IN MARCH 2013, the Michigan Department of Agriculture and Rural Development (MDARD) announced the 56th bovine tuberculosis- affected herd in Michigan, a dairy herd in Saginaw County. Initially, one of the cows from the herd was identified during slaughter surveillance. Testing of lymph nodes from that cow at the National Veterinary Services Laboratory confirmed that she was positive for bovine TB. Further testing of the herd performed at the Diagnostic Center for Population and Animal Health (DCPAH) confirmed other bovine TB positive animals. This was the first positive herd in the southern part of Lower Michigan in more than 30 years.

Since March, DCPAH has continued testing (both gamma interferon bovine TB assays and necropsies) on other animals moved from that farm to other herds, and on those herd mates. On September 16, MDARD confirmed an infected heifer traced to a feedlot that was connected to the Saginaw County index herd. Three affected premises have been identified, one each in Midland, Gratiot, and Arenac counties. As of the writing of this article, MDARD has completed 85 percent of the traces from the index dairy herd. However, if a new TB positive farm comes to light, the trace investigation will expand. DCPAH expects to continue receiving samples through the fall.

Because of the historic prevalence of the disease, bovine TB was once a staple in veterinary education curricula. However, with successful eradication, it began to diminish in texts and in the classroom. It is likely that most veterinarians in practice today were minimally educated about the disease in medicine or in pathology courses while in veterinary school.

Bovine Tuberculosis Primer— Not Just Cows

BOVINE TB IS CAUSED BY Mycobacterium bovis, a bacterium in the Mycobacterium tuberculosis complex of the family Mycobacteriaceae. As the name suggests, cattle are the primary hosts for M. bovis, but it has the largest host range of any of the Mycobacterium tuberculosis group. Most mammals, both wild and domestic, are susceptible including dogs, cats, horses, sheep, goats, pigs, and humans. Some spillover hosts, such as white-tailed deer, can become maintenance hosts if their population density is high. Lesions caused by M. bovis may occur in lymph nodes and lungs but may also occur in other parts of the body, including the intestines. Transmission can occur by the inhalation of aerosols, via ingestion, or through breaks in the skin.

Common symptoms of TB in humans (fevers, night sweats, cough, and weight loss) are not the clinical signs that are seen in cattle or other animals with bovine TB. Often, there are no clinical signs in the early stages of the disease. A herd can appear to be in normal health and yet have multiple animals infected. A Historical Perspective MICHIGAN IS NOT UNFAMILIAR with bovine TB. Almost 20 years ago, in 1994, a bovine TB positive white-tailed deer was harvested by a hunter in Alpena County. In 1998, an interagency project to address eradicating bovine TB in Michigan was established following the detection of a bovine-TB-infected cattle herd. In 2000, the entire state of Michigan lost TB-free status and the state has since methodically taken steps to block transmission of the disease, which is now only endemic in a small four-county area of northeastern Lower Michigan. The Modified Accredited Zone, where the disease still occurs, is Alcona, Alpena, Montmorency, and Oscoda counties. Nationally, Michigan and California are the only states to have a TB status other than “free.”

From Differential to Confirmed Diagnosis

DCPAH and DISEASE OUTBREAKS

WITH ANY INCIDENCE of bovine TB, quick diagnostic testing is needed so that a management plan appropriate for the specific situation can be developed by state and/or regional epidemiologists and implemented as soon as possible. “As a former large animal practitioner, I understand what a difficult situation this is for producers. As a pathologist, I know how serious and insidious this disease is,” said Thomas Mullaney, professor and acting director of DCPAH. “We perform histopathology for any TB-suspect animals that come to DCPAH for a postmortem, but tissues are sent almost immediately to National Veterinary Services Laboratory for culture and PCR. Getting the correct diagnosis as quickly as possible is critical to us and we realize it is critical to all stakeholders—producers, state and federal veterinarians, epidemiologists, and the public. Protecting human and animal health in Michigan is core to our mission.” In fact, DCPAH’s facility was constructed for this kind of testing. “This building was designed and built with the idea of being able to handle animal health disasters,” explains Steven Bolin, section chief of Immunodiagnostics/ Parasitology at DCPAH. “TB was the one we were looking at, but we’ve had other large outbreaks including West Nile virus, eastern equine encephalitis, and epizootic hemorrhagic disease for which this building has proven very useful. The building was specifically designed to handle those. We were trying to find information on every disease of concern and ask, ’What do we need to do if this pops up?’ For those of us working in this building today, bovine tuberculosis is never going to go away.”

December 2013
Courtney Chapin