Saliva test a sample of benefits

Dental cotton rolls are set to join lower exchange and interest rates at the top of sheep farmers' shopping lists.

Soon farmers will be buying them in their thousands to identify which of their sheep have the greatest resistance to internal parasites.

A simple new test means a saliva sample collected using a cotton swab can directly measure one of the main ways sheep naturally neutralise internal parasites.

From the middle of next year, CarLA, a test that detects a naturally occurring antibody that helps protect sheep from infection by worm larvae, will be on the market.

It will be welcomed by sheep farmers keen to identify and select breeding stock with both reduced faecal egg counts (FEC) and high performance.

AgResearch scientist Richard Shaw outlined the technology to the Institute of Motivated Farmers when they met in July. It was also discussed more recently in a paper he presented to this year's conference of the New Zealand Society for Parasitology held in Wanaka.

He says collecting the saliva sample is a simple and straightforward process, and for many will be much more enjoyable than collecting a faecal sample. Just open the sheep's mouth and, using a pair of long-nose forceps, place a dental cotton roll between the inside of the cheek and the bottom jaw. The cotton roll is similar to the one used by dentists.

The cheek pouch is then gently swabbed for 7-10 seconds and the roll removed. Because saliva contains several bacteria the sample will need to be frozen within eight hours.

Since discovering CarLA in 2005, the AgResearch team effort has focused on identifying what association the antibody to CarLA has with FEC.

Almost by accident, the discovery was made by AgResearch scientist Gavin Harrison during attempts to develop an antiparasite vaccine.

The research effort stepped up another gear in 2006 when IgG1 and IgA, two antibody types in sheep saliva, were found to be associated with reduced FEC.

Since then, a large-scale study involving more than 4000 animals in 20 commercial and research sheep flocks throughout the country has been done to determine its heritability and the correlations it has with FEC. More than 10,000 saliva tests were taken during an extensive validation process, with anti-CarLA antibodies found in all breeds tested, including Romney, Coopworth, Perendale, Composites and Merino.

It also confirmed the saliva test is capable of predicting reductions in FEC regardless of which of the various worm species, such as teladorsagia, haemonchus and nematodirus, are the dominant internal parasites in the gut.

CarLA is the result of five years of Ovita-funded research by AgResearch scientists now based in the state-of-the-art Hopkirk Research Institute on the Massey University campus.

Jointly owned on behalf of sheep and beef farmers by Meat & Wool NZ and AgResearch, much of Ovita's funding of the CarLA development has been matched dollar for dollar by the Foundation for Research Science and Technology (FoRST)

The research has also led to a greater understanding of the science underpinning a series of complex interactions that occur during the immune response to the CarLA molecule that follows when animals are fighting a challenge from internal parasites.

In short, the CarLa antibody prevents L3-stage larvae establishing in the animal's gut, resulting in fewer of them maturing and becoming adults. Also, with fewer adults in the gut there will be less pasture contamination. It is the adult parasites that shed the large number of eggs on to pasture which are then eaten by the sheep, continuing the cycle.

Shaw says that when L3s establish in the gut and mature into adults the immune system has to work harder, sapping the animal of some of its energy. When the numbers of L3s are reduced, this energy can be turned into productivity.

Forming a protective coating around the parasite, the CarLA molecule is found on the outer coat of infective L3 nematodes. It is thought to be a complex carbohydrate larval antigen, hence its name CarLA.

The coating is important for the survival of L3-stage nematodes. It acts as a shield from the elements, enabling them to last for many months on pasture. After being eaten by sheep, the parasitic worm larvae shed this protective armour, exposing the CarLA, which then stimulates the sheep's immune system resulting in antibody production.

This process occurs when the larva loses its CarLA coating during the first moult, which happens four to seven days after being ingested. Shaw says the coating is not found on the adult worms.

When sheep produce antibodies to CarLA they bind to the L3s, preventing them from establishing in the gut and causing them to eventually pass straight out in the faeces.

During the past five years, the research has shown lambs with elevated levels of the CarLA-specific antibody in their saliva have 20-30% lower FECs and better growth rates.

Compared with some other productive traits, the heritability of the response, 0.3, is high. It is similar to the heritability of growth and carcase traits and fleece weight.

Highly correlated genetically and phenotypically with FEC, use of the CarLA antibody test as a selection tool will result in reduced FEC, but not quite as rapidly as is achievable when using FEC-based selection. Viewed as complementary technology to FEC, Shaw says the difference is that the CarLA test provides farmers with a direct measure of their sheep's protective immune response.

"Over time this will speed up the development of their ewe flocks' natural resistance to internal parasites."

The test has several advantages over FEC-based selection: it is accurate, fast and easy to do; and there are no adverse correlations between CarLA antibody levels and productivity - sheep with higher antibody levels grow faster and have fewer dags than FEC-selected sheep.

The test is also equally effective on ewes and rams. One obvious application for commercial sheep farmers keen to push along their ewe flocks' resistance to internal parasites is using the test to select their replacement ewe lambs.

One of the advantages of the test is that it can give good results even if sheep are drenched regularly. There is no need to let FEC rise, and production suffer, to measure the CarLA immune response. For the test to be of most value, sheep will still need to be exposed to what Shaw calls a reasonable larval challenge.

"When sheep are drenched regularly, such as every 28 days, pasture larval contamination takes longer to build up, delaying build-up of the CarLA IgA antibody response."

In this case animals need to be tested later rather than earlier in the autumn to get best results.

Sheep can be sampled from five months of age, although the best results will be from April onwards when most animals will have faced a reasonable exposure to L3s. By this stage, sheep are already developing some degree of immunity, and about 30% or more of the flock can be expected to be producing the anti-CarLA antibody.

While more on-farm research and refinement is required, the CarLA test also shows promise as a farm-management tool. While it is early days, Shaw says initial paddock studies suggest the saliva test has real promise in helping farmers to assess the actual larval challenges present on their pastures.

Meanwhile the demand for dental cotton rolls is set to spike sharply.

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