RA

Parasites of the Dog – Core Vocabulary

Lecture Objectives

  • Provide a broad overview of major groups of parasites infecting domestic canines.
  • Emphasize distinguishing characteristics of major parasites.
    • Scientific & common names.
    • Lifecycles, definitive & intermediate hosts.
    • Basic parasitology terminology.
  • Offer a brief overview of the diagnostic methods most often used in small-animal parasitology.

Recommended Web Resources & Tools

  • Companion Animal Parasite Council (CAPC) — general parasite info for cats & dogs.
    • https://www.capcvet.org
  • American Heartworm Society (AHS) — focused information on canine/feline heartworm \textit{Dirofilaria immitis}.
    • https://www.heartwormsociety.org
  • Mobile app: “CAPC Internal Parasites ID – Veterinary Internal Parasite Flashcards.”
    • Species selector (canine, feline, avian, reptile, small mammal).
    • Two study modes (Random / Preview). 400× microscopy images, reference features, version 1.0.1 (minor bug fixes, released 4\text{ mo} ago).

Why Study Parasites?

  • Parasites exist in nearly every environment: households, farms, zoos, aquaria, wildlife, exotic pets, and humans.
  • In the U.S. veterinarians encounter parasites more frequently than physicians, yet zoonotic risk remains high.
  • Enormous economic impact: prevention, treatment, diagnostics constitute a multi-billion-dollar industry.

Core Terminology

  • Parasite: an organism that lives on/within and benefits at the expense of a host.
  • Host: organism harboring a parasite.
    • Definitive host: where parasite reaches sexual maturity / reproduces.
    • Intermediate host: required for development but not sexual maturity.
  • Life cycle: sequence of developmental stages (egg/larva/nymph/adult) plus host transitions.
  • Endoparasite → infection (inside body).
  • Ectoparasite → infestation (on exterior).

“Big 3” Groups of Parasites Infecting Dogs

  • Helminths (worms): trematodes, cestodes, nematodes.
  • Protozoa: flagellates, amoebae, apicomplexans.
  • Arthropods: fleas, ticks, mosquitoes, lice.

Diagnostic Foundations

  • Fecal flotation (simple vs. centrifugal)
    • Differences: amount of feces, speed, efficiency.
    • Detects most nematode & cestode eggs, protozoan cysts/oocysts.
    • Samples can be older or refrigerated.
  • Flotation solutions & specific gravities (SG):
    • Sheather’s sugar 1.27 — broad, esp. coccidians.
    • Zinc sulfate 1.18 — \textit{Toxocara} spp.; good for fragile Giardia.
    • Sodium nitrate (Fecasol) 1.18 — most ascarids.
    • Saturated sodium chloride 1.33 — whipworms.
    • Saturated magnesium sulfate 1.32 — tapeworms.

Nematodes

Toxocara canis — Canine Ascarid

  • Habitat: small intestine.
  • Adult morphology:
    • Females up to 18\text{ cm}, males up to 13\text{ cm}.
    • Three prominent lips.
  • Egg dimensions: 75\times90\,\mu\text{m}; thick shell.
  • Lifecycle pathways:
    1. Eggs shed → larvate in environment.
    2. Larvated egg ingested.
    • Puppies: tracheal migration (liver → heart → lungs → SI) → adults.
    • Adults: somatic migration → encyst in tissues (hypobiosis) → possible later repopulation of SI.
    1. Transplacental: encysted larvae mobilize during pregnancy → fetal infection.
    2. Transmammary (rare but possible if dam infected during lactation).
    3. Paratenic hosts (rodents, earthworms) can harbor encysted L3.
  • Disease (esp. puppies):
    • Pneumonia, vomiting/diarrhea, intestinal obstruction, pot-belly, neurologic signs (aberrant migrations).
  • Diagnosis:
    • Fecal flotation → eggs.
    • Detection of adult antigen.
    • Adult worms sometimes visible in vomitus/feces.
  • Control/Prevention:
    • Daily feces removal; heat > 140^{\circ}\text{F} (60^{\circ}\text{C}) or dilute bleach on hard surfaces.
    • Treat pregnant & nursing females; rodent control.
    • Zoonotic risk (visceral/ocular larva migrans in people).

Ancylostoma caninum — Canine Hookworm

  • Habitat: small intestine.
  • Adult morphology:
    • Length 10–15\text{ mm}, reddish when engorged.
    • Buccal capsule with 3 pairs of teeth; males bursate.
  • Egg dimensions: 56!–!85\times34!–!47\,\mu\text{m}.
  • Lifecycle highlights:
    1. Eggs shed → L1–L3 development in environment → L3 hatches.
    2. Skin penetration by L3 OR oral ingestion.
    3. Puppies: heart–lung migration → SI.
    4. Adults: many larvae become hypobiotic in muscle (“larval leak”); can later repopulate intestine.
    5. Transmammary route: encysted larvae mobilize → milk → puppies.
    6. Paratenic hosts (rodents) possible.
  • Disease spectrum (puppies → adults):
    • Hemorrhagic diarrhea, anemia, poor appetite; may be subclinical.
  • Diagnosis:
    • Fecal flotation (eggs) or adult antigen tests.
  • Control/Prevention:
    • Sanitation (dry, clean kennels; heat, steam, flame; bleach on concrete).
    • Multi-drug resistance & persistent larval leak complicate treatment.
    • Treat dams during pregnancy/lactation.
    • Zoonotic: cutaneous larva migrans.

Dirofilaria immitis — Heartworm

  • Habitat: right ventricle & pulmonary artery.
  • Adult size:
    • Females 10–12\text{ in} (≈25–30\text{ cm}).
    • Males 5–7\text{ in} (≈12–18\text{ cm}), spiral tail.
  • Microfilariae length: 307–322\,\mu\text{m}.
  • Lifecycle (indirect, mosquito-borne):
    1. Microfilariae (mff) circulate in canine blood.
    2. Feeding mosquito ingests mff → develop to L3 in ≈2\text{ wk}.
    3. L3 deposited during next bite; reside in skin → molt to L4.
    4. L4 migrate (abdomen/thorax) → L5 (adolescent) \approx70\text{ d} PI.
    5. L5 reach heart/pulmonary artery & mature 6–7\text{ mo} PI (pre-patent period).
  • Key points:
    • Antigen positive before microfilariae detectable; “occult” infections common (antigen+ / mff−).
    • Single-sex or immature infections may evade detection.
    • Must differentiate mff from other filarids (e.g., \textit{Acanthocheilonema reconditum}).
    • \textit{Wolbachia pipientis} endosymbiont in all stages increases inflammation; doxycycline weakens worms and is integrated into therapy.
  • Pathogenesis / Clinical signs:
    • Villous endarteritis → vascular inflammation, resistance, increased cardiac workload.
    • Decreased cardiac output, exercise intolerance, possible Vena Cava Syndrome (massive worm burden in right atrium/vena cava).
    • Many dogs asymptomatic.
  • Diagnosis:
    • History + clinical signs, point-of-care antigen tests, microfilarial tests (direct smear, Knott’s, filtration).
  • Control/Prevention:
    • Heartworm found in all 50 U.S. states; year-round monthly preventives + annual testing (antigen & mff).
    • Treat diseased dogs to remove adults & mff to reduce community transmission.
    • Documented macrocyclic-lactone resistance in some strains.

Protozoa

Cystoisospora spp. — Coccidia

  • Species in dogs: \textit{C. canis}, \textit{C. ohioensis} complex, etc.
  • Habitat: small-intestinal epithelial cells; intracellular multiplication.
  • Oocyst size range: 17–53\,\mu\text{m} (species-dependent).
  • Direct life cycle:
    1. Unsporulated oocysts passed in feces.
    2. Sporulation in environment → infective sporulated oocyst.
    3. Ingestion → asexual (schizogony) and sexual (gametogony) development in enterocytes.
    4. New oocysts shed via feces.
  • Key concepts:
    • Requires sporulation outside host; therefore, daily sanitation breaks cycle.
    • Highly species-specific; adults often subclinical reservoir for puppies.
  • Disease (primarily young dogs):
    • Diarrhea, enteritis, weight loss, dehydration; adults typically asymptomatic.
  • Diagnosis:
    • Fecal flotation (oocysts), clinical signs, history.
  • Control/Prevention:
    • Daily feces removal; dry, clean kennels; steam/flame; high-ammonia disinfectants; seal floor cracks.
    • Treat adults as well as pups.

Arthropods

Rhipicephalus sanguineus — Brown Dog Tick

  • Family: Ixodidae (hard ticks).
  • Global distribution; present throughout U.S.; thrives indoors (kennels, homes).
  • Three-host tick — dogs commonly serve as all 3 hosts (larva, nymph, adult).
  • Preferred attachment sites: ear pinnae, interdigital skin.
  • Simple metamorphosis stages:
    1. Egg → 6-legged larva → feed → drop & molt.
    2. 8-legged nymph → feed → drop & molt.
    3. Adult → feed/mate on host; engorged female drops to lay eggs.
  • Medical/veterinary importance:
    • Can cause tick paralysis.
    • Vector for multiple pathogens:
    • \textit{Hepatozoon canis} (ingestion of tick required).
    • \textit{Babesia canis}.
    • \textit{Ehrlichia canis}.
    • \textit{Rickettsia rickettsii} (Rocky Mountain spotted fever).
  • Control measures: environmental treatment, topical/oral acaricides, regular inspection & removal.

Summary – Major Parasites of Dogs Covered

  • Nematodes: \textit{Toxocara canis} (ascarid), \textit{Ancylostoma caninum} (hookworm), \textit{Dirofilaria immitis} (heartworm).
  • Protozoa: \textit{Cystoisospora} spp. (coccidia).
  • Arthropods: \textit{Rhipicephalus sanguineus} (brown dog tick).

Ethical / Practical Takeaways

  • Zoonotic potential emphasizes One-Health responsibility; client education on hygiene & preventive medicine is essential.
  • Regular, accurate diagnostics (flotation, antigen, microfilariae tests) prevent advanced disease and limit environmental contamination.
  • Environmental management (feces removal, kennel sanitation, vector control) is as critical as pharmacologic treatment.
  • Drug resistance (hookworms, heartworms) demands conscientious dosing, compliance, and integration of non-drug strategies.