Sheep drenches and drench usage

Drench Classes

Whilst there are a large number of commercial drench treatments available in New Zealand, all products belong to at least one or more drench classes/families (i.e. as a single active or combination drench). Development of new drench families is a complex process and happens rarely. It is therefore important we use best practice principles to ensure the longevity of the families that we have.

AAD’s and SI’s are considered ‘novel’ actives as they are the most recent drenches created. Strategic use of these drenches can help prevent and or delay the development of drench resistance on your farm.

Short acting drenches

Most farms use a short acting combination drench as their routine drench for their fattening lambs and replacement hoggets. This can be in the form of a double or triple (e.g. Troika^®) combination drench. Drenching can start prior to weaning or at weaning, and then continues at about 28-day drenching intervals until winter.

Combination drenches (more than one active) are usually more effective at killing worms and can help delay further resistance development, more than using just a single active drench. Short acting drenches kill the worms that are in the animal at the time of treatment and generally have no residual activity. Once animals start eating grass after drenching, they will get reinfected with larvae from the pasture again (if larvae are present)

The 28-day interval is based on the 21 day pre-patent period for most parasites, plus an extra week to allow some susceptible eggs to pass out of the animal. The pre-patent period is the time it takes for newly ingested L3 larvae to develop into adults and to start laying eggs which then pass out in the faeces.

The 28-day interval is only a guide and the actual start points, intervals and end point of the drenching program should be monitored and adjusted using faecal egg counts, weather information, farm system (use of integrated grazing) and animal performance information to determine the most appropriate routine drenching program for the farming system and for that year.

Strategic drenches

Strategic drenching is the planned use of drenches to manage or delay anthelmintic resistance, to keep routine drenches working effectively for longer. Strategic drenches include a Knockout, Exit and Quarantine drench.

Knockout drench

This is the substitution of a routine lamb drench to one that contains a novel active, prior to optimal conditions for larval survival and development on pasture. It is done to remove resistant parasites which may have survived repeated routine drenching and so prevent their further development on the farm. The Knockout drench used must be able to kill resistant parasites that might have survived the routine drench.

Exit drench after long-acting drenches

Long-acting drenches have persistent activity against certain parasites. This means they can kill the incoming larvae of those parasites for a number of days, reducing the effect they have on the animal and reducing that parasite’s contribution to pasture contamination for longer. However, if some larvae are resistant to the drug, they can survive the long acting treatment when those that are susceptible do not, causing an increase in resistant larvae on pasture if left unchecked. An Exit drench is therefore recommended after using a long-acting drench to remove any parasites that may have survived the treatment.

An effective Exit drench should:

• be a short acting drench,
• be a combination drench, and
• be able to kill any resistant parasites that could survive the long-acting drench.

Quarantine drench

The principle of quarantine treatment is to ensure stock coming on to a property do not bring on any resistant worms with them. All new stock brought onto a farm should be quarantine drenched and be held off pasture with food and water for 24-48 hours in order to empty out most of the eggs. They should then graze contaminated pasture in order to dilute any resistant worms that may have survived the drench. A drench check by faecal egg counting is advised 10 days later.

The quarantine drench used for sheep should:

• contain either monepantel or derquantel (Startect^®), and
• be different from the routine drench being used on the farm.