Which of the following parasites has an indirect life cycle?

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Question restatement: Which of the following parasites has an indirect life cycle?

Option 1: Lucilia cuprina (Australian sheep blowfly). This species typically follows a direct life cycle where eggs are laid on hosts or carrion and develop through larval stages without an obligatory intermediate host. The life cycle does not require a separate organism to complete development, so this option does not illustrate an indirect life cycle.

Option 2: Ascaris suum (pig roundworm). Ascaris suum is classically a direct life cycle parasite in pigs, with eggs developing in the environment and infecting hosts without passing through an intermediate host. There is no obligatory intermediate host involved, so this option does not show an indirect life cycle.

Option 3: Fasciola hepatica (liver fluke). This parasite is well known for its indirect life cycle that requires an intermediate snail host. Eggs hatch in water, miracidia infect snails, multiply and produce cercariae that encyst as metacercariae on vegetation, which are then ingested by the final host. This two-host (snail then vertebrate) sequence is characteristic of an indirect life cycle.

Option 4: Haemonchus contortus (barber pole worm). Haemonchus contortus typically has a direct life cycle: eggs shed in feces develop to infective larvae in the environment, which are then ingested by the grazing host. There is no obligatory intermediate host in its standard life cycle, so this option does not represent an indirect life cycle.

Summary of reasoning across options: The key distinction is whether the parasite requires an intermediate host for development. Direct life cycle parasites complete development within a single host or in the external environment without a secondary host, whereas indirect life cycles involve one or more intermediate hosts (often a snail, insect, or other organism) that are essential for transmission. In this question, only Fasciola hepatica requires such an intermediate host (a freshwater snail) to complete its development, making its life cycle indirect, while the others do not rely on an obligatory intermediate host.

ANSC20003_2025_SM2 ANSC20003 - Topics in Animal Health (ANSC20003) - Mid-semester Exam

Which of the following parasites has an indirect life cycle?

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