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The Indirect Life Cycle

(492) This requires an intermediate host as shown in Fig.11-4. It commences as a direct cycle with the eggs leaving the pig with the first stage larvae developing. The egg containing the larva is eaten by a second host such as an earth worm or a beetle, where it undergoes a further two larval stages before finally becoming infective to the pig. The pig then eats the intermediate host and thus the cycle of reinfection is completed. An indirect cycle always requires another host for development before the larva can infect the pig. Removing or preventing access to the host breaks the cycle of infection.

The length of each cycle is dependent on the temperature and humidity of the environment. Eggs and larvae do not develop in cold conditions and most die in very dry conditions. This survival time outside the pig is important in controlling continuing infections. It also takes a number of days for the larva to develop inside the egg to the infectious stage. If faeces are removed from the environment before this development has been completed then the cycle is broken. The period of time taken for the larva inside the pig to mature to an egg laying adult is called the prepatent period. Fig.11-5 shows the prepatent periods and also the time of survival of the eggs and larvae in the environment.

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