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Questions

  1. What are the factors that affect sheep susceptibility to lice?

  2. How many drench actives should be used in a quarantine drench?

  3. What should the dose rate of spray ons be based on? 

  4. What is the difference between goats that are resilient and those that are resistant to worms?


 

Answers

1. What are the factors that affect sheep susceptibility to lice?

Breed

There are large differences in susceptibility between breeds. Merinos appear to be more susceptible than many other breeds, but all breeds of sheep, including shedding breeds, such as Dorpers and Damaras, can carry lice.

Differences among sheep within breed

Individual sheep also vary in susceptibility. Some sheep do not become infested despite repeated challenge.

Age

Lambs are more susceptible to lice than older sheep. This emphasizes the need to avoid running untreated lambs together with recently treated ewes

Sheep health and nutrition

Heaviest infestations of lice are found on lambs with low growth rates and sheep under stress from poor nutrition or disease.

2. How many drench actives should be used in a quarantine drench?

A combination of no less than 4 unrelated drench actives with at least one of these being the newest drench actives: monepantel (Zolvix®) or derquantel (with abamectin—Startect®) should be used. This can be done using multi-active (combination) and/or single-active products concurrently—up the race with one product, then up the race again with the next. Do not mix different drenches unless the label states you can or under veterinary advice, as different products may be incompatible. If sheep have come from high rainfall (>600 mm) or irrigation areas in eastern states, consider a liver fluke treatment using triclabendazole.

3. What should the dose rate of spray-ons be based on?

Some products stipulate dose rates based on bodyweight, whereas dose rates for others are based on wool length. In mobs where sheep vary widely in bodyweight, cost savings, more efficient application and reduced residues might be possible if the mob is drafted into several weight classes. Otherwise, it would be prudent to set the dose to the weight of the heaviest sheep in the mob. Similarly, where dose rate is determined by length of wool growth, similar sheep should be drafted into treatment groups. If this is not feasible for mobs with mixed shearings or where unshorn young sheep vary by more than two months in ages it would be prudent to treat according to the longest wool length.

4. What is the difference between goats that are resilient and those that are resistant to worms?

Goats that are resilient to worms can grow and produce with less ill effects from worms while goats that are resistant to worms achieve lower worm egg counts by reducing worm development and growth, and egg production by female worms established in the gut. Reduced larval establishment and early expulsion of adult worms are not often observed in goats.