Barber's pole worm

Barber’s pole worm (Haemonchus contortus) is a blood-sucking roundworm of sheep and goats, with minor crossover into cattle. It has a typical roundworm life cycle in that its eggs are passed in a sheep’s faeces, hatch and moult three times in the environment to an L3 form which is infectious and eaten by a sheep. It then develops to an L4 and adult worm in the abomasum.

Clinical disease is primarily seen in weaners and lambing ewes and is more pronounced with poor nutrition. Each L4 and adult worm may remove 30 μl of blood from a sheep per day, which quickly adds up if there is a significant worm burden, rapidly causing anaemia and possibly death when more than 500 worms are present. Clinical signs of Haemonchosis include anaemia due to blood loss with associated lethargy. Affected individuals may flop down when mustered briskly and have extremely pale inner eyelids and gums. All condition types can be affected including big, fat ewes. In more chronic cases you may see 'bottle-jaw' due to loss of blood protein. Diarrhoea is not a feature of the disease unless burdens of other worms are present.

Barber’s pole worm gets its name from the 'barber-pole' colouration of the female worm as its intestine and uterus intertwine. These two to three centimetre long adult female worms can be readily seen on a fresh post-mortem in the abomasal contents of an affected sheep. A sheep with Haemonchosis will have worm egg counts in the thousands as opposed to a sheep affected by the scour worms in which a worm egg count in the low hundreds is deemed significant.

As for other roundworms, to manage Haemonchosis treat with an effective drench, move onto a low-risk paddock and monitor worm egg counts. All broad-spectrum drenches as well as closantel may be effective, although anthelmintic resistance does occur.

Typically, barber’s pole worm’s environmental survival is favoured by warm, humid conditions. Compared with other roundworms, barber’s pole worms have a high fecundity, as evidenced by their high egg counts, and a short life cycle, meaning that worm burdens can build up relatively quickly. The high fecundity, short life cycle and high degree of genetic variation of barber’s pole worm means that it can make rapid adaptive changes and potentially rapidly develop resistance to drenches. This may help explain why we have seen changes in the biology of barber’s pole worm over the past few decades. Originally, barber's pole worm survived cold Victorian winters by arresting its development inside the sheep until environmental conditions were conducive for its larval survival. Modern drenches, which killed the inhibited worms, removed this survival advantage of remaining inhibited over winter. Consequently, selection pressure was applied for the adult barber's pole worm to complete its development through winter, for its eggs to hatch at a lower temperature and for it to have a shorter life cycle in the environment. This helps explain why barber’s pole worm has apparently become a winter parasite in Victoria.

Reference

Emery DL, Hunt PW and Le Jambre LF (2016) Haemonchus contortus: the then and now, and where to from here? Int J Parasitol 46, 755–769