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1. Where are worms when they are 'in refugia'?
2. What 4 characteristics are ideal to assess when classing sheep for resistance/susceptibility to flystrike?
3. How does resistance develop to lice treatment products?
4. How many drench groups are recommended for a quarantine drench for dairy goats compared to meat goats, and why is there a difference?
The term ‘worms’ is used here to refer to all stages of the worm: eggs, larvae, and immature and adult worms. When a drench is given to a mob, the main way that worms within the sheep or goats will be in refugia is when some animals are left undrenched. In contrast, worms on the pasture cannot be exposed to a drench, so are always in refugia. Likewise, on a property basis, worms in mobs not being drenched are in refugia, compared with those in mobs receiving a drench.
Within any population of lice there will be some able to withstand higher concentrations of insecticide than others. If lice are exposed to concentrations of insecticide that do not kill all of them, for example by under-dosing or poor application method, the more resistant individuals survive and breed, gradually increasing the level of resistance in the population.
Sometimes a genotype or mutation with high-level resistance develops enabling lice that carry it to survive properly applied chemical treatments. With continued use of the same chemical group, these more resistant individuals have a selective advantage and come to predominate in the population.
Discuss with your veterinarian which drench groups and how many can be used, their dose rates and withholding periods, including those drench groups not registered for use in goats, but which can be used with an off-label veterinarian’s prescription. The quarantine treatment should ideally consist of: