Sunday, March 13, 2011

Wormer resistance in sheep.

As alpaca breeders, we must learn from other species, to reduce any losses, and years of problems that other breeders of others species have spent sorting these problems out.
 From time to time, I hear of alpacas becoming resistant to some wormers.
This is a serious problem if this happens, because then the wormers that are available are very harsh.
I hear all the time, we do not have worms on our property.
We test for worms, and they come zero.

Take this for a person who has to clear worms out of all breeding animals that are exported. You do have worms, and although they may be at a low level, they still need tobe treated.
Any time your animals come into it, sudden changes of weather, feed, or lack of, weaning, pregnancy, this is the trime, that worms do take over, and without a very keen sense of the body language of your alpacas, they can go downhill very quickly.
Here are a couple of very good articles on this problem

Wormer resistance in sheep
There are three different types of wormers available against gastro-intestinal
parasites in sheep.
These are:
Benzimidazoles (albendazole, fenbendazole, oxfendazole and mebendazole).
Macrocyclic lactones (ivermectin, doramectin and moxidectin).
Resistance to all three types of wormers has been recorded in the UK. In a recent survey in Wales,
82% of farms that took part had benzimidazole or levamisole resistance or both.
Wormer resistance does not suddenly appear on a farm, but the wormer gradually becomes less
Contact details
VLA, Weybridge,
New Haw, Addlestone,
Surrey, KT15 3NB
United Kingdom
Tel: 01932 341111
Fax: 01932 347046
E mail:
Please contact your nearest VLA regional laboratory for advice on tests and services
available. Contact details can be found on the VLA website.
It would be wise to check, once a year, that the wormer you are using is
working well in your lambs. There are two tests for this.
The faecal egg count reduction test
Ten lambs are selected, weighed and marked. Faecal samples (a minimum of 10-15
pellets) are obtained and then the animals are treated for their weight with the wormer
(checking that the dosing equipment is accurate). Faecal worm egg counts from these
sheep are compared with those obtained from the same animals taken a set time after
treatment. The time between samples depends on the type of wormer used. Sufficient
numbers of worm eggs have to be present at the start of this test, for it to be accurate.
Resistance is suspected when the reduction in average egg counts is less than 90%.
For benzimidazoles, faecal samples should be taken 10-14 days after treatment.
For levamisole, faecal samples should be taken 5-6 days after treatment.
For macrocyclic lactones, faecal samples should be taken 14-16 days after treatment.
Doramectin or moxidectin, (the longer acting MLs) should not be used in this test.
Larval development test
VLA offers this test. It uses pooled faecal samples from a number of lambs. If sufficient
worm eggs are present then these are tested for benzimidazole and levamisole resistance.
Macrocyclic lactone resistance cannot be tested by this method at present.
Contact your veterinary practitioner for further advice.
May 2009
Worm Resistance and How To Avoid It Disclaimer

There are many worming pastes and drenches available in Australia to treat the common worms in horses. Most of the newer products on the market are highly effective and safe, however, some worms have the ability to become resistant to worming drugs. If this occurs horses can carry heavy infestations of worms in their gut even though they are wormed on a regular basis.

How does resistance develop?   Top
The horse parasite that is the most effective at generating resistance is the Redworm or Small Strongyle. If a worming product (or a range of worming products that belong to the same family of drug) is used continuously on a property for a long period of time, there is a high likelihood that the redworms on that property may develop resistance to that family of drug, making the products ineffective.

Redworms have developed resistance to the Benzimidazole or BZ group of wormers in many parts of the world. If a horse from a property that has resistant worms is moved to a new property it can carry the resistant worm species with it and infect its new environment.

The macrocyclic lactones or “mectin” family of worming product is the most common class of wormer used in Australia today, including Equimax (abamectin), Equimec (ivermectin) and Equest (moxidectin). These are very good worming agents however, one of the major fears in the horse industry is that resistance will develop to these products and they will lose their high level of efficacy. “Mectin” resistance has already developed in some strains of sheep, cattle and goat parasites and it is feared that horse parasites may eventually be affected.

How can resistance be avoided?   Top
In order to avoid or slow the development and spread of resistance particularly to the “mectin” products, a range of management and worming suggestions has been recommended by leading parasitologists:
  1. Use an effective worming product
  2. Give the correct dose
  3. Rotate worming drug classes on an annual basis
  4. Avoid introducing resistant worms
  5. Use the minimum number of treatments
  6. Maintain good pasture hygeine

Use an effective worming product   Top
Do not use products to which resistance has developed. Continuing to use a product known to have generated resistance will only make the problem worse. Equimax is recommended as the ideal ‘mectin’ product as it treats all equine worms, including tapeworms with no reported cases of resistance.

Using mixtures of drugs (from 2 different families of worming products) has also been recommended to slow the development of resistance. For example, Strategy-T Paste contains a combination of Oxfendazole and Pyrantel that work together synergistically, achieving a much higher efficacy against redworms than when either drug is used alone. Strategy-T is proven to be highly effective against BZ resistant redworms.

Give the correct dose   Top
Modern worming products are usually safe compounds – it is better to slightly overdose than to underdose. Underdosing increases the risk of selecting for resistant strains of worm. If possible horses should be weighed before worming or their weight estimated with a heart girth tape to ensure that their weight is not underestimated.

Be careful to avoid horses spitting out paste wormers and therefore only receiving a part of their dose. Deposit the paste over the back of the tongue, not between the cheeks and the teeth, which makes it easier for the horse to “slobber” the dose out. After dosing, hold the horse’s head up for 15-20 seconds and rub the throat latch area to stimulate the swallowing reflex.

Rotate worming drug classes on an annual basis   Top
A slow (yearly) rotation of worming drug classes has been suggested as a means of slowing the development of resistant worms in sheep, goats and horses. However, it is very important that you rotate to a different chemical family, not to another product within the same family e.g. there is no advantage in rotating from one “mectin” wormer to another “mectin”.

Strategy-T Paste being a combination wormer containing an active ingredient from two different classes of worming compound is an ideal worming product to use in an annual rotation program with the “mectins”.

Avoid introducing resistant worms   Top
New horses arriving on a property can carry worms that are resistant to certain worming products. It is therefore good policy to worm all new arrivals to a property with an effective all-wormer (e.g. Strategy-T Paste or a “Mectin” product) and to keep the new horses off pasture for 48 hours to reduce the risk of introducing resistant worms from another property.

Use the minimum number of treatments   Top
The more frequently horses are wormed the more likely it is that anthelmintic resistance will develop. However, if treatments are too infrequent pasture contamination with worm larvae and eggs will not be controlled. The ideal frequency of worming varies depending on the age of the horse, the housing conditions and the level of contamination of paddocks or yards where re-infection of the horse can occur.

As a rule of thumb stabled horses should be wormed out when they first come into work and again 3-4 weeks later to remove the redworms that have been released from the bowel wall after the first worming. Following these initial treatments a regular worming every 3 months should be sufficient. Horses housed in a paddock or large yard should be wormed every 6-8 weeks, as they are more likely to become re-infested with worms while grazing. It is important to remember that up to 99% of worm larvae exist on the pastures and only 1% are actually in a horse.

Ideally you should consult your own vet for advice to determine how frequently you need to worm on your individual property.

Maintain good pasture hygeine   Top
Pasture and yard hygiene is a very important part of worm control and can decrease the frequency at which you need to worm your horses. Manure should be collected at least once daily from stables and small yards, while in larger yards and small paddocks twice weekly manure collection is recommended.

Frequent manure removal will help to decrease the risk of feed contamination with worm eggs or larvae. Manure should be composted out of reach of horses and fresh, non-composted manure should not be spread onto paddocks grazed by horses. Alternate grazing of pastures with cattle or sheep or prolonged destocking of the pasture will also help to reduce reinfection rates and decrease the frequency of treatments needed.

By following these management practices, using effective wormers and rotating worming classes on an annual basis, we can hopefully slow the development and spread of resistance in redworms to worming compounds. This is particularly important as no new families of worming drug are thought to be close to commercialisation and we need to prolong the effective lifespan of the existing products for as long as possible.
Article courtesy of Dr John Kohnke from ‘Health Care and problems of Horses, 9th edition’ published by Virbac-Vetsearch.

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