9_IHD

Learning Outcomes

• Understand the burden of parasitic infestations on human health

• Define parasites and identify the three types of disease-causing parasites:

  • Helminths

  • Protozoa

  • Ectoparasites (e.g., fleas, lice, mites, ticks)

• Classify helminths and explain common anthelminthic drugs

• Describe the general morphology and life cycles of protozoans

• Explain the sporozoan life cycle stages in humans and mosquitoes

Parasites

Definition: A parasite is an organism that lives on or in a host and derives nourishment from that host

• The bacteria does not have a symbiotic relationship with the host because the host is the only one that get benefits

• Types of parasites:

  • Helminths: Worm-like organisms

  • Protozoa: Single-celled organisms

  • Ectoparasites: External parasites (e.g., fleas, lice, mites and ticks).

Helminths

Overview

→ Large, multicellular organisms, generally visible to the naked eye in their adult stages

• Can be free-living or parasitic

• Adult helminths cannot multiply in humans

  Three major groups of human parasites:

  • Flatworms (Platyhelminths): Includes trematodes (flukes) and cestodes (tapeworms)

  • Thorny-headed worms (Acanthocephalins): Adult forms reside in the gastrointestinal tract

  • Roundworms (Nematodes): Adult forms are found in various body systems (GIT, blood, lymphatics, or SC tissues); larval stages can cause diverse diseases

Helminths: Flatworms & Acanthocephalins

• Tapeworm (Cestodes)

  • Example: Taenia species

    

• Flukes (Trematodes)

  • Example: Fasciola species

    

  Thorny-headed worm

  • Example: Moniliformis moniliformis

    

Roundworms

These kinds of helminths are more commonly recognised

• Hookworm: Example: Ancylostoma duodenale

• Pinworm: Example: Enterobius vermicularis (found in the sigmoid colon)

• Roundworm: Example: Ascaris lumbricoides

• Whipworm: Example: Trichuris trichiura

Helminth Infestations

Overveiw

• Affect 2 billion+ people worldwide

• Major issue in veterinary medicine

• Significant impact on animal welfare, public health, and food safety

Epidemiology

• Polyinfection (Multiple parasite infections at once) is common in tropical regions.

Transmission & Lifecycle

• Invade the host via skin or GI tract

• Develop into well-developed parasites but they cannot replicate in the host

• Helminths vary in: lifecycle, body structure, development, physiology, localisation in the host and drug susceptibility

Anthelminthic Drugs

Protozoa

Overview

Definition: Microscopic, unicellular organisms either free-living or parasitic.

• Protozoa can multiply in humans, enabling significant infection potential from a single organism.

  Transmission routes:

  • Faecal-Oral Route: Contaminated food/water or person-to-person contact.

    • Mainly for protozoa that live in the human’s intestine

  • Arthropod Vectors: Mosquitoes or sand flies transmit blood or tissue-dwelling protozoa.

    • Mainly for protozoa that live in the blood/tissues

Classification of Protozoa

• Based on locomotion (their mode of movement):

Common Protozoa

• Leishmania

• Entamoeba histolytica

• Plasmodium falciparum

• Balantidium

General Features

Definition:
Protozoa are unicellular, eukaryotic microorganisms that can be free-living or parasitic, capable of surviving in water, soil, or within a host.

• Locomotion:

  • Cilia

  • Flagella

  • Pseudopodia

  • None

• Reproduction:

  • Both sexual and asexual.

• Encystment:

  • Fromation of cysts (dormant, resistant form)

  • Helps protozoa survive in harsh environments

    • Like hibernation, encystment allows these organisms to conserve energy and resources until conditions become more favorable for growth and reproduction.

  • Plays a role in transmission to new hosts

• Nutrition Types:

  Protozoa are heterotrophic

  • Holozoic - ingest solid food by phagocytosis

  • Saprozoic - Absorb dissolved nutrients

Protozoal Asexual Reproduction

Protozoa primarily reproduce through asexual methods, which allow rapid population growth.

Types of asexual reproduction:

1. Binary Fission (Longitudinal): Producing two equal daughter cells.

2. Budding: Producing unequal daughter cells (smaller bud detaches).

3. Schizogony: Multiple fission, leading to numerous daughter cells.

Plasmodia [Haemosporidia]

→ Protozoa causing malaria

Overview

• Humans are affected by 5 plasmodium species

• All species attack RBC’s

• All species cause Malaria

  • One of the most significant parasitic diseases globally 🌍

Species / Types of Malaria

Parasite and Vector

Malaria is a caused by the invasion of parasites, and a crucial step in the disease is the vector (transmission agent)

• Vector: Infected Anopheles (mosquitos).

  • When feeding on humans

• Infection: Red blood cells may contain P. falciparum malaria parasites.

Global Burden of Malaria

Epidemiology (WHO Report 2023)

• 263 million cases of malaria globally

• 597,000 deaths reported

• 94% of deaths occur in sub-Saharan Africa

• Children under 5 years account for 80% of malaria deaths

Geographical Distribution

• Eradicated in temperate regions,

• Endemic in parts of Africa, Asia, and South America.

COVID-19 Impact

• From 2000 to 2019, malaria mortality rates fell by 60% globally

• However, progress stagnated during the COVID-19 pandemic due to:

  • Disruption of healthcare services

  • Delayed mosquito control programs

  • Reduced access to antimalarial treatments

Plasmodia - Exoerythrocytic Cycle

*Occurs in liver

1. Infected female Anopheles (mosquito) bites a human and injects sporozoites from their salivary glands

   • Sporozoites enter the blood stream

2. Sporozoites travel to the liver and penetrate/infect liver cells (takes ~30 min).

3. Asexual multiplication (schizogony) occurs in the liver; lasts 5-16 days.

4. Schizonts rupture (end of primary cycle)

   • releasing merozoites, which can reinfect liver (secondary cycle).

Plasmodia - Erythrocytic Cycle

*Occurs in the bloodstream

5. Merozoites invade erythrocytes (RBC’s).

6. Asexual reproduction (erythrocytic schizogony) continues:

   • Released merozoites reinfect other RBC’s.

   • Incubation period (before clinical signs of malaria) involves ~ 2 exoerythrocytic and 1/2 erythrocytic cycles.

• RBC’s burst after a specific period:

  • 48 (P. vivax, P. ovale) or 72 hours (P. malariae).

  • Malarial fever results from toxins released by the rupture of RBC’s.

Plasmodia - Sexual Cycle

7. Some merozoites produce sexual gametocytes which do not develop further in red blood cells.

8. Uninfected Anopheles mosquitoes take infected blood, allowing gametocyte maturation into sperm and egg in the mosquito gut.

*Gamates mature in the female gut

Plasmodia - Sporogonic Cycle

9. Male (microgamete) and female (macrogamete) gametes fuse after fertilization.

10. Forms a motile zygote (oökinete). Oökinete penetrates the stomach wall of the mosquito.

11. Forms an oöcyst, which undergoes meiotic and mitotic divisions (sporogony)

12. Oocyst ruptures and releases sporozoites that migrate to salivary glands.

Plasmodium falciparum Morphology

Plasmodium falciparum is the most dangerous species of Plasmodium and causes malignant tertian malaria. Its morphology varies at different stages in its life cycle, from sporozoites to gametocytes.

1. Sporozoite (Infective Stage)

• Location: Found in the mosquito salivary glands and injected into the human bloodstream during a mosquito bite.

• Function: Infects liver cells (hepatocytes) to begin the exoerythrocytic cycle.

2. Merozoite (Liver to Bloodstream Stage)

• Location: Released from the liver into the bloodstream.

• Function: Infects red blood cells (RBCs) and starts the erythrocytic cycle.

3. Trophozoite (Early Blood Stage)

• Location: Inside infected RBCs, feeding and growing.

• Function: Trophozoites actively feed on hemoglobin. They are also responsible for the cyclic fever pattern in malaria.

4. Schizont (Mature Blood Stage)

• Location: Inside RBCs, containing many merozoites ready to be released.

• Function: Divides by schizogony to produce multiple merozoites.

5. Gametocytes (Sexual Stage)

• Location: Found in the blood, but not inside RBCs. Male gametocytes are smaller and round; Female gametocytes are larger and crescent-shaped.

• Function: Ready to be ingested by an Anopheles mosquito during a blood meal to continue the life cycle in the mosquito.

Quick Identification Tip

• Trophozoite (ring form) in RBCs = Plasmodium falciparum

• Gametocyte = Crescent shape

• Multiple merozoites inside RBCs = Schizont

• Oöcyst on the exterior stomach wall of the mosquito.

Epidemiology & Pathology - P. vivax

• Most widespread, species of Plasmodium

• Accounts for 43% of global malarial cases.

• Once common in parts of Northern Europe before it was eradicated

• Causes Benign Tertian Malaria

  • Characterised by recurring fever every 48 hours

  • Symptoms: mild but debilitating

• Untreated infections can persist for up to 5 years due to liver reservoirs, causing relapses.

Epidemiology & Pathology - P. malariae & P. ovale

• P. malariae:

  • Localisation: Tropics and sub-tropics

  • 7% of malarial cases, causes quartan malaria.

  • Causes a quartan Malaria (recurring fever every 72 hours)

  • Symptoms mild in adults but can cause nephrotic syndrome in children, persisting up to 40 years if untreated.

• P. ovale:

  • Least common Parasite: less than 1% of cases

  • Localisation: West Africa mostly

  • Causes ovale tertian malaria; benign relapses may occur

  • Specific natural vector has not been identified

Epidemiology & Pathology - P. falciparum

• Causes 50% of global cases.

• Most lethal

• Localisation: Widespread in the Tropics and Sub-Tropics

• Causes malignant tertian malaria

  • Infected RBC’s grow to have prejected knobs which can cause obstructions in the blood vessels

    • Thrombosis and/or local ischaemia

  • Cerebral Malaria

  • Blackwater fever

• Lacks a secondary exoerythrocytic stage; no relapses.

• Attacks last ~ 36 Hrs and may overlap

Controls of Malaria

Past Approaches

• Eliminating mosquito breeding grounds through:

  • Water drainage, covering wells, spraying insecticides, personal protection.

• Treatment of infected individuals to remove reservoir

• WHO eradication efforts from 1955-1970 using DDT and other insecticides ended in failure, transitioning to control strategies.

Present

• Modern measures include:

  • Long-lasting insecticide-treated nets.

  • Indoor residual spraying.

  • Presumptive treatments in infants, young children, and pregnant women.

  • Access to effective treatments for febrile children and capacity for early epidemic response.

  • Education and communication

  • The ability to detect, prepare, and respond to early warnings of epidemics

Vector Control

• Vector control is vital for reducing transmission, with:

  • Indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs).

• Increasing resistance to insecticides poses challenges to control strategies.