Question 1

Which Brucella species is the most pathogenic to humans?

Accepted Answer

Introduction: Brucellosis is a zoonosis caused by Brucella spp. Human disease severity varies by species due to differences in virulence and host preference. Given Data / Assumptions: We compare pathogenicity of common Brucella species in humans. Species include B. melitensis, B. abortus, B. suis, and others. Concept / Approach: B. melitensis (from goats/sheep) is most virulent to humans, causing acute, severe disease; B. abortus (cattle) and B. suis (pigs) are typically less severe; B. canis is generally milder. Step-by-Step Solution: Consider human clinical severity rankings. Recognize B. melitensis as the most pathogenic for humans. Select Brucella melitensis as the correct option. Verification / Alternative check: Standard epidemiology sources rank B. melitensis highest in human virulence. Why Other Options Are Wrong: B. abortus / B. suis: Cause human disease but generally less severe than B. melitensis. B. canis / B. ovis: Typically milder or rarely cause human illness. Common Pitfalls: Assuming prevalence in livestock equates to human virulence; virulence and host tropism differ among species. Final Answer: Brucella melitensis is the most pathogenic to humans.

Question 2

Which disease can be identified by the milk ring test in dairy surveillance?

Accepted Answer

Introduction: The milk ring test is a simple screening assay used in dairy herds to detect antibodies against certain pathogens in bulk milk samples. Given Data / Assumptions: Question targets the specific disease detected by the milk ring test. Common cattle diseases listed as options. Concept / Approach: The milk ring test detects anti-Brucella antibodies (primarily B. abortus) in milk. Antigen binds antibodies and forms a colored ring at the cream layer when positive. Step-by-Step Solution: Recall the purpose and antigen used in milk ring testing. Link the test to Brucella antibody detection in milk. Choose Brucellosis as the correct answer. Verification / Alternative check: Veterinary surveillance manuals specify the milk ring test for brucellosis screening in herds. Why Other Options Are Wrong: Salmonellosis / Bovine tuberculosis / Leptospirosis: Not detected by the milk ring test; require other diagnostics. All of these: Incorrect because the test is specific for brucellosis screening. Common Pitfalls: Assuming all common cattle diseases are detected by one milk-based assay; tests are disease-specific. Final Answer: Brucellosis is identified by the milk ring test.

Question 3

In brucellosis serology, the 2-mercaptoethanol (2-ME) agglutination test is primarily used to detect which antibody class?

Accepted Answer

Introduction: Brucellosis serology employs agglutination tests to differentiate antibody classes and disease phase. The 2-ME test helps distinguish between early and chronic/ongoing infection. Given Data / Assumptions: 2-ME reduces disulfide bonds in pentameric antibodies. We must identify which class remains agglutinating after treatment. Concept / Approach: 2-ME disrupts IgM (pentamer) more readily than IgG (monomer). Persistence of agglutination after 2-ME treatment indicates IgG, commonly associated with chronic or ongoing infection. Step-by-Step Solution: Apply chemical effect: 2-ME reduces IgM activity. Assess remaining agglutination: due to IgG. Conclude that the test is used to detect IgG. Verification / Alternative check: Serology protocols define 2-ME positivity as evidence of IgG antibodies. Why Other Options Are Wrong: IgM: Largely inactivated by 2-ME; not the intended readout. IgA / IgE / IgD: Not the principal targets in standard brucellosis agglutination workflows. Common Pitfalls: Confusing the standard agglutination test (detects mostly IgM) with the 2-ME modified test (reveals IgG). Final Answer: The 2-ME agglutination test primarily detects IgG.

Question 4

Morphologically, Brucella species are best described as:

Accepted Answer

Introduction: Accurate morphological description aids in presumptive identification of bacteria in the laboratory. Given Data / Assumptions: Brucella morphology is being queried. Microscopy typically guides early classification. Concept / Approach: Brucella are small, gram-negative coccobacilli, appearing as very short rods sometimes resembling cocci, especially in clinical smears. Step-by-Step Solution: Recall typical gram stain appearance. Differentiate from true cocci and long rods. Select coccobacilli as the most accurate descriptor. Verification / Alternative check: Textbook descriptions consistently list Brucella as coccobacilli. Why Other Options Are Wrong: Cocci / Rods: Too generic and not precise for Brucella. Very short rods: Descriptive but not the proper morphological term. Spiral-shaped bacilli: Not applicable to Brucella. Common Pitfalls: Equating “very short rods” with the formal term; coccobacilli is the correct classification. Final Answer: Brucella are coccobacilli.

Question 5

In brucellosis, the standard agglutination test (SAT) mainly detects which antibody class?

Accepted Answer

Introduction: Serologic diagnosis of brucellosis often begins with agglutination tests. Understanding which antibody class is primarily detected helps interpret disease phase. Given Data / Assumptions: SAT uses whole-cell antigen to measure agglutinating antibodies. We must identify the predominant antibody class detected. Concept / Approach: Because of its pentameric structure, IgM agglutinates efficiently and dominates early serologic responses detected by SAT. Differentiation from IgG is performed by 2-ME testing when needed. Step-by-Step Solution: Recall SAT principle: lattice formation favors IgM. Early infection shows high IgM titers; IgG rises later. Therefore, SAT mainly reflects IgM. Verification / Alternative check: Diagnostic algorithms pair SAT with 2-ME: loss of titer with 2-ME implies IgM, persistence implies IgG, confirming SAT's initial IgM dominance. Why Other Options Are Wrong: IgG: Detected better after 2-ME modification or via other assays. IgA / IgE / IgD: Not primary targets in standard brucellosis SAT. Common Pitfalls: Confusing SAT (IgM-dominant) with 2-ME (used to demonstrate IgG persistence). Final Answer: The SAT primarily detects IgM antibodies.

Question 6

How do humans commonly acquire Brucella infection?

Accepted Answer

Introduction: Brucellosis is a classic occupational and food-borne zoonosis. Understanding transmission routes informs prevention strategies. Given Data / Assumptions: Possible exposures include food, direct animal contact, and dairy products. Human-to-human spread is exceedingly rare. Concept / Approach: Brucella infects via ingestion (unpasteurized milk/dairy, undercooked meat), direct inoculation through cuts or mucosa during animal handling, and less often via aerosols in laboratory settings. Combined, these make multiple routes viable. Step-by-Step Solution: Recognize food-borne risk: unpasteurized milk and contaminated meat. Acknowledge occupational risk: direct contact with animal tissues. Choose the inclusive answer covering major routes. Verification / Alternative check: Epidemiologic data consistently implicate all listed routes except routine person-to-person spread. Why Other Options Are Wrong: Each single route alone: Incomplete; multiple routes cause infection. Airborne person-to-person spread: Not a common route for brucellosis. Common Pitfalls: Overlooking unpasteurized dairy as a key exposure; ignoring occupational hazards in abattoirs/vets. Final Answer: All of these routes (meat, direct contact, infected milk) can transmit Brucella to humans.

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