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USMLE Step 2 (Fach) / Infectious disease (Lektion)
- Rocky Mountain spotted fever Pathogen: Rickettsia rickettsii (aerobic, gram-negative, obligate intracellular bacteria ) Epidemiology: Rocky mountains, southeastern, and south central US Reservoir: dogs, rodents, ticks Route of transmission: bite of Dermacentor variabilis (dog tick) Clinical features (incubation period ∼ 7 days, or 2–12 days)- Fever, headache, myalgia, malaise, conjunctivitis, nausea, and abdominal pain- Blanching macular rash (90% of cases): begins on wrists and ankles → spreads to trunk, palms, and soles → becomes petechial and/or hemorrhagic in 50% of cases- Hepatomegaly, splenomegaly- Noncardiogenic pulmonary edema with ARDS Diagnosis: empiric diagnosis based on clinical and epidemiological features Treatment: doxycycline
- Louse-borne diseases Trench fever (five-day fever) Definition: a type of louse-borne relapsing fever caused by Bartonella quintanaEtiology: Bartonella quintanaTransmission: feces of infected body louse (vector)Epidemiology: most commonly affects people who are at risk of body louse infestation (e.g., homeless)Clinical features:- Recurrent fever episodes that last ∼ 5 days- Nonspecific, flu-like symptoms (malaise, headache, nausea, vomiting)- Bone pain, splenomegaly, and maculopapular rash of the trunk may occurDiagnosis: isolation of Bartonella confirms the diagnosisSerology testing or PCR are often necessaryTreatment: doxycycline and gentamicin Epidemic typhusDefinition: an exanthematous typhus fever caused by Rickettsia prowazekiiEtiology: Rickettsia prowazekiiTransmission: feces of infected body louse (vector)Epidemiology: extremely rare in the US; occurs in Rwanda, Ethiopia, Asia, and rural parts of South America [6]Clinical features:- Abrupt onset of fever, severe headache, malaise, myalgia, abdominal pain, and nausea- After 4-5 days: maculopapular or petechial rash that spreads from the trunk to the extremitiesDiagnosis: Serology, Positive Weil-Felix reactionTreatment: doxycycline or chloramphenicol Louse-borne relapsing feverDefinition: a louse-borne infection caused by Borrelia recurrentis (not to be confused with tick-borne relapsing fever)Etiology: Borrelia recurrentisEpidemiology: most common in areas affected by overcrowding, war, refugee campsClinical features:- Recurring episodes of high fever (39-43°C) lasting 3-6 days followed by an afebrile period of 7 daysOther symptoms include headache, arthralgia, nausea, confusion, macular rashDiagnostics: Detection of Borrelia in peripheral blood smear, bone marrow, or CSF with Wright or Giemsa stainLaboratory studies: normocytic anemia, thrombocytopenia , ↑ ESRTreatment: Doxycycline or ceftriaxone
- Mechanisms of foodborne illness Enterotoxin ingested:- Staphylococcus aureus- Bacillus cereus- Quick onset (1-6 hours)- Vomiting predominant Enterotoxin made in intestine- Clostridium perfringens- ETEC/STEC- Vibrio cholerae- Delayed onset (> 1 day)- Watery/bloody diarrhea Bacterial epithelial invasion- Campylobacter jejuni- Nontyphoidal salmonella- Listeria monocytogenes- Variable onset- Watery/bloody diarrhea- Fever- Systemic illness (Listeria)