The worm life cycle describes the developmental changes worms undergo to produce the next generation of worms and to spread infection to new hosts, or re-infect the same hosts. Adult and immature life stages are involved.
Animals that harbour the adult and pre-adult life stages of worms are called the definitive or final hosts, whereas animals (including insects or snails) harbouring immature life stages are known as the intermediate hosts.
Worm life cycles that involve only a definitive host are said to be direct life cycles whereas those that use an intermediate host in addition to the definitive host, are said to be indirect life cycles. In a direct life cycle, infection is spread to new hosts by life stages that live on pasture in a non-parasitic (‘free living’) life stage that, after a period of development, is ingested by the grazing animal. Indirect life cycles have immature life stages parasitic in other hosts, but the spread of infection to new hosts may require non-parasitic life forms out in the environment, or predation.
Cattle and other animals can be both the definitive hosts for roundworms, tapeworms and fluke, and intermediate hosts for some larval tapeworms. Roundworms of the digestive tract usually have direct life cycles whereas tapeworms and flukes have indirect life cycles.
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:
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.
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.
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.