Infective stage of Hook worm is :

Question

Accepted Answer

The correct option is 4:

The infective stage of the hookworm, which includes species like *Necator americanus* and *Ancylostoma duodenale*, is the **Filariform larva**. The hookworm life cycle is critical to understand here. Hookworm eggs are passed in human feces and hatch in warm, moist soil into the non-infective first-stage Rhabditiform larva (L1). This larva, which has a rhabditiform (club-shaped) esophagus, feeds on soil bacteria and molts into the second-stage Rhabditiform larva (L2). The L2 then molts to become the Filariform larva (L3), which is the crucial transition to the infective stage. The L3 is non-feeding, elongated and slender (filariform), retains the sheath from the L2 molt, and is highly motile. This stage is uniquely adapted to survive in the environment while awaiting a host and is capable of actively penetrating the unbroken skin—most commonly on the feet—to initiate the human infection, making it the definitive infective stage.

The incorrect option is 1:

The Rhabditiform larva, often designated as L1 or L2, represents the free-living, non-infective, and feeding stages of the hookworm in the external environment. This larva has a muscular esophagus and a prominent buccal capsule, which are necessary for its diet of bacteria and organic matter in the soil. It is a necessary developmental stage but is not structurally equipped to penetrate the tough outer layers of human skin. Therefore, while it is a part of the hookworm's early life cycle, it is not the stage that can successfully transfer the infection to a human host.

The incorrect option is 2:

The Metacercaria larva is an entirely different parasitic form belonging to the phylum Platyhelminthes, specifically the class **Trematoda** (flukes), such as *Clonorchis sinensis* (Chinese liver fluke). This stage is characterized as a dormant, encysted larval form typically found embedded within a secondary intermediate host, such as fish, shellfish, or aquatic vegetation. It is the stage that is infective to humans and is acquired by ingestion, not through skin penetration. Its association with flukes rather than nematodes (hookworms) makes it an incorrect choice.

The incorrect option is 3:

The Cercaria larva is another stage found in the life cycle of **Trematodes** (flukes), not hookworms. It develops inside the primary intermediate host, which is usually a snail. The cercaria is a free-swimming, tadpole-like larva, possessing a distinct head and a muscular tail for propulsion in water. The cercaria generally leaves the snail and actively searches for a second intermediate host or directly for the definitive host, where it then transforms into the infective metacercaria or schistosomule stage, confirming it is not the hookworm's infective stage.