Introduction to Parasitology

Parasitology

• Parasitology is the study of parasites.
• Medical parasitology studies parasites of medical importance that can cause disease in humans.
• A parasite is an organism entirely dependent on another larger organism (host) for shelter and/or nutrition.
• Symbiosis is a close and prolonged association between two organisms of different species. It can be commensalistic (one benefits, the other not affected), parasitic (one benefits, the other is harmed), or mutualistic (both benefit).
• Parasites can be permanent (e.g., head lice) or temporary (e.g., bed bugs).
• Ectoparasites live on the outside of the host (e.g., head lice).
• Endoparasites live inside the host (e.g., Entamoeba histolytica).
• Parasites can be facultative (able to live free living and as parasites), obligate (must live as a parasite), or coprozic (spurious, passing through the host's digestive tract).
• A host is the organism that harbors a parasite.
• Definitive host: harbors the adult, most developed stage of the parasite.
• Intermediate host: harbors sexually immature or larval stages of a parasite.
• Reservoir host: holds the same parasite species as a human, acting as a source of infection.
• Habitat: the tissue or organ where the adult parasite exists in the definitive host.
• Infective stage: The stage of parasite capable of causing infection (entering the host and continuing development).
• Diagnostic stage: the stage of a parasite that can be detected in specimens (stool, blood, urine, sputum, CSF) for diagnosis.
• Vectors are arthropods that transport parasites from infected to uninfected hosts.
• Biological vectors: necessary to complete the life cycle of a parasite.
• Mechanical vectors: passive carriers of parasites.
• Infection: invasion of the body by any pathogenic organism (except arthropods).
• Infestation: establishment of arthropods on or within a host.
• Autoinfection: infected individuals act as a source of infection for themselves.
• Zoonosis: animal diseases transmissible to humans.

Classification of Medically Important Parasites

• Parasites are classified according to international code taxonomy.
• Classification often includes Kingdom, Phylum, Class, Order, Family, Genus, and Species.
• Some parasites have further divisions: Sub-order, Superfamily, Subspecies.
• The scientific name of a parasite consists of the genus and species names (e.g., Plasmodium falciparum). The genus name is one word; the species name is two words.

Entamoeba histolytica (Amoebiasis)

• Disease: Amoebiasis
• Mode of locomotion: Pseudopodia (false feet)
• Geographic distribution: Cosmopolitan, common in tropical and subtropical countries with poor sanitation.
• Habitat: Lumen of the large intestine; pathogenic because it can invade the intestinal wall.
• Reservoir: Primarily humans, also dogs, pigs, monkeys.
• Morphology: 2 forms: Cysts and Trophozoites.
• Infective stage: ingested in contaminated water/food.
• Pathogenic stage: Trophozoite stage, causing ulcerations and possible tissue invasion.
• Clinical picture: Dysentery (bloody/mucous diarrhea), severe abdominal pain, tenesmus.
• Complications: Intestinal (peritonitis, appendicitis, hemorrhage), Extraintestinal (liver abscess, complications involving lung, brain, and skin).

Trypanosoma (Trypanosomiasis/Sleeping Sickness)

• Disease: Trypanosomiasis
• Mode of locomotion: Undulating movement of flagella and undulating membrane.
• Geographic distribution: Cosmopolitan.
• Habitat: Blood plasma.
• Intermediate host: Tsetse fly.
• Definitive host: Vertebrates, including humans.
• Morphology: Spindle-shaped.
• Reproduction: Asexual, longitudinal binary fission.
• Transmission: Tsetse fly bite, rarely mother-to-child, accidental needle-sticking, rarely sexual contact.
• Infection and symptoms: Initial stage (haemo-lymphatic stage) includes fever, headache, enlarged lymph nodes, and itching. Later (CNS involvement) includes behavior changes, confusion, sensory problems, sleep cycle disturbance.
• It is usually fatal without treatment.
• Other related diseases: Chagas disease/American trypanosomiasis (bite from triatomine bugs/kissing bugs).

Toxoplasma gondii (Toxoplasmosis)

• Disease: Toxoplasmosis
• Geographic distribution: Worldwide.
• Transmission: Ingesting raw, undercooked meat from sheep/cows containing viable trophozoites; consuming water/food contaminated with cat feces; transplacental transmission; blood transfusion/organ transmission.
• Infection in healthy immunologically competent people is usually asymptomatic.
• Severe illness in immunocompromised individuals and congenital infections can lead to significant complications/blindness in newborns.
• Symptoms may include fever, headache, splenomegally, lymphadenopathy, hydrocephalus; in severe cases this can lead to toxoplasmosis encephalitis.

Malaria (Plasmodium spp.)

• Disease: Malaria
• Geographic distribution: Primarily Africa, Asia, and Latin America, previously widespread.
• Causative agent: Protozoan parasites of the genus Plasmodium.
• Transmission: Bite from the female Anopheles mosquito.
• Reproduction: Asexual & sexual reproduction
• Asexual reproduction occurs in humans (sporozoites multiply to produce trophozoites/merozoites).
• Sexual reproduction in mosquite
• Symptoms: Episodes of chills, fever, sweating, anemia, often leading to cerebral malaria and death.

Intestinal Flukes (e.g., Heterophyes heterophyes)

• Heterophyes heterophyes is a fluke that affects humans.
• Definitive host: is typically humans, often ingesting undercooked fish.
• Intermediate host: freshwater snails and fish. These snails and fish then act as a source of the infection.
• Symptoms: inflammation, diarrhea, tissue-damage, neuro-logical disorders and often fatalities.

Ficolia Hepatica (Liver Flukes)

• Disease: infection of the liver
• Transmission: Occurs by ingestion of undercooked freshwater fish
• Pathology: The fluke causes extensive mechanical injury to the liver tissue.
• Symptoms: May involve symptoms of fever and abdominal pain; severe cases may lead to biliary obstructions.

Bilharzia/Schistosomiasis (Blood Flukes)

• Disease of the venous system.
• Transmission: Direct skin penetration from contaminated water.
• Definitive host: Humans
• Intermediate host: Snails
• Symptoms: intestinal/urinary blood, and other issues.
• Pathology: Eggs cause tissue damage in intestinal or urinary bladder, liver damage.

Balantidium coli (Ciliates)

• Disease: Balantidiasis an intestinal parasitic infection.
• Geographic distribution: Worldwide. It is found mostly in pigs and other animals, but humans can be infected by ingesting food/water contaminated with the parasite.
• Habitat: Large intestine of humans, pigs, and non-human primates.
• Symptoms: diarrhea, possibly dysentery, sometimes asymptomatic. This infection generally does not involve locations external to the intestines.

Trichomonas vaginalis

• Disease: Trichomoniasis.
• Habitat: In the vagina and cervix of females, and in the anterior urethra of males.
• Transmission: Commonly sexually transmitted.
• Symptoms: Mild irritation to severe inflammation, foul-smelling discharge, painful urination, dyspareunia.