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Risk Factors for Worm Infestation
All sheep are not the same, even if exposed to the same pasture that may be contaminated with worms. Sheep responses can be affected by:
- Age - young animals carry more worms and suffer more severe disease than older animals, partly as they have lower acquired immunity. Older animals are more resistant to worms as they have acquired some immunity through previous exposure.
- Nutrition - good nutrition is essential to help animals deal effectively with worm burdens
- Condition - pregnant and lactating animals have lowered immunity, thus they are less able to deal with worm infections
- Stress - stressed sheep have lower immunity and, as with pregnant and lactating animals, are less able to deal with infections.
- Breed/genetics - some animals have greater innate immunity, meaning they are better able to mount a challenge to worm burdens.
Other factors affecting worm burdens are:
- Pasture management - animals grazing heavily infected pastures will pick up greater numbers of worms
- Climate - Weather can affect both worm numbers and species present in a pasture. Warm and wet weather favours worm survival on pasture.
Specific examples of risk factors found on farms that predispose sheep to higher worm burdens are:
- Stress associated with management actions such as clipping, dipping, transport, gathering, handling, social isolation, etc.
- Stress associated with periods of indoor housing in comparison to pasture managed sheep (but depends on exposure)
- Stress associated with weaning in young lambs compared to animals that remain with their mothers for longer
- Being pregnant or lactating
- Subordinate animals that may have a lower feed intake, and are more likely to graze less-favoured areas of pasture with higher worm contamination
- Chronic lameness and other disease conditions resulting in pain and reduced feed intake
- Periods of poor feed availability and malnutrition
- Coping with periods of poor weather (very cold or especially hot weather), particularly when there is little shelter or shade.