Hundreds of sheep on up to 30 West Otago farms are believed to have died from a disease that causes ewes to abort.
Salmonella Brandenburg cases have surged, especially in the area between Waitahuna and Clydevale, and mortality at some farms has hit triple figures.
Confirming the details yesterday, Balclutha-based veterinary surgeon John Smart said it was far from an epidemic but some farms were caught in "a bit of a disaster".
Reports suggested 25 to 30 farms were affected in the latest spike of a blight that has lingered in Otago and Southland since 1997, Dr Smart said.
It had not infected all animals on all affected farms, but "when you are dealing with triple-figure losses at some places, then that's significant for individual farmers".
It was natural for disease to have have peaks and troughs.
It would be difficult to pin the latest increase on the climate, while there was very little stock movement in the area at this time of year.
Vaccination rates varied between farms as some considered it a costly insurance against a disease that might not be prevalent one year but which reared its head the next, Dr Smart said.
The biggest source of infection was from aborting ewes, but it could survive in places such as covered sheep yards for up to a year.
It could also be spread by livestock, farm dogs and machinery, or through water and the droppings of black-backed gulls.
Factors identified as predisposing ewes to Salmonella Brandenburg include feed changes, multiple-bearing ewes in late pregnancy, depressed immune systems, stress from the weather, stock movement or time spent in yards.
"It will continue to be something that we have to be aware of - and it's transmittable to man, so just about anywhere it can be passed to animals it can be passed to people."
Public Health South medical officer of health Derek Bell said the most recent statistics suggested there was no corresponding upsurge in Salmonella Brandenburg cases in people living in the area.
There were no reported cases in the past two weeks.
"There is a good degree of awareness in the rural community about this and other such diseases, so it is good news to see that something as transmittable as this does not appear to have been picked up by people."
A Ministry of Agriculture and Forestry spokesman confirmed monitoring and reporting of the disease was left to local veterinarians.