Hundreds of sheep in Otago and Southland have died following an outbreak of Salmonella.
The bacteria causes pregnant ewes to abort their foetuses and may cause the death of the ewe.
The strain is salmonella brandenburg and it can be transferred to humans.
Clutha Vets veterinarian John Smart says about 30 farms are affected in the Balclutha area and he knows of others further south.
He says many farmers never report salmonella so the problem could be bigger than the 1000 or so sheep which he knows of that have died.
Mr Smart says there could more cases once lambing gets underway fully next month.
He says farmers should try to separate affected stock or spread flocks over a wider area.
Mr Smart says it's important farmers maintain vigourous hygiene practices of their own as salmonella can be spread to humans.
The Southern District Health Board says reported salmonella levels in humans are not of concern at this point.