Bovine Tuberculosis

The Organism

Bovine tuberculosis is caused by Mycobacterium bovis, a member of the same bacterial group that causes human tuberculosis. It occurs mainly in cattle, bison, deer, and elk, and can infect people [2].

It grows slowly and causes a chronic infection. The median time from tuberculosis entering a US herd to its detection at slaughter is about five years [3].

M. bovis is naturally resistant to pyrazinamide, one of the four standard drugs used to treat human tuberculosis [4].

Where It Lives

The national eradication program began in 1917 and has nearly eliminated tuberculosis from US livestock; the estimated prevalence is 7 per 1 million cattle screened [1]. As of September 1, 2026, every state is accredited-free except Michigan, which is split between a modified accredited zone and an accredited-free zone [5].

The main remaining source is free-ranging white-tailed deer in northeastern Michigan. Michigan’s modified accredited zone covers Alcona, Alpena, Montmorency, and Oscoda counties. In 2026, infected cattle herds were also found outside that zone, in Charlevoix, Presque Isle, and Iosco counties, and a positive deer was found in Crawford County in November 2025 [6]. Additional infected herds in Iosco County in September 2026 led the state to propose new testing areas in Arenac, Iosco and Ogemaw counties [7].

Outside Michigan, infection appears sporadically, most often in dairy herds, and is traced through animal movements [1, 8].

How It Spreads

Tuberculosis spreads mainly through nose-to-nose contact between animals and through contaminated feed and water. Herds usually become infected either by buying an infected animal or through exposure to infected wildlife [1]. People can also spread it to cattle [1].

Because infected cattle usually look healthy for years, people buy, sell, and move them normally before anyone knows. When researchers trace a slaughter case back, they find more infected animals in the herd of origin about 65% of the time [3].

People are infected mainly by drinking unpasteurized milk or eating raw-milk dairy products, and less often by breathing in the bacteria or through wounds while handling infected animals or carcasses [2].

Infection Pattern

Infected cattle typically show no signs. Clinical signs indicate advanced disease affecting several organ systems and are rare [1].

The infection forms tubercles, or firm nodules, in the head and chest lymph nodes and in the lungs. Inspectors find these lesions at slaughter [3]. Because producers usually see nothing on the ranch, they detect tuberculosis through inspection and testing rather than observation.

What to Do If You Suspect Tuberculosis

On the ranch, suspicion usually comes from outside: a notice that an animal you sold or slaughtered had lesions, or that cattle you bought came from an infected herd. Cooperating with the trace investigation and holding animals on the premises until state and federal officials advise [1, 3] leads to the least operational setbacks.

If a cow is wasting or coughing without a diagnosable cause, your veterinarian may consider tuberculosis testing. An accredited veterinarian or regulatory official must perform testing [1].

Never drink raw milk or eat raw-milk cheese from any species, especially from at-risk dairy cattle near modified accredited zones. Anyone in close contact with at-risk animals (cattle, bison, deer, elk) who develops a lingering cough, fever, night sweats, or weight loss should see a doctor and mention the animal contact [2].

Diagnostic Differential

At slaughter, tuberculosis lesions in the lymph nodes and lungs can look like other granulomatous or nodular infections, including Valley fever, actinobacillosis, abscesses, and some tumors. Only laboratory testing distinguishes them, so suspicious lesions are submitted for differentiation [3].

On skin testing, cattle exposed to Mycobacterium avium and other environmental mycobacteria can react without having tuberculosis. The comparative cervical test distinguishes these reactions [3].

Diagnosis

Live-animal testing is in two stages. The caudal fold tuberculin test, an injection in the skin fold at the base of the tail read three days later, is the screening test. A responder then undergoes a confirmatory test ordered by the regulatory veterinarian: the comparative cervical test, which distinguishes M. bovis from reactions to M. avium, or the gamma interferon blood test [1, 3]. Gamma interferon can also be run alongside the caudal fold test in high-risk herds to catch more infections, but it can produce more false positives [3].

Most infected herds are found at slaughter. Adult cattle and bison carcasses are inspected for lesions, suspicious granulomas excised and shipped to the National Veterinary Services Laboratories for histopathology, PCR, and culture [1, 3]. The national standard requires submitting at least one suspicious lesion per 2,000 adult cattle slaughtered. Actual submission rates fell from one per 830 in 2011 to one per 1,401 in 2017, and slaughter surveillance was estimated in 2009 to detect about 21% of infected animals [3].

Risk to Humans

M. bovis causes under 2% of US tuberculosis, fewer than 230 cases a year. The most commonly reported source is unpasteurized milk and raw-milk dairy products. Ranchers, dairy workers, slaughterhouse and butcher workers, and hunters are at higher risk [2].

In California and other US studies, the main source has been fresh cheese made from unpasteurized milk, often from Mexico. In California from 2003 to 2011, M. bovis caused 742 of 18,752 tuberculosis cases (4%), and its share of culture-positive cases rose from 3.4% to 5.4%. It caused 22% of culture-positive tuberculosis in children under 15, and 12.8% of adult patients died before completing treatment [9]. In New York City from 2001 to 2004, 83% of interviewed M. bovis patients had eaten Mexican-produced cheeses while living in the US, and 78% did not know whether their dairy products were pasteurized [10].

M. bovis disease is often outside the lungs: in the New York series, 60% of patients had extrapulmonary disease only [10]. It is treated much like other tuberculosis, but it resists pyrazinamide [2, 4]. Hunters exposed to blood or tissue from wild deer, elk, or bison should report the exposure to a doctor [2].

Treatment and Control

Tuberculosis is not treated in livestock [1]. No vaccine is used in US cattle.

When a herd is confirmed infected, it is quarantined and either depopulated with federal indemnity or placed on a test-and-removal plan, in which the whole herd is tested repeatedly, and reactors are removed. A whole-herd test covers all cattle and bison 12 months or older, plus younger animals not born in the herd [7, 11]. An outbreak investigation will then trace and test herds that bought from or sold to the infected herd [1].

Prevention on the ranch:

• Buy from accredited or tested herds, and isolate and test purchased breeding cattle, particularly dairy animals.
• Keep official identification and movement records to speed tracing and limit how many herds are affected.
• Keep cattle feed and water away from deer and elk, which can carry tuberculosis.
• Do not feed raw milk to calves from herds of unknown status.

What Tuberculosis Costs Producers

For affected herds, costs include quarantine, repeated whole-herd testing, loss of reactors, and, in some cases, the loss of the whole herd. USDA has stated that depopulating affected herds is often no longer feasible because of herd size, public perception, and the availability of indemnity funding, so large dairies are commonly placed on test-and-removal plans instead [12].

The higher cost is to the state. If two or more affected herds are found in an accredited-free state or zone within 48 months, it loses accredited-free status. A single affected herd does not automatically cost the state its status: if the herd is depopulated and the epidemiologic investigation finds no spread within 90 days, the state may keep it. If the herd is not depopulated, the state must put an approved eradication plan in place within six months or lose its status. Regaining it takes at least two to three years after the last affected herd is depopulated, and up to five years otherwise [11]. Losing status triggers testing requirements for cattle moving out of the entire state or zone.

One infected herd also draws in its neighbors and trading partners. Traces and tests every herd that bought from or sold to it [1, 3].

Where to Report a Case in the Southwest

Bovine tuberculosis is a federal program disease and is notifiable nationally. It is reportable in animals and people in all three states.

Documented Incidence in the Southwest

Arizona, New Mexico, and Texas are all accredited free as of September 1, 2026 [5]. Recent infections in the region have occurred in dairy herds, linked by ownership and cattle movement, and managed under test-and-removal plans rather than depopulation [8, 12]. Nationally, affected animals fell from 64 in fiscal 2021 to 29 in 2022 and seven in 2023 [12]. Cattle imported from Mexico must be tuberculosis tested, with requirements set by the exporting region’s classification: Sonora is classified Level II, and most of Chihuahua Level III, with parts of 13 municipalities at Level V [20].

Arizona

Accredited free. No affected herd appears in the most recent published federal herd summaries [8, 12]. Cattle crossing from Sonora at Douglas and Nogales enter under the Level II testing requirements [20].

New Mexico

Accredited free. Dairy herds detected from 2019 to 2022 were under test-and-removal plans in 2023 and 2024; one of which was epidemiologically linked to a Texas herd [8, 12]. In March 2026, a New Mexico herd later found to be infected was identified as the source of two loads of cows shipped to a dairy in Ionia County, Michigan; the Michigan herd tested negative and was released in April 2026 [6].

Texas

Accredited free. Dairy herds detected from 2019 to 2022 were under test-and-removal plans in 2023 and 2024 [8, 12].

References

1. Bovine tuberculosis in cattle. USDA APHIS, modified 6 May 2026. https://www.aphis.usda.gov/livestock-poultry-disease/cattle/bovine-tuberculosis-cattle↑ a b c d e f g h i j k l
2. Mycobacterium bovis (bovine tuberculosis) in humans. US Centers for Disease Control and Prevention, Division of Tuberculosis Elimination, August 2011. https://stacks.cdc.gov/view/cdc/75872/cdc_75872_DS1.pdf↑ a b c d e f
3. National bovine tuberculosis surveillance plan. USDA APHIS Veterinary Services, October 2021. https://www.aphis.usda.gov/sites/default/files/tbsurveillanceplan.pdf↑ a b c d e f g h i j k
4. Kurbatova EV, Cavanaugh JS, Dalton T, Click E, Cegielski JP. Epidemiology of pyrazinamide-resistant tuberculosis in the United States, 1999–2009. Clinical Infectious Diseases 2013;57(8):1081–1093. https://stacks.cdc.gov/view/cdc/34431/cdc_34431_DS1.pdf↑ a b
5. Status of current eradication programs. USDA APHIS, tuberculosis status updated 1 September 2026. https://www.aphis.usda.gov/livestock-poultry-disease/status-eradication-programs↑ a b
6. Bovine tuberculosis eradication program quarterly update, 13 July 2026. Michigan Department of Agriculture and Rural Development. https://www.michigan.gov/mdard/-/media/Project/Websites/mdard/documents/boilerplate-reports/fy2026/MDARDLegisRptTBProgramJuly2026QtrlyUpdate.pdf↑ a b
7. MDARD announces public comment period for proposed bovine tuberculosis testing areas. Michigan Department of Agriculture and Rural Development, 14 October 2026. https://www.michigan.gov/mdard/about/media/pressreleases/2026/10/14/mdard-announces-public-comment-period-for-proposed-bovine-tuberculosis-testing-areas↑ a b
8. Affected herd summary, fiscal year 2024, quarter 1. USDA APHIS. https://www.aphis.usda.gov/sites/default/files/affected-herd-summary-q1-fy2024.pdf↑ a b c d e
9. Gallivan M, Shah N, Flood J. Epidemiology of human Mycobacterium bovis disease, California, USA, 2003–2011. Emerging Infectious Diseases 2015;21(3). https://wwwnc.cdc.gov/eid/article/21/3/14-1539_article↑
10. Human tuberculosis caused by Mycobacterium bovis — New York City, 2001–2004. MMWR 2005;54(24). https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5424a4.htm↑ a b
11. 9 CFR Part 77: Tuberculosis. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-9/chapter-I/subchapter-C/part-77↑ a b
12. USDA national tuberculosis eradication program update. USDA APHIS Veterinary Services, presented to the USAHA Subcommittee on Tuberculosis, October 2023. https://usaha.org/upload/Committee/CattleBison/USDA_TB_Eradication_Update_Lyons.pdf↑ a b c d e f
13. Arizona Administrative Code § R3-2-402: Mandatory disease reporting by veterinarians and veterinary laboratories. https://www.law.cornell.edu/regulations/arizona/Ariz-Admin-Code-SS-R3-2-402↑
14. Reportable diseases list for health care providers, effective 2 June 2025. Arizona Department of Health Services. https://www.azdhs.gov/documents/preparedness/epidemiology-disease-control/communicable-disease-reporting/reportable-diseases-list.pdf↑
15. 21.30.4.9 NMAC: New Mexico reportable disease list. New Mexico Livestock Board. https://www.srca.nm.gov/parts/title21/21.030.0004.pdf↑
16. 7.4.3.13 NMAC: Notifiable conditions and reporting timeframes. New Mexico Department of Health. https://www.srca.nm.gov/parts/title07/07.004.0003.pdf↑
17. 4 TAC § 45.3: Reportable and actionable disease list. Texas Animal Health Commission. https://www.law.cornell.edu/regulations/texas/4-Tex-Admin-Code-SS-45-3↑
18. Reportable diseases. Texas Animal Health Commission. https://www.tahc.texas.gov/regs/reportable.html↑
19. Texas notifiable conditions, 2025. Texas Department of State Health Services. https://www.dshs.texas.gov/sites/default/files/IDCU/investigation/Reporting-forms/notifiable-conditions-2025.pdf↑
20. TB classifications of Mexican regions. USDA APHIS, 1 April 2026. https://www.aphis.usda.gov/sites/default/files/aphis-tb-classifications-mexican-regions.pdf↑ a b