Relapsing fever borreliosis — DTM&H MCQ

Correct answer: A — Louse-borne relapsing fever

The correct answer is A, Louse-borne relapsing fever. The combination of recurring high fevers separated by clear afebrile intervals plus direct visualisation of spirochaetes on a thick blood film taken during the febrile episode is the classical diagnostic pattern for relapsing fever caused by Borrelia recurrentis, transmitted by the human body louse in conditions of overcrowding, poverty and forced migration seen in East African refugee populations. Diagnosis is confirmed by microscopy of thick or thin Giemsa-stained blood films taken specifically during a febrile episode, since spirochaete numbers fall sharply between relapses. This organism is large enough to be seen directly on light microscopy, unlike the other spirochaetal diseases, which is why it is the pathogen "discovered" on a routine thick film done for suspected malaria. Why the other options are wrong: C. Malaria: Causes cyclical fever but the parasite seen on thick film is an intracellular protozoan within red cells, not a free spirochaete, so the film description excludes it. D. Brucellosis: Produces undulant fever but is diagnosed by blood culture or serology; brucella organisms are small intracellular coccobacilli, never visible as spirochaetes on a blood film. E. Typhoid fever: Causes a stepwise, sustained fever rather than sharply relapsing fever, and is diagnosed by blood or stool culture for Salmonella Typhi, not by seeing organisms on a film. B. Leptospirosis: Also spirochaetal but organisms are too thin to be seen on standard light microscopy and require dark-field or PCR/serology; the epidemiology (rodent urine exposure, not lice, overcrowding, refugee camps) also does not fit. Key point: Visible spirochaetes on a blood film taken during a febrile spike, in a patient from a louse-infested, overcrowded setting, is diagnostic of louse-borne relapsing fever.

Reference: MSF Medical Guidelines, Louse-borne relapsing fever (LBRF), 2023 update: 'The diagnosis is confirmed by detection of Borrelia in thick or thin blood films (Giemsa stain). Blood samples must be collected during febrile periods.' https://medicalguidelines.msf.org/en/viewport/CG/english/louse-borne-relapsing-fever-lbrf-16689949.html