Effects of molybdenum intake on primary infection and subsequent challenge by the nematode parasite Trichostrongylus colubriformis in weaned Merino lambs

In a study designed to test the effect of molybdenum (Mo) on resistance to trichostrongylosis, the Mo content of the diet during primary infection of 8-month-old Merino lambs with Trichostrongylus colubriformis was found to affect the number of worm eggs found in the faeces during challenge and the total number of worms in the jejunum after six weeks of challenge infection. The optimal intake of Mo in this experiment was in the range of 4-8 mg sheep(-1) day(-1), approximately equivalent to feeding a diet containing 6-10 mg Mo kg(-1) dry matter (DM) and to 0.15-0.30 mg Mo kg(-1) liveweight. Lambs fed Mo at this rate showed a 90 per cent reduction in faecal egg count and total worm count six weeks after challenge compared with lambs fed quantities of Mo outside this range. The mechanism by which Mo exerted these effects was not defined, although the interactions of molybdenum, worm establishment and faecal egg count suggested that this trace element may be acting via an effect on the host's acquired immune response. This hypothesis is supported by the observed enhancement of immune responses (intestinal antibody and granulocyte numbers and in vitro worm-specific proliferation of lymphocytes) associated with Mo intake of 0.15-0.3 mg kg(-1) Lwt.