Nematode parasites are not a major cause of illthrift in adult ewes in New Zealand

Illthrift, defined as low body condition score (BCS), is a problem in adult ewes on many farms in New Zealand, and nematode parasites are often considered a significant contributing factor. Here we test the hypothesis that, over the lambing-lactation period, parasites are a significant causal factor in ewe illthrift, and that anthelmintic administration to low BCS ewes results in a greater improvement in condition than treatment of other ewes. Faecal nematode egg count (FEC) prior to lambing and ewe liveweight change over the study period were poorly correlated, and both were independent of pre-lambing BCS, as was ewe response to anthelmintic treatment. Some ewes lost, or gained, considerable amounts of weight independent of their BCS, the number and size of lambs reared, and anthelmintic treatment. We find no evidence that ewes of low BCS have higher FECs, or a larger response to anthelmintic treatment than ewes in better condition.