Infective stage of Hookworm is:

Question

Accepted Answer

The correct option is 2: Filariform larva

The infective stage for the two main human hookworm species, *Necator americanus* and *Ancylostoma duodenale*, is the **Filariform larva**, specifically the third-stage larva (L3). The life cycle begins when eggs, passed in the feces of an infected person, hatch in warm, moist soil into the non-infective Rhabditiform larvae (L1). After feeding on bacteria and molting twice over about a week in the soil, the L1 and L2 larvae transition into the non-feeding, sheathed, and much more slender L3 Filariform larva. This stage is crucial because it is structurally adapted to be the transmission agent; it actively migrates to the soil surface and can survive for several weeks while awaiting a host. Infection is initiated when this Filariform larva comes into contact with and penetrates human skin, typically through the bare feet, causing the characteristic itchy papular rash known as "ground itch" or cutaneous larva migrans. Once inside the host's body, the larva enters the circulatory system to migrate to the lungs, is coughed up and swallowed, and finally matures into the adult worm in the small intestine.

The incorrect option is 1: Rhabditiform larva

The Rhabditiform larva is the first-stage (L1) larva that hatches from the hookworm egg in the external environment, such as the soil. It is the non-infective, free-living, feeding stage in the hookworm life cycle, characterized by a distinct rhabditoid esophagus. This larva must undergo development and two molts in the soil to transform into the infective Filariform stage. Therefore, while it is the first larval form in the sequence, it cannot penetrate human skin and transmit the infection. Its primary function is to grow and mature in the soil until environmental conditions or time trigger its development into the infective L3 stage.

The incorrect option is 3: Cercaria larva

The Cercaria larva is a life cycle stage belonging to the **Trematodes (flukes)**, which are parasitic flatworms, and not to the hookworms (nematodes). This is a free-swimming, typically tailed larval stage that develops within the intermediate host, which is usually a freshwater snail. The Cercaria is released from the snail into the water and is the stage that actively penetrates the skin of the definitive host, as is famously seen in the life cycle of the schistosomes (blood flukes). For other flukes like *Fasciola*, the Cercaria encysts to form a Metacercaria, but in all cases, it is entirely separate from the nematode-based hookworm life cycle.

The incorrect option is 4: Metacercaria larva

The Metacercaria larva, like the Cercaria, is a stage unique to the life cycle of **Trematodes (flukes)** and does not occur in hookworms. It is essentially the encysted or resting stage of the fluke life cycle, which the Cercaria forms after leaving the snail. This tough, walled cyst is found either encysted on aquatic plants (like watercress for the liver fluke, *Fasciola*) or within a second intermediate host (like fish or crabs). The Metacercaria is the infective stage for the definitive host when it is ingested via contaminated food or water, marking a pathway fundamentally different from the skin penetration used by hookworm Filariform larvae.