Before nematode ("roundworm") parasite control measures can be considered, it is important to understand the life cycle of these worms. The life cycle of most nematodes includes periods spent within the host and in the environment.
Figure 1. [http://www.acsrpc.org]
The host goat ingests infective stage-3 larvae while grazing. These larvae mature into adult worms, then mate in the host, and the females lay eggs that are passed in the feces. The eggs hatch and develop to infective larvae while remaining in the feces. The infective larvae then move out of the feces onto the surrounding forage(Figure 2) where they can be consumed during grazing thus completing the cycle. Some excellent animations of the life cycles of different parasitic worms in goats are available at http://www.goatbiology.com/animations/parasites.html .
The time from ingestion of infective larvae to egg-laying adults, called the prepatent period, is roughly about three weeks depending on the worm species and weather conitions. However, the time for development from egg to infective larvae can be as short as 7-10 days, especially during the summer months. Therefore,transmission and continual pasture contamination can be quite rapid. During the colder months, however, larval development on pasture is delayed and may take up to a month or two to reach the infective larvae stage. Thus pasture contamination and reinfection is minimized. The infective larvae have a protective sheath, making them relatively resistant to adverse environmental conditions. They can survive for months, thus extending transmission potential. As long as the temperature and moisture conditions remain warm and wet, especially following periods of substantial rainfall, development and survival continues and pasture contamination accumulates. But if the temperature gets too hot/cold and/or the moisture conditions become dry, development and survival are threatened and pasture contamination decreases. Transmission of parasites can be reduced by implementing control measures to eliminate the worms from the goat and/or reduce the chances that infective larvae have to reinfect the goat. Depending on the worm species, the time of the year that is most favorable for transmission varies. This will be addressed below.