The Ministry for Primary Industries is investigating an unexplained increase in the number of anaemic calves in Northland.
Vets in Northland report there have been an unusually high number of calves getting sick and dying from anaemia and that the cause is currently unproven.
However, vets are on the lookout for the parasite theileria, which they believe could be linked to the cases.
There is speculation that environmental conditions in Northland have led to a seasonal increase in the number of ticks, which spread the parasite.
MPI is releasing advice for farmers on its website.