The unpredictably abundant rainfall in much of the north of the state and extending into the mid north, Eyre Peninsula and Yorke Peninsula this summer has provided ideal conditions for opportunistic parasites such as Barbers pole worm (Haemonchus contortus) to abound.
Sheep in pastoral areas are the most vulnerable as they don’t have acquired immunity afforded sheep repeatedly exposed to worms. However, Haemonchus are not discriminating and readily infest and kill sheep and goats regardless of their age, ethnicity, or spatial distribution.
The exceptional summer rainfall has enabled Haemonchus to appear in areas where it is not commonly detected. Its unique blood sucking capability combined with prolific egg production enables a rapid build-up leading to high mortality rates in mobs of sheep in as little as three weeks from a significant rainfall event. Survival of the worm in the environment is reliant upon warmth, shade and moisture and so lighter soil types where moisture persists is a preferred habitat. However, it primarily relies on surviving and multiplying in the gut of undrenched sheep.
Infected sheep usually have white conjunctiva and gums due to anaemia and bottle jaw is frequently seen. Lethargy, reluctance to move and sudden death are the other most common signs. There is also an increased likelihood of death when animals are stressed by droving or yarding.
Both the adult and larval Haemonchus are quite susceptible to most broad-spectrum drench groups in South Australia due to its spasmodic occurrence. Some drenches also provide persistent activity preventing re-infection in sheep for up to six weeks. This can be an advantage where sheep cannot be moved to low worm risk paddocks following drenching. However, the use of persistent activity chemicals is generally discouraged due to the risk of drench resistance developing.
Vaccines are also an effective means of control, but these are only cost effective and warranted in areas where Haemonchus frequently occur such as northern NSW.
Worm egg counting (WEC) is the most cost-effective means of monitoring the need for drenching and achieved by submitting 3-4 fresh faecal pellets in individual sandwich bags from each of 15-20 sheep per mob to your animal health advisor or direct to a WEC lab. Worm egg counting can also be used to determine which drench groups are effective in your flock by re-sampling 10 days after drenching to see if any worm eggs remain.
If worms are detected the infection is likely to include multiple species and so it is advised to use an effective triple active drench to cover these. For further details discuss with your preferred Advisor on sheep health.
Dr Colin Trengove
Pro Ag Consulting
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