Watch the video extract from the ParaBoss Conference 2018 on the ‘Development of immunity to worms’, presented by Dr John Webb Ware and Dr Caroline Jacobson.
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 in goats 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:
- Worm development and growth of established female adult worms
- 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.
Tools