Economically, gastrointestinal parasites of cattle are among the most important causes of disease and production loss in cattle in Australia, especially in southern or temperate Australia (Sackett and others, 2006).
As with other grazing livestock, use of anthelmintics plays an important role in worm control, although integrated parasite management, including nutrition and grazing management, is vigorously promoted, but variably adopted.
In this paper we consider the impact of worms on weaners, the usefulness of worm egg counts, drench resistance in Australia, and the effect of route of administration on drench efficacy.
There have been two very interesting sets of Meat and Livestock Australia (MLA)-sponsored field trials with extensively grazed weaner beef cattle in south eastern Australia in recent years.
One was in the Central Tablelands of NSW (Eppleston, 2011) and the other was in Victoria (Rolls and Webb-Ware, 2011). My summaries follow.
The Central Tablelands trials were conducted in 2008/2009 on six farms, using weaner heifers. Basically the treatments were 'no drench', 'traditional' and 'worm-free', although only one farm had a 'no treatment' group. The heifers were followed for 12 months from weaning at around 7 months old in autumn. 'Worm free' animals were those suppressively drenched with moxidectin (Cydectin long acting injection) every 3 months. 'Traditional' (common practice) drenching was usually two short acting drenches; one at weaning plus a second one later. Approximate weight deficits over 12 months compared to 'worm-free' cattle were 60kg for the 'no drench' cattle and 30 kg for those receiving the 'traditional' treatments. Most of the deficit was in first 6 months post weaning. Worm egg counts (WECs) and pepsinogen estimations were relatively poor predictors of the impact of worms on production.
No resistance, as determined by worm egg count reduction tests, to benzimidazole group (BZ) drenches or full dose ivermectin was found but one herd had resistance to levamisole and 2 of 2 to half dose ivermectin.
In Victoria Rolls and Webb Ware (2011) surveyed weaner cattle on 16 farms in higher rainfall areas of Victoria. To examine the impact of worms, 'no drench' was compared with two drenches: ivermectin (IVM) or a benzimidazole (BZ) or a levamisole (LEV) drench: one at weaning (May), and the next in July. When weighed in November, the 'no drench' groups were 14-26 kg less than drenched groups. They also found that there were significant weight differences between treatments even at low WECs, e.g. as low as 50 eggs per gram (epg).
Worm egg count reduction tests were done on 10 of the farms. BZ resistance was found on one farm, LEV resistance on six, and IVM resistance on five. No abamectin resistance was found. Resistance was mostly in Cooperia, but IVM-resistant Ostertagia were found on two farms.
The study also considered minerals: selenium, copper and cobalt. In short, selenium deficiency was common, and there were significant production responses to supplementation with that mineral.
My 'take home messages' from these trials:
Generally, for both trials:
Most of the weight deficits occurred in the first 6 months post weaning
WECs are a poor predictor of weight loss due to worms in beef cattle, at least from weaning (7-8 months of age) onwards.
Anthelmintic resistance appears to be present, and not uncommon.
Do not rely solely on WECs in cattle. Also monitor productivity. If in doubt consider a 'diagnostic' drench: drench and then monitor a few cattle in a mob.
Producers need to have a good worm control strategy for weaners in higher rainfall areas. A WEC monitoring-based treatment program, which may be suitable for sheep and goats, is likely to be unsuitable for beef cattle, if one aims to optimize productivity.
Do not forget grazing management (lower worm-risk paddocks for weaners). Rolls and Webb Ware opined that a short-acting drench combined with good grazing management might be as efficacious as treating with a long-acting moxidectin injectable drench.
Cattle worm resistance has been known to occur in Australia for 20-30 years or more (e.g. Eagleson and others, 1986 and 1992), but was mostly regarded as a curiosity.
Resistance surveys in New Zealand, including that by Waghorn and others (2006), heightened interest in re-evaluating the situation in Australia.
The results reported by Waghorn and others (2006) are summarised in the figure below.
Notes. Source: Waghorn and others, 2006. Sixty two farms surveyed. BZ=benzimidazole, LEV=levamisole.
A number of trials have been conducted in Australia since then, some of the results of which have been published.
The figure below summarises results from Victorian (Rendell, 2006) and Western Australia (trials by Cotter, Besier and others (Cotter, 2012)).
Notes. Victorian trials: Rendell, 2010. Western Australian trials: Cotter, 2012. Ost = Ostertagia (small brown stomach worm). Coo = Cooperia (small intestinal worm). BZ = benzimidazole ('white'). LEV = levamisole. ML = macrocyclic lactone ('ML', 'mectin'). Oral formulations used, except injectable ivermectin in WA trials.
What can producers do now?
Check drench efficacy: periodically conduct a worm egg count reduction test when drenching cattle. This simply involves a worm egg count on 10-20 animals (preferably 15-20) at the time of drenching a mob, followed by repeat worm testing 14 days later, ideally with larval cultures and differentiation being done as well. If individual animals cannot be identified and sampled, bulk samples (collecting from many dung pats; >20) before and after testing is still worthwhile.
EFFICACY OF DRENCHES AND ROUTE OF ADMINISTRATION
Leathwick and Miller's (2012) recent paper further confirmed that route of administration of anthelmintics in cattle can have a marked effect on efficacy, and possibly also selection for drench resistance.
The efficacy of moxidectin administered by different routes - oral vs injectable (subcutaneous) vs topical (pour-on) - was compared. The authors state that moxidectin was chosen as the active solely because it was readily available in all three formulations and because it is an anthelmintic molecule for which considerable efficacy and pharmacokinetic data has been published.
Faecal egg count reduction tests were done on 14 commercial farms throughout New Zealand. On each farm, groups of 15 calves, with naturally acquired infections of gastrointestinal nematode parasites, were sampled for faecal nematode egg count and then treated with ivermectin administered orally, or with moxidectin administered either by the oral, subcutaneous injection or topical route. Samples were collected again 14 days after treatment. Efficacy was calculated as the percentage reduction in group mean egg count between the pre- and post-treatment samples. (Farmers were reluctant to have untreated controls).
To compare the variability of the different treatments, efficacy was also calculated for individual animals. To estimate plasma-moxidectin concentrations, untreated control groups were run on four farms and five animals from each of the control and all of the moxidectin-treated groups were bled at various times.
Averaged across all tests, the reduction in faecal egg count was significantly greater after treatment with moxidectin oral (91.1%) than following treatment with moxidectin injection (55.5%) or with moxidectin pour-on (51.3%).
Low efficacies in these trials were invariably against Cooperia oncophora. Efficacy against Ostertagia, where it was present in sufficient numbers, was generally high.
The oral treatments were significantly less variable in efficacy than the injection and pour-on treatments.
Moxidectin concentrations in plasma following treatment were highest with subcutaneous injection, next highest with oral administration (significantly lower than post injection and significantly higher than post-pour-on) and lowest following pour-on administration.
There was no evidence of transfer of moxidectin to untreated animals through licking.
Based on these results, along with those of other studies, the authors proposed that oral administration of macrocyclic lactone anthelmintics results in higher concentrations of active reaching the target worms in the gastrointestinal tract than following either administration by injection or by pour-on.
Their final comments at the end of the discussion section were:
"The results indicate that oral administration of MLs may be the most efficient at achieving high efficacy against some nematodes, especially Cooperia species. Further, the implications of the likely lower, and more variable, concentrations of ML reaching target sites following administration by the injection and pour-on routes on the selection for anthelmintic resistance warrant further investigation."
Dawbuts recently (December 2012) published a special newsletter on cattle drench resistance in feedlots in the USA and Australia.
This is interesting reading and can be found at Dawbuts website
It has also been published in a WormMail and can be found at WormMail
As with sheep, there is broad consensus among the experts that broad-spectrum drenches for cattle ideally should contain at least two unrelated actives. At the time of writing there is one such product on the market in Australia, Eclipse (Merial), which contains abamectin and levamisole. This however is only available at this stage as a pour-on. It has also been argued that abamectin + BZ might be a better choice than abamectin +levamisole, given the greater efficacy of BZs against Ostertagia compared to levamisole.
Gastrointestinal parasites are an important cause of loss in beef cattle in much of Australia, especially higher rainfall areas.
Much of the loss is in weaners, and in these animals most of the impact from worms likely occurs in the first 6 months post-weaning.
Worm egg counts and pepsinogen may be unreliable predictors of production loses from worms in cattle. While high egg counts are likely to be significant, low egg counts may not be insignificant. Measures of productivity (e.g. weight gains) and visual assessment should be used as well.
An appropriate worming strategy for weaners in higher rainfall areas appears to be better than an approach based on WEC egg count monitoring, even though the latter approach is often applicable in small ruminants.
Drench resistance is no longer uncommon in Australia and is likely to become an increasingly important problem. Hopefully cattle producers can learn earlier rather than later from the drench resistance problem in sheep production.
Producers should make regular drench efficacy testing a part of their management, noting not only the active used, but also the formulation and route of administration.
Macrocyclic lactones are the most commonly used cattle drenches in Australia, with pour-ons generally being preferred.
Topical MLs are easy and convenient, and may also help with management of some ectoparasites as well as endoparasites. The frequent use of topical MLs to specifically control ectoparasite (ticks, flies) is of concern because of the potential to select strongly for cattle worm resistance.
There is evidence that pour-on and injectable drenches can be somewhat less effective than oral drenches. Lower efficacy may result in higher selection for drench resistance.
BZ and LEV-based broad-spectrum drenches for cattle are commonly available as oral formulations in Australia. This is not the case for MLs. A recent check showed that the only oral formulations of ML cattle drenches were those containing ivermectin plus triclabendazole, a flukicide (i.e. Fasimec, Triclamec).
From a technical point of view, notwithstanding higher initial cost, it could be argued that the 'best' broad-spectrum cattle drench (everything else being equal) is an oral formulation of a drench containing multiple actives, with the actives having similar persistency and spectra of activity.
Some related WormMails (cattle: drench resistance; route of administration of anthelmintics)