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Questions

  1. Does the immunity developed by sheep against worms continue in the absence of infection?

2. Why worm test the lamb/weaner group separately from the ewe group if both age groups are pastured in the same paddock?

3. Why is it that a lice treatment can appear to be successful even though resistant lice are present?

4. Flies - where do they go in winter?


Temperatures consistently below 16 degrees are needed for a complete break in the fly season
Temperatures consistently below 16 degrees are needed for a complete break in the fly season

Answers

1. Does the immunity developed by sheep against worms continue in the absence of infection?    

Given the right conditions, sheep will develop immunity to all the major worm species. But there are differences in the persistence of this immunity in the absence of continuous infection. Immunity to the black scour worm will generally persist for a number of months even in the absence of infection whereas immunity to barber’s pole is much shorter-lived and may only persist for a few weeks in the absence of infection.

2. Why worm test the lamb/weaner group separately from the ewe group if both age groups are pastured in the same paddock?

Separate worm tests are important because the ewes may have low counts and the lambs/ weaners high counts.  A WormTest on a random sample from the paddock may report a low count whereas, in fact, the worm burden in the lamb/weaner group may be high.

Immunity to worms develops with age. Generally, older sheep have greater immunity than younger sheep except for the ewe during late pregnancy and lactation. If the ewes are still lactating, their immunity may be temporarily suppressed and they could be wormy, but after lactation has ceased, immunity quickly returns, and ewes rapidly lose their infections. Lambs and weaners, on the other hand, are unlikely to have developed substantial worm resistance.

If you have mixed age groups in a paddock, then, for accurate results, test each class/age group individually.

3. Why is it that a lice treatment can appear to be successful even though resistant lice are present? 

Sometimes, when resistance is present, treatment suppresses lice, but does not completely eradicate them, i.e. the treatment removed the majority of the infestation and the remaining “suppressed’ group are most likely to be resistant to the treatment. The suppressed infestations are difficult to detect and for this reason, the treatment appears to be successful. This ‘hidden’ population can increase the chance of resistant lice spreading between flocks, particularly on purchased or agisted sheep. 

4. Flies - where do they go in winter?

The parasite survives to the following spring by overwintering as ‘arrested’ larvae in the soil.

Winter daily maximum air temperatures must drop below 16°C for most days in an 8-week period to provide a complete break in the fly season. In these circumstances, no adult flies are present during winter, but the parasite survives to the following spring by overwintering as ‘arrested’ larvae in the soil.