Pathophysiology
1) cause is infection with Lyssavirus 2) virus attacks unmyelinated peripheral neurons and travels to spinal cord (retrograde at rate of 1-2 cm/day) 3) once it reaches spinal cord, it travels rapidly to brain 4) from brain, virus travels systemically to other tissues, especially the salivary glands 5) transmitted via infected secretions, especially the saliva
Signs and Symptoms
Prodrome – 1) fever 2) anorexia 3) headache 4) anxiety 5) paresthesias at wound site Systemic manifestations – 6) dysphagia and frothing at the mouth 7) pharyngeal spasm 8) hydrophobia 9) acrophobia 10) agitation 11) confusion and hallucinations with periods of lucid behavior 12) paralysis 13) coma 14) death
Characteristic Test Findings
Laboratory – 1) mild leukocytosis Lumbar puncture – 2) mild lymphocytic meningitis
Histology/Gross Pathology
1) single-stranded, linear enveloped, RNA virus 2) brain parenchyma has areas of necrosis 3) formation of Negri bodies (cytoplasmic inclusions inside neurons) 4) virus binds acetylcholine receptors
Inheritance/Epidemiology
1) exposure is via a bite, saliva on an open wound, or inhalations 2) incubation period can be weeks to years 3) 2-4 cases in USA each year 4) USA reservoirs are raccoons (50%), skunks (20%), and bats (10-15%) 5) endemic in southeast Asia with 50,000 deaths each year, mostly caused by bites from rabid dogs
Treatment
1) if the animal who has bitten the patient can be watched, there is no need for vaccine 2) if animal is suspicious for symptoms or cannot be observed, rabies immunoglobulin is given close to the bite site and then rabies vaccine is given in the deltoid at 0, 3, 7, 14, and 21 days 3) bite should be thoroughly cleaned with soap, water, and Betadine 4) site of bite should be elevated and immobilized if possible 5) if traveling to rural areas of southeast Asia, consider taking the vaccine prophylactically
Tips for USMLE
1) if a 34 year-old spelunker reports a bat bite on her hand while on an exploration 6 weeks ago and now has paresthesias in her fingers on that side, headache, and fever, think rabies 2) if question mentions Negri bodies, think rabies 3) occasionally presents as ascending paralysis, especially if cause is a dog bite in Southeast Asia
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