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Worm control in horses — Part 1 — it's all changed

Petrea Wait, District Veterinarian, Local Land Services Monaro

August 2018

Posted Flock & Herd December 2018

Once upon a time, every horse owner was taught that all horses should be wormed every six to eight weeks for the rest of their lives. This information was based on worm control programs designed over 40 years ago, when the most common and dangerous worm type in horses was the large strongyles (blood worms or red worms). Many things have changed since then, and that worming strategy is no longer recommended, and in fact, is causing more problems than it solves. 

The advent of ivermectin-based drenches has almost wiped out the large strongyles, but these have been replaced by the small strongyles as the worm type of greatest concern. Because this worm type has a different life cycle, the frequent use of drenches has led to the development of drench resistance in small strongyles, as well as other worm species. Drench resistance is caused by internal parasites developing inherited tolerance to commonly-used drenches and results in animals with worms that don’t respond to drenching. As a result, integrated worm control programs have been developed that enable worm control without the need to rely on frequent use of drenches.

The cornerstone of new worm control methods is performing worm egg counts/ faecal egg counts, the results of which are used to determine if your horse has worms, how many worms and what type of worms. It is now recognized that in adult horses 80 percent of the worm burden is carried by 20 percent of the horses in a population. An adult horse’s immune system keeps worm numbers in check, and a low worm burden is actually an advantage as it keeps the immune system primed to recognize and deal with worms as they are picked up from the pasture. Another advantage of not treating with drench when your horse has a low worm burden is that it means that there are worms in the horses environment that have not been exposed to certain drenches, helping to prevent the development of drench resistance.

It is the 20 percent of horses that carry high worm burdens that need to be identified and targeted for worm treatment as they are responsible for contaminating the pasture with worm larvae and may experience clinical disease. Many vet practices offer worm testing as part of their services. Horse worm test kits are also available from your Local Land Services office. Up to 10 horses can be tested, with a single sample costing $15.60 and up to 10 horses tested for $58.27.

Worm egg counts should be undertaken 2-4 times per year, more frequently for juvenile, aged, unwell horses, or those kept in large herds, particularly where the horses in the mob change often, such as at agistment centers and studs. Horses can then be classified based on the result as low egg shedders (<200 eggs per gram), moderate egg shedders (200-500epg) or high egg shedders (>500epg). The test can also tell you the type of worms present, be they large strongyles, small strongyles and / or ascarids. Pin worms, tape worms and bots are not identified reliably by a worm test.

Once you know how many worms your horse has and what type of worms, a drench needs to be chosen. There are four main categories of active ingredients in horse drenches:

Due to the development of resistance in worms to individual active ingredients, it is now recommended that combination worming products are used that combine active ingredients from two or more of the above groups. As praziquantel is really only effective against tapeworms, products that contain this ingredient should not be considered a combination, unless a third ingredient is present.

The time of year should also be considered when choosing a worming product. All horses should receive a broad-spectrum wormer that includes ivermectin and praziquantel in late Autumn to ensure that tapeworm and bots are eliminated at a time when horses are unlikely to become re-infested.  By contrast, a drench containing a “mectin” should be avoided in summer as bots are in the fly stage of their lifecycle and not present in the horse’s gut to be susceptible to wormer.

The age of the horse also needs to be taken into account when deciding if worming is required. Young horses are far more susceptible to worms than adults and require more frequent treatment. They are also susceptible to different worm species, particularly the ascarid worms, and drenching programs need to consider this difference. As foals and weanlings do not have fully developed immune systems the test and treat method of strategic worming is not suitable for this group. In the first year of life it is recommended to treat at 10-12 weeks of age, 5-6 months of age and at 9-12 months of age. For yearlings, worm counts are recommended to be undertaken every 6-8 weeks and treatment given as indicated.

The last, but possibly most important part of a parasite reduction program, is the management of the pasture to ensure that horses are not becoming re-infested with worms.  Pasture management aims to reduce the amount of worm eggs in the environment and has been found to be up to five times more effective than worming alone. Practices to achieve this may include:


Part two can be found here


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