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USMLE Step 1 typically plays with you by throwing a case in which the patient is living in the southwest desert states such as California, Colorado, New Mexico, Arizona, Nevada, and Utah.

Your Differential Diagnosis should include the following:

  • Yersinia pestis (Plague)
  • Coccidioides immitis (San Joaquin Valley Fever)
  • Hanta virus

To add more difficulty they typically give a pneumonia-like or flu-like syndrome and inhalational mode of transmission both can be seen in all of these infections!
What is even more confusing is that both plague and Hanta virus may be transmitted from rodent feces and urine such as mice!

Distinguishing between these are the following points:

  • Plague: bipolar staining yersinia, coagulase positive, V, W, and F-1 antigens, very high fever, conjunctivits, and hemorrhagic lymph nodes (buboes) with high mortality.
  • Coccidioides: is unlike the two others is usually self limited unless in immune compramised patients. Other clues may include the desert bumps which are erythema nodosum and calcific pulmonary lesions and dimorphic fundus with unique spherules in the tissue form.
  • Hanta virus: Progression to adult respiratory distress syndrome, then respiratory failure, then death.
 
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