Long-Acting Worm Treatments- the Good, the Bad and the Ghastly

Long-acting treatments (injectables and capsules) were introduced as a convenient way of delivering drenches (anthelmintics and endectocides[i]) to livestock over a prolonged period. Producers need to be cautious though, because as well as selecting for resistance, long-acting treatments usually do not work as well as when first released more than 20 years ago. Use of long-acting formulations should be considered when other methods of worm control (paddock management, mob management, short-acting treatments) are not appropriate.

The keys to using long-acting products are:

• only use in the right circumstances and
• follow current best-practice methods of primer and exit drench treatments.

Capsules

At the time of introduction of capsules in the mid-1980s, the mectin class of drenches had only just been introduced and the focus was on extending the useful life of the white drenches (benzimidazoles or BZs). This was based on the findings that:

1. prolonged exposure to white drenches killed some resistant worms by paralysing their pharynx for long enough for them to starve
2. larval worms were more susceptible to the effects of benzimidazoles than adult worms
3. white drenches kill worm eggs, decreasing the amount of pasture contamination.

Capsules were developed for lambs, adult sheep and cattle, but in Australia, only the lamb and sheep capsules were marketed. Each capsule contained a dose (2.3g for lambs, 4.62g for sheep) of albendazole, divided into 6 pellets. Each capsule had wings that were held in place by a moisture-sensitive band for administration, but expanded once they were in place in the rumen. The product had a label claim to prevent pasture contamination with nematode (roundworm) eggs for 100 days.

Productivity benefits were measured in treated sheep and were summarised as:

• 5kg extra weight in ewes over the lambing period,
• 5kg extra weight in lambs at weaning as well as
• 5% increase in the number of lambs weaned[ii].

The manufacturers anticipated that prolonged exposure to the same active ingredient would select rapidly for resistance in the worms and this may have been the case. In response to this, other active ingredients, notably ivermectin and abamectin, were added, initially alone (Ivomec Maximiser) and then in combination with albendazole in the newer capsules e.g. Dynamax.

By this time, the most common worms already had some resistance to ivermectin and abamectin. However, due to the additive effect of the combination on efficacy and the ability of the BZs to kill worm eggs and larvae, they remained useful for preventing pasture contamination. The manufacturer announced that capsules will be discontinued in 2022, but expects that current stocks will last for existing users until the end of 2023 season.

Injectables

First introduced as a sheep product in the mid-1990s (Cydectin LA), moxidectin in a long-acting injectable 100mg/mL formulation was adapted for cattle as well. There are currently several different products with this formulation registered in Australia for sheep and cattle.

Other long-acting formulations of ivermectin and eprinomectin for cattle have been released overseas but were never available in Australia.

When first released over 20 years ago, long-acting moxidectin was capable of killing adult worms in the livestock, then preventing re-infection with larvae for up to 100-120 days (depending on product and worm type). Subsequent escalation of resistance means that on most properties today, a ‘primer’ drench is required to kill adult worms already present in the animals. In sheep, the 2 major worms that survive treatment with single-active moxidectin are barber’s pole worm and brown stomach worm, while in cattle, Cooperia (small intestinal worm) and barber’s pole worm are likely to survive.

Once the primer has killed most of the existing worm population, the injectable moxidectin circulates around the body for the remainder of the payout period and prevents re-infestation with new larvae.  This period is highly-variable, but worm egg counts can be used to check that:

1. the primer and LA injection have been effective in killing the initial infection and
2. the LA injection is still keeping worm numbers low. This also provides an idea of how long the LA treatment will last in subsequent years.

In sheep, the recommended times for testing are 30, 60 and 90 days post-injection, but this program can be modified to fit in with other management dates such as lamb-marking and weaning. In cattle, it is recommended to test treated mobs of cattle at the end of the payout period (up to 112 days for brown stomach worm, 120 days for barber’s pole worm), but may be much shorter. Three to 4 months is a good time to conduct follow-up testing in at-risk mobs.

Long-acting moxidectin products for cattle are also registered to control cattle ticks for up to 51 days. An exit drench should be used after this treatment as well, as it will select for resistance in internal parasites.

Primer treatment

In sheep, it is recommended to use a primer that kills 95% or more of the worms present. The primer should be selected based on resistance testing on farm, or one of the new active ingredients (monepantel, derquantel) in a combination product. In cattle, oral levamisole is usually used as an effective primer.

Tailcutter or exit drench

Once the larval worms have overcome the circulating moxidectin and established in the animals’ gastrointestinal tract, the eggs they lay will be partially or totally resistant to moxidectin. To prevent these resistant worms from becoming the dominant population on the property, a ‘tailcutter’ drench, also known as an ‘exit’ drench, is used at the end of the payout period. This can be timed for when worm egg counts rise back above a pre-determined threshold, often set as 100 to 150 eggs per gram (epg). In cattle, the appropriate threshold is expected to be about half that of sheep.

The exit drench will be a drench that is known to kill the type of worm surviving the long-acting treatment, preferably from a different class of chemical to the moxidectin. For sheep this is usually a product containing one of the ‘new’ active ingredients (monepantel or derquantel), while in cattle an oral treatment containing levamisole is used to good effect.

Ewes pre-lambing

Moxidectin is a highly lipophilic (fat-loving) molecule and is excreted into ewe’s milk. Injection of ewes with long-acting product pre-lambing results in both shorter duration of activity due to moxidectin being shed in milk and sublethal doses transferred to lambs. This will result in more rapid selection of resistant worms. It is recommended that ewes be injected at least 4 weeks prior to start of lambing.

The ‘ghastly’- when things go from bad to worse

Despite their convenience in allowing high-stocking rates, long-acting treatments are not without risk. Cases of high mortalities in sheep flocks have occurred only weeks after the use of moxidectin injection without primers, while onset of almost total resistance within 5 years has occurred on properties that have not used tailcutter drenches. Use in pre-lambing ewes can also result in short duration of activity and selection for onset of resistance.

A further complication is off-label use in goats which can result in high levels of residues in meat, threatening international trade.

Because of this, producers should always use long-acting treatments strictly according to label directions and consult wormboss or their local accredited advisor for a whole farm worm control program.

Summary

Long-acting treatments including capsules and injectable products can be used to allow efficient use of pasture in high-stocking rate systems, particularly when they have high levels of pasture contamination. When used effectively with a primer and tailcutter, they can be part of a sustainable worm control program. However, overuse, use too close to lambing and neglecting to use primers and tailcutters have resulted in problems with poor production and mortality, as well as rapid onset of resistance.

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[i] Endectocides target both internal (endo) and external (ecto) parasites. Mectins (macrocyclic lactones) fall into this category, but their efficacy against external parasites is limited in oral and injectable formulations.

Anthelmintics target helminths (flukes, tapeworms and roundworms). Albendazole is in this category and has good activity against susceptible roundworms but will kill only adult fluke and has only partial activity against tapeworms. See product label labels for details of claims and use only as directed.

[ii] Farquharson and Webster (1999) Assessment of Ivomec Maximiser administered to sheep on commercial properties, Proceedings, ASV Conference Hobart 16-21 May