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1. What is a primer drench, and when is it used?
2. What is the difference between worm resistance and drench resistance?
3. Why are lice usually seen in winter?
4. Why are blowflies not usually seen in winter?
A primer drench is a drench that removes any worm survivors of a long-acting (LA) drench treatment.
A primer drench is usually an effective short-acting drench (preferably a combination) that does not include the same group of drench actives as the long-acting product.
Its purpose is to provide support to the LA drench in an attempt to remove 100% of the worms present in the host at the time of the treatment so as not to further contribute to drench resistance.
A primer drench is applied at the same time as the long-acting product. The clue is in the name.
Livestock can be resistant to parasites such as worms, and worms can develop resistance to drug treatments such as anthelmintics.
Being ‘resistant to worms’ is a description about the host (sheep and goats), and its ability to control (resist) a parasitic worm population from establishing, developing and reproducing within it.
Drench resistance is about the worm itself and its ability to live through a drug treatment such as a dose of anthelmintic. A drench treatment can also be thought of in terms of a selection pressure applied to the worm population, as the ability to develop resistance is genetically based. Any worms able to resist the effects of the treatment are described as ‘drench resistant’.
Goats primarily retard the growth of worms and reduce the number of eggs they produce, whereas sheep are also able to reduce the establishment of larvae in the gut and expel already established adult worms.
During the colder months of the year, lice numbers tend to build-up more rapidly than during summer when it is a battle for lice to survive the extremes of temperature, solar radiation and moisture, and much of the lice population dies.
Shearing that occurs mostly in spring and summer, directly removes up to 50% of the lice population with the fleece. The remainder is left exposed in the short wool, and many more subsequently die. Lethal temperatures over summer kill lice eggs and adults, and thunderstorms can cause reductions in louse numbers of more than 90%.
The optimum temperature for lice survival is 37-39ËšC, which is the approximate skin temperature of sheep under most conditions. Lice can regulate their temperature conditions by moving up and down the wool fibres, but the extremes of heat in summer often cause temperatures near the skin to be about 45°C if the wool is very short, and they die. So it takes time for the population to build to detectable levels after the set-backs of summer.
In many areas of Australia, it is usually too cold and too windy. Also, in many places this year, it is too dry.
The risk of flystrike increases when temperatures rise above 17°C, wind speeds are moderate (<30 km per hour), and the fleece remains moist over a few days of rain.