When it comes to pre-breeding preparations, sheep producers are typically to the fore when it comes to administering health treatments such as mineral supplementation, liver fluke treatment, etc.
Unfortunately, needless worm control treatment of healthy mature ewes continues to crop up and, in doing so, increases the rate at which anthelmintic resistance is likely to develop, along with making an avoidable monetary and labour investment.
Worm control initiative
The rate at which resistance is developing on sheep farms is a huge cause of concern, with resistance now prevalent to the commonly used anthelmintic classes of Group 1-BZ (benzimidazoles or white drench), Group 2-LV (levamisole or yellow drench) and Group 3-ML (macrocyclic lactones or clear drench) products.
Teagasc teamed up with industry partners at the end of 2020 to highlight and combat the alarming rate at which anthelmintic resistance is developing on farms.
The initiative is being carried out in tandem with the Department of Agriculture and UCD, with support from Chanelle, Elanco, Norbrook and Zoetis.
healthy mature sheep will have developed natural immunity to worms and, as such, do not require routine worm treatment
This article is the fourth of a series exploring anthelmintic resistance with previous articles, which can be found here, covering worm treatment pre- and-post-lambing, nematodirus control and understanding pasture contamination.
Getting back to unwarranted worm treatment around breeding, as mentioned earlier, healthy mature sheep will have developed natural immunity to worms and, as such, do not require routine worm treatment.
The exception to this is the tiny number of flocks located in an area with a known haemonchus contortus (barbers pole worm) issue or treatments administered as part of a quarantine protocol.
Immunocompromised sheep may also be harbouring a higher worm burden and could benefit from treatment.
Teagasc sheep specialist Ciaran Lynch points out that it is important to identify why there may be sheep in the flock with compromised immunity.
It is important to differentiate the cause, as you also have to ask the question are these sheep really fit for breeding?
“The typical causes for compromised immunity include underlying health issues, inadequate body condition and delayed development of natural immunity in two-tooth hoggets which have reared lambs and faced nutritional challenges.
“It is important to differentiate the cause, as you also have to ask the question are these sheep really fit for breeding?
“Yes, of course worm treatment is necessary where it is required, but if it has been triggered by continual health issues or unexplainable factors, would these ewes be better served joining a cull group post-treatment? If treatment is required, then it should be confined to just the sheep that need it.”
In many cases at this time of year, sheep are treated for worms as a consequence of combination worm and flukicide products being used.
When questioned on their use, some farmers cite that they feel they are getting better value in purchasing a combination product and incorrectly assume that there is surely merit in treating for worms.
The added downside to such a scenario is that many combination products only offer protection against mature liver fluke parasites when the threat at present is typically from early immature and mature liver fluke parasites. Thus, there may be a double blow of unnecessary worm control and inadequate treatment for liver fluke.
Table 1 details the active classes of flukicides available as a standalone product and the stage of liver fluke each targets.
Acute liver fluke caused by the ingestion of large numbers of early immature larvae poses the greatest risk from September through to December.
This will transition to sub-acute fluke over the immediate months ahead and it is commonplace for acute fluke and subacute fluke to present a challenge at the same time.
Trichlabendazole products are the obvious choice, as they cover all stages
Therefore, it is important during this timeframe that products are used that at least cover immature and mature fluke stages of the life cycle.
Trichlabendazole products are the obvious choice, as they cover all stages, but some producers have resistance issues due to overuse and therefore caution may need to be exercised.
Using products that target only mature liver fluke stages during this timeframe leaves sheep heavily exposed.
The use of oxyclozanide-based products is generally advised against for fluke on the basis that it is the only ingredient class which has been shown to be effective against rumen fluke
Closantel and rafoxanide products target mature and immature flukicide, while nitroxynil--based products target mature fluke and are 50% to 90% effective against parasites aged six to nine weeks old. This is marketed through Trodax with production of this product ceased.
The use of oxyclozanide-based products is generally advised against for fluke on the basis that it is the only ingredient class which has been shown to be effective against rumen fluke and, therefore, there is an appetite at industry level to protect its use for this cause.
There is a perception that two-tooth hoggets that have reared lambs will need treatment for worms for a sustained period, but once they have been well looked after during lactation, these sheep will generally be capable of developing immunity relatively quickly once they have been weaned.
The decision to treat this cohort of sheep should be based on how sheep are performing and their condition.
Consideration should be given to aspects such as delayed weaning or insufficient grass supplies or poor grass quality being responsible for hoggets falling behind target
Ciaran says that farmers are generally focused on keeping up to date with worm control in finishing lambs and care should be taken not to forget treatments in replacement ewe lambs
Ciaran adds that not administering unnecessary treatments will help to maintain a population of susceptible worms in the environment and help prevent resistant worms multiplying over time and becoming dominant.
Ciaran says that farmers are generally focused on keeping up to date with worm control in finishing lambs and care should be taken not to forget treatments in replacement ewe lambs being joined with rams and also ram lambs running with breeding ewes as these animals may need subsequent treatment.
For flocks with lambs still on hand, it is not too late to carry out a drench test to determine the efficacy of anthelmintic classes.
The level of pasture contamination peaks from July to October. There are differences in the type of worm present but there is still a good opportunity to carry out a drench test.
This information will be invaluable for the coming season and putting a robust worm control programme in place.
The procedure for carrying out a test is laid out as follows:
1 Contact an approved lab to receive a sampling kit.
2 Select a group of 15 lambs at random and hold in a clean pen. Adhere to the following instructions:
3 Dose lambs with the chosen product, taking care to:
4 Post/deliver the sample as soon as possible. This should preferably be on the day of sampling and early in the week to avoid samples sitting in postal services over the weekend. Do not forget to include the relevant details, such as contact number, herd number, etc. If a delay is unavoidable prior to posting, place samples in a refrigerator/cool box. Do not freeze or leave samples in direct sunlight, as this will render the sample useless.
5 A post-dosing sample is needed for a repeat faecal egg count to check the efficacy of the product. This means that steps two to four will need to be repeated. The timing of the retest depends on the product as follows:
6 Calculate the percentage reduction as follows:
Note: the initial egg count would need to be in excess of 200 epg (ie a worm burden is present) to draw conclusions regarding product efficacy, if count is lower repeat at the next dosing interval.
7 Consult your vet or adviser to assist in interpreting the results and discussing control measures.