A 28-year-old male farmer from rural Maharashtra presents to the emergency department with a 10-day history of fever, restlessness, and progressive neurological deterioration. He reports being bitten by a stray dog 6 weeks ago while working in the fields, but did not seek medical attention at that time. On examination, he is agitated, has excessive salivation, and exhibits hydrophobia—he refuses to drink water despite severe thirst. His temperature is 38.5°C, and he shows signs of autonomic instability with tachycardia (HR 110/min) and irregular respirations. CSF analysis shows lymphocytic pleocytosis with normal glucose and elevated protein. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Rabies Virus Infection ### Key Clinical Features Supporting Diagnosis **Key Point:** The combination of a dog bite 6 weeks prior, hydrophobia, excessive salivation, agitation, and progressive encephalitis is pathognomonic for rabies. ### Pathognomonic Signs in This Case 1. **Hydrophobia** — fear and inability to swallow water due to pharyngeal spasm; occurs in ~50% of cases but is highly specific 2. **Excessive salivation (ptyalism)** — due to autonomic dysfunction 3. **Incubation period** — 6 weeks is typical; ranges from 2 weeks to several months (rarely years) 4. **Animal exposure** — dog bite in endemic region (rural Maharashtra) 5. **Prodromal phase** — fever, restlessness, anxiety preceding neurological signs ### Laboratory Findings | Feature | Rabies | HSE | JEV | TBM | | --- | --- | --- | --- | --- | | **CSF Glucose** | Normal | Normal/Low | Normal | Low (<40% serum) | | **CSF Cell Type** | Lymphocytic | Lymphocytic | Lymphocytic | Lymphocytic | | **CSF Protein** | Elevated | Elevated | Elevated | Very elevated | | **RBC in CSF** | Rare | Often present | Rare | Rare | | **Hydrophobia** | Yes (50%) | No | No | No | | **Salivation** | Excessive | No | No | No | **High-Yield:** Hydrophobia + salivation + encephalitis + animal bite = rabies until proven otherwise. ### Diagnostic Confirmation 1. **Negri bodies** — pathognomonic intracytoplasmic inclusions in hippocampus (found in ~50% of cases; NOT required for diagnosis) 2. **Direct fluorescent antibody (DFA) test** — gold standard; detects rabies antigen in corneal impression smears or brain tissue 3. **RT-PCR** — highly sensitive; detects viral RNA in saliva, CSF, or skin biopsy 4. **Immunohistochemistry** — post-mortem confirmation ### Why Rabies Is Nearly 100% Fatal Once Symptomatic **Clinical Pearl:** Once clinical signs appear, rabies is almost universally fatal (case fatality rate >99.5%). The Milwaukee Protocol (induced coma with antivirals) has shown rare survival but remains experimental. Prevention via post-exposure prophylaxis (PEP) within 48 hours is the only effective intervention. ### Management Implications **Mnemonic: RABIES PEP** — **R**abies immunoglobulin (RIG), **A**ctive vaccination (HDCV/PCECV), **B**iological safety, **I**ncision wound care, **E**xposure assessment, **S**upport care; **P**ost-exposure, **E**arly intervention, **P**revention Since this patient is already symptomatic (10 days of fever + hydrophobia), PEP is no longer effective. Supportive care and palliative measures are indicated. **Warning:** Do NOT delay diagnosis waiting for confirmatory tests. Clinical presentation + exposure history is sufficient to initiate supportive care and infection control measures.