Factsheets - Sheep & Beef
The following fact sheets have been prepared by Anexa FVC Veterinarians as a guide to topics of interest. For specific information please contact your local vet.

Drenches and Drench Resistance

Drenches and Drench Resistance
Drench resistance is when previously susceptible worms in the animal survive a correctly applied, standard dose of drench. The resistant worms do not die, they survive and breed and are not controlled by the drench. Therefore the resistant worms gradually make up an increasing proportion of the worm population on the farm.
Worms resistant to one drench in a ‘family’ will be resistant to some extent to other drenches in the same family (side-resistant). Worms can also be resistant to more than one family (multiple resistance). Once developed on a property, resistance is usually permanent. There is also evidence that resistance to all drench families is increasing in NZ.

Risk factors for drench resistance
This section refers to sheep though similar principles apply to cattle.
Using long acting drench products pre-lambing carries a high risk in selecting for worm resistance
Docking treatment of ewes carries moderate risk and should not be necessary when ewes are well fed.
Routine drenching of all ewes increases the selection for resistant worms.
Generally it is better to only treat high risk animals such as ewes carrying multiples and ewes in poor condition - leave the rest un-drenched. This way there are always some susceptible worms to dilute and compete with the resistant worms.
Use narrow spectrum products such as Closantel to treat specific problems such as Barbers Pole.
Welfare can be compromised if animals go un-drenched when they are stressed by feed shortages, late pregnancy or other diseases.

Preventative drenching repeatedly from weaning is the basis of worm control in New Zealand Sheep farms. The aim is to control pasture contamination by treating the most at risk group. This minimises worm challenge to all sheep in the autumn and prevents production limiting disease in these lambs.
Intervals between drenches should be 28 days where possible to allow susceptible worms to re-infect the host and produce eggs. This results in a mixture of resistant and susceptible eggs and larvae on pasture. Care must be taken with this in the Barber’s Pole season.
Mixed grazing with older animals reduces pasture challenge and mixes any resistant eggs from the lambs with susceptible eggs from the un-drenched older animals.
Clean pasture carries a high risk for resistance as all eggs deposited on pasture by drenched lambs will be resistant. This risk can be reduced by leaving the heaviest 5% of lambs un-drenched, or mixing with un-drenched sheep before or after drenching.
Trading lambs carry a high risk of introducing resistance. As more than 60% of sheep farms have drench resistance of some sort, when large numbers of lambs are bought in from a number of sources, importing resistance is likely. Strict quarantine drenching with a triple combination and holding off pasture for 24 hours is vital.

Drench Products:
Single active product use selects more rapidly for drench resistance than the use of double or triple actives. ‘Saving’ a combination product for when the single actives have stopped working is not good practice.
Ineffective product - when drench resistance is established on a farm, continued use of an ineffective product will rapidly increase the frequency of drench resistant worms. Likewise products that are out of date given at the incorrect dose or not mixed properly will all select for resistance.
FECRT - the only way to definitively establish which drenches are working on your farm is to perform a Fecal Egg Count Resistant Test. This should be repeated every three to five years depending on results.
Drench Check- carrying out a drench test is a simple way of ensuring a particular product has worked on a specific group. A pooled sample composed of around fifteen different animals is checked for worm eggs ten days after drenching. With all products there should be no worm eggs present at this stage.

Date Added: Monday, 22nd February 2016