News & Advice

Worms and the weather

Nov 29, 2020 | Beef cattle, Dry stock, Sheep

By Lucy Scott, Veterinarian, Anexa Vets Raglan

In the Waikato, we tend to drench our youngstock on a 28-day cycle, but is this really necessary? In some seasons, it may need to be a shorter or longer cycle…

The lifecycle of a ruminant internal parasite consists of egg, larvae and worms. While there are many types of worm, the basic lifecycle is the same. Worm eggs pass out onto pasture in animal dung. The larvae develop in the deposited dung and become infective (L3) to animals grazing on pasture.

The number of larvae on pasture is affected by the weather. Warm moist conditions speed up larval development, resulting in greater numbers of eggs developing to become infective larvae and the cold, heat or dry slows down the development.

A temperature of 20-25 degrees creates the fastest growing larvae, but they also need water, so dry weather slows down their growth due to dehydration. This is why during drought weather we don’t often see clinical parasitism.

Many eggs and larvae are killed off during drought, except the Barbers pole worm (haemonchus contortus) of sheep are slightly different in that it can survive the heat. Its lifecycle can also be shorter than other parasites.

So on average, it takes around 21-28 days from when an animal eats a worm larva to when worm eggs appear in dung samples. This is called the pre-patent period, and we don’t recommend drenching 21 daily routinely as this can speed up resistance development (unless in a barbers pole risk area). We recommend doing faecal egg counts at the 28-day mark to see if drenching is required, sometimes we can push out drenching periods and save $ and labour.

Remember, if you are using long-acting drenches, this prepatent period is added on to the length of action of the drench.

Contact your local Anexa clinic for a free faecal egg count kit to help you start drenching strategically.

Share This