Question
A 13-year-old boy from a farming village presents with a slowly enlarging painless swelling in his left armpit for about 2–3 weeks. He has difficulty lifting his arm fully. A course of oral antibiotics did not help. He also reports that he previously had swollen lymph nodes in the neck and groin that disappeared on their own, along with occasional joint aches.
On examination, he is otherwise well. Imaging studies are normal. A lymph node biopsy is done, and special silver staining reveals small bacilli.
Which organism is the most likely cause?
a. Bartonella henselae
b. Brucella melitensis
c. Mycobacterium tuberculosis
d. Toxoplasma gondii
e. Epstein–Barr virus
Answer
a. Bartonella henselae
Explanation
This is a classic presentation of cat scratch disease, caused by Bartonella henselae.
Key clinical clues:
- Subacute painless lymphadenopathy (especially axillary)
- Poor response to routine antibiotics
- History of transient lymph node enlargement elsewhere
- Systemically well patient
- Intermittent arthralgia
- Warthin–Starry stain positive (silver stain showing bacilli)
Why Bartonella henselae?
- Gram-negative bacillus
- Transmitted via cat scratches or bites
- Causes regional lymphadenopathy (often axillary if upper limb inoculation)
- Diagnosis: Warthin–Starry silver stain showing organisms in lymph node tissue
Why others are wrong
- Brucella → systemic illness with fever, sweats, hepatosplenomegaly; not localized lymphadenopathy pattern
- Mycobacterium tuberculosis → diagnosed by Ziehl–Neelsen stain (acid-fast bacilli)
- Toxoplasma gondii → diagnosed by serology or Giemsa stain, usually in immunocompromised
- Epstein–Barr virus → causes generalized lymphadenopathy + hepatosplenomegaly, diagnosed serologically (Monospot), not by biopsy staining
Cheat Sheet (Exam-Oriented)
Cat Scratch Disease
- Organism: Bartonella henselae
- Transmission: Cat scratch/bite
- Pathology: Granulomatous lymphadenitis
- Stain: Warthin–Starry (silver stain)
- Features:
- Local lymphadenopathy (axillary common)
- Mild fever, malaise
- Self-limiting
- Treatment:
- Usually supportive
- Azithromycin if needed
Flash Cards
Q1. What organism causes cat scratch disease?
A: Bartonella henselae
Explanation: Gram-negative bacillus transmitted via cats.
Q2. Which stain is used to identify Bartonella?
A: Warthin–Starry stain
Explanation: Silver stain highlights small bacilli.
Q3. Typical lymph node involvement in cat scratch disease?
A: Regional (often axillary)
Explanation: Depends on site of inoculation.
Q4. How is TB identified histologically?
A: Ziehl–Neelsen stain
Explanation: Detects acid-fast bacilli.
MCQs (High-Yield + Challenging)
MCQ 1
A patient presents with axillary lymphadenopathy after a kitten scratch. Which stain confirms diagnosis?
a. Ziehl–Neelsen
b. PAS stain
c. Warthin–Starry
d. Gram stain
Answer: c. Warthin–Starry
Explanation: Bartonella is best visualized with silver staining.
MCQ 2
Which of the following is FALSE regarding cat scratch disease?
a. Caused by Bartonella henselae
b. Typically presents with generalized lymphadenopathy
c. Often self-limiting
d. May show granulomatous inflammation
Answer: b. Typically presents with generalized lymphadenopathy
Explanation: It causes regional, not generalized lymphadenopathy.
MCQ 3
A biopsy shows organisms on silver stain. Which organism is most likely?
a. Mycobacterium tuberculosis
b. Bartonella henselae
c. Toxoplasma gondii
d. EBV
Answer: b. Bartonella henselae
Explanation: Silver stain (Warthin–Starry) highlights Bartonella.
MCQ 4
Which infection is most associated with exposure to unpasteurized dairy products?
a. Bartonella henselae
b. EBV
c. Brucella
d. Toxoplasma
Answer: c. Brucella
Explanation: Brucellosis → zoonotic, dairy exposure.
MCQ 5
Which organism is acid-fast?
a. Bartonella henselae
b. Mycobacterium tuberculosis
c. Brucella
d. EBV
Answer: b. Mycobacterium tuberculosis
Explanation: Identified with Ziehl–Neelsen stain.
MCQ 6
A patient has lymphadenopathy, hepatosplenomegaly, and atypical lymphocytes. Most likely diagnosis?
a. Bartonella infection
b. EBV infection
c. Brucellosis
d. TB
Answer: b. EBV infection
Explanation: Classic infectious mononucleosis.
Summary for Quick Exam Revision
Cat scratch disease is a self-limiting infection caused by Bartonella henselae, typically transmitted via scratches or bites from cats. It presents with subacute regional lymphadenopathy, most commonly axillary, often following an upper limb inoculation. Patients are usually systemically well, with mild constitutional symptoms such as fever and malaise. Diagnosis is confirmed histologically using Warthin–Starry silver staining, which highlights the causative bacilli. This differentiates it from tuberculosis (Ziehl–Neelsen stain), toxoplasmosis (Giemsa stain/serology), and EBV infection (clinical + serology). The disease may show granulomatous inflammation on biopsy and typically resolves spontaneously, though macrolides like azithromycin may be used in symptomatic cases. Recognizing the combination of localized lymphadenopathy, poor antibiotic response, and characteristic staining is key for exam success.