Cat Scratch Disease

1. Overview
2. Pathogen and Transmission
3. Epidemiology
4. Pathophysiology
5. Clinical Manifestations — Typical CSD
6. Clinical Manifestations — Atypical and Severe Forms
7. Clinical Manifestations — Immunocompromised Patients
8. Diagnosis
9. Treatment
10. Prevention
11. References
12. Connections
13. Featured Videos

Overview

Cat scratch disease (CSD) is a zoonotic bacterial infection caused by Bartonella henselae, transmitted to humans primarily through the scratch or bite of an infected cat — most often a kitten. It is the most common cause of chronic regional lymphadenopathy in children and young adults worldwide, and one of the most frequently misdiagnosed causes of unexplained lymph node swelling in any age group.

In immunocompetent individuals, CSD is almost always a benign, self-limited illness that resolves spontaneously within 2–4 months. The lymph nodes swell, become tender, and may suppurate, but serious complications are uncommon. However, in immunocompromised patients — particularly those with HIV/AIDS and CD4 counts below 100 cells/μL — Bartonella henselae causes severe disseminated disease including bacillary angiomatosis (vascular proliferative lesions that mimic Kaposi sarcoma) and peliosis hepatis (blood-filled cysts in the liver), which can be life-threatening without prolonged antibiotic therapy.

Approximately 12,000 cases of CSD are diagnosed annually in the United States, with estimates suggesting the true burden is 2–3 times higher due to mild cases that are never formally diagnosed. The peak incidence occurs in autumn and winter, coinciding with kitten season.

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Pathogen and Transmission

Bartonella henselae is a small, fastidious, gram-negative rod. Key characteristics:

• Obligate intracellular tendencies: replicates within endothelial cells, erythrocytes, and macrophages
• No classic toxins: disease is mediated by host immune response and vascular proliferative effects of bacterial proteins
• Slow culture growth: grows very slowly on blood agar under elevated CO2; cultures take 5–6 weeks; not a practical diagnostic test

The Cat-to-Flea-to-Cat cycle: Bartonella henselae circulates in the bloodstream of infected cats (cats remain asymptomatic and bacteremic for months); the cat flea (Ctenocephalides felis) ingests the organism in a blood meal and sheds bacteria in flea feces; flea feces contaminate cat claws and fur; when a cat scratches or bites a human, flea-contaminated material inoculates the wound.

• Direct inoculation route: scratch or bite introduces flea feces from the cat's claws; cat saliva is NOT thought to be the direct source of transmission
• Kitten risk: kittens are more often bacteremic than adult cats and more likely to scratch; kittens younger than one year pose the highest transmission risk
• Flea control is key: eliminating fleas breaks the transmission cycle; cats themselves do not require antibiotic treatment

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Epidemiology

• Incidence: approximately 12,000 diagnosed cases per year in the USA; estimated 24,000–40,000 true cases when accounting for mild undiagnosed illness
• Age: most common in children and young adults; 80% of cases occur in patients under 21 years
• Sex: slight male predominance
• Season: peak autumn–winter (September–January), coinciding with kitten adoption season and increased indoor cat contact
• Geography: worldwide; no specific geographic clustering unlike tick-borne diseases; any region with cats and cat fleas
• Cat contact: approximately 90% of patients have a cat contact history; the majority involve scratches from kittens under 1 year old
• Risk factors for atypical/severe CSD: immunocompromise (HIV/AIDS, organ transplant, cancer chemotherapy, corticosteroids); advanced age; primary exposure through eye contact (Parinaud's oculoglandular syndrome)

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Pathophysiology

After skin inoculation through a scratch or bite, Bartonella henselae replicates locally at the wound site, producing a small papule or pustule. Bacteria then travel via lymphatics to regional lymph nodes, where they trigger a characteristic immune response.

Histopathological sequence: initial follicular hyperplasia → granuloma formation (stellate, necrotizing granulomas with central suppurative necrosis surrounded by palisading histiocytes) → neutrophilic microabscess formation within granulomas → eventual fibrosis and resolution.

The stellate suppurative granuloma is pathognomonic when found in a lymph node biopsy or aspirate, combined with supporting epidemiology (cat contact, skin inoculation site).

In immunocompetent hosts: the immune response successfully contains the infection within the regional lymph nodes, producing the characteristic self-limited lymphadenopathy.

In immunocompromised hosts (especially HIV with CD4 <100):

• Bartonella causes uncontrolled angioproliferation — stimulation of vascular endothelial growth factor (VEGF) and other angiogenic factors → vascular proliferative lesions (bacillary angiomatosis)
• Bacteria can also seed the liver and spleen → blood-filled sinusoidal cysts (peliosis hepatis) with perisinusoidal bacterial aggregates

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Clinical Manifestations — Typical CSD

The classic presentation unfolds in a characteristic sequence:

Stage 1 — Inoculation Papule (3–14 days after scratch)

A small, non-tender red papule appears at the original scratch site. This may progress to a pustule and then crust over. By the time the patient presents with lymphadenopathy, this initial skin lesion may have already healed and the patient may not notice or remember it. Ask specifically about recent cat scratches on the skin draining to the enlarged lymph node region — this temporal sequence is diagnostically very helpful.

Stage 2 — Regional Lymphadenopathy (1–7 weeks after scratch)

One to three regional lymph nodes draining the scratch site enlarge — typically to 1–5 cm, occasionally larger. Characteristics:

• Location: axillary lymph nodes are most common (upper extremity scratches); followed by cervical, submandibular, preauricular (face/head scratches), and inguinal (lower extremity scratches)
• Texture: nodes are warm, tender, and firm; overlying skin may become erythematous
• Suppuration: occurs in 10–40% of cases — the node becomes fluctuant and may spontaneously drain; a fluctuant node should be aspirated (NOT surgically excised — excision risks chronic fistula formation)
• Constitutional symptoms: present in approximately 50–60% of cases — low-grade fever (rarely >39°C), fatigue, decreased appetite, malaise
• Duration: lymphadenopathy resolves spontaneously over 2–4 months; rarely persists longer; no treatment accelerates this meaningfully in most cases

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Clinical Manifestations — Atypical and Severe Forms

Approximately 10–15% of cases present atypically. Key forms to recognize:

Parinaud's Oculoglandular Syndrome

Occurs when B. henselae is inoculated via the conjunctiva (typically from rubbing eyes after handling an infected cat or flea). Presents as unilateral granulomatous conjunctivitis (red, inflamed conjunctiva with a yellowish granuloma visible on the bulbar or palpebral conjunctiva) plus ipsilateral preauricular or submandibular lymphadenopathy. This triad — conjunctivitis + granuloma + regional adenopathy — is called oculoglandular syndrome. Self-limited in most cases; treat like CSD; ophthalmology referral for severe cases.

Neuroretinitis

Unilateral visual disturbance — blurred vision, scotoma, decreased acuity — from optic disc edema and macular star formation. The macular star (stellate hard exudates radiating from the fovea like a starfish pattern, visible on fundoscopy) is the characteristic finding. Highly associated with CSD and largely pathognomonic when combined with disc swelling. Most patients recover vision substantially, though slowly over weeks to months. Treat with azithromycin + rifampin for 4–6 weeks.

Hepatosplenic CSD

Predominantly in children; presents with prolonged fever, hepatosplenomegaly, and abdominal pain — without prominent lymphadenopathy (which can make CSD non-obvious). CT shows multiple low-density microabscesses in the liver and spleen — the characteristic CT pattern is described as "Swiss cheese" lesions in liver and spleen. Usually self-limited; treat with azithromycin + rifampin.

Encephalopathy

Rare; occurs predominantly in children; presents with altered mental status, seizures, agitation, or combativeness; CSF usually normal; EEG may show generalized slowing. Mechanism unclear; self-limited in most cases over 1–6 months. No clear antibiotic benefit demonstrated.

Osteolytic Lesions

Bone involvement (osteomyelitis) is a rare extranodal manifestation; may cause bone pain and lytic lesions visible on imaging; more common in children.

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Clinical Manifestations — Immunocompromised Patients

In patients with HIV/AIDS (particularly CD4 <100 cells/μL), organ transplant recipients, or those on prolonged immunosuppressive therapy, Bartonella henselae (and B. quintana, from lice) causes distinct severe syndromes:

Bacillary Angiomatosis (BA)

Vascular proliferative lesions that can involve the skin, mucous membranes, bones, brain, and visceral organs. Skin lesions are the most visible: cherry-red or violaceous papules and nodules that bleed easily on contact. Critically, BA skin lesions closely mimic Kaposi sarcoma (KS) in appearance in HIV patients — distinguishing them requires biopsy. Warthin-Starry silver stain shows masses of small bacilli; the histology of BA shows lobular vascular proliferation with neutrophilic inflammation, distinctly different from the spindle-cell pattern of KS. BA responds dramatically to antibiotics, unlike KS — this distinction is treatment-defining. Without treatment, BA is progressive and potentially fatal.

Bartonella Bacteremia

Fever and bacteremia, often without localizing signs; blood culture positive (Bartonella grows very slowly; dedicated Bartonella blood culture bottles required in specialized labs); most common in HIV patients and homeless individuals (B. quintana, from body lice).

Peliosis Hepatis

Blood-filled cystic spaces (pelioses) replacing normal hepatic sinusoids plus perisinusoidal bacterial aggregates. Presents with fever, hepatomegaly, and elevated alkaline phosphatase. Diagnosed by liver biopsy (Warthin-Starry stain); CT/MRI may show hypodense lesions. Life-threatening if untreated.

All Bartonella infections in immunocompromised patients require prolonged antibiotic therapy (months) and can relapse without maintenance suppression until immune reconstitution.

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Diagnosis

Diagnosis of typical CSD is most often clinical plus serological in a patient with characteristic presentation.

Serology (IFA or EIA)

IgM and IgG antibodies to Bartonella henselae. IgG serology has sensitivity approximately 90% and specificity approximately 94% for CSD. IgM may be undetectable at presentation. A single elevated titer (IgG ≥1:64 by IFA) in the right clinical context is supportive; titers typically peak at 1:256–1:1024. Cross-reactivity with B. quintana and rare cross-reactions with Chlamydia and Coxiella exist — interpret in clinical context.

PCR (Lymph Node Aspirate or Tissue Biopsy)

Highest specificity; confirms active infection; can speciate (identify B. henselae vs. B. quintana); sensitivity 43–76% on aspirate material. Preferred when diagnosis is uncertain and invasive sampling is being done anyway.

Histopathology (Lymph Node Biopsy)

Stellate necrotizing granulomas with suppurative centers plus Warthin-Starry silver stain demonstrating small pleomorphic bacilli. Pathognomonic combination for CSD. Invasive but definitive.

Blood Cultures (Immunocompromised Patients)

Bartonella requires lysis-centrifugation blood culture technique; standard BACTEC/BACT-Alert bottles usually negative; incubate 5–6 weeks; sensitivity approximately 15–28% for immunocompetent patients, higher in bacteremic HIV patients. Specialized labs may use shell-vial culture. PCR of blood is faster for bacteremia diagnosis.

Imaging

• Ultrasound: lymph node suppuration (fluctuance) can be assessed; guide aspiration
• CT abdomen/chest: hepatosplenic microabscesses; mediastinal/mesenteric adenopathy; BA visceral lesions in HIV patients
• MRI brain: BA brain lesions; encephalopathy workup

No blood test is 100% sensitive; diagnosis is often clinical in classic presentations (child or young adult + recent kitten contact + scratch papule + regional lymphadenopathy).

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Treatment

Typical CSD (Immunocompetent)

Observation is appropriate for most cases. The illness is self-limited and resolves spontaneously. Antibiotic data show modest benefit — azithromycin (5-day course: 500 mg day 1, then 250 mg days 2–5) has demonstrated accelerated reduction in lymph node size in one RCT but does not significantly shorten duration of illness. Still, azithromycin is reasonable in the first 1–2 weeks if the patient wants treatment or has significant symptoms.

Symptomatic Lymph Node Management

• For large, tender, or fluctuant lymph nodes: needle aspiration relieves pain and speeds drainage; use a large-bore needle (18-gauge); may need to repeat
• Do NOT surgically excise suppurative CSD lymph nodes — excision significantly increases the risk of chronic draining fistula, which is very difficult to close

Atypical/Systemic CSD (Hepatosplenic, Neuroretinitis, Bone, Encephalopathy)

Azithromycin 500 mg/day × 5–10 days plus rifampin 300 mg BID × 10–14 days. Prolonged therapy may be needed for hepatosplenic disease.

Neuroretinitis

Azithromycin + rifampin × 4–6 weeks; ophthalmology co-management.

Bacillary Angiomatosis (Immunocompromised)

• First-line: erythromycin 500 mg four times daily × 3–4 months OR doxycycline 100 mg BID × 3–4 months. Doxycycline has slightly better bioavailability and may be preferred in stable patients.
• Duration: minimum 3 months; many experts treat for 4 months; relapse is common with shorter courses
• In HIV: relapse will occur until CD4 count improves substantially with antiretroviral therapy (ART); maintain suppressive Bartonella therapy until CD4 >200 for at least 3–6 months

Peliosis Hepatis and Bacteremia (Immunocompromised)

Same regimen as BA (doxycycline or erythromycin) × 4 months minimum. ART for HIV patients is essential.

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Prevention

Cat Ownership Safety

• Adopt adult cats rather than kittens if household includes immunocompromised individuals
• Maintain strict flea control for cats using veterinarian-recommended products (topical fipronil, selamectin, imidacloprid — monthly application; flea collars are less effective)
• Wash hands after handling cats; before touching eyes
• Trim cat claws regularly to reduce scratch depth
• Do not allow cats to lick open wounds, mucous membranes, or skin breaks
• Do not allow cats to sleep with immunocompromised household members
• Immunocompromised individuals should avoid rough play that risks scratches

For HIV and Immunocompromised Patients

CDC and IDSA guidelines do not recommend getting rid of cats, but recommend the above precautions and vigilance for symptoms of CSD or bacillary angiomatosis.

Cat Treatment

Cats with Bartonella bacteremia are asymptomatic and well. Testing and treating cats for Bartonella is NOT recommended — cats cannot be reliably cured, and cat treatment does not reduce human risk if flea control is maintained.

Flea Prevention

Flea prevention is the key intervention. B. henselae circulates cat-to-cat via fleas; eliminating fleas breaks the transmission cycle effectively.

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References

1. Margileth AM. Cat scratch disease: a therapeutic dilemma. Vet Clin North Am Small Anim Pract. 1987;17(1):15-28. PMID: 3823566
2. Florin TA, Zaoutis TE, Zaoutis LB. Beyond cat scratch disease: widening spectrum of Bartonella henselae infection. Pediatrics. 2008;121(5):e1413-1425. PMID: 18443019
3. Jacomo V, Kelly PJ, Raoult D. Natural history of Bartonella infections (an exception to Koch's postulate). Clin Diagn Lab Immunol. 2002;9(1):8-18. PMID: 11777823
4. Resto-Ruiz S, Burgess A, Anderson BE. The role of the host immune response in pathogenesis of Bartonella henselae. DNA Cell Biol. 2003;22(6):431-440. PMID: 12906746
5. Bass JW, et al. Prospective randomized double blind placebo-controlled evaluation of azithromycin for treatment of cat-scratch disease. Pediatr Infect Dis J. 1998;17(6):447-452. PMID: 9655532
6. Rolain JM, Brouqui P, Koehler JE, Maguina C, Dolan MJ, Raoult D. Recommendations for treatment of human infections caused by Bartonella species. Antimicrob Agents Chemother. 2004;48(6):1921-1933. PMID: 15155180
7. Koehler JE, et al. Molecular epidemiology of Bartonella infections in patients with bacillary angiomatosis-peliosis. N Engl J Med. 1997;337(26):1876-1883. PMID: 9407154
8. Maguina C, et al. Bartonellosis (Carrion's disease) in the modern era. Clin Infect Dis. 2001;33(6):772-779. PMID: 11512081
9. Wear DJ, et al. Cat scratch disease: a bacterial infection. Science. 1983;221(4618):1403-1405. PMID: 6612349
10. Cunningham ET Jr, Koehler JE. Ocular bartonellosis. Am J Ophthalmol. 2000;130(3):340-349. PMID: 11020415
11. Hansmann Y, et al. Diagnosis of cat scratch disease with detection of Bartonella henselae by PCR: a study of patients with lymph node enlargement. J Clin Microbiol. 2005;43(8):3800-3806. PMID: 16081913
12. Vermeulen MJ, et al. Serological testing for Bartonella henselae infections in The Netherlands: clinical evaluation of immunofluorescence assay and ELISA. Clin Microbiol Infect. 2007;13(6):627-634. PMID: 17371536

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Connections

• Bartonellosis
• Toxoplasmosis
• Leptospirosis
• Leishmaniasis
• Rabies
• Brucellosis
• Chagas Disease

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Featured Videos

Medical Education — Cat scratch disease: causes, symptoms, and clinical presentation of Bartonella henselae infection.

Infectious Disease — Bartonella henselae pathogenesis, flea transmission cycle, and cat bacteremia explained.

Clinical Pearls — Diagnosing cat scratch disease: lymphadenopathy workup, serology, and when to biopsy.

Zoonotic Disease — Cat-transmitted infections: CSD, toxoplasmosis, ringworm, and prevention strategies.

HIV Medicine — Bacillary angiomatosis: Bartonella in immunocompromised patients vs. Kaposi sarcoma differentiation.

Ophthalmology — Parinaud's oculoglandular syndrome: conjunctival granuloma and preauricular adenopathy in CSD.

Treatment Guide — Managing cat scratch disease: observation vs. azithromycin, lymph node aspiration, and when to treat.

Prevention — Flea control for cat owners: breaking the Bartonella transmission cycle and protecting immunocompromised family members.

Retinal Disease — Neuroretinitis and macular star formation in cat scratch disease: fundoscopy findings and prognosis.

Microbiology — Bartonella species: henselae vs. quintana, reservoir hosts, and spectrum of human disease.

Pediatric ID — Hepatosplenic cat scratch disease: CT Swiss cheese lesions, prolonged fever, and treatment in children.

Clinical Diagnosis — Differential diagnosis of regional lymphadenopathy: CSD, lymphoma, tuberculosis, and other causes.

Hepatology — Peliosis hepatis in Bartonella infection: imaging, histopathology, and antibiotic management.

Pediatrics — Cat scratch disease in children: recognizing CSD as a cause of prolonged fever and swollen lymph nodes.

Microbiology — Gram-negative intracellular bacteria: Bartonella, Brucella, and Rickettsia compared.

Public Health — Zoonotic disease prevention for pet owners: safe cat and dog handling to reduce infection risk.

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