Salmonella is "slamming" cows on dairy farms in the South, Gore vet Karen Nicholson says.
Dr Nicholson, of Combined Vet Services, said she had heard "horrible" stories of the disease killing cows in southern Southland.
The disease had been "particularly bad" this season and it was an abnormal pattern, she said.
Cows ate the bacteria and, if they were under stress, the cows struggled to fight it in their guts and became sick, often dehydrated with acute diarrhoea. Death was the worst-case scenario.
Salmonella bacteria could enter a farm by an infected cow carrying it.
At a DairyNZ field day in Eastern Southland this month, farmers asked Dr Nicholson if the wet spring attracting more waterfowl to the farms could be the cause of the spread.
"We don't know. We think that maybe that birds are responsible, but we also think that there's probably carrier cows as well ... we just don't know," she said.
Ministry of Primary Industries had launched a survey to investigate why some farms got it and others did not.
