Clinical microbiology — McFadyean's reaction is employed for the presumptive diagnosis of which infectious disease in humans and animals?

Introduction / Context:
Rapid, presumptive identification of highly pathogenic bacteria is crucial in clinical microbiology and veterinary practice. McFadyean's reaction is a classic staining-based test that helps quickly flag suspected cases of anthrax, caused by Bacillus anthracis, before confirmatory culture or molecular methods are completed.

Given Data / Assumptions:

• The question asks for the disease best identified presumptively by McFadyean's reaction.
• We consider routine, historical microbiological methods used on clinical or animal blood smears.
• We distinguish between different bacterial diseases with distinct diagnostic tests.

Concept / Approach:
McFadyean's reaction uses polychrome methylene blue staining of a peripheral blood smear from a suspected case. In anthrax, Bacillus anthracis organisms may show a prominent purplish capsule against a blue bacillary body when stained with this dye. The capsule consists of poly-D-glutamic acid and is best visualized by this special stain, enabling a rapid presumptive diagnosis in outbreaks among livestock or potential human cases with compatible clinical features.

Step-by-Step Solution:

Verification / Alternative check:
Additional supportive tests include culture producing non-hemolytic, irregular, "Medusa head" colonies and susceptibility to gamma phage. Modern PCR assays for toxin and capsule genes (pagA, cap) provide definitive confirmation.

Why Other Options Are Wrong:

• Tetanus: Caused by Clostridium tetani; diagnosis is clinical; no McFadyean utility.
• Typhoid: Caused by Salmonella Typhi; diagnosed by culture/serology; not by capsule staining in blood smears.
• All of these: Overbroad; McFadyean's reaction is specific for suspect anthrax.

Common Pitfalls:
Mistaking general capsule stains (e.g., India ink) for McFadyean's special stain; also assuming a positive stain is confirmatory—culture or molecular tests are still needed.

Final Answer:
anthrax