Oct. 1 2021 08:00 AM

A small but growing number of tuberculosis cases in cattle have been linked to humans.

Lombard is a veterinary epidemiologist with UDSA’s Field Epidemiology Investigation Services and Patton is the assistant state veterinarian for the Wisconsin Department of Agriculture, Trade and Consumer Protection.

While control programs have been very successful, tuberculosis has not been eradicated from the U.S. dairy herd.

Bovine tuberculosis (Mycobacterium bovis, or bovine TB) has long been recognized to transmit between cattle and from cattle or cattle products to humans. However, the spread of TB from humans to animals is now under the microscope. As dairy producers, you already employ many biosecurity practices, but have you ever considered your employees, or even yourself, as a risk to your cows?

The COVID-19 pandemic has been a reminder to us all about the importance of human-to-animal transmission. Infected humans have been linked to SARS-CoV-2 virus infections in pets, zoo animals, and mink. Although TB is a bacterial disease and not a virus, it is another example of a zoonotic disease that can be transmitted between humans and animals.

Most people infected with TB have the human-adapted species, Mycobacterium tuberculosis or M. tb, but some people become infected with the bovine type, Mycobacterium bovis or M. bovis. These are the people most at risk of spreading the disease to cattle.

Historically, TB has most commonly been introduced to cattle herds through the movement of infected cattle or exposure to infected wildlife. No matter how it is introduced, once in a herd, M. bovis can cause severe economic and psychological stress to cattle owners.

Humans as the source

Although human-to-cattle transmission of TB has been speculated as a means of introducing TB into a herd for a long time, humans are now recognized as a potential risk. In fact, in the past decade, there have been more than 20 papers published worldwide describing human-to-cattle TB infection. In the U.S., however, there have only been two cases reported, and those papers were published more than 50 years ago and lacked solid laboratory confirmation of humans as the source of infection.

In a recent publication, three TB cases in cattle herds were reported in the U.S. (on.hoards.com/3TBcases) where all evidence points to humans as the source of infection. Advances in diagnostics and the use of genetic sequencing of bacteria to ‘fingerprint’ the specific strain of bacteria has modernized the way we are able to conduct investigations and link infections from other herds, wildlife, or human introductions of TB.

This technology was critical in determining the source of infection in two of the three cases. The third case involved a calf with human TB (M. tb). Although this species of TB is not a significant concern for transmission in U.S. cattle herds, humans are the only known source for cattle infected with M. tb.

The first case involved a dairy employee in North Dakota who was diagnosed with the bovine strain of TB. The North Dakota Department of Health and the North Dakota Department of Agriculture’s State Board of Animal Health worked with the dairy herd owner to test the employees and cattle.

The TB strain in the diagnosed employee and an infected cow were a match. The employee was born in Mexico, where bovine TB is much more common in cows, and the individual likely acquired the infection from eating raw dairy products in Mexico.

The second case took place on a Wisconsin dairy farm where an employee was diagnosed with bovine TB. Communication between the Wisconsin Division of Public Health and the Wisconsin Department of Agriculture, Trade and Consumer Protection resulted in human and animal testing. After two rounds of herd testing, no infected cattle were detected.

Three years later, a cull cow from the dairy was detected with bovine TB through slaughter surveillance testing. The herd was quarantined and placed on a test and remove program. After multiple rounds of herd testing, 11 additional cows were detected with a bovine TB strain matching the employee’s strain of bovine TB.

This worker was from Latin America, where in some areas bovine TB is a more common cause of TB in humans. The delay between detection of the employee’s infection and cattle infection was likely due to the insidious nature of the bacteria and the limited ability of diagnostic tests to detect early infection.

The third case involved a dairy crossbred calf from New Mexico that was shipped to Texas. The animal was TB tested after shipment, as required by Texas state law. In this case, the calf was found to be infected with human TB (M. tb) as opposed to bovine TB. Although a genetic match was not identified in a worker or the human TB database, it is assumed that the calf became infected from an infected human.

The goal is eradication

These three cases prompted the formation of the National Milk Producers Federation’s (NMPF) TB Working Group comprised of animal and public health officials, personnel from USDA and the Centers for Disease Control and Prevention, industry representatives, and producers. The goal of the working group is to raise awareness and provide resources and best practices for state animal and public health officials, dairy producers, and veterinarians.

Meanwhile, the U.S. Bovine TB Eradication Program has been in place for more than 100 years. Although it has been extremely successful, the country has not managed to eradicate this disease from our cattle populations. In fact, we still identify as many as 10 newly infected herds each year.

Approximately half of these cases can be linked to a strain of bovine TB found in Michigan, where the disease is endemic in the wild white-tailed deer population. Of the remaining cases, the source for 50% is never determined, even with the use of genetic diagnostic tools. Without a known source of infection, it is challenging to implement the best strategies to mitigate those risks.

These three cases highlight the importance of considering humans as a potential source for introduction of TB into dairy herds. The inclusion of genetic sequencing in both human and animal TB cases will greatly improve our ability to conduct trace investigations and determine sources of introduction in the future.

Understand your farm’s risks

What does that mean for dairy producers or managers? Although a challenging question, it is the primary reason NMPF formed the TB Working Group. We are working to develop recommendations and best practices for prevention and eradication.

One frequently asked question is whether dairy producers should require TB testing for their employees. No single plan will fit every operation, and creating the right plan for your farm should involve an evaluation of risks, including the risk of human introduction of bovine TB into your herd.

Although the working group is still developing recommendations and best practices, you can immediately take steps to ensure the safety of your employees and your herd. Work with your veterinarian, state animal health officials, and public health officials to understand your risks for bovine TB introduction and consider ways in which you may be able to reduce those risks for your operation.

For more information on bovine tuberculosis, please visit the USDA APHIS National Tuberculosis Eradication Program website at on.hoards.com/TB-erad-program.