parasites

parasite

An organism that lives in or on a host organism and benefits at the host's expense, often causing harm.

parasite direct life cycles

• swithching b/w multiple hosts

• asexual and sexual reporoduction

• transmission of parasites in same species

indirect life cycle

• transmission of parasites from 1 host to next in different species

• heterogenous parasites

• definitive host: parasites reach sexual maturity

• intermediate host: parasite development but where parasites do not reach sexual maturity.

helminths

• Nematodes (Roundworms)

  • Cylindrical, unsegmented

  • GI and tissue infections

  • Examples: Ascaris, Enterobius (pinworm), Strongyloides

• Cestodes (Tapeworms)

  • Flat, segmented

  • Absorb nutrients through skin

  • Examples: Taenia, Echinococcus

• Trematodes (Flukes)

  • Flat, leaf-shaped

  • Infect blood, liver, lungs

  • Examples: Schistosoma, Fasciola

arthropods

Characteristics: Exoskeleton, segmented body, jointed limbs

Roles in Disease:

1. Ectoparasites (live on host)

   • Direct irritation + secondary infection

   • Examples: Mites (Scabies), lice, fleas, ticks

2. Vectors (transmit disease)

   • Mechanical (carry pathogens): e.g., flies

   • Biological (pathogen develops in vector):

     • Mosquitoes – malaria, dengue

     • Ticks – Lyme disease, babesiosis

     • Fleas – plague

protozoa

unicellular euk organisms that can cause diseases in humans and animals, typically spreading through water or food.

• asexual reproduction usually: binary fission

• movement: pseudopodia, cilia, flagella. gliding

giardia

• Location: small intestine

• Reproduction: (longitudinal) binary fission;

• Motility: by using flagella;

• Infection: Ingest cysts (contaminated water/food)

• Excystation: Cysts release trophozoites in the jejunum (triggered by bile & pH ~7.8)

• Encystation: Trophozoites form cysts again

• Cysts excreted in feces → survive months in environment

Morphology:

• Trophozoite

  • Size: 9–21 µm

  • Pear-shaped (rounded front, pointed back)

  • Flattened ventral surface with adhesive disc

  • Two nuclei

  • Active, feeding stage

• Cyst

  • Size: 8–14 × 7–10 µm

  • Oval shape

  • Four nuclei, axonemes (flagella parts), disc fragments

  • Infective, resistant stage

Toxoplasma gondii - life cycle and stages in environment

Life Cycle

• Indirect life cycle (2 hosts)

  • Definitive host: Cats (sexual & asexual stages in small intestine)

  • Intermediate host: Any warm-blooded animal (asexual stages in any nucleated cell)

    • Sporozoites → tachyzoites → invade & multiply → burst host cells

    • Tachyzoites convert to bradyzoites → form tissue cysts

      • Found in brain, muscles, visceral organs

      • Persist for life of host

      • Occasionally rupture

Stages in Environment

• Cats shed unsporulated oocysts in feces

• Sporogony occurs in environment (1–5 days)

• Sporulated oocyst: 2 sporocysts, each with 4 sporozoites

• Oocysts can survive 12–18 months, resist disinfectants

Toxoplasma gondii - infection routes

• Ingestion of sporulated oocysts

  • From contaminated food/water

  • Main route for herbivores

• Ingestion of tissue cysts

  • From raw/undercooked meat

  • Affects carnivores & omnivores

• Tachyzoite transmission

  • Transplacental (mother to fetus)

  • Milk (goats)

  • Blood transfusion, organ transplant

  • Lab accidents

Toxoplasma gondii - Disease & Risks

• Immunocompetent hosts: Control infection

• Immunocompromised hosts (e.g. AIDS):

  • Reactivation of cysts → tachyzoites → severe disease (e.g., encephalitis)

• Congenital toxoplasmosis:

  • Transmission to fetus → abortion or severe pathology

• Zoonotic potential:

  • Affects humans, sea mammals, marsupials

  • Behavioral changes seen in mice, possibly humans

Toxoplasma gondii - survival and inactivation

• Oocysts: survive up to 18 months

• Tissue cysts in meat:

  • Survive up to 2 months at 4–6°C

  • Killed below –12°C or above 65°C

Plasmodium (Malaria) - Transmission & Hosts

• Transmitted by: Female Anopheles mosquitoes

• Indirect life cycle:

  • Definitive host: Mosquito (sexual reproduction)

  • Intermediate host: Human (asexual reproduction)

Plasmodium (Malaria)- Life Cycle in Humans (Asexual Stage)

1. Mosquito bite → injects sporozoites

2. Sporozoites → travel to liver → invade hepatocytes

3. Multiply asexually → liver cells burst → release merozoites

4. Merozoites → invade red blood cells (RBCs)

   • Multiply asexually → RBCs burst → more merozoites released

   • Some develop into gametocytes (male & female

Plasmodium (malaria) - Life Cycle in Mosquito (Sexual Stage)

• Mosquito bites infected human → ingests gametocytes

• In mosquito gut:

  • Gametocytes → gametes → fertilize → zygote

  • Zygote → ookinete → becomes oocyst

  • Oocyst releases sporozoites → migrate to salivary glands

• Ready to infect another human during next bite

Helminths (Worms)

• Phylum Platyhelminthes (flatworms)

• Phylum Nematoda (roundworms)

Phylum Platyhelminthes (flatworms) - classes

• Trematoda (Flukes)

  • Fasciola hepatica

  • Opistorchis felineus

  • Clonorchis sinensis

  • Schistosoma spp.

  • Paragonimus westerman

• Cestoda (Tapeworms)

  • Taenia solium, T. saginata

  • Echinococcus granulosus

  • Dipylidium caninum

Phylum Platyhelminthes (flatworms) - general features

• Body: flat (dorso-ventrally), bilaterally symmetrical, no body cavity

• Organs in parenchyma (specialized connective tissue)

• Incomplete or absent digestive tract

• Suckers/hooks for attachment

• Mostly hermaphroditic (except Schistosoma)

Class Trematoda (Flukes)

• Short, flat (2–7.5 cm)

• Have oral & ventral suckers

• Incomplete digestive tract

• Life cycle: indirect, needs at least 1 intermediate host

• Examples:

  • Clonorchis sinensis (liver fluke)

  • Schistosoma spp. (blood fluke – dioecious)

  • Fasciola hepatica, Opisthorchis spp., Paragonimus westermani

Class Cestoda (Tapeworms)

• Long (mm to meters), segmented body

• Segments = proglottids; head = scolex (with hooks/suckers)

• No digestive tract

• Hermaphroditic

• Life cycle: indirect, larval stage infects tissues

• Examples:

  • Taenia solium, T. saginata, Echinococcus granulosus

  • Hymenolepis nana, Dipylidium caninum, Diphyllobothrium latum

Anatomic Locations in the Host

how hosts become affected

Phylum Nematoda (Roundworms) - classes

• Ascaris lumbricoides

• Ancylostoma caninum (hookworms)

Ascaris lumbricoides morphological features

• Adults:

  • Large cylindrical worms, up to 35 cm in length.

  • Anterior end has 3 prominent lips.

  • Feed on intestinal contents and epithelial cells.

• Eggs:

  • Oval, yellow-brown, and have a thick mammillated shell.

  • Contains one cell inside.

  • Can survive in the environment for 5–7 years

Ascaris lumbricoides Biology / Life Cycle

1. Female worms produce hundreds of thousands of eggs per day.

2. Eggs are passed in feces.

3. In the environment, embryonated eggs develop in 10–15 days.

4. Host becomes infected by ingesting embryonated eggs.

5. Inside host:

   • Eggs hatch in small intestine.

   • Larvae migrate: intestine → liver → heart → lungs → alveoli → pharynx → swallowed → small intestine.

   • Mature into adult worms in small intestine.

Ascaris lumbricoides anatomical location

• Primary: Small intestine.

• Others: Bile ducts, pancreatic ducts, stomach.

• May be seen vomited, or exit through mouth/nose in heavy infections

Ascaris lumbricoides Infection Mode

Ingestion of embryonated eggs from contaminated food, water, or hands.

Ancylostoma caninum (Hookworm) - Morphological Features

• Adults:

  • Anterior end is dorsally bent (hook-shaped).

  • Large buccal capsule with 3 pairs of sharp teeth.

  • Feed by biting the intestinal mucosa, creating wounds.

  • Frequent change of feeding sites (4–6 times/day).

• Eggs:

  • Thin-shelled, morula stage (many cells inside).

  • About 16,000–20,000 eggs/day laid by females.

Ancylostoma caninum (Hookworm) - Biology / Life Cycle

1. Eggs passed in dog feces.

2. In environment:

   • Eggs develop: L1 → L2 → L3.

   • L3 is the infective stage.

3. Infection routes:

   • Transcutaneous: L3 penetrates skin → migrates to lungs → trachea → pharynx → swallowed → small intestine.

   • Oral ingestion: Direct maturation in the small intestine.

   • Transmammary: Arrested larvae reactivated during pregnancy → passed to puppies through milk.

   • Via paratenic hosts (mice, rats, birds).

4. Some larvae go into hypobiosis (dormant stage), reactivating later.

Ancylostoma caninum (Hookworm) - Anatomic Location

• Dogs: Adult worms in small intestine – cause severe blood loss.

• Humans:

  • Adults may cause eosinophilic enteritis.

  • Larvae cause cutaneous larva migrans (CLM), folliculitis, etc

Ancylostoma caninum (Hookworm) - infection mode

• Skin penetration (walking barefoot on contaminated ground).

• Ingestion of infective larvae or paratenic hosts.

• Transmammary transmission in dogs.

Phylum Arthropoda - general morphological features

• Body:

  • Segmented body covered by a protective exoskeleton (made of chitin).

• Legs:

  • Jointed appendages (arthron = joint, podos = foot).

• Reproduction:

  • Most are dioecious (separate sexes).

• Development:

  • Molting (ecdysis) is necessary due to the rigid exoskeleton.

• Metamorphosis:

  • Hemimetabolous: Incomplete metamorphosis (egg → nymph → adult).

  • Holometabolous: Complete metamorphosis (egg → larva → pupa → adult)

Pediculus humanus capitis (Head Louse) - morphology

• Size: Up to 3.6 mm.

• Body shape: Dorso-ventrally flattened.

• Segments: Head, thorax, abdomen.

• Legs: Three pairs (adapted to cling to hair shafts)

Pediculus humanus capitis (Head Louse) - biology/life cycle

• Eggs (nits) are glued to hair shafts.

• Eggs hatch in ~7 days.

• Three nymph stages (look like small adults).

• Total life cycle: 3–4 weeks (egg to egg).

• Do not survive long off the host

Pediculus humanus capitis (Head Louse) - anatomical location and infection mode

Anatomic Location

• Scalp hair, especially near the nape of the neck and behind ears.

Mode of Infestation

• Mainly via head-to-head contact.

• Also via contaminated items: combs, hats, headphones, brushes, etc.

Sarcoptes scabiei - Essential Morphological Features

• Size: Up to 450 µm (microscopic).

• Body: Rounded, compact, unsegmented.

• Legs:

  • 4 pairs of short legs in adults and nymphs.

  • Legs are stubby and suited for burrowing.

• Mouthparts: Adapted for skin tunneling.

• Eyes: Absent.

• Cuticle: Covered with spines and bristles.

Sarcoptes scabiei - Biology & Life Cycle

• Species: One species – Sarcoptes scabiei – with host-specific variants (e.g., var. hominis in humans).

• Hosts: Wide range – humans, pigs, dogs, cattle, goats, sheep, etc.

  • Infestations in non-preferred hosts are short-lived and non-reproductive.

1. Egg

2. Larva (6 legs)

3. Nymph (8 legs)

4. Adult (8 legs)

• Gravid females:

  • Burrow tunnels in the stratum corneum (outer layer of skin).

  • Lay 1–3 eggs/day.

  • Live for 1–2 months.

• All life cycle stages occur on the host (no environmental stages required).

• Transmission:

  • Direct contact (15–20 minutes of close skin-to-skin contact).

  • Indirect: Via contaminated bedding, clothing, towels – less efficient.

• Immune response:

  • Immediate and delayed hypersensitivity to mite secretions and feces.

  • Leads to intense itching (pruritus), especially at night.

Sarcoptes scabiei - anatomical location and host infection

• Mites live and reproduce in epidermal tunnels.

• Commonly affected areas:

  • Webbing of fingers

  • Wrists

  • Elbows

  • Armpits

  • Waistline

  • Genital area

• Human-to-human contact (most common route).

• Close contact in families, schools, or care facilities.

• Fomites (e.g., towels, bedding) may spread mites indirectly.