Brucellosis in Cattle: Testing, Vaccination, and Regulations (2026 Guide)

1. Introduction: Why Brucellosis Still Matters in 2026

Although the United States achieved Class Free status for cattle brucellosis in most states by the early 2000s, the disease is not gone. A persistent wildlife reservoir in the Greater Yellowstone Area (elk and bison), combined with global trade and the movement of cattle across borders, keeps brucellosis a real threat in 2026. Outbreaks in Montana, Wyoming, and Idaho over the last five years have reminded producers that vigilance, testing, and vaccination remain essential.

Beyond the herd-level impact, brucellosis is a zoonotic disease — meaning it can infect humans — making it a public health as well as an agricultural concern.

2. What Is Brucellosis?

Brucellosis in cattle is caused primarily by Brucella abortus, a gram-negative bacterium that lives inside host cells. Related species affect other animals: B. melitensis (goats/sheep), B. suis (swine), and B. canis (dogs). All can potentially infect humans.

The bacterium has a strong affinity for reproductive tissues, especially the pregnant uterus, placenta, and mammary gland, which is why abortion in the last trimester is the hallmark sign.

3. How Brucellosis Spreads

Brucella is shed in enormous numbers at calving or abortion. The main routes of transmission are:

• Ingestion of contaminated placenta, fetal fluids, or feed/water.
• Licking of aborted fetuses or newborn calves — very common in curious cattle.
• Contaminated milk — calves nursing from infected cows.
• Venereal transmission — less common but possible via natural service.
• Wildlife exposure — infected elk and bison in the Greater Yellowstone Area.
• Contaminated fomites — boots, trailers, equipment.

4. Clinical Signs in Cattle

Brucellosis is often silent until reproductive losses appear. Key signs include:

• Late-term abortion (5th to 9th month of pregnancy) — the classic sign.
• Retained placenta and metritis after abortion or calving.
• Stillbirths and weak calves at term.
• Reduced fertility — repeat breeders, extended calving intervals.
• Milk yield drop in dairy cows.
• Hygroma — fluid swelling on knees or hocks (in chronic cases).
• Bulls: Orchitis, epididymitis, reduced fertility.

5. Zoonotic Risk: Undulant Fever in Humans

Humans typically contract brucellosis by handling infected tissues without protection, consuming unpasteurized dairy, or through mucosal splashes. The human disease is called undulant fever because of its recurring fever pattern.

Symptoms include prolonged fever, night sweats, joint pain, fatigue, and depression. It can become chronic and difficult to treat, requiring weeks of combination antibiotic therapy.

6. Testing Methods and Protocols

Reliable diagnosis relies on serologic tests, culture, and PCR. Multiple tests are often used in combination.

When to Test

• Before movement across state lines (as required by regulation).
• Before sale at auction markets.
• At change of ownership.
• Following any abortion investigation.
• Routine surveillance in Designated Surveillance Areas (DSAs).

7. Vaccination: RB51 Program

The RB51 vaccine is the current USDA-approved vaccine for cattle brucellosis. It replaced the older Strain 19 vaccine because RB51 does not interfere with standard serologic tests, making surveillance easier.

RB51 Vaccination Facts

• Who administers: Only a USDA-accredited veterinarian or state/federal animal health official.
• Age: Heifer calves between 4 and 12 months of age.
• Route: Subcutaneous injection.
• Identification: Vaccinated animals receive an official tattoo in the right ear and a metal or RFID orange "Brucellosis Vaccination" tag.
• Duration of immunity: Generally lifelong protection against clinical disease, though not 100% sterile immunity.

8. Regulations and Reporting Requirements

Brucellosis in cattle falls under a coordinated federal-state framework managed by USDA APHIS and state veterinarians. Key regulations in 2026 include:

Interstate Movement Requirements

• Test-eligible cattle (typically sexually intact cattle 18 months or older) must have a negative brucellosis test within 30 days of interstate movement — unless from a Class Free state and meeting exemption criteria.
• All interstate movement requires a Certificate of Veterinary Inspection (CVI).
• Official individual identification (RFID or approved ear tag) is required.

Designated Surveillance Areas (DSAs)

In the Greater Yellowstone Area (parts of Montana, Wyoming, and Idaho), enhanced surveillance rules apply because of the wildlife reservoir. Cattle in DSAs are subject to:

• Mandatory herd testing on an annual or biennial basis.
• Testing before movement out of the DSA.
• Mandatory RB51 vaccination of heifer calves.

State Status Categories

Reporting

Brucellosis is a federally reportable disease under the National Animal Health Reporting System. Any suspicion must be reported to the state veterinarian or USDA APHIS Area Veterinarian in Charge within 24 hours.

9. Biosecurity and Prevention

Preventing brucellosis introduction is far cheaper than dealing with an outbreak.

Top Biosecurity Practices

• Buy from known-negative herds and require test results before purchase.
• Quarantine new arrivals for at least 30 days and retest before mixing.
• Vaccinate heifer calves with RB51 in DSAs or high-risk areas.
• Prevent wildlife contact — fencing, hay storage, and elk-cattle separation during elk calving season.
• Handle abortions safely — wear PPE, disinfect the area, and submit fetuses for diagnosis.
• Clean and disinfect calving areas after every use.
• Do not feed unpasteurized milk to calves from untested cows.

10. Economic Impact

A brucellosis outbreak can be devastating financially, with losses continuing long after the diagnosis.

11. Frequently Asked Questions