Introduction to Parasitology Quiz

Introduction to Parasitology

• Parasitology is the study of parasites and their interactions with hosts
• Parasites can be endoparasites (inside the host) or ectoparasites (outside the host)

Symbiosis

• Symbiosis: close association of organisms of different species
• Eugene Odum's (1959) concept: determine symbiosis by figuring out each relationship's effect on population growth
• Types of symbiosis:
  • Commensalism: one species benefits, the other is not affected
  • Mutualism: both species benefit
  • Exploitation: one species benefits, the other is harmed

Parasitism

• Parasite: an organism that lives in/on another organism (host) and feeds on its tissues/fluids
• Characteristics of parasites:
  • Lives in/on a host
  • Gets part or all of its needs from the host
  • Smaller than the host
  • Has greater reproductive potential than the host
  • Can harm the host
• Taxonomic categories of parasites:
  • Helminths (worms)
  • Protozoa
  • Arthropods
• Not all arthropods are parasites; some are hosts to parasites

Categories of Parasites

• Location:
  • Ectoparasites (e.g., ticks, blowfly maggots)
  • Endoparasites (e.g., heartworm, tapeworm)
• Dependency:
  • Obligate: cannot live outside the host (e.g., ticks, tapeworms)
  • Facultative: can survive outside the host (e.g., brain-eating amoeba)

Life Cycles of Parasites

• Direct life cycle: parasite transmits from one definitive host to another
• Indirect life cycle: parasite develops in an intermediate host before reaching the definitive host

Parasites and Human Health

• Leading causes of death in the US: non-communicable diseases (NCDs)
• Leading causes of death in low-income countries: infectious diseases (e.g., malaria, diarrheal diseases)
• United Nations' Sustainable Development Goals (SDGs) aim to end epidemics of AIDS, TB, malaria, and neglected tropical diseases (NTDs) by 2030

Neglected Tropical Diseases (NTDs)

• 4 categories of pathogens:
  • Protozoa (e.g., malaria)
  • Helminths (e.g., hookworm)
  • Bacteria (e.g., leprosy)
  • Viruses (e.g., dengue fever)
• Features of NTDs:
  • Rural poverty
  • Ancient diseases
  • Chronic disability
  • Disfigurement rather than death

Case Studies

• Clonorchis sinensis (oriental liver fluke):
  • Life cycle: similar to Fasciola hepatica
  • Morphology: adult worms live in bile ducts, eggs have operculum, shoulders, and abopercular knob
  • Diagnosis: detect eggs in feces or detect fibrosis in bile ducts via ultrasound
  • Pathology: adults in bile ducts can cause damage, leading to fibrosis and cholangitis
• Opisthorchis viverrini (cat liver fluke):
  • Life cycle: similar to Clonorchis sinensis
  • Morphology: adult worms have lobed testes, unlike Clonorchis sinensis
  • Diagnosis: detect eggs in feces or detect fibrosis in bile ducts via ultrasound
  • Pathology: adults in bile ducts can cause damage, leading to cholangiocarcinoma
• Fasciola hepatica (sheep liver fluke):
  • Life cycle: involves snail intermediate host
  • Morphology: adult worms have highly branched caeca and testes
  • Diagnosis: detect eggs in feces
  • Pathology: juveniles in liver can cause acute fascioliasis, leading to liver necrosis and fibrosis
• Paragonimus westermani (oriental lung fluke):
  • Life cycle: involves snail and crustacean intermediate hosts
  • Morphology: adult worms have red/brown color and no shoulders
  • Diagnosis: detect eggs in sputum
  • Pathology: juveniles can cause ulcers in ectopic sites, and adults in lungs can cause chronic damage and fibrosis### Treatment
• Praziquantel (PZQ) is a drug used to treat platyhelminthes infections, including flatworms like Clonorchis, Opisthorchis, and Paragonimus.
• PZQ is a very safe drug that doesn't cause mutations or cancer.
• The exact mode of action of PZQ is still unclear, but it causes worms to undergo rapid, sustained muscular contraction, leading to paralysis and eventually being flushed out of the host.
• PZQ disrupts voltage-gated calcium 2+ ion channels in the tegument, leading to an influx of Ca2+ ions and disruption of worm functions.

Fasciola hepatica

• PZQ does not work against Fasciola hepatica, possibly due to the thicker tegument of these worms.
• Triclabendazole (TCBZ) is used to treat Fasciola hepatica infections, causing swelling of the tegument, mitochondria, and disruption of the cytoskeleton and vesicle movement.

Control Strategies

• Kill adult worms using PZQ.
• Reduce environmental contamination by reducing egg output and improving sanitation, including not using night soil in agriculture.
• Control snail populations through physical removal, molluscicides like copper sulfate or sodium pentachlorophenate, but beware of ecotoxicity.

Schistosomiasis

• Schistosomiasis is a disease caused by blood flukes (trematodes) that live in the circulatory system, affecting over 200 million people worldwide.
• There are three main species of schistosomes that infect humans: Schistosoma mansoni, S. haematobium, and S. japonicum.

Schistosome Life Cycle

• Humans are the definitive host, and eggs are produced by adult worms and shed in urine or feces.
• Eggs hatch into miracidia, which penetrate snails, where they develop into sporocysts and cercariae.
• Cercariae penetrate human skin, migrate to the bloodstream, and develop into adults.

Characteristics of Schistosomes

• Schistosomes are dioecious (having separate male and female individuals).
• They live in blood vessels, produce non-operculated eggs, and have no redia or metacercaria stage.

Egg Stage

• Eggs have distinct spines (lateral, terminal, or very small).
• Hatching is stimulated by fresh water, and miracidia escape through a suture rupture.

Larval Stages

• Miracidium: infects snails, becomes sporocysts, and transforms into daughter sporocysts.
• Sporocyst: produces cercariae through asexual reproduction.
• Cercaria: swims, penetrates human skin, and develops into schistosomula.

Adult Stage

• Adults live in the hepatic portal vein, mate, and produce eggs.
• Morphology: dioecious, lacks pharynx, and has a gynecophoral canal.
• Habitat: S. haematobium lives in venous plexus of urinary bladder, S. mansoni in veins of large intestine, and S. japonicum in veins of small intestine.

Glucose Uptake

• Schistosomes take up glucose from the bloodstream using glucose transporters (GTPs).
• GTPs are integral membrane proteins that span the membrane, allowing glucose uptake.
• SGTP1 and SGTP4 are specific to schistosomes and are potential drug targets.

Exit of Eggs from Host

• Adults move to smaller veins, females lay eggs near the intestine or bladder, and then move back to the male.
• Eggs utilize endothelial cells to migrate to the lumen of the gut or bladder.
• SEA (soluble egg antigen) stimulates an immune response, leading to granuloma formation around the egg.

Pathology

• Cercarial dermatitis: cercariae penetrate skin, causing an itchy rash.
• Eggs trapped in the gut/bladder wall, liver, or spleen cause granuloma formation, leading to tissue destruction and organ dysfunction.
• Symmers pipestem fibrosis: fibrotic tissue builds up around liver veins, impeding circulation and leading to increased pressure and hepatomegaly.

Diagnosis, Treatment, and Control

• Identify eggs in feces or urine using microscopy.
• Chemotherapy: PZQ is effective against all three human schistosome species, but resistance is emerging.
• Alternative treatments: artemether, a promising antimalarial drug, and nanomedicine, which targets specific receptors on schistosome cells.### Antibodies
• Produced by the host to recognize and interact with a specific molecular target with high affinity and high specificity
• Research involves bioconjugation of antibodies with nanoparticles to target SGTP proteins (schistosome glucose transporters) in the tegument
• Therapy binds specifically to SGTP, delivering a concentrated dose of the drug to the worm

Control Measures for Schistosomiasis

• Chemotherapy: uses a combination of PZQ, etc.
• Sanitation: isolate fecal material and urine to prevent contamination
• Snail control: prevent direct contact with water
• Education
• Vaccines: integrated with existing chemotherapy, targeting parasite muscle proteins and enzymes

Schistosomiasis Vaccine Development

• Vaccine candidate: Glutathione-S-transferase (GST)
• Results in a 40-60% reduction in worm burden
• No cross-reactivity with human GST, showing promise in phase II clinical trials
• Schistosomiasis Control Initiative: provides PZQ to populations in sub-Saharan Africa

Cestodes (Tapeworms)

Taxonomy

• Domain: Eukarya
• Kingdom: Animalia
• Phylum: Platyhelminthes (flatworms)
• Class: Cestoda (tapeworms)

Generalized Life Cycle

• Most species require at least two hosts
• Variation depends on species

Adult Morphology

• Scolex: head-like structure with nervous structures and attachment organs
• Neck: undifferentiated zone with stem cells
• Strobila: chain of proglottids (segments)

Reproductive Structures

• Male: protandry, with testes and cirrus
• Female: ovary, vitelline glands, and uterus
• Monoecious: each proglottid develops both male and female organs

Eggs

• Released through apolysis (ending blindly) or anapolysis (through uterine pore)
• Morphology: oncosphere (cestode larva) with six hooks, protective layers for survival outside the intestine

Treatment

• Praziquantel: damages tegument
• Niclosamide: disrupts oxidative phosphorylation (ATP production)

Order Cyclophyllidea

• Development: cysticercoid, cysticercus, coenurus, and hydatid metacestodes
• Features: acetabulate scolex, compact vitelline glands, apolysis

Specific Cestode Species

• Taenia solium, Taenia saginata, Echinococcus species, Hymenolepis nana: life cycle, morphology, pathology, treatment, and control measures