Immunity is the means by which sheep become resistant to worm infection. The immune response by sheep is complex and requires exposure to a sufficient level of infective worm larvae.
Immunity can be either innate or acquired:
- Innate immunity is displayed in the first exposure of sheep to infective worm larvae and does not typically play a large role in resistance to worms.
- Acquired immunity develops in sheep after they have been exposed to worms. Most adult sheep have good acquired immunity to worms, whereas lambs up to 4 months of age do not.
With the availability now of the Barbervax® vaccine, sheep can also acquire enhanced immunity specifically against barber’s pole worms.
More information.
Development of immunity
Immunity to worms is acquired in stages that produce the following responses by sheep.
Watch the video extract from the ParaBoss Conference 2018 on the ‘Development of immunity to worms in sheep in different rainfall pattern zones’, presented by Dr Caroline Jacobson and Dr John Webb Ware.
Immunity is rarely completely effective against worms, so even immune sheep carry some worms. The time taken for immunity to develop can range from weeks to months and depends on:
- The age and nutritional status of the sheep: older sheep develop immunity more quickly and adequate protein nutrition also hastens acquisition of immunity.
- Previous exposure to worms: exposure to worms is needed for immunity to develop and to be maintained. Immunity acquired from previous exposure can be maintained for a considerable period and this speeds development of immunity to new infections.
- The number of worm larvae on pasture: there must be enough worm larvae to activate the immune response; low levels of exposure are insufficient to allow sheep to acquire good immunity.
When a sheep develops immunity to one species of worm it sometimes assists immunity to other worm species. For example, immunity to barber’s pole worm that resides in the 4th stomach (abomasum) helps with immunity to small brown stomach worm (and vice versa). However, immunity to black scour worm, which resides in the small intestine, does little to help immunity against these other worms.
In response to a worm infection, nutrients are diverted from growth and wool production to developing an immune response to worms. When selecting worm-resistant sheep, it is important to make sure these sheep are also productive.
Factors affecting immunity
- Sex: dry adult ewes have better immunity than wethers, which have better immunity than rams.
- Pregnancy and lactation: immunity in ewes is severely weakened during late pregnancy (up to 3 weeks before lambing) and early lactation (up to 8 weeks after lambing). This is due to the peri-parturient relaxation of resistance (PPRR). During this period, ewes can be as susceptible to worm infection as young sheep. Immunity is recovered in the late stages of lactation and is completely recovered after lambs are weaned.
- Breeding: using rams with negative worm egg count Australian Sheep Breeding Values (WEC ASBV) will increase immunity and resistance to worm infection (see breeding for worm resistance). Some sheep breeds are more resistant to worm infection, but there is large variation in resistance within each of the commonly used breeds in Australia.
- Poor nutrition: sheep need adequate nutrition (preferably adequate protein; see nutrition page) to develop and maintain immunity to worms. Sheep with higher growth rates that attain target weights or condition score more quickly tend to have better immunity. Severe weight loss or poor health because of other diseases will diminish immunity to worms.
- Stress: weaning and transport can reduce immunity to worms.
Immunity to different worm species
Given the right conditions, immunity develops to all major worm species. Importantly, there are some major differences in the period for which immunity persists against some of these major worm species.
Sheep have been vaccinated against worms experimentally. Since 2014 there has been a commercially available vaccine specific for barber’s pole worm: Barbervax®.
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