PROTOZOA

 INTRODUCTION

Single cell eukaryote belongs to the kingdom Protista

 (Greek protos: first; zoon: animal)

GENERAL FEATURES

• The single protozoa cell performs all functions. 
• Most of the protozoa are completely non-pathogenic but few may cause major diseases such as malaria, leishmaniasis and sleeping sickness. 
• Protozoa like Cryptosporidium parvum and Toxoplasma gondii are being recognized as opportunistic pathogens (parasites (especially protozoa) that normally do not cause disease in healthy individuals but can produce serious infections when the host’s immune system is weakened)  in patients affected with human immunodeficiency virus (HIV) and in those undergoing immunosuppressive therapy.
• Protozoa exhibit wide range of size (1- 150 µm), shape and structure; yet all possess essential common features

STRUCTURE

A typical protozoan cell is bounded by a trilaminar unit membrane, supported by a sheet of contractile fibrils enabling the cell to move and change in shape. 

CYTOPLASM

Cytoplasm has 2 portions

1. Ectoplasm

• Outer clear part of the cell
• Helps the organism to:
  • Move using pseudopodia
  • Capture and engulf food
  • Breathe (respiration)
  • Remove waste materials
  • Protect the cell

2. Endoplasm

• Inner granular part of the cytoplasm
• Contains the nucleus and cell organelles such as:
  • Golgi bodies
  • Endoplasmic reticulum
  • Food vacuoles
  • Contractile vacuoles (Maintain osmotic balance by removing excess water from the cell)

NUCLEUS

Usually single nucleus, but some protozoa may have two or many nuclei.

The nucleus containsNucleolus or karyosome & Chromatin material

Chromatin arrangement may be distributed along periphery (peripheral chromatin) or as condensed mass around the karyosome.

IMPORTANT TERMINOLOGY IN PROTOZOOLOGY

1. Chromatoid Body: Extranuclear chromatin material (e.g., cyst of Entamoeba histolytica)
2. Karyosome: DNA containing body, situated peripherally or centrally within the nucleus (e.g., Entamoeba histolytica & Entamoeba coli).
3. Kinetoplast: Extra DNA present outside nucleus (e.g., Trypanosomes). Flagellum originates near the kinetoplast. Point of origin of flagellum is called as basal body.
4. Cilia: Fine hair-like structures covering the body which helps in movement (e.g., Balantidium coli)
5. Trophozoite: Active feeding and growing stage of protozoa. Obtains nutrition by diffusion, pinocytosis & phagocytosis.

REPRODUCTION

Reproduction usually occurs asexually in protozoans; however, sexual reproduction occurs in ciliates and sporozoans.

Asexual Reproduction

1. Binary fission: 

•  A single parasite divides either longitudinally or transversally into two or more equal number of parasites. 
• Mitotic division of nucleus is followed by division of the cytoplasm.
•  In amebae, division occurs along any plane, but in flagellates, division is along longitudinal axis and in ciliates, in the transverse plane

2. Multiple fission or schizogony: Plasmodium exhibits schizogony, in which nucleus undergoes several successive divisions within the schizont to produce large number of merozoites

3. Endodyogeny: Some protozoa like Toxoplasma, multiply by internal budding, resulting in the formation of two daughter cells.

Sexual Reproduction

1. Conjugation: In ciliates, the sexual process is conjugation, in which two organisms join together and reciprocally exchange nuclear material (e.g. Balantidium coli).

2. Gametogony or syngamy: In Sporozoa, male and female gametocytes are produced, which after fertilization form the zygote, which gives rise to numerous sporozoites by sporogony (e.g. Plasmodium).

LIFE CYCLE

Single host: Protozoa like intestinal flagellates and ciliates require only one host, within which they multiply asexually in trophic stage and transfer from one host to another by the cystic form. 

Second host: In some protozoa like Plasmodium, asexual method of reproduction occurs in one host (man) and sexual method of reproduction in another host (mosquito).

CLASSIFICATION OF PROTOZOA

Protozoa of medical importance belong to:

• Kingdom: Protista
• Subkingdom: Protozoa

They are divided into 4 phyla:

1. Sarcomastigophora
2. Apicomplexa
3. Microspora
4. Ciliophora

I. Phylum Sarcomastigophora

Divided into 2 subphyla based on movement:

1. Sarcodina

• Move using pseudopodia (false feet)
• No permanent locomotory organs
• Example: Amoebae

2. Mastigophora

• Move using flagella
• Example:
  • Trypanosoma
  • Trichomonas

Amoebae

• Can change shape
• Move using pseudopodia
• Simple in structure

Types of Amoebae

1. Amoebae of alimentary canal

• Important example:
  • Entamoeba histolytica
• Causes:
  • Intestinal amoebiasis
  • Extraintestinal amoebiasis

2. Free-living amoebae

Found in soil and water.

Important types:

• Naegleria
• Acanthamoeba

Can cause:

• Eye infections
• Fatal meningoencephalitis

Flagellates

• Have whip-like flagella for movement
• Some possess undulating membrane

Types of Flagellates

1. Kinetoplastida

• Possess kinetoplast
• Single flagellum arises from kinetoplast
• Blood parasites transmitted by insects

Examples:

• Trypanosoma
• Leishmania

Cause:

• Systemic or local infections

2. Flagellates without kinetoplast

• Have multiple flagella
• Mostly intestinal parasites

Examples:

• Giardia
• Trichomonas

II. Phylum Apicomplexa

(Formerly called Sporozoa)

Characteristics

• Possess apical complex for attachment to host cells
• Tissue parasites
• Complex life cycle with:
  • Sexual stage
  • Asexual stage

Examples

• Malarial parasites (Plasmodium)
• Toxoplasma
• Sarcocystis
• Isospora
• Cryptosporidium
• Babesia
• Pneumocystis jirovecii

III. Phylum Ciliophora

Characteristics

• Move using cilia
• Cilia cover entire body

Important parasite

• Balantidium coli

Cause:

• Rarely causes dysentery

IV. Phylum Microspora

Characteristics

• Very small intracellular parasites
• Commonly infect immunodeficient people
• Rarely infect healthy individuals

INTRODUCTION TO MEDICAL PARASITOLOGY

 Medical Parasitology is the branch of science that studies "Parasites infecting human & Diseases caused by these parasites"

Medical parasitology is broadly divided into:

1. Protozoology – Study of protozoa (single-celled parasites)

2. Helminthology – Study of parasitic worm
   

• 

  1681 – Antonie van Leeuwenhoek (Holland)

  • Introduced the single-lens microscope

  • First observed Giardia in his own stool

• 1870 – Louis Pasteur

  • Published the first scientific study on protozoal disease

  • His work helped in understanding disease control and prevention

• 1878 – Patrick Manson

  • Discovered the role of mosquitoes in filariasis

  • First evidence of vector transmission

• 1880 – Alphonse Laveran (Algeria)

  • Discovered the malarial parasite

• 1897 – Ronald Ross (India: Secunderabad & Calcutta)

  • Proved transmission of malaria by mosquitoes

• After these discoveries, many vector-borne diseases were identified.

Modern Era

• By the mid-20th century, major advances occurred due to:

  • Antibiotics and chemotherapy

  • Insecticides and antiparasitic drugs

  • Improved living conditions

• These developments greatly reduced infectious diseases.

  PARASITES 

  Parasites are living organisms that depend on a living host for nourishment and survival, they multiply or develop inside the host

  The term parasite is mainly used for:

  • Protozoa – unicellular organisms

  • Helminths – multicellular parasitic worms

  Classification of Parasites

  1. Based on Habitat

  Ectoparasites

  • Live on the body surface of the host

  • Do not penetrate tissues

  • Examples: Lice, ticks, mites

  • Infestation → term used for ectoparasitic infection

  Endoparasites

  • Live inside the body of the host

  • Cause infection

  • Most disease-causing protozoa and helminths are endoparasites

  Free-living forms

  • Non-parasitic stages that live independently of the host

  • Example: Cyst stage of Naegleria fowleri

  2. Classification of Endoparasites

  Obligate Parasites

  • Cannot survive without a host

  • Examples:

    • Plasmodium

    • Toxoplasma gondii

  Facultative Parasites

  • Can live as free-living or parasitic forms

  • Example:

    • Naegleria fowleri

  Accidental Parasites

  • Infect an unusual host

  • Example:

    • Echinococcus granulosus infecting humans → hydatid cyst

  Aberrant (Wandering) Parasites

  • Infect a host where further development does not occur

  • Example:

    • Toxocara canis (dog roundworm) infecting humans

HOST

 A host is an organism that harbors the parasite, provides nourishment and shelter. It is usually larger than the parasite.

Types of Hosts

1. Definitive Host

• Host in which the adult parasite lives

• Sexual reproduction occurs

• Example:

  • Mosquito → definitive host for Plasmodium (malaria)

• In most human parasitic infections, man is the definitive host

  • Examples: Filariasis, roundworm, hookworm

2. Intermediate Host

• Host in which the larval stage lives or asexual multiplication occurs

• Some parasites require two intermediate hosts:

  • First intermediate host

  • Second intermediate host

• Example:

  • Human → intermediate host in malaria

3. Paratenic Host (Transport Host)

• Host in which the larval stage remains alive

• No further development occurs

• Helps in transmission of infection

• Example:

  • Fish → paratenic host for plerocercoid larva of Diphyllobothrium latum

4. Reservoir Host

• Host that maintains the parasite in nature

• Acts as a continuous source of infection in endemic areas

• Example:

  • Dog → reservoir host for hydatid disease

5. Accidental Host

• Host that is not the usual host for the parasite

• Example:

  • Man → accidental host for cystic echinococcosis

 LIFE CYCLE OF PARASITES

1. Direct life cycle: When a parasite requires only single host to complete its development, it is called as direct life cycle, e.g. Entamoeba histolytica requires only a human host to complete its life cycle

2. Indirect life cycle: When a parasite requires 2 or more species of host to complete its development. e.g. malarial parasite requires both human host and mosquito to complete its life cycle

SOURCES OF INFECTION 

1. Contaminated Soil and Water

• Soil

  • Ingestion of embryonated eggs

    • Ascaris (roundworm), Trichuris (whipworm)

  • Larvae penetrating skin

    • Hookworm

• Water

  • Ingestion of infective cysts

    • Entamoeba histolytica, Giardia

  • Swallowing water containing intermediate host

    • Cyclops → Dracunculus medinensis (guinea worm)

  • Larvae penetrating skin

    • Cercariae of Schistosoma

  • Free-living parasites entering directly

    • Naegleria fowleri → enters through nasopharynx

2. Food

• Contaminated food/vegetables containing:

  • Amebic cysts

  • Toxoplasma oocysts

  • Echinococcus eggs

• Raw or undercooked meat:

  • Measly pork → Cysticercus cellulosae (larva of Taenia solium)

3. Vectors

Vector: An agent (usually arthropod) that transmits parasites between hosts.

Example:

• Female Anopheles mosquito → malaria

Types of Vectors

A. Biological Vectors (True vectors)

• Parasite develops or multiplies inside the vector

• Essential for parasite life cycle

Examples:

• Mosquito → Malaria, Filariasis

• Sandfly → Kala-azar

• Tsetse fly → Sleeping sickness

• Reduviid bug → Chagas disease

• Tick → Babesiosis

• 
  
  
  

Extrinsic incubation period:
Time between parasite entry into vector and its ability to transmit infection.

B. Mechanical Vectors

• Only carry parasite physically

• No development occurs in vector

Example:

• Housefly → Amebiasis

4. Animals

• Domestic animals

  • Cow → Taenia saginata, Sarcocystis

5. Carrier

• Person harboring parasite without symptoms

• Can transmit infection to others

• Seen in:

  • Anthroponotic infections

  • Congenital (vertical) transmission

6. Self-infection (Autoinfection)

• Finger-to-mouth → Pinworm (Enterobius)

• Internal reinfection → Strongyloides

MODES OF INFECTION

1. Oral Transmission

• Most common mode

• Through contaminated:

  • Food, water, fingers, fomites

• Infective forms:

  • Cysts, eggs, larvae

• Example:

  • Entamoeba histolytica

2. Skin Transmission

• Larvae penetrate intact skin

• Examples:

  • Hookworm → walking barefoot

  • Schistosomiasis → cercariae in water

3. Vector Transmission

• Through insect bite

• Examples:

  • Malaria → Anopheles mosquito

  • Filariasis → Culex mosquito

4. Direct Transmission

• Person-to-person contact

• Examples:

  • Kissing → gingival amebae

  • Sexual contact → trichomoniasis

5. Vertical Transmission

• Mother to fetus

• Examples:

  • Malaria

  • Toxoplasmosis

6. Iatrogenic Transmission

• Medical procedures

• Examples:

  • Transfusion malaria

  • Organ transplant → toxoplasmosis

PATHOGENESIS 

• Parasitic infections may be:

  • Asymptomatic

  • Symptomatic

• Some parasites live as commensals

  • Example: E. histolytica (non-invasive form)

Types of Clinical Disease

• Acute

• Subacute

• Chronic

• Latent

• Recurrent

MECHANISMS OF PATHOGENESIS

1. Lytic Necrosis

• Enzymes destroy host tissues

• Example:

  • E. histolytica → amebic ulcers

2. Trauma

• Mechanical damage at attachment site

• Example:

  • Hookworm → intestinal bleeding

3. Allergic Manifestations

• Due to immune response

• Examples:

  • Eosinophilic pneumonia → Ascaris

  • Anaphylactic shock → ruptured hydatid cyst

4. Physical Obstruction

• Worm masses block organs

• Examples:

  • Intestinal obstruction → roundworm

  • Cerebral malaria → P. falciparum

5. Inflammatory Reaction

• Chronic inflammation and fibrosis

• Examples:

  • Lymphadenitis → filariasis

  • Bladder granuloma → Schistosoma haematobium

6. Neoplasia

• Some parasites associated with cancer

• Examples:

  • Clonorchis sinensis → bile duct carcinoma

  • Schistosoma haematobium → bladder cancer

7. Space-Occupying Lesions

• Cyst formation compressing tissues

• Example:

  • Hydatid cyst

IMMUNITY IN PARASITIC INFECTIONS

1. General Features

• Parasites elicit both humoral (antibody) and cell-mediated immune responses

• Immunity against parasites is less efficient than against bacteria or viruses due to:

  1. Large and complex parasite structure → immune system cannot target all protective antigens

  2. Many protozoa are intracellular, avoiding immune attack

  3. Helminths and protozoa in body cavities → protected from immune system

• Premunition: Partial immunity dependent on residual parasite population; host becomes susceptible once parasite is fully cleared

2. Immune Response

• Antibodies

  • Different classes produced against parasites

  • IgM → indicates current infection

  • IgE → high in helminth infections

• Cellular response

  • Eosinophilia is typical in helminth infections (local and systemic)

• Host specificity

  • Parasites often infect specific hosts (e.g., malaria parasites of humans, birds, rodents)

3. Immune Evasion by Parasites

• Antigenic variation: e.g., Trypanosomes, Plasmodium → evade antibodies

• Antigenic disguise: parasite surface mimics host molecules → immune system cannot recognize

• Immunodeficiency: parasites can damage immune organs (e.g., visceral leishmaniasis)

• Opportunistic infections occur in immunodeficient hosts (e.g., AIDS)

  • Examples: Pneumocystis jirovecii, Toxoplasma gondii

4. Vaccination

• No effective human vaccines yet due to:

  • Complex life cycles

  • Antigenic variation

  • Adaptive responses

• Progress: identification of protective antigens in malaria and other parasites

LABORATORY DIAGNOSIS OF PARASITES

1. Methods

• Microscopy – most common

• Culture

• Serological tests

• Skin tests

• Molecular methods (PCR)

• Animal inoculation / Xenodiagnosis

• Imaging

• Hematology

2. Specimens for Examination

• Stool, blood, urine, sputum, CSF, tissue/aspirates, genital specimens

3. Specimen-wise Findings

Stool

• Detect intestinal parasites: Giardia, Entamoeba, Ascaris, Ancylostoma

• Cysts, trophozoites, eggs, larvae

• Examples:

  • E. histolytica, Giardia → cysts/trophozoites

  • Ascaris, Taenia → eggs

  • Strongyloides → larvae

Blood

• Detect blood-borne parasites

• Example: Malaria → morphological stages of Plasmodium

Urine

• Schistosoma haematobium → lateral-spined eggs

• Trichomonas vaginalis → trophozoites

• Wuchereria bancrofti → microfilariae in chylous urine

Sputum

• Paragonimus westermani → eggs

• Occasional larvae: Strongyloides, Ascaris

CSF

• Protozoa: Trypanosoma brucei, Naegleria, Acanthamoeba, Balamuthia, Angiostrongylus

Tissue & Aspirates

• Muscle biopsy → Trichinella larvae, Schistosoma eggs

• Brain → Naegleria, Acanthamoeba

• Bone marrow / spleen → Leishmania (Kala-azar)

• Intestinal aspirates → Giardia trophozoites

• Liver pus → E. histolytica trophozoites

Genital Specimens

• Vaginal/urethral discharge → T. vaginalis trophozoites

• Anal swabs → Enterobius eggs