intro to parasitology

DEFINITION OF TERMS

  • Parasitic diseases:

    • Major global health issue, particularly in economically disadvantaged countries.

    • Cause some of the most serious health problems locally and worldwide.

PARASITOLOGY

  • The area of biology focused on the dependence of one living organism on another.

  • A study of animal parasites.

MEDICAL PARASITOLOGY

  • Concentrates on animal parasites of humans and their medical significance, as well as their role in human communities.

TROPICAL MEDICINE

  • A branch of medicine dealing with tropical diseases and other medical problems specific to tropical regions.

  • Diseases are primarily found in tropical areas.

TROPICAL DISEASE

  • Indigenous or endemic to tropical areas but can also be sporadic or epidemic in non-tropical regions.

  • NOTES:

    • The Philippines is a tropical country, making it susceptible to specific parasitic diseases.

    • Certain diseases seen in other countries are not present in the Philippines.

MALARIA

  • A significant parasitic disease endemic to specific regions in the Philippines.

  • It kills approximately half a billion people each year.

BIOLOGIC RELATIONSHIPS

SYMBIOSIS

  • Defined as “living together” of unlike organisms.

    • E.g., humans and lower forms of animals.

  • May involve protection or benefits to one or both partners.

  • Close associations primarily for food acquisition by the involved members (Markell & Voge's).

COMMENSALISM

  • Defined as “eating at the same table.”

  • Two species live together; one benefits without harming or benefiting the other.

  • Example:

    • Entamoeba coli:

    • A non-pathogenic organism; normal flora found in the intestinal lumen.

    • Derives nourishment from food without harming the host.

MUTUALISM

  • Two organisms mutually benefit each other.

  • Example:

    • Termites and flagellates:

    • Termites provide housing for flagellates, which help termites digest wood for food.

PARASITISM

  • One organism (the parasite) lives in or on another (the host), depending on it for survival, usually at the expense of the host.

    • This results in one being benefited while the other is harmed.

PARASITE

  • An organism that lives on or in a host, which is typically of a larger size, provides physical protection, and serves as nourishment.

LOCATION OR HABITAT

  • Endoparasite

    • Lives inside the host's body.

    • Presence leads to infection.

  • Ectoparasite

    • Lives on the surface of the host's body.

    • Presence results in infestation (e.g., lice or ticks).

  • Erratic

    • Found in an organ that is not typically its habitat.

  • Example:

    • Ascaris usually infects the gut lumen but may also appear in the brain or liver.

MODE OF DEVELOPMENT

  • Obligate Parasites

    • Require a host at certain life cycle stages for development and reproduction (e.g., tapeworms); cannot survive outside the specific host.

  • Facultative Parasites

    • Can exist freely and become parasitic when the opportunity arises.

HOST SPECIFICITY

  • Accidental/Incidental

    • Establishes itself in a host where it does not ordinarily reside.

  • Permanent

    • Stays in or on the host for its entire life.

  • Temporary

    • Resides in the host for a short duration.

  • Spurious/Coprozoic

    • Free-living organism that passes through the digestive tract without infection.

ROLE IN LIFE CYCLE OF THE PARASITE

  • Definitive/Final

    • Host where the parasite reaches sexual maturity (e.g., Taeniasis).

  • Intermediate

    • Host harboring the asexual or larval stage of the parasite (e.g., pigs or cattle for Taenia spp.).

  • Paratenic

    • Host does not allow further development of the parasite.

  • Reservoir Hosts

    • Carry the parasite, aiding in spreading the infection human hosts, usually unaffected by the disease.

EXAMPLE OF RESERVOIR HOSTS

  • Pigs – Balantidium coli

  • Field rats – Paragonimus westermani

  • Cats – Brugia malayi

VECTORS

  • Essential for transmitting the parasite from one host to another.

  • Biologic Vector

    • Transmits the parasite after it has completed its development within the host (e.g., Aedes mosquito transferring Plasmodium).

  • Mechanical/Phoretic Vector

    • Only transports the parasite without development (e.g., mosquitoes carrying parasites on body).

EXPOSURE AND INFECTION

  • Pathogens

    • Harmful parasites that can inflict mechanical injury to hosts and cause disease.

  • Carrier

    • Harbors a pathogen without showing symptoms, may be symptomatic or asymptomatic (e.g., asymptomatic carrier of Entamoeba histolytica).

  • Exposure

    • Process of introducing an infecting agent (e.g., Plasmodium falciparum transmitted via mosquito bite).

  • Infection

    • Establishment of an endoparasite within the host (e.g., Anopheles mosquito injecting Plasmodium falciparum).

  • Infestation

    • Presence of an ectoparasite on the host, external to the body.

INCUBATION PERIOD

  • Clinical Incubation Period

    • Time from infection to the appearance of symptoms (e.g., time from Plasmodium infection to fever onset).

  • Biologic Incubation Period

    • Duration from infection to demonstrable infection evidence.

  • Autoinfection

    • Infected individuals become their source of infection (e.g., Enterobiasis).

EXAMPLE OF AUTOINFECTION

  • Enterobius vermicularis (pinworms):

    • Gravid females release eggs, causing irritation.

    • Eggs can mature outside the body; autoinfection occurs if ingested.

  • Superinfection/Hyperinfection

    • Refers to re-infection of already infected individuals, leading to severe infection.

SOURCES OF INFECTION

  • Common Portal of Entry: Mouth

  • Primary Sources:

    • Contaminated Soil

    • Contaminated Water

CONTAMINATION DETAILS

  • Hygiene in developing countries is often linked to socioeconomic status.

  • Lack of sanitation leads to egg and cyst contamination in the environment.

SOIL-TRANSMITTED HELMINTHS (STHs)

  • Examples:

    • Hookworms

    • Ascaris lumbricoides

    • Trichuris trichiura

    • Strongyloides stercoralis

  • STHs are prevalent in children due to poor hygiene practices.

FOOD CONTAMINATION

  • Risks:

    • Intake of undercooked or raw freshwater fish leading to fluke infections.

    • E.g., Kinilaw or sashimi associated with tapeworm infection.

VECTORS AND TRANSMISSION

  • Arthropods as vectors (e.g., mosquitoes for malaria, rats for parasites like Hymenolepis nana).

  • Autoinfection also occurs with various parasites.

MODES OF TRANSMISSION

  • Mouth: Most common mode; can involve ingestion of infective larval stages or contaminated water.

  • Skin Penetration: E.g., hookworms and Schistosoma species.

  • Bites: Arthropods transmitting diseases (e.g., malaria).

  • Congenital Transmission: E.g., Toxoplasma gondii crossing the placental barrier.

  • Inhalation: E.g., exposure to airborne eggs of Enterobius vermicularis.

  • Sexual Intercourse: E.g., Trichomonas vaginalis.

NOMENCLATURE

  • Classified under the International Code of Zoological Nomenclature:

    • Phylum, Classes, Orders, Families, Genera, Species, with categories such as Suborder, Superfamily, and Subspecies.

  • Names Latinized; family names use -idae suffix.

EXAMPLE OF SCIENTIFIC NAMING

  • Ascaris lumbricoides (italicized or underlined)

LIFE CYCLE

  • Simple Life Cycle:

    • Process: Ova → Larva → Adult

    • Example: Ascaris lumbricoides

COMPLICATED LIFE CYCLES

  • Involve 2+ hosts, parasite sexual maturity in definitive hosts.

  • Examples include Schistosoma spp.

EPIDEMIOLOGIC MEASURES

  • Epidemiology: Study of disease patterns and occurrences.

  • Incidence: New infections in a population over a specific period.

  • Prevalence: Percentage of infected individuals at a specific time.

  • Cumulative Prevalence: Percentage of individuals hosting at least one parasite.

  • Intensity of Infection: Refers to the worm burden per infected person.

  • Morbidity: Clinical consequences of infections affecting individual well-being.

TREATMENT

  • Deworming: Utilization of anti-helminthic drugs for individuals or community health programs.

  • Cure Rate: Percentage of previously infected individuals found egg negative post-treatment.

  • Egg Reduction Rate (ERR): Percentage decline in egg counts after treatment.

  • Selective Treatment: Targeted deworming based on diagnosis or infection intensity.

  • Targeted Treatment: Group-level deworming based on defined characteristics.

  • Preventive Chemotherapy: Large-scale drug administration to reduce morbidity and transmission of helminth infections in high-risk groups.

  • Efficacy: Drug effectiveness under ideal conditions.

  • Effectiveness: Drug effectiveness in specific host contexts (e.g., immunocompetence).

  • Drug Resistance: Loss of susceptibility to drugs in parasite populations previously responsive.

PREVENTION AND CONTROL

  • Strategies to prevent parasite infection/infestation

  • Morbidity Control: Aims to avoid illness from infections.

  • IEC: Health education for promoting healthy practices.

  • Environmental Management: Modifications to prevent vector propagation.

  • Environmental Sanitation: Interventions to minimize health risks through waste management, clean water, housing safety, etc.

SANITATION

  • Ensures access to safe disposal of human waste and clean drinking water.

ERADICATION VS ELIMINATION

  • Eradication: Permanent global reduction of infections to zero due to deliberate actions (e.g., smallpox).

  • Elimination: Zero incidence of a particular disease in defined areas due to deliberate efforts (e.g., Polio).

IMPORTANT GROUPS OF ANIMAL PARASITES

PROTOZOA

  • Single-celled organisms, classified into classes like:

    • Sarcodontia:

    • E.g., Giardia lamblia, Dientamoeba fragilis

    • Sarcodina:

    • E.g., Entamoeba histolytica

    • Ciliophora:

    • E.g., Balantidium coli

    • Apicomplexa:

    • E.g., Plasmodium, tissue parasites with complex life cycles.

    • Microsporidia: Minute intracellular parasites.

PLATYHELMINTHES (FLATWORMS)

  • Multicellular, hermaphroditic organisms (except schistosomes) with classes like:

    • Trematoda (Flukes): Leaf-shaped parasites.

    • Cestoda (Tapeworms): Segmented body with scolex.

NEMATODA (ROUNDWORMS)

  • Cylindrical worms with stiff cuticle, separate sexes, and well-developed digestive tract.

ACANTHOCEPHALA (THORNY-HEADED WORMS)

  • Endoparasitic with a hook-bearing retractable proboscis, lacking a digestive tract.

ARTHROPODA (INSECTS, SPIDERS, MITES, TICKS)

  • Segmented animals with exoskeletons, including:

    • Crustacea: Aquatic forms; some are intermediate hosts.

    • Arachnida: Scorpions, spiders, ticks.

    • Insecta: Various orders with distinct characteristics.

REFERENCES

  • Belizario, V. Y. Jr. and De Leon, W. U. (2015). Medical Parasitology in the Philippines (3rd ed.). University of the Philippines Press.

  • John, D. & Petri, W. (2010). Markell and Voge's Medical Parasitology (9th ed.). Saunders Elsevier.

  • Velasco, L. (January 2025). Introduction to Medical Parasitology [PPT].