Parasitology Exam 1

Mutualism vs. Commensalism vs. Parasitism

• Mutualism: Symbiotic relationship where both species benefit.

• Commensalism: Symbiotic relationship where one species benefits while the other is unaffected.

• Parasitism: Relationship where one organism benefits at the explicit expense or harm of the host.

Ectoparasite vs. Endoparasite

• Ectoparasite: A parasite that lives on the exterior surface of its host (e.g., ticks, fleas, lice).

• Endoparasite: A parasite that lives inside the body tissues or organ systems of its host (e.g., roundworms, tapeworms).

Obligatory vs. Facultative Parasite

• Obligatory Parasite: Entirely dependent on a host to complete its life cycle (e.g., ticks, roundworms).

• Facultative Parasite: Capable of living independently in the environment, but can act parasitically given the opportunity (e.g., blowflies).

Permanent vs. Temporary Parasite

• Permanent Parasite: Remains attached to or inside the host for its entire adult lifespan (e.g., roundworms).

• Temporary Parasite: Visits the host briefly to feed and then detaches into the environment (e.g., blood-feeding ticks).

Definitive Host

The host species in which a parasite reaches biological maturity and undergoes sexual reproduction.

Intermediate Host

An essential host species in which a parasite undergoes mandatory pre-adult developmental stages or asexual reproduction.

It lives there before reaching sexual maturity

Paratenic (Transport) Host

An optional host used for transfer or survival, in which the parasite accumulates and remains viable without undergoing developmental changes or multiplication.

Reservoir Host

An animal population that naturally harbors and maintains a parasite infection in nature, serving as an infection source for other susceptible species.

paratenic is only a temporary transport vehicle where the parasite survives without developing. reservoir is actively maintains the parasite's life cycle and serves as a continuous source of infection for other species

Direct vs. Indirect Life Cycle

• Direct Life Cycle: Involves only a definitive host without requiring intermediate or paratenic hosts.

• Indirect Life Cycle: Requires one or more intermediate hosts for essential developmental stages before reaching the definitive host.

Biological Vector vs. Mechanical Vector

• Biological Vector: A vector in which the parasite develops or multiplies (functions as an intermediate host).

• Mechanical Vector: A transport vector that carries parasites passively on its exterior without any parasite development or multiplication (e.g., houseflies carrying eggs on legs).

biological vector is actually infected, the mechanical vector carries the parasite without actually being infected

Prepatent Period (PPP)

The time period elapsed between initial host infection and the earliest detectable appearance of parasite offspring in bodily fluids or feces.

Patent Period (PP)

The duration of time during which a mature parasite actively produces offspring detectable in host feces or bodily fluids.

Clinical Incubation Period

The time interval between initial parasite infection and the onset of clinical signs or symptoms in the host.

Helminth-Induced Immune Response

Helminth (worm) infections characteristically provoke a Type 2 immune response, involving basophils, eosinophils, mast cells, M2 macrophages, and Th2 cytokines (similar to allergic responses).

Bystander Response (Hygiene Hypothesis)

The concept that a lack of early childhood exposure to parasitic worms and infections leads to immune dysregulation, predisposing individuals to hyper-reactive allergic diseases.

Parasite Countermeasures Against Host Immunity

1. Altering surface glycoproteins to evade antibody recognition.

2. Residing intracellularly inside host immune or somatic cells.

3. Absorbing host-produced molecules to disguise themselves.

4. Directly suppressing host immune responses.

Key Anatomical Features of Nematodes

• Body shape: Vermiform (worm-like).

• Cavity: Pseudocoelomate structure with pressurized fluid cavity.

• Locomotion: Circular muscle layer producing S-shaped movement.

• Digestive system: Complete tract (mouth to anus).

• Reproduction: Dioecious (separate sexes, females larger).

Alae

Lateral, wing-like cuticular broad projections located near the anterior neck region of certain nematodes (e.g., the broad, heart-shaped alae of Toxocara cati).

Toxocara canis — Primary Transmission Routes

1. Direct ingestion of infective eggs.

2. Ingestion of infected paratenic hosts (e.g., rodents, birds).

3. Transplacental transmission to unborn fetuses.

4. Transmammary transmission to nursing puppies.

Toxocara canis — Sequential Steps of Tracheal Migration

1. Host ingests an infective egg containing an L3 larva.

2. Gastric acid removes the outer protein coat and thins the egg wall.

3. Egg hatches in the small intestine, and larva penetrates the intestinal mucosa.

4. Larva enters venules and travels via portal circulation to liver, heart, and pulmonary arteries.

5. Larva breaks through alveolar walls into bronchial tree and ascends the trachea.

6. Host coughs and swallows larva back into the small intestine, where it matures into an adult.

what is tracheal migration:

Tracheal migration means the larvae specifically migrate through the lungs → up the trachea → get swallowed → return to the intestine to mature.

• Tracheal migration = lung/trachea route back to gut

Toxocara canis — Sequential Steps of Somatic Migration

1. Host ingests infective egg, which hatches in the small intestine and enters bloodstream.

2. Larva passes through lung capillaries into systemic circulation without breaking into alveoli.

3. Larva distributes to somatic tissues (muscles, organs) and enters an encysted/dormant state.

4. In pregnant females, dormant larvae reactivate to cross the placenta or migrate to mammary glands.

What is somatic migration:

Somatic migration means larvae migrate into or remain in body tissues outside the gut, such as muscle, liver, or other organs. They may become arrested/dormant there rather than immediately returning to the intestine.

• Somatic migration = migration into body tissues

Toxocara canis — Host Age Determinant for Migration

• Puppies < $3\,\text{months}$ old: Predominantly undergo tracheal migration, leading directly to patent intestinal infections.

• Dogs > $3\,\text{months}$ old: Predominantly undergo somatic migration, causing dormant tissue encystment.

Toxocara canis — Transplacental vs. Transmammary Routes in Pups

• Transplacental: Larvae cross placenta, arrest in fetal liver, and complete tracheal migration after birth.

• Transmammary: Larvae pass in milk directly to pup's gut, undergoing direct development without migration.

Toxocara cati — Key Distinctions from Toxocara canis

• Lacks transplacental transmission entirely.

• Transmammary transmission occurs predominantly during acute maternal infections. (meaning this is most likely to happen when the mother has a recent/active infection with migrating larvae, rather than just an old, dormant infection.)

• Possesses broad, heart-shaped cervical alae.

Baylisascaris procyonis — Life Cycle Steps

1. Definitive host (raccoon) sheds eggs in latrines, which become infective in $\sim 30\,\text{days}$.

2. Paratenic hosts (rodents, birds) ingest eggs; larvae migrate to tissue/CNS causing severe dysfunction.

3. Raccoons ingest infected paratenic hosts (or eggs directly).

4. Larvae undergo direct development in raccoon small intestine without somatic migration.

Ascaris suum / Ascaris lumbricoides — Host & Clinical Manifestations

• Hosts: Ascaris suum (pigs), Ascaris lumbricoides (humans).

• Pigs: Respiratory distress ("thumps") and liver lesions ("milk spots").

• Humans: Ascaris pneumonitis (Loeffler's pneumonia) (pneumonia = infection of the lungs), intestinal obstruction, and malnutrition.

Parascaris equorum — Clinical Impact

Infects foals ($3\text{ to }9\,\text{months}$ of age), causing coughing, eosinophilia, diarrhea, failure to thrive, and potentially fatal intestinal obstruction or perforation (colic).

Visceral Larva Migrans (VLM)

Human disease caused by migrating Toxocara or Baylisascaris larvae in liver and lungs; manifests as fever, hepatomegaly (enlarged liver), and eosinophilia. Most common in children < $5\,\text{years}$ exhibiting pica/geophagia.

Yes this is caused by raccoon, dog, or cat roundworms. we are just the paratenic/accidental hosts, they migrate through our organs but do not mature into adult worms

pica/geophagia means wanting to eat non-food substances like dirt or clay or something. geophagia is just a type of pica.

hepatomegaly means an enlarged liver

eosinophilia means increased eosinophils (white blood cells) in the blood

Ocular Larva Migrans (OLM)

Human zoonotic disease where Toxocara or Baylisascaris larvae migrate to the eye, causing unilateral visual loss or granulomas. Typically occurs in children aged $7\text{ to }8+\,\text{years}$ without pica.

Neural Larva Migrans (NLM)

Severe zoonotic disease caused primarily by Baylisascaris procyonis (raccoon roundworm) larvae migrating and growing in brain and spinal cord tissue, leading to severe neurological damage.

Anisakis species — Life Cycle & Transmission

• Definitive Host: Marine mammals.

• Intermediate Host: Crustaceans.

• Paratenic Host: Fish and squid.

• Human Infection: Ingestion of raw/undercooked seafood (sushi, ceviche), causing acute abdominal pain, nausea, and vomiting within $1\text{ to }14\,\text{days}$ post-ingestion.

they are nematodes, which are a type of helminth

Hookworm Pathology & Feeding Rate

The primary clinical disease caused by hookworms is severe anemia. Ancylostoma caninum feeds at a rate of approximately $0.5\,\text{mL}$ blood per worm daily ($0.25\,\text{mL}$ for Uncinaria and Necator).

Hookworms are also helminths/nematodes

Cutaneous Larva Migrans (CLM)

Serpentine skin lesions ("creeping eruption" / "plumber's itch") caused by animal hookworm larvae penetrating human skin and migrating through epidermal tracks.

Larval Leak

A phenomenon in hookworm infections where dormant somatic larvae encysted in host tissues spontaneously reactivate and leak into the intestinal lumen, establishing a patent infection.

“Larval leak” is classically associated with hookworms, especially Ancylostoma caninum in dogs.

Trichuris Species — Morphology & Attachment

Whipworms inhabit the large intestine/colon. They have a thin anterior esophageal neck that "sows" into the mucosal lining, while the thicker posterior end remains free in the lumen.

whipworms are also helminths/nematodes

Strongyloides — Heterogonic vs. Homogonic Life Cycle

• Heterogonic Cycle: Free-living environmental cycle containing both adult male and female worms reproducing sexually.

• Homogonic Cycle: Parasitic cycle inside host containing only female worms reproducing via parthenogenesis. (pathogenesis meaning female only reproduction without fertilization being needed)

also helminths/nematodes

Soil-Transmitted Helminths ("Unholy Trinity")

The collective name given to the three major soil-transmitted intestinal nematodes: Ascarids (Roundworms), Hookworms, and Whipworms.

Host Range

The number or range of different host species a parasite can infect (e.g., narrow vs. broad host range).

Zoonotic Disease

A disease that is naturally transmitted from animals to humans under natural conditions.

Epidemiology

The study of all ecological aspects of a disease, explaining its transmission, distribution, prevalence, and incidence in a population.

Three Primary Factors in Parasite Transmission

1. Source of infection (environmental contamination, reservoir hosts)

2. Mode of transmission

3. Presence of a susceptible host

Major Modes of Parasite Transmission

• Ingestion of infective eggs or cysts from food/water

• Ingestion of infective larvae

• Skin penetration by infective larvae

• Ingestion of infected intermediate or paratenic hosts

• Transmission by blood-feeding vectors

• Transplacental or transmammary transmission

• Sexual transmission

Treatment vs. Control

• Treatment: Eliminating an active parasite infection from the host.

• Control: Preventing parasite transmission, infection, and reinfection.

Major Helminth Groups

1. Nematodes (roundworms)

2. Trematodes (flukes)

3. Cestodes (tapeworms)

Type 1 vs. Type 2 Immune Response

• Type 1 Response: Triggered by bacteria, viruses, protozoa, and fungi; involves NK cells, Tc cells, and Th1/Th17 cytokines.

• Type 2 Response: Triggered by helminths (worms); involves basophils, eosinophils, mast cells, M2 macrophages, and Th2 cytokines (similar to allergic reactions).

General Nematode Development & Molting

Nematode eggs generally begin as uninfective in the environment and progress through four distinct larval molting stages ($L1$ to $L4$) before reaching adulthood.

Ascarid General Characteristics

• Nematodes possessing three prominent lips around the mouth.

• May feature cervical alae (wing-like cuticular broad projections near the head).

Toxocara canis — Primary Hosts & Intestinal Site

• Hosts: Dogs and wild canids.

• Location: Small intestine.

Toxocara canis — Clinical Disease Signs

Dull hair coat, pot-bellied abdominal distention, poor appetite, loose stool, verminous pneumonia, and intestinal obstruction.

Toxocara canis — Infection via Paratenic Host

When a canine ingests an infected paratenic host (e.g., rodent or bird), larvae undergo direct development in the small intestine without undergoing tracheal migration.

Ascaris suum / Ascaris lumbricoides — Host & Clinical Manifestations

• Hosts: Ascaris suum (pigs), Ascaris lumbricoides (humans).

• Pigs: Respiratory distress ("thumps") and liver lesions ("milk spots").

• Humans: Ascaris pneumonitis (Loeffler's pneumonia), intestinal obstruction, and malnutrition.

Parascaris equorum — Susceptibility & Clinical Impact

• Hosts: Horses (primarily foals $3\text{ to }9\,\text{months}$ old; begin developing immunity around $6\,\text{months}$).

• Clinical Signs: Coughing, eosinophilia, diarrhea, poor appetite, failure to thrive, and potentially fatal intestinal obstruction or perforation (colic).

Large Animal Parasite Control Measures

1. Deworm pregnant mares and sows prior to giving birth.

2. Thoroughly wash animals and place in clean pens prior to giving birth.

3. Keep stalls and pens clean after delivery with proper sanitation.

Zoonotic Roundworms and Associated Human Syndromes

• Toxocara canis: VLM, OLM, NLM

• Toxocara cati: VLM, OLM

• Baylisascaris procyonis: VLM, OLM, NLM

• Ascaris suum: Intestinal infection

cati not doing NLM apparently is notable distinction here I think

Hookworm Species and Primary Hosts

• Ancylostoma caninum: Dogs, foxes, coyotes

• Ancylostoma tubaeforme: Cats

• Ancylostoma braziliense: Dogs, foxes, cats

• Uncinaria stenocephala: Foxes, dogs, cats

• Bunostomum spp.: Ruminants

• Ancylostoma duodenale: Humans

• Necator americanus: Humans (Note: All animal species of Ancylostoma are zoonotic).

General Hookworm Life Cycle & Transmission Routes

1. Unembryonated eggs pass in feces, hatch into L1, and develop into infective L3 larvae in the environment.

2. Infection routes: Ingestion (direct development), skin penetration (tracheal migration), transmammary (direct development), transplacental, or ingestion of paratenic hosts.

Ancylostoma duodenale vs. Necator americanus Transmission

• Ancylostoma duodenale: Transmitted via ingestion, skin penetration, transplacental, transmammary, and paratenic hosts.

• Necator americanus: Transmitted primarily via skin penetration.

Factors Influencing Hookworm Disease Severity

Host age, nutritional status, level of environmental exposure to L3 larvae, and host immunity.

Canine Hookworm — Clinical Disease Categories

• Peracute: Severe blood loss/fatality in nursing pups (~10 days PI via transmammary route).

• Acute: Older pups/dogs with heavy infections; lethargy, anemia, soft to liquid dark tarry feces.

• Chronic: Subclinical to chronic weakness, emaciation, and anemia.

• Secondary: Older, malnourished or debilitated animals.

Human Hookworm Disease & Severe Pathologies

• Mild/Moderate: Pneumonitis, progressive iron-deficiency anemia, abdominal pain, loss of appetite, geophagy.

• Severe/Heavy: Blood loss up to $200\,\text{mL/day}$, severe protein deficiency, dry skin/hair, edema, pot-belly in children, delayed puberty, heart failure, death.

Eosinophilic Enteritis

A zoonotic human infection caused by Ancylostoma caninum where larvae migrate to the lower small intestine and colon, leading to recurrent abdominal pain, eosinophilia, elevated IgE, bowel thickening, and focal mucosal ulceration.

Trichuris Species and Primary Hosts

• Trichuris vulpis: Dogs, foxes

• Trichuris ovis: Sheep, goats

• Trichuris suis: Swine

• Trichuris discolor: Cattle

• Trichuris campanula: Cats (rare)

• Trichuris trichiura: Humans, primates

Whipworm Life Cycle & Prepatent Period

Direct life cycle; eggs develop to the infective stage in the environment in $\sim 1\,\text{month}$. Infection occurs via ingestion without extraintestinal migration. Prepatent period (PPP) is $2\text{ to }3\,\text{months}$.

Whipworm Clinical Disease & Treatment

• Clinical Signs: Diarrhea, abdominal discomfort, rectal prolapse, loss of appetite, weight loss, dull hair coat, and cognitive impairment in humans.

• Treatment: Dogs require treatment once a month for 3 months (due to long PPP); humans require a 3-day course of albendazole.

Strongyloides Species and Primary Hosts

• Strongyloides stercoralis: Dogs, cats, humans, foxes

• Strongyloides ransomi: Swine

• Strongyloides papillosus: Ruminants

• Strongyloides westeri: Horses

Strongyloides — Autoinfection & Animal Pathology

• Autoinfection: Larvae penetrate the intestinal mucosa or perianal skin without leaving the host, establishing longstanding infections.

• Pathology: Worms penetrate deeply into small intestinal mucosa causing catarrhal to necrotic enteritis, protein-losing enteropathy, and watery/mucoid to hemorrhagic diarrhea in young animals.

Major Intestinal Locations of the "Unholy Trinity"

• Ascarids (Roundworms): Small intestine

• Hookworms: Small intestine

• Whipworms (Trichuris): Large intestine / colon