The worm is called Haemonchus contortus, but you’ll probably know it by another name: Barber’s Pole.

Female Barber’s Pole worms are about 2-3cm long, so although they’re quite small they are visible with the naked eye. They live inside the fourth stomach (abomasum) of sheep and goats, where they are attached to the stomach wall. 

Interestingly, if you look closely at an adult female worm, you’ll see a spiral of red and white stripes – a lot like the striped poles often found outside barber shops. This is the reason behind the name.

All of this makes for interesting dinner party conversation (perhaps only for vets), but those red and white stripes actually tell us two important things about the Barber’s Pole worm.

Firstly, the red stripe. This part of the spiral is essentially the worm’s digestive tract, and it is red because it contains blood from the sheep the worm is living in. This demonstrates the first important point – that Barber’s Pole worms are blood-suckers.

I sometimes talk to farmers who expect that sheep with a heavy burden of Barber’s Pole worms will have diarrhoea. In actual fact, Barber’s Pole worms don’t cause the irritation to the digestive tract that some other worms do, so diarrhoea isn’t a feature of disease.  Instead, anaemia (a low red blood cell count) is the main clinical sign.

Since red blood cells transport oxygen around the body, anaemia results in poor tissue oxygenation. Affected sheep become lethargic and weak, and often lag behind the mob if mustered. 

Severely affected sheep may have a “bottle jaw” – a swelling under the chin. With a high enough Barber’s Pole burden anaemia can develop rapidly, and in many cases affected sheep are simply found dead.

What about the white stripe? This part of the spiral is the worm’s reproductive tract, and it is white because it contains eggs. This is a clue to the second important point – Barber’s Pole worms lay a huge number of eggs.

Each female worm can lay up to 10 000 eggs per day. These are passed out in the sheep’s droppings, where they sit on pasture awaiting the right conditions to hatch.  If it’s warm enough and wet enough (4-10 days of >18C max temp, and >10-15mL rainfall) these eggs hatch and eventually develop into something called “L3 larvae”. 

These hatched larvae can then be picked up in a mouthful of pasture when sheep graze the paddock. They attach to the stomach lining, start sucking blood, develop into adult worms, begin to lay eggs, which hatch into larvae, and so the cycle continues…

Because Barber’s Pole worms lay so many eggs, paddocks can become “contaminated” with worm larvae very quickly, especially in a wet spring or summer. It’s these risky paddocks which must be avoided by susceptible sheep, such as weaners or pre-lambing ewes.

The number of larvae on pasture can be reduced by spelling the paddock or by grazing it with cattle.