Choice of product for treatment of cattle worms requires consideration of a number of factors:
First determine whether it is roundworms or fluke that you wish to control. Roundworms in cattle are Australia wide. Liver fluke is common throughout Australia with the exception of Western Australia, the Northern Territory and parts of South Australia and Queensland. However, even on infected properties, transmission of liver fluke among stock is confined to areas of moisture where the intermediate freshwater host snails occur, and larval stages of the parasite can develop and survive (see the fluke life cycle).
The active ingredient(s) within an anthelmintic may have activity against parasites for which the treatment is not primarily intended. For example, it is preferable that worm control treatments in cattle be considered as a separate treatment to lice or tick control because individual treatments are likely to be needed at different times of the year. Consider other parasites when choosing products. Where possible, choose a product to address only the parasites of concern at the time of treatment, to reduce the risk of resistance developing from unnecessary treatments of non-target species.
Drench resistance to different chemical actives has been reported in different roundworm and fluke species Australia-wide. Liver fluke has been confirmed as resistant to the active triclabendazole. There is likely to be varying resistance in small intestinal worm (Cooperia species) to the ML group, with the most widespread resistance being to the older and most used drug versions (e.g. ivermectin). Although resistance has recently been reported in Victoria, on the great majority of properties ML’s still appear to be fully effective against small brown stomach worm (Ostertagia ostertagi). Established resistance does exist in small brown stomach worm to benzimidazole (white) and levamisole (clear) groups. Choosing a drench product with a combination of active ingredients may be needed to bring full effectiveness against these species.
Short-acting formulations available for cattle include the orals, injectables and the 14 to 28 day persistency pour-ons. Longer acting agents include greater length persistency pour-ons.
Indiscriminate use of long-acting products can increase the risk of developing resistance through unnecessary exposure of worm populations to a particular class of chemical. In general, orals, injectables or the short-acting pour-ons are preferred, with persistent products reserved for specific purposes where there are no other efficient management options. Persistent products provide a long time during which ingested resistant larvae can survive and reproduce. There is little need to use mid-length or long-acting treatments in cattle older than 15 to 18 months of age (provided they have adequate nutrition and are housed at an appropriate stocking rate), as most will have developed a strong immunity to worms.
It is essential to choose a drench with an appropriate withholding period (WHP) and export slaughter interval (ESI), according to the time left before the animals may go to slaughter, or their milk may be used for human consumption.