Almost all sheep have internal parasites. They often don’t cause any disease and sheep can look normal or even in good condition. However if worms get out of control, sheep can suffer from both subclinical disease (signs that can be measured but not seen visually) or clinical disease (signs range from very subtle to obvious, including death).
Table 1: Effects of worms on production of sheep
A sheep with a zero worm burden has no compromise to its production, wool, lamb or meat as a result. There is an impact on production from the lowest worm burden through reduced appetite, refocus of immune system to the parasite, damage to the gastrointestinal system and potential protein and blood loss.
The visual effects (clinical signs) are apparent when the worm burden becomes high and there is substantial reduction in production and a negative impact on the health of the animal. It is the non-visual (sub-clinical) effects which accounts for the majority of production loss in a sheep enterprise as it can go un-noticed over a long period of time.
COMMON WORMS AND LIFECYCLES
All roundworms begin as eggs. These eggs are produced by female adult roundworms living in the gut of the animal.
These eggs are passed out in the animal’s dung onto the ground.
The eggs hatch into larvae, given the right environmental conditions (sufficient moisture levels and temperature).
The larvae develop and become infective. They move up blades of grass in water droplets, where they can be eaten by grazing sheep.
Once ingested, the larvae enter the gut of the animal and either develop into adults quickly, or they may stay in an “inhibited” stage until a later time (for example, during a summer drought or winter cold). When the external conditions are favourable for worm survival, the inhibited larvae develop into adults.
The table below offers a summary of the most important roundworms of sheep in NZ
Affects all sheep irrespective of condition score, age or breed
Clinical and Sub-clinical production losses
Death through scouring
Rapid loss of condition and depressed growth rates, particularly in pre-weaned lambs.
Reduction in meat and wool production
WHAT IS DRENCH RESISTANCE?
Drench resistance is caused by internal parasites developing inherited tolerance to commonly used drenches. It results in animals with worms that don’t respond to drenching.
Theoretically, drench resistance occurs once a population of a species of worm can survive a dose of a drench that would have previously killed it. Initially resistant worms are rare in a population of worms. When a sheep is treated the resistant worms survive and, if they find a mate, can reproduce. The resultant offspring are resistant and if they survive as larvae on the pasture and infect another sheep. Over time, and with continued treatment, the overall resistance level to the treatment within the worm population increases.
Drench resistance is a major concern for NZ sheep farmers, resulting in production losses and increasing costs to control parasites.
THE CURRENT DRENCH RESISTANCE STATUS IN NZ SHEEP
2016-2017 national FECRT data from Gribbles Veterinary Laboratories