Brucellosis (Brucella spp.)
2025 Case Definition

Background

Brucellosis is a zoonotic disease caused by certain bacteria in the Brucella genus categorized as brucellosis-causing Brucella species (BBS). Other species of Brucella, including the former Ochrobactrum genus, are considered non-brucellosis causing Brucella species (nBBS), as they have not been shown to cause brucellosis disease.¹ There are multiple BBS (listed with preferred animal host) known to infect humans, including but not limited to: B. abortus (cattle), B. melitensis (goats, sheep, camels), B. suis (pigs), B. canis (dogs), and B. neotomae (wood rats).

Clinical Criteria

• Acute or insidious onset of fever, AND
• Two or more of the following signs and symptoms:
  • Night sweats
  • Arthralgia
  • Headache
  • Fatigue
  • Anorexia
  • Myalgia
  • Weight loss
  • Arthritis
  • Spondylitis
  • Meningitis, encephalitis, or other neurologic abnormalities
  • Discitis or osteomyelitis
  • Abscesses
  • Focal organ involvement (including, but not limited to: endocarditis, orchitis/epididymitis, hepatomegaly, splenomegaly)

Laboratory Criteria

Confirmatory Laboratory Evidence:

Category 1:

• Identification of a Brucella isolate as a brucellosis-causing Brucella species (BBS) by methods specific for BBS (i.e., PCR assay with documented specificity for BBS and/or biochemical tests and/or whole genome sequencing of Brucella isolate).

Category 2:

• Evidence of fourfold or greater rise in Brucella antibody titer between acute and convalescent serum specimens obtained at least 2 weeks apart.*

Presumptive Laboratory Evidence:

• Brucella total antibody titer ≥1:160 by standard tube agglutination (SAT) or Brucella microagglutination test in one or more serum samples obtained after onset of symptoms.

Supportive Laboratory Evidence:

• Detection of Brucella IgG antibodies by ELISA in a sample collected at least 2 weeks after onset of symptoms.

Note: The categorical labels used here to stratify laboratory evidence are intended to support the standardization of case classifications for public health surveillance. The categorical labels should not be used to interpret the utility or validity of any laboratory test methodology.

* To ensure consistency with laboratory methodologies, it is recommended that paired sera testing for the purposes of confirmatory classification be conducted within the same laboratory.

Epidemiologic Linkage

• Direct contact with body fluids or tissue from a confirmed human case of brucellosis, OR
• Veterinary occupational exposure to Brucella vaccine (i.e., needle stick, mucous membrane exposure), OR
• Laboratory exposure to brucellosis-causing Brucella species (BBS), OR
• Direct contact to an animal diagnosed with a Brucella infection (or their fluids), as determined by a state or federal animal health official, including potential aerosol exposure, OR
• Shared one of the following exposures with a confirmed human case of brucellosis:
  • Consumption of dairy products from a common source that were unpasteurized or of unknown pasteurization, particularly from countries lacking domestic animal health programs, OR
  • Consumption or handling of undercooked meat or carcass of an animal from a herd or of a species with a known or suspected history of Brucella, OR
  • Slaughtering, dressing, butchering, or having other direct contact with animals or animal tissues possibly infected with Brucella.

Criteria to Distinguish a New Case from an Existing Case

Public health authorities should enumerate new cases of brucellosis in the following instances:

• A person should be enumerated as a case if not previously enumerated as a case, OR
• A person who was previously enumerated as a confirmed or probable case that meets confirmatory laboratory evidence category 1, AND has an event date at least twelve months after completion of adequate antimicrobial therapy, AND has new or ongoing risk factors for brucellosis exposure, OR
•  A person who was previously enumerated as a confirmed or probable case that meets confirmatory laboratory evidence category 1 AND determined to be infected with a different brucellosis-causing Brucella species (BBS) or strain than prior infection.

A person should not be enumerated as a new case if previously enumerated as a case AND there is evidence the new report is due to one of the following: brucellosis relapse, chronic infection, or delayed convalescence.

Suspect

• Meets confirmatory laboratory evidence category 2, OR
• Meets presumptive laboratory evidence, OR
• Meets supportive laboratory evidence, OR
• Meets vital records criteria.

Probable

• Meets clinical criteria AND presumptive laboratory evidence, OR
• Meets clinical criteria AND meets epidemiologic linkage criteria.

Confirmed

• Meets confirmatory laboratory evidence category 1, OR
• Meets clinical criteria AND confirmatory laboratory evidence category 2.

Vital Records Criteria 

• Death certificate lists brucellosis as a cause of death or a significant condition contributing to death.

• Brucellosis (Brucella spp.) | 2010 Case Definition
• Brucellosis (Brucella spp.) | 1997 Case Definition