Parasitology Notes

Intro to Parasitology Definitions

  • Facultative parasite: Does not require a host to be parasitic.
  • Opportunistic infection: Pathogens that take advantage of an opportunity not normally available, such as a weak immune system.
  • Parasitism: An organism that spends its life eating from a host, harming the host in the process.
  • Ectoparasite: Outside the host.
  • Endoparasite: Inside the host.
  • Phoresis: Two organisms traveling together, one being carried by the other.
  • Commensalism: One partner benefits, the other is not affected.
  • Mutualism: Both organisms benefit.
  • Hypobiosis: Arrested development and/or hibernation.

Key Concepts

  • Parasites are usually smaller than their hosts.
  • They reproduce at a faster rate.
  • They typically do not kill their host, as they want to live.
  • Parasites reduce the biological fitness of their hosts.

Modes of Infection

  • Oral ingestion.
  • Skin penetration.
  • Injection by vectors.
  • Transplacental or transmammary transmission.

Host Types

  • Intermediate Host (IH): Required; necessary for asexual development of the parasite.
  • Definitive Host (DH): The parasite continues sexual development in this host.
  • Paratenic Host (PH): Non-required; a transfer host where no parasitic development occurs.
  • Aberrant Host: Not the typical host; development is slow or incomplete; may be a dead end for the parasite.
  • Dead-End Host: Does not allow transmission to the definitive host and prevents development completion.

Life Cycles

  • Direct Cycle: Involves only a definitive host (DH).
  • Indirect Cycle: Requires two or more hosts, including an intermediate host (IH).
  • Sexual Reproduction: Leads to genetic diversity and a longer life cycle, involving at least two parasites.
  • Asexual Reproduction: No genetic diversity, a short life cycle, and only one parasite needed.

Parasites in Veterinary Medicine

  • Helminths (Parasitic worms):
    • Phylum Nematoda: Roundworms.
    • Phylum Platyhelminthes:
      • Class Cestodes: Tapeworms.
      • Class Trematoda: Endoparasitic flukes.
      • Class Monogenean: Ectoparasitic flukes.
    • Phylum Acanthocephala: Thorny-headed worms.
  • Kingdom Protozoa:
    • Flagellates (Mastigophora).
    • Ciliates.
    • Apicomplexa.
  • Kingdom Animalia:
    • Myxozoa.

Fitness/Survival Strategies

  • Hypobiosis: Arrested development to survive unfavorable conditions.
  • Predilection Site: Preferred site in/on the host for the parasite.
  • Aberrant Site: Abnormal location in the host for the parasite.
  • Trophic Transmission: Linked by food chain; may involve a loss of predator avoidance, as seen in the "Zombie Snail" (Leucochloridium paradoxum), a parasitic flatworm (trematode) that manipulates gastropods to attract the definitive host.
  • Toxoplasma gondii: Only reproduces sexually in cats; causes rodents to lose their fear of cats.

Host Balance

  • Parasite x Host x Environment interactions.
  • -iasis suffix: Indicates the presence of a parasite (often subclinical).
  • -osis suffix: Indicates a disease caused by a parasite (e.g., Coccidiasis vs. Coccidiosis).
  • Parasites CAN benefit the host in some circumstances, such as promoting healthy skin or providing an immune boost.

Key Characteristics of Nematodes (Roundworms)

  • Males and females are separate sexes.
  • Significant size variation among species.
  • Oviparous: Egg-laying.
  • Viviparous: Live young (some lay eggs containing live young).
  • Offspring must leave the host - either free-living or parasitic.
  • Elongated/cylindrical body shape.
  • Alimentary canal present.
  • Do not multiply within the host.
  • Mouth often has teeth for drawing blood, causing anemia.
  • Female: Possesses a vulva.
  • Male: Possesses a bursa.

Nematode Life Cycle

  • Egg - L1 - L2 - L3 - L4 - L5 - Adult
  • Pre-L1: Microfilaria (mff) - a stage after the egg but before L1; can be considered L1 of its own stage.
  • L5: Immature adult.
  • Insert Host (L3):
    • Final/Definitive host
    • Intermediate host
    • Vectors, paratenic hosts, etc.
  • Mode of infection:
    • Consumption.
    • Percutaneous (skin penetration).
    • Transmammary (through milk).
    • Transplacental (in utero).
    • Direct: Infected larva (L3).
    • Infected larva inside an egg (usually not L1; could be L2).
  • Indirect:
    • L1 infective to intermediate host.
    • L3 infectious to the final host.
  • Diagnostic Stage: Egg or L1.
  • Dead Animal: Adults are full-grown and visible.
  • Means of Survival:
    • Offspring enter the host.
    • Survival of eggs or larvae due to thick shells.
    • Paratenic host.
    • Transmammary, transplacental transmission to naive hosts (e.g., puppies).
    • Periparturient rise: Egg increase in/around birth.
    • Avoiding the immune system, crowding, and bad weather via hypobiosis (arrested development).

Species: Ancylostoma caninum (Hookworms)

  • Mode of Infection:
    • Transmammary.
    • Direct life cycle.
    • Puppy ingests L3.
  • Predilection Site:
    • Adults in the small intestines, larvae in lungs.
  • Pre-patent period (ppp): 2-3 weeks - period between ingestion of L3 and reaching the adult stage.
  • Puppy Clinical Signs:
    • Acute or chronic hemorrhagic anemia, bloody diarrhea, respiratory disease (larval migration).
    • Immediate treatment needed, as they may not test positive initially.
  • Zoonotic: Cutaneous larval migrants.

Peri-parturient Rise

  • Producing eggs in/around lambing.
  • Immune exclusion: L3's eliminated in 48 hours due to strong immunity from previous exposure.
  • Delayed rejection: L3's enter mucosa and eventually die.
  • Naive Sheep: More susceptible.
  • Crowding: Can lead to arrested development.
  • Adult excretion.
  • Older L3: Slower development.
  • Inhibited - "take a nap"; immune projection - arrests and facilitates feedings.
  • Small L4 or adults.
  • Few eggs.
  • Spring Time! L4 resume development causing a rise in egg count and new L3.
  • Causes"self-cure" Premonition

Epidemiology

  • Knowing how a species perpetuates allows targeted intervention.

Life Cycle (modifications)

  • Lambing causes an increase of eggs in the feces (solid line) due to a decreased immune system; adults increase egg production when consumed.
  • Starts in early parturition and ends in lactation.
  • Accumulation of L3 in the environment depends on temperature/environment; eggs prefer to grow to L3 in the spring rather than winter.
  • Lambs consume large amounts of eggs (dotted line).
  • Immune system is decreased due to pregnancy.
  • Peak goes down because lambs are beginning to get some immunity.
  • Adult eggs already around telling other not to grow.
  • Animals on pasture in the spring, in the barn in winter meaning grazing animals eat L3, and more grazing results in more chances of exposure.
  • Maximize nutrition: Pasture rotation every 30 days (Q30days) depends on weather, time of year, and type.
  • Accumulation of larvae: Egg-L3 takes a few days to a few weeks, and L3 lifespan is a few weeks to 3+ months, resulting in L3 accumulation.

Peri-Parturient Rise/Pastures: Prevention/Intervention

  • Rotate pasture.
  • Halfway through lambing/late spring-early summer [April-June].
  • Deworm ewes.
  • Halfway through lambing/late spring early summer [April-June].
  • Increase Protein: 22% for 4 weeks.

Cestodes (Tapeworms)

  • Key Characteristics:
    • Flat body without a body cavity.
    • Scolex: Head with 4 suckers and a rostellum (armed or unarmed with hooks).
    • Larva's main goal in the intermediate host is to form.
    • Strobila: Body composed of proglottids (segments).
    • Proglottids: Segments, with mature ones further away from the head.
    • Each proglottid is hermaphroditic, containing vitelline glands and a genital pore.
    • Indirect life cycle.
    • No alimentary canal.

Types of Cestodes

  • Cyclophyllidea: Scolex with suckers; 1 intermediate host (IH); adult in final host (FH); eggs in IH develop into larvae; segments have an oncosphere (hexacanth embryo).
    • Taenia solium:
      • From eating undercooked pork.
      • Intermediate Host: Pigs eat human feces with hexacanth embryo, forming a larva in muscle/tongue; may have clinical signs.
      • Final Host: Humans; adults in intestines causing no clinical signs; final host produces eggs (proglottids/oncosphere) in feces; typically not pathogenic in the final host.
      • Neurocysticercosis: Humans eating human feces; aberrant and dead-end host in larval stage; larva goes to the brain.
  • Pseudophyllidea: Scolex with bothria; 2 intermediate hosts - first a crustacean; eggs contain oncosphere with cilia for motility.

Treatment for Cestodes

  • Do not usually cause problems in the final host.
  • No clinical signs.
  • Why treat?
    • Aesthetic purposes and zoonotic potential.

Trematodes (Endoparasite Flukes)

  • Key Characteristics:
    • Dorso-ventrally flattened, leaf-like shape.
    • Oral and ventral suckers for attachment and feeding.
    • Hermaphroditic (can cross-reproduce).
    • Indirect life cycle (no direct life cycle).
    • No alimentary canal.
    • Broad host spectrum except for gastropods (1st IH).
    • Diagnosed via sedimentation due to heavy, operculated eggs.
    • Location is species-dependent: rumen, liver, lungs, or blood.
  • Complex life cycle and is host-specific for certain snails.

Life Cycles (Trematodes)

Final HostIntermediate Host #1Intermediate Host #2
Fasciola hepaticaSnail
Sheep LIVER fluke
  • Infects the bile duct.
  • Egg → Miracidium, Sporocyst → Redia → Cercaria → Metacercaria.
  • Signs: Bottle jaw and anemia.
  • Pathology: Pipestem liver w/ chronic infection.
  • Final Host: Ruminant (zoonotic).
  • Schistosoma: Known as swimmer's itch with signs of skin rash (cercarial dermatitis) and is zoonotic.
  • Alaria americana (Diplostomatidae): Lives in small intestines as mature worms and infects carnivorous mammals using IH and PH.

Monogenea (Ectoparasite Worms)

  • Key Characteristics:
    • Larval stage is the Oncomiracidium (ciliated).
    • Distinct opisthaptor.
    • Gyrodactylus: Viviparous (egg-laying), extremely pathogenic.
    • Attach via hooks and poorly developed suckers.
    • Hermaphroditic.
    • Simple digestive system.
    • Large in size (2-3cm).
    • No true body cavity.
  • Nutrients are absorbed via the integument.

Acanthocephala (Thorny-Headed Worms)

  • Key Characteristics:
    • Large (>10cm).
    • Sister group of rotifers.
    • Freshwater zooplankton.
    • Sexually dioecious (male and female).
    • Complex life cycles.
    • Not host-specific.
    • Not pathogenic.
    • Diagnosed by eggs in the feces or adults in the small intestines.
    • St. Kitts mongoose/monkey as dead-end hosts.

Cnidaria (Myxozoa)

  • Key Characteristics:
    • Parasitic jelly fish.
    • Obligate parasites.
    • Indirect life cycle.
    • Polar capsules.
    • Vermiform stage.
    • 2 Hosts: Fish and annelid worm.
    • Spores: Myxospore (diagnostic) and actinospore.
    • Predilection Site depends on the species.

Definitions: Protozoa

  • Protozoa: Single-celled eukaryotes that feed on organic matter, now discouraged as a term, replaced by Protist.
  • Protist: Eukaryotic organism with cells containing nuclei, and is NOT an animal, plant, or fungi.
  • Zoite: "Little creature."
    • Sporozoite: Infective form.
    • Trophozoite: Feeds and grows.
    • Tachyzoite: Divides rapidly.
    • Bradyzoite: Divides slowly.
    • Merozoite: Many.

Protozoa Flagellates

  • Locomotion via flagella.
  • Examples:
    • Giardia: Traveler's diarrhea, chronic infection (GI parasite).
    • Histomonas: Important in poultry (GI parasite) causing blackheads, lethargy, yellow diarrhea, and weight loss infecting the liver and cecum, transmitted in nematode eggs or earthworms containing eggs (hyperparasite).
    • Trypanosomes: Sleeping sickness and Chagas disease (blood parasite).
  • Myxobolus cerebralis: Whirling disease (multivalvulida), histozoic (pathogenic - in muscles), coelozoic (NOT pathogenic -- gallbladder).
  • Kudoa: Human GI problems (food poisoning, melting fish)

Key Characteristics (Protozoa)

  • Unicellular & Eukaryotic: possess a nucleus, endoplasmic reticulum, mitochondria, Golgi body, and lysosomes.
  • Anaerobic or Aerobic.
  • Feed on bacteria via osmotrophy or phagocytosis.
  • Cytostome: "Mouth-like" aperture.
  • Many do not cause disease.
  • Grouped by movement: Pseudopodia, Flagella, Gliding movements, Cilia.
  • Asexual reproduction: Large exposure and significant damage.
  • Host-specific or broad spectrum and can be zoonotic causing chronic infections.

Reproduction (Protozoa)

  • Asexual reproduction:
    • Binary fission: Budding.
    • Schizogony: Serious cell damage due to exponential increase in numbers and multiple divisions of the nucleus and cell leading to being VERY pathogenic.
  • Sexual Reproduction:
    • Merozoites.
    • Macrogametocyte to Macrogamete.
    • Microgametocyte to Microgamete.
    • Microgamete fertilizes macrogamete forming a zygote into an oocyst, is NOT pathogenic.

Ciliates

  • Locomotion via cilia.
  • Examples:
    • Balantidium sp.: Direct life cycle; transmission contained to water; zoonotic; pathogenic in humans but asymptomatic in pigs.
    • White spot disease (Ichthyophthirius multifiliis).

Apicomplexa

  • Locomotion via gliding.
  • Reproduction via sexual & asexual mechanisms.
  • Mostly intracellular.
  • Examples:
    • Order Eucoccidiorida (Zoonotic):
      • Parasites of epithelial cells.
      • Asexual stage causes damage.
      • Diagnostic stage: Oocyst.
      • Ex: Eimeria –Oocysts w/ 4 sporocyst w/ two sporozoites and releases the sporozoites via stieda body & Goussia: Oocysts w/ 4 sporocyst w/ two sporozoites and releases the sporozoites via a longitudinal suture causing goo build-up in swim bladder causing.
      • One host.
    • Two Hosts:
      • Sarcocystis: Causes Equine Protozoal Myeoencephalitis (EPM);
        • DH: Opossum (aberrant host: horse) forming cysts in the muscles.
      • Toxoplasma: Alters IH behavior to facilitate its way to the DH; IH: Mouse, Obligate intracellular
        • Zoonotic
  • Order Prioplasmordia:
    • Parasites of blood cells.
    • Asexual and Sexual reproduction and Vectors.
    • Examples:
      • Babesia microti: Texas cattle fever.
        • 2 hosts: Rodent and tick.
        • DH (sexual reproduction): Tick.
        • IH (asexual reproduction): Mammal.
        • Clinical Signs: Anemia, is Zoonotic.
      • Theileria: East coast fever.