Introduction to parasitology
UNIT I 1.1 Introduction to medical parasitology 1.2 General characteristics, morphology, classification of Protozoa and Helminth.
PARASITOLOGY
Dr Pramila Singh
9/4/202310 min read
UNIT I
1.1 Introduction to Medical Parasitology
1.2 General characteristics, morphology, and classification of Protozoa and Helminth.
1.1 Introduction to Medical Parasitology
Parasitology is a branch of biology that deals with parasites that affect animals. A parasite is an organism that lives on or in a larger species to get food from them. Species that provide space to live and food (nutrients) to survive the parasite are called host organisms. There are three classes of parasites. These are protozoa, helminths, and ectoparasites. Medical Parasitology deals with parasites that infect humans to cause diseases. Medical parasitology includes the study of parasitic protozoa, parasitic helminths (worms), and arthropods. Arthropods that cause disease or act as vectors for pathogens are included in the medical parasitology.
Definitions
1. Ectoparasites: Parasites that live on the host body surface are called ectoparasites.
2. Endoparasites: Parasites that live inside the host body for nourishment and complete their life cycle.
3. Facultative parasites: Facultative parasites live either as parasites or as free-living organisms depending upon the availability of suitable host or environment conditions.
4. Obligate parasite (Holo parasites): Parasites that depends upon the host for their survival and reproduction are called an obligate parasite.
5. Opportunist parasites: Parasites present inside the host body and multiply when the immunity of the host body decreases.
6. Definitive host: The host that provides an environment for the reproductive stage of the parasite is called a definitive host or primary host.
7. Paratenic host: A host that provides an environment for parasite to survive during their inactive phase (dormant condition) is called paratenic host.
1.2.1 General characteristics, morphology, and classification of protozoa
Definition: Protozoa are eukaryotic single-celled animals under the kingdom Protista. Organism under prostita kingdom does not have tissue differentiation. They are also placed in the kingdom Animalia. Examples: Amoeba, Euglena, Paramecium, Leishmania, etc.
General characteristics of protozoa
Habitat: Aquatic organism, free-living or parasite.
Structure: Eukaryotic single-cell animal, Lack of cell wall, body surrounded by plasma membrane or pellicle.
Shape: Oval-shaped, spindle-shaped, bell-shaped, or irregular-shaped organism.
Organelles function: The functions of protozoa organelles are the same as the functions of higher animals' organs,
Vacuoles: Contractile vacuoles are present in protozoa cells found in the fresh water. Contractile vacuoles excrete metabolic waste from protozoa cells.
Nucleus: Normally one nucleus is in the center of the protozoa cell.
Nutrition: Heterotrophic and Holozoic nutrition,
Reserved food: Glycogen.
Locomotion: Pseudopodia, cilia, or flagella are the locomotive organs of protozoa. Some protozoa do not have any locomotive organs.
Reproduction: Both sexual reproduction and asexual reproduction. Asexual reproduction includes cell division by binary fission, schizogony, or budding. Sexual reproduction includes gamogony.
Respiration: Through plasma membrane or pellicle,
Excretion: Through plasma membrane or pellicle,
Life cycle: Cyst or Oocyst and trophozoite.
Morphology of Protozoa
The morphology of protozoa can be studied under the following headings
Size: There is variation in size and shape. Their size may vary from 1 micro-meter to 2 mm. The size of parasitic protozoa inside the human body varies from 1 micro-meter to 150 micro-meter.
Intracellular Structure: They are single-celled eukaryotic animals. Protozoa cells consist of cytoplasm and a nucleus covered with plasma membrane.
Cytoplasm: In most protozoa, cytoplasm is divided into two portions: ectoplasm and endoplasm. An outer transparent gel-like portion of cytoplasm in protozoa cells is called ectoplasm. The inner portion of cytoplasm containing organelles is called endoplasm. Organelles present in the cytoplasm are the endoplasmic reticulum, mitochondria, Golgi complex, food vacuole, and contractile vacuole. Organelles are visible in the light microscope.
Nucleus: It is present inside the endoplasm of protozoa cells. One protozoa cell may have a single nucleus or multiple nuclei. In several protozoa one large nucleus called macronucleus and one small nucleus called micronucleus are present. Macronucleus control metabolism in protozoa cells. The micronucleus is for reproductive activities.
Plasma membrane: The plasma membrane separates protozoa cellular contents from the external environment. It provides protection to cells and controls the exchange of substances between cellular contents and the external environment. Some protozoa have hard outer covering. It is called a pellicle.
Locomotion organs: Protozoa have locomotion organs. There are three types of locomotion organs in protozoa. These are pseudopodia, flagella and cilia. Protozoa have any one of these locomotion organs.
Cyst: A cyst is a phase of the protozoa life cycle. Cysts can survive in unfavorable conditions such as dry weather and low nutrition.
Classification of protozoa
Protozoa are placed in the kingdom Animalia and sub-kingdom Protista. On the basis of their locomotion organs, they are classified under the following four groups.
Amoeba: Amoeba proteus
Ciliates: Paramecium
Flagellates: Giardia intestinalis
Sporozoa:Plasmodium.
Amoeba: Amoeba proteus: Example Entamoeba histolytica.
A. General characteristics
Habitats: They live in moist soil, sea, and fresh water.
Locmotion: Pseudopodia
Shape: No definite shape and size.
Vacuoles: Contractile vacuoles are present in amoeba lives in fresh water.
Reproduction: Asexual, Binary fission
B. Structure of amoeba:
C. Disease caused by amoeba: Amoebiasis or Amoebic dysentery or frequent stool with blood and mucus, and inflammation in liver.
Ciliates: Example: Paramecium
A. General characteristics of Ciliates
Habitat: Aquatic
Shape: Fixed shape due to pellicle.
Locomotion: Cilia
Vacuoles: Contractile vacuoles.
Reproduction: Transverse division.
B. Diseases: They do not cause any disease.
Ciliates: Example: Balantidium coli
A. General characteristics of Ciliates
Habitat: Large intestine of humans and other mammals
Shape: 50 to 100 micrometers in length. Oval or pear-shaped, Body covered in cilia,
Locomotion: Cilia,
Vacuoles: Contractile vacuoles.
Reproduction: Binary fission
Life cycle: Both trophozoite and cyst.
Nutrition: Heterotrophic organism.
Transmission: Transmitted through ingestion of cyst.
B. Diseases: Infect the large intestine of the human to cause disease balantidiasis. Symptoms are diarrhea, abdominal pain, nausea, vomiting, and occasionally dysentery.
Flagellates: Example: Giardia intestinalis, Trichomonas, Trypanosoma, etc.
A. General characteristics
Habitats: Aquatic
Locomotion: Flagella
Shape: definite shape due to pellicle.
Vacuoles: Contractile vacuoles are present.
Reproduction: Asexual, Binary fission
B. Disease caused by amoeba: Amoebiasis or Amoebic dysentery.
Sporozoa: Plasmodium, Myxidium, Nosema, Globidium
A. General characteristics
Habitats: Aquatic.
Locomotion: No specialized organ for locomotion
Shape: definite shape due to pellicle.
Vacuoles: Contractile vacuoles are present.
Reproduction: Sporozoite formation
B. Disease: Plasmodium causes malaria.
1.2.2 General characteristics, morphology, and classification of Helminth (Worms).
Classification of helminths (Parasitic Worm)
On the basis of their morphology, helminths are classified into the following three classes
Flukes (Trematodes): Trematodes are a group of flatworms that belong to the phylum Platyhelmenthis. Adult flukes are leaf-shaped and unsegmented worms. They have oral and ventral suckers. Suckers attach worms to the host’s cell and absorb nutrition. They have complex life cycles. They have multiple hosts: vertebrates and invertebrates Examples: Schistosoma mansoni, Fasciola hepatica.
Tapeworms (Cestodes): Cestodes are group of parasitic flatworm that belongs to the phylum Platyhelminthes. They are flat-segmented worms with ribbon-like bodies. They contain a hook or sucker for attachment with the host’s intestine. It does not have digestive system. It absorbs nutrition from the host. Examples: Taenia saginata and Teenia solium.
Roundworms (Nematodes): They are cylindrical unsegmented worms with tapering at both ends. They have a complete digestive system with mouth and anus. Examples: Ascaris lumbricoides, Enterobius vermicularis,
General characteristics of flukes (Trematodes): The following are the general characteristics of Trematodes
Habitat: Primary host human and secondary host snail. fish and crab in fresh water.
Shape: Flat body, leaf-like structure. Non-segmented body without body cavity.
Size: From a few mm to several cm in length.
Tegument: The outer surface of trematodes is called tegument. Tegument absorb nutrition from the host and protect trematodes from the host immunity system.
Sucker: Trematodes have oral and ventral suckers. The oral sucker is located at the anterior end. It attaches to the host tissue. The ventral sucker helps in attachment and locomotion.
Digestive system: Simple digestive system with mouth, pharynx, and branched intestine. No anus.
Sex: Hermaphrodite means they have both male and female sex in individual trematode bodies.
Reproductive system: They form a large number of eggs by internal fertilization. These eggs are excreted from the host body through host feces.
Excretory system: Ammonia is the main excretory product excreted through excretory pores.
Life cycle: The complex life cycle consists of intermediate host snails in fresh water. Asexual reproduction occurs inside the intermediate host up to the larva stage. Vertebrates are the final host. The adult stage of trematodes is in vertebrates.
Parasitic lifestyle: Trematodes are obligate parasites. They can infect humans, mammals, birds, fish, and invertebrates.
Pathogenic effect: They can damage tissues, blood vessels, organs, etc. They cause abdominal pain, fever, anemia, and organ function problems.
Morphology of flukes (Trematodes): The following are the general features of the morphology of trematodes
Body shape: Unsegmented, flat leaf-like body. It can move inside the host’s body or reside in the host's body cavities or organs.
External surface: Covering of Trematodes body is called tegument. It is a specialized layer of fused cells that protects the trematode body from the host immunity system and absorbs nutrients from the host.
Oral and ventral suckers: Trematodes have specialized suckers for feeding and attachment. The oral sucker is located at the anterior part of the trematodes. It surrounds the mouth of Trematodes. It attaches the trematodes to the host tissues. Ventral suckers are located at the ventral side of the trematodes body. It helps in locomotion and additional attachment.
Digestive system: The digestive system consists of the mouth, muscular pharynx, and branched intestine. Intestine branches are present throughout the trematode body. It absorbs nutrition from the host. The digestive system has no anus in trematodes.
Reproductive system: Hermaphrodite means they have both male and female sex in individual trematode body. Reproductive organs include testes, ovaries, seminal vesicles, and complex duct systems.
Excretory system: Trematodes have an excretory canal that opens outside the body through excretory pores. Excretory canals regulate osmotic pressure inside the trematode's body and excrete metabolic waste as ammonia.
General characteristics of Tapeworms (Cestodes): The following are some general characteristics of the tapeworm:
Habitat: Adult tapeworm in the Intestine of humans (primary/definitive host)and larva tapeworm in animals (Secondary host).
Body shape: Tapeworms have a long ribbon-like segmented body. Segment size increases with maturity. Matured segment is located at the posterior end of the tapeworm body. Each segment is called proglottids.
Sizes: Varies from a few mm to several meters.
Hooks and suckers: Anterior end of the tapeworm has a structure called a scolex. Scolex has hooks or suckers or both.
Absorption surface: The body surface of the tapeworm is covered with a specialized structure called microtriches (microvilli). Microtriches are small projections on the body,s surface, It increases the surface area for nutrient absorption.
Body cavity: No body cavity.
Digestive system: Tapeworms do not have a digestive system. They absorb nutrients directly from their body surface.
Respiration: Anaerobic respiration.
Reproductive system: Hermaphrodite means they have both male and female sex in individual trematode body. Reproductive organs are located in each proglottids. Both self-fertilization and cross-fertilization occur. Tapeworms produce a large number of eggs released from mature proglottids. The eggs are excreted from the host body with feces.
Excretory system: Excretory system is found.
Nervous system: Nervous system is found.
Life cycle: Tapeworms have a complex life cycle. Animals and arthropods are intermediate hosts. Eggs released by adult tapeworms enter into the intermediate host. Eggs form larvae inside the intermediate host. The intermediate host is consumed by the definitive host. Vertebrates are definitive host of Tapeworms. Larva develops inside the intestine of the definitive host to form adult tapeworms.
Morphology of Tapeworms (Cestodes): The following are morphology features of the Tapeworm:
Body structure: Tapeworms have a long ribbonlike segmented body. Segment size increases with maturity. Matured segment is located at the posterior end of the tapeworm body. Each segment is called proglottids.
Body surface: The body surface of tapeworm is covered with a specialized structure called tegument. The tegument absorbs nutrition from the host intestine wall. Its body surface has numerous microvilli. Microvilli increase the surface area of the tapeworm body surface.
Proglottids: The body of a tapeworm is made up of numerous proglottids. Each proglottid has both male and female reproductive organs. Proglotiids mature slowly. The oldest proglottids are located at the posterior end of the tapeworm. The mature proglottid is filled with eggs.
Scolex: Scolex is the anterior end of the tapeworm. It acts as an organ for attachment with the intestine wall of the host. Scolex has hooks and suckers. Hooks and suckers help in attachment to the host intestine wall.
Reproductive system: Tapeworm has a complex reproductive system. Each proglottid has reproductive organs both male and female reproductive system. The male reproductive system consists of testes and vas deferens. The female reproductive system consists of three ovaries, an oviduct, and the uterus. Fertilisation occurs in proglottid.
Male and female reproductive organs cannot be differentiated in the immature proglottid. Male and female reproductive organs can be differentiated in mature proglottids.
General characteristics of Roundworms (Nematodes): The following are the general characteristics of roundworms.
Habitats: The plants, animals, soil, fresh water, salt water, etc. They are present everywhere in the earth's ecosystem.
Body structure: Cylindrical, unsegmented, body with tapering at both ends. It is called a roundworm because its cross-section structure is circular. The body is covered with a tough flexible cuticle to protect the body.
Size: Most of the tapeworms are microscopic. Some may grow up to several meters long.
Body cavity: Roundworm has a body cavity filled with fluid. The body cavity is called pseudocoel. It is filled with pseudocoel fluid. This fluid helps to maintain body shape. Roundworm has no skeletal system.
Mobility: Rondworms move by contraction of their longitudinal muscles. They do not have circular muscles.
Digestive system: Roundworm has a complete digestive system with a mouth and anus. It consists of the mouth, esophagus, intestine and anus. The mouth has teeth and cutting plates. It feeds on organic matter such as bacteria, fungi, algae, small vertebrates, and plants.
Reproduction: Roundworm exists as separate male and female roundworms. Female roundworm's size is bigger than male roundworms. Sexual reproduction occurs. Fertilization takes place in the female roundworm body i.e. internal fertilization.
Nervous system: It has a simple nervous system. Nerve tissues form a ring around the pharynx. It has sensory organs to sense the touch and temperature.
Respiratory system: Absent. Gaseous exchange occurs through the body surface.
Circulatory system: Absent. Fluids in the body cavity act as the circulatory system.
Excretory system: Poorly developed. Ammonia is an excretory product.
Parasitic nature: Several roundworms infect humans, animals, and plants.
Environmental importance: It breaks down the organic matter and contributes to the nutrient cycle.
Economic impact: Some roundworms are harmful to agriculture.
Morphology of Roundworms (Nematodes): The following are the morphology features of roundworms.
Body structure: It has unsegmented, elongated, cylindrical shaped tubular bod
Body wall: The body wall consists of several layers. The outermost layer is the cuticle. A cuticle is a hard non-living material that acts as a protective covering. The epidermis is present under the cuticle layer.
Symmetry: Bilateral symmetry.
Body cavity: It has a body cavity called pseudocoelom. It is filled with fluid located between the wall and the digestive system.
Digestive system: It has a complete digestive system with a mouth and anus. The mouth has teeth for feeding. The digestive system consists of the muscular pharynx, intestine, and anus.
Nervous system: Simple nervous system. It has a nerve ring around the pharynx.
Reproductive system: Male round worn has curved posterior end with copulatory organ. The female roundworm has a straight posterior end.
Sensory organs: It has sensory papillae on the body surface to detect changes in the environment. They can sense touch and temperature changes.
Author: Dr Pramila Singh
