Traditionally, worm control in dairy heifers involved regular and routine anthelmintic treatments, for example every four weeks, with four to six treatments per season.
It was recommended that treatment was followed by moving to a ‘safe’ pasture, which was considered to have a low burden of worms.
While such regimes have controlled gastrointestinal worms reasonably well, they have led to the survival of a minority population of resistant worms.
These surviving resistant worms subsequently parent the next generation of worms, leading to a majority population of resistant worms, where the original anthelmintic will no longer have an effect.
Think of this as inadvertent ‘genetic selection’ for the resistant genes, similar to how we select for specific traits in cattle.
More recently, the concept of ‘refugia’ has been proposed as the most important factor to slow the development of anthelmintic resistance.
The number of worms in refugia is the number that are not exposed to the drench.
Here the term ‘worms’ refers to all stages of the worm life cycle (eggs, larvae, immature and adult worm).
There are three main ways that worms can be in refugia (that is, not exposed to a drench):
- Un-drenched heifers, in a mob that have received drench.
- Inhibited larvae of some worm species, when they are not susceptible to a drench.
- Worms on the pasture cannot be exposed to a drench, so are always in refugia.
Worms ingested after a drench will dilute any remaining resistant worms, so these do not become a significant proportion of the total worm population in the heifer.
These susceptible worms contribute to the next generation of worms, delaying the development of resistance.
However, there is a conflict between managing drench resistance and gaining good worm control.
Maintaining too many worms in refugia to dilute drench-resistant worms also means more worms to infect heifers after drenching.
There needs to be a compromise to prolong the use of effective drenches.
Maintaining worms in refugia
In practice, we need to re-think the way worm burdens are managed.
In the past, drenching programs focused on elimination of infective stages.
Instead, we need to focus on maximising the contribution made by the susceptible worms to the next generation.
There are a few strategies to help achieve this:
- Treat and stay: Putting anthelmintic-treated heifers back on to known contaminated pasture for a short period, to expose them to susceptible worms in refugia. Unfortunately, the long duration of action of some anthelmintics will result in susceptible worms being killed and only resistant worms surviving to produce the next generation of worms.
- Move then treat: Heifers are placed onto a ‘safe’ pasture at least one week prior to drench treatment. This allows some pasture contamination by unexposed worms, to provide a small population in refugia.
- Selective treatment: Of heifers that show evidence of worm infestation (diarrhoea, ill-thriftiness, poor weight gain) and leave some heifers untreated.
- Avoid using: Long-acting anthelmintics or those that kill all developmental stages of worms in the animal. These anthelmintics will kill susceptible worms in refugia.
- Avoid treating: Adult cows that have developed immunity. These animals are largely resistant to worm infestations but will still have a worm burden to some degree. It is controversial as to whether such burdens affect productivity in early lactation.
Delaying drench resistance
In addition to maintaining worms in refugia, other practices to delay the development of drench resistance include:
- Avoid drenching more frequently than once monthly, unless there is a specific need, in consultation with your veterinarian.
- Quarantine all introduced stock and drench on arrival.
- Use effective combination drenches.
- Weigh animals prior to drenching to establish an appropriate dose.
- Administer the correct dose using the correct technique for the product.
Monitoring worm burdens
A commonly used strategy for managing worm burdens is ‘monitor and treat’.
Faecal egg counts, body condition, live weight gain, pasture length and predicted larval contamination of pasture are used to decide when to treat heifers.
A faecal egg count (FEC) estimates the worm burden in an animal. They can be carried out on individual faecal samples or pooled to provide an assessment of a group of animals.
It is recommended that FECs are performed prior to treatment to determine if drenching is necessary. FECs can also be used to assess the effectiveness of a drench.
Testing for drench resistance
A ‘drench check’ is usually carried out around 10 days after a drench treatment and should yield zero eggs in the faeces of 10 randomly selected treated animals.
If the results indicate a worm burden, then further testing may be required to check for the development of resistance.
Discuss the use of FECs and drench testing with your veterinarian.
This article has been adapted from an original Apiam article published in Dairy News Australia in November 2017.
