Trypanosoma

The Helminths

The Helminths: Characteristics

• The parasitic worms• Multicellular organisms• Digestive, circulatory, nervous,

excretory, and reproductive systems• Spend part or all of their lives in

humans• Invade and live in tissues

The Helminths: life cycles

• Complex life cycles• May have multiple hosts• Larval stages: developmental

– Include eggs

• Adult stages– Worms

• Sexual reproduction

The Helminths: life cycles

Helminth taxonomy

Two phyla:• Platyhelminthes (the flatworms)

– Trematodes– Cestodes

• Nematoda (the roundworms)

Trematodes

• Also known as flukes• Flat shaped body• Attach to tissues via suckers• Genus of public health importance:

Schistosoma

Cestodes• Also known as the tapeworms• Intestinal parasites

– Lack a digestive system; absorb nutrients from their host

– Attach to tissues via suckers

• Genus of public health importance: Ehinococcus and Taenia

Nematodes• Also known as the roundworms• Two types

– Egg is infective– Larva is infective

• Genus of public health importance– Ascaris– Necator (hookworm)– Trichinella

NematodesAscaris Necator

Trichinella

Key Concepts

• Eukaryotic cells• Can be single celled or multicellular• Feed on bacteria and small nutrients• Complex life cycles

– Distinct life stages that undergo structural changes

– Some stages are reproductive– Different stages take place in different

places

Key Concepts

• The protozoa (protists)– Single celled eukaryotes– Environmental stages inhabit water or soil– Most are aerobic– Consume food in several ways

• The helminths– Multicellular eukaryotes– The parasitic worms– Multicellular organisms– Digestive, circulatory, nervous, excretory, and

reproductive systems– Spend part or all of their lives in humans– Invade and live in tissues

Key Concepts

The protozoa:• Archaezoa• Microspora• Amoebozoa• Apicomplexa• Ciliophora• Euglenozoa

The helminths:• Platyhelminthes

(the flatworms)– Trematodes– Cestodes

• Nematoda (the roundworms)