he Montana Department of Livestock (MDOL) has confirmed that an animal from a Gallatin County livestock herd within Montana’s Designated Surveillance Area (DSA) tested positive for brucellosis, marking the second detection of the disease in Montana livestock in 2025.
State officials said the affected herd has been placed under quarantine and an epidemiological investigation is underway to determine the source of the infection and ensure the disease has not spread to other herds.
A brucellosis investigation typically includes reviewing animal movement and conducting contact tracing with other livestock that may have had exposure to the affected herd. Investigations can extend as far as three years into a herd’s history. In this case, MDOL officials said the scope of the investigation is relatively limited because the herd does not utilize shared grazing, has few adjacent herds, and participates in voluntary annual whole herd testing.
Voluntary whole herd testing played a key role in the early detection of the disease. The brucellosis-positive animal was identified as a suspect during voluntary fall testing. The animal was pregnant at the time of detection and was removed from the herd before calving or aborting, which significantly reduced the risk of disease transmission within the herd.
Early detection also allowed MDOL to minimize the length and impact of the required quarantine, with no expected disruption to the operation’s routine management practices or livestock movements.
“Detections of brucellosis are inevitably burdensome for an operation,” said State Veterinarian Dr. Tahnee Szymanski. “Our goal at MDOL is that the work that DSA producers do on an annual basis can be leveraged towards a highly tailored and focused investigation that reduces the impact to an operation when brucellosis is found.”
Montana’s Designated Surveillance Area was established due to the ongoing risk of brucellosis spillover from infected wildlife, particularly elk, in the Greater Yellowstone Area. Brucellosis can cause serious reproductive issues in livestock, including abortions, stillbirths and neonatal mortality.
MDOL officials said program testing requirements are designed to ensure cases are detected before animals leave the DSA, helping to protect animal health and maintain confidence among Montana’s livestock trading partners. Early detection and containment are also intended to reduce movement restrictions for Montana cattle, particularly those originating in the DSA.
This Gallatin County herd is the 15th brucellosis-affected herd confirmed in Montana since the DSA was implemented in 2010. The most recent prior detection occurred earlier this year in Beaverhead County, also within the DSA.
While the source of infection for the Gallatin County case has not yet been determined, MDOL officials said DNA genotyping and epidemiological investigations have shown that the previous 14 cases were linked to transmission from wild elk. Past investigations have also confirmed that brucellosis has not spread to neighboring herds through fence-line contact.
By DNU Staff