The growing concern of anthelmintic resistance is not just a worry for sheep farmers. A number of cattle and dairy farmers are reporting that they are unsatisfied with the results achieved after worming cattle.
Regardless of the product of choice, it is important that the right dose of wormer is given correctly to ensure maximum efficacy. Recent surveys have shown that farmers could significantly delay the development of wormer resistance on their farms by worming correctly (Shalaby, 2013) Of critical importance is that the choice of wormer has no residual effect on the food product being produced thus choice of product must suit stage of lactation, for example, a zero milk withhold product allowing milk to enter the tank as normal.
What is Anthelmintic resistance?
A worm is considered to be resistant if it can survive exposure to the standard recommended dose of that particular chemical group of wormer*. Wormer resistance is heritable - the worm’s ability to survive is passed on to its offspring. Thus we need to prevent this happening.
*this refers to all presentations of anthelmintics: including drenches, pour-ons, boluses and injectables.
What has happened?
The original guidelines for worm control were drawn up nearly thirty years ago. Since then the pattern of parasite infections has changed, as has the type of products available. All of these affect the way that we treat worm infections. Climate change has brought wetter, milder weather fronts and increased grazing seasons with higher stocking rates has encouraged greater exposure to stock. Since temperature and moisture levels affect the development of infective larvae, this has also changed the pattern of worm populations. Due to milder winters, worms that do not survive well in very cold climates are now being found throughout Ireland. Infection by Cooperia (the small intestinal worm) is more common in cattle during the spring and early summer, whilst infection with Ostertagia (the small brown stomach worm) and Dictocaulus (lungworm) is often seen during the late summer and early autumn.
What can be done?
Bulk milk test screening can give an indication of level of infestation in the herd and can help in monitoring the burden level. Faecal sampling gives greater feedback on individual animals and helps establish profiles for individual or small group treatments by age or stage of lactations. Lung washes conducted by vets are also becoming popular to monitor lung worm burden. Results from any sampling taken on farm should form part of the consultation with your veterinary advisor on herd heath planning and preventative medicine use.
A recent European Medicine Agency report, dated 21st April 2017 (EMA/CVMP/EWP/573536/2013) highlights findings and recommendations to reduce the issues surrounding anthelmintic resistance. It highlights that long-acting drugs (e.g. long acting boluses) and pour-on formulations pose a particular risk suggesting that Pour-on formulations may be associated with substantial variation in drug exposure, i.e. there may be the risk of suboptimal treatment due to grooming behaviour, dirty coat or weather conditions therefor adding to the development of resistance.
Another recommendation is the introduction of targeted selective treatment at farm level which ideally should include a post treatment check-up, This may have economic benefits and labour benefits based on routine screening.
Farmers and their advisors should be prepared for possible legislation changes if the area of resistance keeps developing, for the time being it is important that farmers know what that are trying to treat, choose the correct product and ensure that the application of the product is accurate and does not pose a threat to the development of anthelmintic resistance.