Animal Health

What’s in a drench?

By Gemma Chuck

Last month we discussed the common gastrointestinal worms and how they can affect dairy heifers. This month we focus on how gastrointestinal worms can be managed through chemical control.

The term ‘drench’ refers to the anti-parasitic group of chemicals called anthelmintics. It is quite an inaccurate term as it implies either thoroughly wetting the animal or giving something orally.

In the past this could have reflected the oral and pour-on methods of applying the anthelmintic but now they are available in injectable forms, the term seems a little out-dated. Flukicides are a separate group of anti-parasitic and are specifically designed for the control of liver fluke

The majority of anthelmintics registered for use in dairy cattle are considered broad-spectrum, meaning they are effective against a wide range of gastrointestinal worms and at different stages of their lifecycles. There are three classes of anthelmintic available for use:

1. Benzimidazoles (the ‘white’ drenches)

First discovered in the 1960s, the white drenches were highly effective against adult and immature gastrointestinal worms.

Unfortunately, their overuse has led to a resistance of gastrointestinal worms on up to 50 per cent of properties in some countries. The species of worms relevant to this problem in dairy cattle include Ostertagia spp., Cooperia spp., Haemonchus spp. and Trichostrongylus spp.

They are generally administered orally and have little to no residual effect. This means that a single treatment will kill the parasites present in the animal at the time of treatment, but it will not protect the animal against re-infestations.

More recently, the benzimidazoles have been used in combination drenches. An example of a white drench registered for use in dairy cattle is fenbendazole (Panacur®, Coopers Animal Health).

2. Levamisole (the ‘clear’ drenches)

Levamisole has been used for the control of gastrointestinal worms in dairy cattle for approximately five decades. It is effective against mature worms and larval stages of some species, although is ineffective against the arrested stage of Ostergia ostertagii which limits its sole use in certain regions of Australia during at-risk periods.

Like the white drenches, resistance is common particularly from Ostertagia spp. but also Trichostrongylus spp. Levamisole is generally available as an oral drench or a pour-on.

Pour-on products containing levamisole should be used with caution in hot weather as toxicity can develop due to rapid absorption. Levamisole has minimal residual effect but is frequently used in combination drenches.

3. Macrocyclic Lactones (the ‘mectin’ drenches)

In the 1980s the first macrocyclic lactone, ivermectin, was introduced and it revolutionised the control of veterinary parasites in livestock, horses and pets. They are highly effective against a broad-spectrum of gastrointestinal worms but are also classed as endectocides, meaning they are effective against ectoparasites as well (lice, mites, ticks).

However, they are ineffective against flukes (trematodes) or tapeworms (cestodes). Unfortunately, resistance of gastrointestinal worms to the macrocyclic lactones has been found in many species, including cattle.

In Australia, the worm species that have shown resistance include Cooperia spp, Ostertagia spp, Trichostrongylus spp and Haemonchus spp. The ‘mectin drenches’ can be administered orally, by injection or as a pour-on. They have a much longer residual activity than the other classes of anthelmintic, meaning they offer a period of protection after a single treatment.

This is due to them being stored in the body fat after administration and then released back into the blood over several weeks or months. Examples of macrocyclic lactones registered for use in dairy cattle include doramectin (Dectomax®, Zoetis), moxidectin (Cydectin®, Virbac) and eprinomectin (Ivomec® Eprinex®, Merial).

Combination drenches

In the past few years, the macrocyclic lactones have been used in combination drenches with the benzimidazoles and/or levamisole.

Research has shown that using effective combinations of two or more classes of anthelmintic at the same time helps prevent and slow the development of resistance.

By the concurrent administration of multiple anthelmintics, gastrointestinal parasites that have shown resistance to one of the classes of anthelmintic can still be targeted.

The best time to use a combination drench is before resistance has developed and while the effectiveness of the individual active ingredients is still high. Combination drenches registered for use in dairy cattle include Eclipse® (Merial) and Trifecta® (Coopers Animal Health).

Ensure your choice of anthelmintic is registered for use in dairy cattle and observe when and how the product should be administered and what the withholding period is. Some products are registered for use in dairy heifers and dry cows, but not for lactating animals.

As discussed, gastrointestinal parasites have shown resistance to all classes of anthelmintic on a global scale.

Whilst the use of combination drenches will help slow this process, we need to look at alternative methods to prevent the unnecessary use of drenches. Next month we will discuss non-chemical gastrointestinal parasite control in dairy cattle.

• Dr Gemma Chuck is a veterinary adviser at Apiam Animal Health.