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.

Wormwise Principles

Wormwise Principles
After coming across two situations in the last few weeks, which appear to be drench resistance, here is a timely reminder of the principle of sustainable worm management.

In one case it was a double active product that was not working on grazed dairy heifers, and the other was potentially a case of resistance to a triple combination in bought-in yearling bulls. Both the drenches used were purchased from Rural Retailers; which differ from the Vet Only products available from veterinary clinics.

In New Zealand products can be sold after only undergoing simple safety trials, and their efficacy does not have to be proven. Despite containing the same active agents, products can vary in their efficacy depending on the carrier agents, formulation etc. For example, consider the difference in the persistence of Cydectin injection for lungworm at 28 days, yet it lasts 42 days when given as a pour on. Once resistant worms have been brought onto a property they will persist on pasture even after the problem has been brought under control in the livestock, by suitable drenching. No drench completely eliminates the effects of pasture larval challenge.

The purpose of any worm management programme is to maintain or enhance profitability by:
- minimising contamination of pasture with infective worm larvae
- minimising uptake of infective larvae by susceptible stock
- monitoring the success of worm management strategies

Worm management strategies may include:
1) Knowing what is happening with worms on your farm with faecal egg counts (FEC), larval cultures and faecal egg count reduction tests (FECRTs). Worm numbers vary throughout the year with peaks in spring and autumn. Worms can survive months or even years on pasture. Cold weather slows worm development but does not kill them, direct sunlight will dry out and kill some eggs and larvae.

2) Manipulation of pasture and stock management plans to reduce exposure of animals to worms at key times. Most worm larvae are in the bottom 2 centimetres of pasture, so keeping pastures longer decreases risk. Fodder crops and tannin rich pasture, such as plantains, will reduce the challenge.

3) Stocking rates - the size of the worm problem largely depends on stocking density, so techniques can be used to mitigate this such as grazing young stock ahead of older animals, or alternating species i.e. using adult cattle to ‘clean’ pasture for lambs.

4) Ensuring animals are well fed and have adequate mineral status will reduce stress. Other stress factors that reduce immunity are concurrent diseases such as BVD, coccidiosis and pneumonia.

5) Genetics - there are differences between individuals and breeds in their resistance to worms; these differences are inherited and can be selected. Research is ongoing into breeding for resistant or tolerant animals and will become an important selection trait in the future.
Wormwise Principles Continued...

6) Drenching- there are numerous factors to consider;
• Adult stock should not require routine drenching, though in times of stress e.g. ewes in late pregnancy or early lactation, this may need modifying.
• Worms are only one reason stock may be thin or scouring, make sure you know what you are treating.
• Drenching intervals should seldom be less than 28 days (except in Barbers Pole or Cooperia outbreaks).
• Aim to keep drenching to a minimum.
• Consider stock age, class, condition, feeding levels and stress.
• Drenching needs to be combined with all the other worm management strategies mentioned above.

Agreed Wormwise Principles
• Healthy animals harbour worms and always will; eradication is neither an appropriate goal or achievable.

• Well-fed animals are less affected by worms than those under nutritional stress.

• Older animals are generally less susceptible to worms than younger ones, and at times can be used to reduce the number of infective larvae on pasture.

• Animals vary in their genetic susceptibility to worms.

• When breeding for a characteristic, intensive selection pressure will result in a more rapid change. This applies both to livestock and worms.

• Breeding for a single trait leads to a more rapid change than breeding for a selection of traits.

• Most of the year there are more worms, in various life stages, on pasture than inside the animals.

• Drench is a finite resource and should be used to give the best sustainable benefit to the farmer.

• The way in which drench is used and worms managed can change the rate at which resistant worms are selected.
Each farm is unique and effective worm management depends on knowing which worms are present and their resistance.
Long acting drenches may hasten the development of drench resistance if they deliver a low (sub-optimal) level of active ingredient over an extended period. Once present on a farm, worm resistance to drench is permanent.

Date Added: Wednesday, 2nd December 2015