[Prevention of gastrointestinal nematodes in calves in Belgium].

The prevention of infections with gastrointestinal nematodes has become an indispensable part of modern farm management in Belgium. It is important that for the choice of the correct preventive strategy the epidemiological situation on the farm is considered. Anthelmintics are mainly used in Belgium for the prevention of gastrointestinal nematodes. The mid-summer rise of the herbage infections can be avoided by either the continuous use or the regular administration of anthelmintics at the beginning of the grazing season. During the last 10 years controlled released devices (CRD) have been developed and made it possible to treat animals before they are turned-out. There are two types of CRD: the pulse and slow release devices, with the oxfendazole pulse release bolus (OPRB) and the morantel slow release trilaminate device (MSR) being used respectively in Belgium. In 9 field trials on commercial farms the effects of different preventive methods were studied. The most efficient preventive methods were the OPRB at turn-out or ivermectin at 3, 8 and 13 weeks post turn-out. Although the MSRT at turn-out and levamisole at 3, 6 and 9 weeks post turn-out apparently has less effect on the parasitological (faecal egg-counts) and serological (pepsinogen, gastrin and Ostertagia ELISA-antibody titers) parameters examined, both systems prevented clinical symptoms. However, the constant use of preventive methods could interfere with the development of immunity against gastrointestinal parasites and/or increase the development of resistance against anthelmintics.