Mode of Infection

Modes of Infection and Transmission

Parasites can enter and infect hosts through various routes depending on the type of parasite, its life stage, and environmental factors. Major modes include:

1. Ingestion

• Definition:
  Infection by swallowing infective stages like eggs, cysts, oocysts, or larvae through contaminated sources.
• Sources:
  • Contaminated feed, water, meat, aquatic vegetation.
  • Intermediate or paratenic hosts.
  • Colostrum and milk.
• Examples:
  • Fasciola hepatica (from aquatic plants).
  • Toxoplasma gondii (from undercooked meat).
  • Eimeria spp. (from contaminated feed/water).

2. Transmammary Transmission

• Definition:
  Parasite is transmitted from mother to offspring via colostrum or milk during nursing.
• Key Features:
  • Important in neonatal infections.
  • Seen especially in canines and felines.
• Examples:
  • Toxocara canis larvae transmitted via milk to puppies.
  • Ancylostoma caninum (hookworm) larvae transmission.

3. Skin Penetration (Percutaneous Infection)

• Definition:
  Parasite directly penetrates the skin to enter the host.
• Key Features:
  • Often involves infective larval stages.
  • Infection sites can develop irritation or allergic reactions.
• Examples:
  • 3rd stage larvae of hookworms (Ancylostoma spp.).
  • Cercariae of Schistosoma spp.
  • Larvae of myiasis-causing flies (e.g., Dermatobia hominis).

4. Inoculation

• Definition:
  Parasites are introduced directly into the bloodstream by vectors (usually arthropods).
• Key Features:
  • Transmission via bite of infected vector.
  • Important route for blood parasites.
• Examples:
  • Trypanosoma evansi via biting flies.
  • Leishmania spp. via sandflies.

5. Direct Contact

• Definition:
  Physical contact leads to the transfer of parasites between hosts.
• Key Features:
  • Common in ectoparasitic infestations.
• Examples:
  • Infestation by mites (Sarcoptes scabiei), lice (Haematopinus spp.), fleas (Ctenocephalides spp.).

6. Venereal Transmission

• Definition:
  Parasites are transmitted during sexual intercourse (coitus).
• Key Features:
  • Important in veterinary medicine (especially livestock reproduction).
• Examples:
  • Tritrichomonas foetus (causes bovine trichomoniasis).
  • Trypanosoma equiperdum (causes dourine in horses).

7. Congenital Transmission

• Definition:
  Parasites are passed from mother to fetus via the placenta.
• Key Features:
  • Leads to in utero infections.
  • May result in abortion, stillbirth, or congenital defects.
• Examples:
  • Toxocara canis larvae transmission in dogs.
  • Ancylostoma caninum (hookworm larvae).

8. Auto-infection

• Definition:
  The host re-infects itself, often through internal mechanisms like reverse peristalsis.
• Key Features:
  • Parasite completes its cycle within the same individual without external exposure.
  • Leads to heavy parasite burden.
• Examples:
  • Taenia solium (autoinfection by ingestion of eggs).
  • Hymenolepis nana (dwarf tapeworm).
  • Cryptosporidium spp. (intestinal protozoa).

9. Inhalation

• Definition:
  Parasite infective stages are inhaled into the respiratory tract.
• Key Features:
  • Involves cysts or oocysts suspended in dust or aerosols.
• Examples:
  • Cryptosporidium spp. (airborne oocysts in barns or animal sheds)

Parasitic Life Stages

Parasites have life cycles characterized by distinct phases inside the host, crucial for diagnosis and disease management.

1. Prepatent Period

• Definition:
  The period between initial infection and the first detection of the parasite stages (eggs, larvae, microfilariae, cysts) in the host’s body or excretions.
• Key Features:
  • No observable parasitic stages yet, but infection is ongoing.
  • Important for early diagnosis.
• Example:
  • Eimeria bovis (coccidia in cattle): 19–22 days.

2. Patent Period

• Definition:
  The period during which parasite stages are present, observable, and can be detected from the host.
• Key Features:
  • Active shedding or circulation of parasite forms (e.g., eggs in feces, microfilariae in blood).
  • Helps in confirmatory diagnosis.
• Example:
  • Eimeria bovis infection: Oocysts discharged for 2–3 weeks after prepatent period.

Master Summary Table

Topic	Mode/Stage	Key Features	Examples
Modes of Infection	Ingestion	Swallowing infective stages	Fasciola, Toxoplasma
	Transmammary	Through mother’s milk	Toxocara canis
	Skin Penetration	Direct skin invasion	Hookworm larvae, Schistosoma
	Inoculation	Bloodborne via vector	Trypanosoma evansi, Leishmania
	Direct Contact	Physical contact	Mites, lice, fleas
	Venereal	Through sexual intercourse	Tritrichomonas foetus
	Congenital	Through placenta	Toxocara canis
	Auto-infection	Self-reinfection	Hymenolepis nana, Cryptosporidium
	Inhalation	Breathing in infective stages	Cryptosporidium spp.
Parasitic Life Stages	Prepatent Period	Infection present, no detection yet	Eimeria bovis: 19–22 days
	Patent Period	Observable parasitic stages	Eimeria bovis: 2–3 weeks