Worm risk when rain comes

“Once it rains… if temperatures are still mild, worm populations can explode.” 

A high worm challenge will put pressure on young stock, so monitor the likely challenge by watching the weather and ideally faecal egg counting, she advises.

“Be ready to drench as worm burdens rise.”

While adult animals are generally more tolerant of worms, any in low body condition are already stressed so may also benefit from drenching.

Field warns against reaching for longer-acting drenches such as moxidectin without assessing which worm species are present.

“Long-acting drenches such as moxidectin are higher risk for resistance, and are often in a single active formulation, which is not recommended best practice; combination drenches are advised.

“Also, Moxidectin does not have the same long-acting activity against Teladorsagia (Ostertagia spp) and Trichostrongylus as it does against Barber’s Pole.”

If conditions are very warm – around 18 deg C - and wet, that’s when Barber’s Pole can reproduce rapidly.  A FEC and larval culture should be done to find out whether it’s prevalent.

“This will give you an idea of which worms to target. If Barber’s Pole is a good proportion of the worm population, then a long-acting drench is usually recommended.”

Fewer worms will have survived on pasture during the dry so particular care with drench use afterwards is needed to minimise the risk of selecting for resistance, stresses Field. 

Any worms surviving the drench are likely to be resistant and if worm numbers on pasture are still low because of the dry conditions there’s less competition or dilution from drench susceptible strains when the drenched stock are turned onto it. As the resistant eggs hatch, the majority of larvae ingested will be resistant. Once in the gut, they’ll mature to produce thousands more resistant eggs, resulting in a rapid rise in the resistant population on farm.

To avoid this scenario, Field recommends:

Drenching with a fully effective drench. “If you don’t know the resistance status of your farm use a triple combination drench, such as Coopers Alliance.”

If you do need to use a long-acting drench for Barber’s Pole, be sure to use an exit drench after this, such as a triple combination.

Note which parts of the farm were grazed by stock given the long-acting product and graze undrenched adult stock on them next to dilute any resistant worms that may have been shed.