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The signs of worm infection in sheep often go unnoticed until the problem becomes severe. As you can see in figure 1, by the time the signs of worms (clinical effects) the producer has experienced significant losses in productivity and breaks in wool.
Ewes
Around the time of lambing, ewes lose their acquired resistance to worms. This occurs from 2-4 weeks before lambing to 4 to 9 weeks after lambing, but may persist up to 14 weeks or more in individual ewes. This is brought on by the metabolic demands of the foetus and lactation and is not thought to be caused by the priority of the ewe to sustain the pregnancy and produce milk, which calls for preferential partitioning of protein. This phenomenon is called ‘periparturient relaxation of resistance’ or PPRR.
Because of their high egg output, lambing and lactating ewes require special attention given to their worm control. Many producers drench ewes with a long-acting drench one month before lambing commences. This is a good strategy to prevent pasture larval contamination, particularly when there is already contaminated pasture due to long grass, rain and cool weather. These conditions favour larval survival and will allow a build-up of larval numbers on pasture to dangerous levels.
Survival of barber's pole worm infective larvae on pasture at various daily maximum temperatures and 60% relative humidity
Source: Modeled from death rate of the L3 population in 'Simulation of pasture larval populations of Haemonchus contortus' by IA Berger. PR Benyon & WH Southcott. Proceedings of the Australian Society of Animal Production (1972) 9:38
Figure 1: Graph showing mortality of Haemonchus contortus larvae at different temperatures on pasture
High risk pastures are those that have been recently grazed by young stock (under two years of age), lactating ewes or sheep in low body condition score. These pastures are likely to have high larval worm burdens and so stock placed on it will pick up lots of worms. It is best to think of preparing a pasture for young stock (especially weaners) several months ahead of the weaning date.
Rams
Rams are sometimes forgotten animals when it comes to managing worms. However because they are often run in high stocking densities in paddocks without rotation they are exposed to high levels of worm larvae. Moreover, because of their high metabolic rate they tend to accumulate worm burdens that impact on their ability to produce quality sperm. Worm egg counts should be conducted at least 6 weeks prior to joining and rams drenched if required. Regular monitoring for the remainder of the year is also essential.
Find out more about Diseases that affect rams >
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