LANSING, Mich. — A northern Michigan dairy herd is quarantined after animals tested positive last week for bovine tuberculosis (TB).

State agricultural officials said the disease was found in a medium-sized Alpena County dairy herd during routine testing conducted by the Michigan Department of Agriculture and Rural Development (MDARD) and the USDA.

“This herd got the disease presumably from deer overlapping with cattle on pasture, drinking water sources or feed,” said Dr. Steve Halstead, chief animal health official for the state of Michigan. “We have steps in place to try to minimize that exposure, but it’s not an exact science.”

The herd of more than 100 milking cattle is quarantined and no cattle may enter or leave the farm premises until testing clears it, according to Halstead.

“We are working with the owner to get them back to TB-free status,” he added. “We will test the whole herd on a regular basis and remove any that react to the test. We will do that until we no longer have positive animals show up in the herd. Then there is a period of time we have to wait before we can release the herd.”

Since 1998, MDARD and USDA have detected 55 TB-positive cattle herds and four privately owned cervid operations in the northern section of Michigan’s Lower Peninsula (L.P.). Of those, more than one-third of the farms are in Alpena County, which is located in the state’s modified accredited zone.

Bovine TB is an infectious disease caused when bacteria attack the respiratory system. It is close to being eradicated in the United States, but still poses a significant risk to domestic livestock in areas where they coexist with wildlife.

Since the bovine TB eradication effort began, all of Michigan’s 14,000 cattle farms have undergone TB testing. Investigation is done by a team comprised of representatives from several agencies.

Routine testing helps officials discover the disease early, but one infected animal can affect the entire herd.

During testing, veterinarians inject a small amount of tuberculin into the cow’s tail and 72 hours later, they revisit the animals to see if the injection reacted with swelling or redness.

If so, a secondary test is performed. This may be a blood test or another type of skin test.

Halstead said the infected herd should not impact Michigan’s TB status.

“We expect as long as we have TB in deer, we are going to have spill-over in cattle occasionally,” he said. “There is nothing about this herd that is outside of the normal understanding of the disease. It’s tragic for the herd and the family, but it should not have any impact on our program.”

Federal officials have designated most of the L.P. as free of bovine tuberculosis. A high-risk zone remains in Alcona, Alpena, Montmorency and Oscoda counties in the northeastern L.P.

There are more than 200 cattle farms in Alpena County and 1,000 cattle farms in the 11 northernmost counties of the L.P.