Stock Sense with Paul McKee Ravensdown Technical Manager

Seasonal patterns in worm populations will see ostertagia numbers peaking on our pastures in the late autumn-early winter period.

Cattle grazing these pastures through winter in turn can acquire large numbers of these worms. A unique ability ostertagia have in controlling their life cycle means there are two types – Type I and Type II ostertagiosis – of disease, which if not controlled can severely reduce animal health and result in costly stock losses.

Normal circumstances will see ingested larvae develop through to adult worms just as in other worm species do, with their associated impacts on animal health and production. This is Type I ostertagiosis. For reasons not well understood, there are possible environmental and animal factors that cause ostertagia larvae to halt their development and enter an inhibited dormant phase.

Again, unknown triggers in spring can causes large numbers of these larvae to simultaneously ‘wake up’ and reemerge on the surface of the abomasum. This process can cause considerable damage to the abomasal wall leading to severe illness and rapid death, a condition is referred to as Type II ostertagiosis.

While many drenches have registered claims for activity against ostertagia, there are some differences that farmers need to be aware of when choosing what treatment to use in spring.

Importantly, ensure the product you use will control not only the active ostertagia, but also the inhibited form. The most effective drench actives for controlling all phases of ostertagia are the endectocide-macrocyclic lactones – called MLs – including moxidectin and abamectin. These actives combined with a good preventative worm programme means outbreaks of Type I and Type II ostertagiosis are now relatively uncommon in New Zealand. On the other hand, levamisole is known to not be effective enough in controlling inhibited ostertagia. Injections and pour-ons that provide persistent activity against ostertagia may be better than short-acting oral formulations using white and clear drenches.

With much of the discussion in recent times being around the control of ML-resistant cooperia worms in cattle, there is a chance we will forget the importance of ostertagia control. Ostertagia worms should still be considered the most significant parasitic worm within New Zealand cattle.