Worm Life Cycles and Life Stages

Roundworms typically have five developmental stages, namely, the first stage larvae (L1), the second stage larvae (L2), the third stage ‘infective larvae’ (L3), the pre-adult or fourth stage larvae (L4) and the adult stage. In most cases, the third stage infects the definitive host, and the later stages are parasitic. In most cases roundworms do not multiply within the host, so increases in worm burdens come from acquisition of extra infective larvae. Continued egg production increases the numbers of infecting larvae in the external environment.

Infection of the definitive host can occur in a number of ways:

• By ingestion of infective larvae, or eggs or cysts containing infective stages during grazing.
• By active penetration of feet and lower legs by infective larvae on pastures.
• From maternal infection during pregnancy and lactation.
• As a result of feeding activities of insects.

Once in the definitive host, development of the infective life stage proceeds to the adult stage with or without a migratory phase through organs and tissues of the host, until the final site is reached.

Roundworm: gastro-intestinal nematode life cycle

Roundworm: lungworm life cycle

Flukes have complex life cycles involving a short lived non-parasitic life stage, multiple life stages in a freshwater snail intermediate host, and a waiting life stage on vegetation growing in water. The stages in the freshwater snails undergo massive multiplication of numbers.

Fluke life cycle

Tapeworms have less complex life cycles than flukes, but invariably they involve an intermediate host in which larval development takes place. When cattle are the prey intermediate host for some larval tapeworms, infection of the definitive predator host involves ingestion of the infected carcass or organs.

The time from entry of the infective stage to reproductive maturity and egg laying is known as the pre-patent period.

Cattle tapeworm life cycle: adult worms in cattle (herbivorous)

Larval tapeworm life cycle: hydatids, beef measles and bladder worm (predator-prey)