To WormMail mailing list (recip. undisclosed). WRML.20120606. sheep immunity to worms (McClure).
immunity, sheep, worms, McClure,
Following is a nice overview of sheep immunity to worms by Dr Susan McClure.
This appeared recently in ‘Plains Talk’ (Issue 2, February 2012), edited by Edward Joshua (NSW DPI). The current issue can be found here:
Susan McClure is a Research Scientist/Veterinarian working with the Central West LHPA in NSW. http://www.lhpa.org.au/districts/centralwest
Previously she was a research scientist working for CSIRO. e.g. see here: http://www.publish.csiro.au/?act=view_file&file_id=EA03002.pdf
Sheep immunity to worms – Dr Susan McClure
"Sustainable worm control programs should aim to optimize host immunity as one element of the armory. Immunity (or host resistance) is the ability of the sheep to block infection with worms and reject established worms. The outcome will be protection of susceptible stock and reduction of pasture contamination in successive seasons.
Immunity can be either innate
immunity is the inherent ability of some sheep and breeds to resist infection with worms.
immunity develops in sheep after they have been exposed to worms. Most adult sheep have good acquired immunity to worms, whereas lambs do not.
Acquired immunity takes several months to develop following exposure to worms. During this time animals, such as lambs, will be vulnerable to infection and disease. It is important to increase the immunity of young lambs to worms, especially before weaning. The sooner they become immune, the less they will need to be drenched. Immunity to worms can be improved by implementing some of the other arms of sustainable worm control. These include good grazing management & nutrition, breeding programs and reducing the number of drenches.
Immunity is rarely complete, so even immune sheep carry a few worms. However, immunity reduces worm numbers and production losses. How long it takes for immunity to develop depends on the age, liveweight and nutritional status of the sheep, whether it has been exposed to worms previously and the number of worm larvae it is exposed to.
Immunity to worms affects worm burdens in 3 stages:
* Fewer incoming worm larvae establish and become adults.
* Established female worms lay fewer eggs.
* Established adult worms are rejected by the sheep.
Exposure to worms is needed for immunity to develop and be maintained. This exposure must be large enough to stimulate the immune response, but not so large as to overwhelm the animals and cause production losses or disease. This is especially a problem with Barber’s Pole Worm. Lambs that are continuously drenched from birth in the absence of vaccination cannot develop immunity, and adult sheep moved from worm-free districts are similarly naïve and therefore susceptible. When a sheep develops immunity to one species of worm, especially Black Scour Worm, it acquires partial immunity to other worm species as well.
The development of an immune response to worms requires extra nutrients. In well-fed animals the development of immunity occurs over the first 6 weeks for Black Scour Worm, but takes longer for Barber’s Pole. In young worm-infected sheep nutrients are diverted from muscle and wool growth to developing an immune response to the worms. Once immunity has been established it is less sensitive to malnutrition, as the mechanism is a rapid hypersensitivity response which ejects incoming larvae very quickly. An exception is in ewes at peak lactation, where the demands of milk compete successfully and immunity wanes unless nutrition is improved.
Thus immunity to worms is influenced by a number of factors (predisposing factors). The first approach to sustainable control is to identify and remove the predisposing factors which slow down the development of immunity.
Individual animal-related predisposing factors include:
* Age – Adult sheep have better acquired immunity than lambs and recently weaned sheep, although this may be because of other associated factors such as available fat reserves rather than age itself.
* Sex – Dry adult ewes seem to have better immunity than males.
* Pregnancy and lactation – During late pregnancy and early lactation immunity in ewes wanes. This can be largely countered by feeding a higher quality diet, in terms of digestibility, bypass protein and/or readily available carbohydrate.
*Liveweight – In general, animals less than 23 kg at first exposure to worms have poorer immunity than heavier animals unless fed a very high quality diet (essentially a monogastric diet). Lambs less than 15 kg do not develop any degree of immunity. The exception is neonatal lambs, which appear to develop good immunity quickly if infected within a week of birth.
* Health – Sheep in poor health because of other diseases will have reduced immunity to worms. However, there is some evidence that concurrent mild inflammation of the gut can hasten the development of immunity to Black Scour worm. In this case the initiators of the gut inflammation were navy beans or polyunsaturated fatty acids from sunflowers (high in omega -6), while polyunsaturated fatty acids from fish oil (high in omega-3) suppressed gut inflammation and impaired immunity (i.e. increased worm egg counts).
* Breed and sire line – Some breeds and sire lines have better immunity to worms than others. However, some caution should be used when applying results of genetic selection beyond the managemental and environmental conditions under which selection was made. (This applies to other parameters as well as worm resistance).
External predisposing factors
* Inadequate nutrition – Sheep need adequate nutrition to develop and maintain immunity to worms, and in many districts problems are observed only in weaners when the feed goes off. Young growing sheep should be given priority access to high quality pasture or nutritional supplements when first exposed to worm larvae. Protein, energy, vitamins and minerals are all important for the optimal development of immunity. Poor seasons or grazing management may result in previously immune sheep becoming more susceptible to worms.
* Stress – Weaning, transport and inclement weather can all reduce immunity to worms. Weaning onto clean pasture limits pick-up of worms at a time when the lamb is particularly vulnerable.
Immune sheep are a good insurance against the developing resistance of worms to drenches, and are especially useful in breeding or wool enterprises. In addition, animals managed so as to be capable of developing good immunity to worms are also better able to mount immune responses to other diseases."
IGNORAMUS, n. A person unacquainted with certain kinds of knowledge familiar to yourself, and having certain other kinds that you know nothing about. – Ambrose Bierce, Devil’s Dictionary