Sheep

Immunity of sheep to worm infection

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 immunity against barber's pole worms. More information.

Development of immunity

Immunity to worms is acquired in stages that produce the following responses by sheep.

  1. Firstly, fewer worm larvae eaten with the pasture are able to establish in the gut and become adults. In sheep with poor immunity, 50–60% of the worm larvae are able to become adults in the gut. In sheep with good immunity this establishment rate may be as low as 5–10%.
  2. Secondly, the immune response is able to reduce the number of eggs laid by established female worms.
  3. Finally, the immune response develops to a stage where established adult worms are expelled from the gut of the sheep over a period of days to weeks.

Immunity is rarely completely effective against worms, so even immune sheep carry a few 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) and black scour worm. 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 that 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 6–8 weeks after lambing). 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.

Immunity developed against black scour worm is able to persist, even in the absence of infection, for months. This is an important reason to provide good management to young sheep so that they better develop immunity.

Immunity developed against barber’s pole worm is more short-lived and may only last for weeks in the absence of infection.

Sheep have been vaccinated against worms experimentally. Since 2014 there has been a commercially available vaccine specific for barber's pole worm: Barbervax®.


Goats

Immunity of goats to worm infection

Immunity is the means by which goats become resistant to worm infection. The immune response by goats is complex and requires exposure to a sufficient level of infective worm larvae, and is not as well developed as with other livestock.

Immunity can be either innate or acquired:

  • Innate immunity is displayed in the first exposure to infective worm larvae and does not typically play a large role in resistance to worms.
  • Acquired immunity develops after exposure to worms. Adult goats do not necessarily develop the same age-related immunity as sheep.

Development of immunity

Sheep and goats, because of their divergent evolutionary histories, have developed different strategies to regulate their worm burdens. Sheep as grazers are continuously exposed to infective larvae on pasture and rely on an immune response to reduce worm establishment growth, and egg laying, and to expel worms. Goats, however, are browsers or intermediate browsers and rely on feeding strategies to avoid ingestion of larvae. Goats feed on a higher diversity of plants than sheep, are able to more rapidly breakdown toxins in plants, and seem able to self-medicate. By browsing, goats are feeding on vegetation free from larval contamination, as infective larvae remain mostly where humidity is higher around the base of the plant and up to about 10 cm from ground level.

Immunity to worms in goats is acquired in stages and results in reduction in:

  1. Worm development and growth of established female adult worms
  2. Female fertility and egg production

Goats, unlike sheep, seem unable to reduce the establishment of infective worm larvae or to expel adult worms (self-cure) from the gut.

Immunity is rarely completely effective against worms, especially in goats compared to sheep, so even immune goats carry some worms. The time taken for immunity to develop ranges from weeks to months or it may never develop. It depends on:

  • Age and nutritional status: older goats are more likely to develop immunity more quickly. 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. However, this response varies with worm type, being least for barber’s pole worm.
  • 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 the acquisition of good immunity.

In response to a worm infection, nutrients are diverted from growth, milk and fibre production to developing an immune response to worms. When selecting for worm resistance, it is important to make sure these goats are also productive. The heritability of a single worm egg count (WEC) in cashmere goats in the United Kingdom with natural infections of mainly brown stomach worms, is approximately 0.17, although with repeated WEC the heritability rose to 0.32. Under Australian conditions, heritability of WEC in fibre producing goats varies with age and type of infection but has been reported to be as high as (0.22) at five months of age during natural challenge with black scour worm.

Factors affecting immunity

  • Class of goat:
    • Age: young goats to about 18 months are more susceptible than older goats.
    • Sex: bucks are more susceptible than dry does or wethers.
    • Pregnancy and lactation: immunity in does is weakened during late pregnancy and early lactation. During this period, does can be as susceptible to worm infection as their young kids.
  • Breeding: using bucks with negative WEC Estimated Breeding Values (WEC EBV) will increase immunity and resistance to worm infection (see breeding for worm resistance).
  • Poor nutrition: goats need adequate nutrition (preferably adequate protein; see nutrition page) to develop and maintain immunity to worms.
  • Animals 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.