Introduction to Medical Parasitology

This lecture covers the fundamental aspects of medical parasitology.
The student will be able to describe different definitions of parasitism.
The student will be able to explain general concepts of parasitology.
The student will be able to distinguish between various types of hosts.
The lecture will examine the host-parasite relationship, including different types, definitions, and real-world examples.
The lecture will detail the impact parasites have on their hosts.

Seven out of ten major tropical diseases are parasitic.
These include African trypanosomiasis, Dengue, Leishmaniasis, Malaria, Schistosomiasis, Tuberculosis, Chagas disease, Leprosy, Lymphatic filariasis, and Onchocerciasis.
Increased global travel facilitates the spread of parasitic diseases.
The rising prevalence of immunosuppression weakens host defenses against parasites.
Environmental modifications contribute to the expansion of parasite habitats and vectors.

Parasite: An organism that lives temporarily or permanently inside or on another larger organism called a host, for shelter and/or nourishment.
Parasitology: The study of parasites, including helminths, protozoa, and arthropods that serve as vectors.
Medical Parasitology: The study of disease-causing parasites affecting humans.

Definitive host: The host in which the parasite reaches sexual maturity and reproduction occurs.
Intermediate host: The host in which the parasite develops into its immature or larval form, and in which no sexual reproduction happens.
Reservoir host: An animal host that carries the same parasite species as humans, which can still transmit infection to humans.
Carrier host: A host that harbors a parasite but does not exhibit any clinical signs or symptoms.

Vector: An arthropod that carries a parasite from an infected to a non-infected host.
Biological vectors: Vectors in which the parasite must undergo a part of its life cycle.
Mechanical vectors: Vectors in which the parasite does not undergo any part of its life cycle – it only carries the parasite to spread it.

Habitat: The tissue or organ in which an adult parasite resides in a host.
Infection: Invasion of a host's body by any pathogenic microorganism (excluding arthropods).
Infestation: The establishment of arthropods in or upon a host.

Prepatent (Biological) Incubation Period: The time from initial infection with a parasite until its detection in diagnostic material.
Clinical Incubation Period: The period between infection exposure and the appearance of initial symptoms.
Autoinfection: A situation where an infected individual serves as a source for further infection and/or hyperinfection affecting the same individual.

Infective Stage: The parasite stage capable of infecting and continuing their development in a new host.
Diagnostic Stage: The parasite stage detectable in bodily fluids (stool, blood, urine, etc.).

Understanding geographical distribution clarifies conditions that favor a parasite's survival.
This information facilitates the development and implementation of effective preventive and control strategies.

Parasite morphology is vital in diagnosing infections.
Differentiation between pathogenic and non-pathogenic organisms relies on morphological characteristics.

Parasite life cycles can be simple (involving one host) or complex (involving one or more intermediate hosts).
A parasite's life cycle is categorized into phases occurring inside and outside the human body.
The phases occurring inside the human body reveal the pathology of the parasite and aids the selection of treatment and diagnostic strategies.
The phases occurring outside the human body give insights for epidemiological studies, prevention, and control strategies.

Direct transmission: The parasite does not require a biological vector or intermediate host. Transmission includes:
- ingestion by mouth
- inhalation during respiration
- sexual contact (or blood transfusion)
- direct skin penetration
- congenital (transplacental)
- transmammary (breast milk)
- autoinfection
Indirect transmission: The parasite requires a biological vector and/or intermediate host.

Carrier state: A balanced host-parasite relationship where parasite damage is countered by host repair.
Disease state: Imperfect balance, where the parasite dominates due to host factors (low immunity) or parasite characteristics (high virulence).
Parasite destruction: The host overcomes the parasite.

Opportunistic parasites: Cause mild illness in healthy individuals but severe disease in immunocompromised hosts. (e.g., Pneumocystis carinii, Toxoplasmosis gondii, Isospora belli).

Consumption of host nutrients (e.g., Diphyllobothrium latum and vitamin B12).
Obstruction of bodily passages (e.g., Ascaris).
Bleeding (e.g., Schistosoma).
Tissue destruction (e.g., Leishmania).
Organ compression (e.g., Hydatid cysts).
Secondary infection (e.g., Dracunculus).
Transmission of pathogens (e.g., lice and Rickettsia).
Predisposition to cancer (e.g., Schistosomiasis).
Immunological reactions (inflammation, fibrosis, allergy, anaphylactic shock, tissue destruction, protection from reinfection).

Susceptibility to parasitic infections depends on both host and parasite factors.
Host factors: Genetic makeup, age, sex, immunity level, malnutrition, intensity/frequency of infections, lifestyle/occupation, co-existing diseases/conditions that weaken immunity (e.g., AIDS).
Parasite factors: Strain of the parasite, parasite load, site of infection in the body, process of metabolism, nature of waste products/toxins during growth/reproduction.

Animal parasites are classified according to the International Code of Zoological Nomenclature.
Each parasite belongs to a phylum, class, order, family, genus, and species (e.g., Ascaris lumbricoides).

Medically important parasites are members of the Animalia kingdom and four phyla:
- Protozoa (unicellular)
- Platyhelminthes (flatworms)
- Nemathelminthes (roundworms)
- Arthropoda (segmented bodies with exoskeletons)

Infection: Invasion of the body by pathogenic organisms (except arthropods).
Infestation: Establishment of arthropods on/within a host.
Habitat: The host tissue/organ where the adult parasite exists.
Diagnostic Stage: Parasite stage detectable in host bodily fluids (stool, blood, urine, etc.).
Infective Stage: The parasite stage that transmits infection to a definitive host.