Bartonella, Francisella and Pasteurella
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Direct answer
Bartonella, Francisella and Pasteurella are small Gram-negative bacilli that cause zoonotic infections acquired from animals, insects or animal bites. Bartonella henselae causes cat-scratch disease, Francisella tularensis causes tularaemia through contact with rabbits, ticks or aerosols, and Pasteurella multocida causes infection after cat and dog bites. Bartonella and Francisella grow poorly on routine media, so the exposure history guides the diagnosis.
What you must remember
- Bartonella henselae: a fastidious Gram-negative bacillus carried by cats and spread by scratches or bites, with fleas implicated in transmission between cats.
- Cat-scratch disease: tender regional lymphadenopathy, often in the forearm, head or neck, that usually resolves on its own.
- Bacillary angiomatosis: vascular proliferative lesions in immunosuppressed people, especially advanced HIV infection, along with peliosis hepatis.
- Trench fever: caused by Bartonella quintana and transmitted by body lice.
- Francisella tularensis: a tiny Gram-negative coccobacillus that needs cysteine-enriched media and is a Category A bioterrorism agent.
- Tularaemia: the ulceroglandular form is commonest, with an ulcer and regional lymph nodes, while pneumonic and typhoidal forms are severe.
- Pasteurella multocida: a capsulated, oxidase-positive coccobacillus with bipolar staining, normally present in the oral flora of cats and dogs.
- Treatment: streptomycin or gentamicin for tularaemia, amoxicillin-clavulanate for infected bites, and azithromycin often used in severe cat-scratch disease.
Working out an animal-linked infection
Each of these organisms produces a different pattern, so the history and the site of infection narrow the list quickly. The steps below move from exposure to treatment.
- Exposure history: ask about cat scratches or kitten bites, handling of rabbits or hares, tick or deer fly bites, and dog or cat bites.
- Pattern of disease: regional lymphadenopathy after a scratch points to Bartonella, an ulcer with regional nodes points to Francisella, and a rapidly spreading cellulitic bite wound points to Pasteurella.
- Culture and stains: Pasteurella grows readily on blood agar, but Francisella needs cysteine-enriched media and Bartonella needs prolonged incubation, so the laboratory must be told the suspected organism.
- Serology and PCR: immunofluorescence for Bartonella and microagglutination for Francisella support the diagnosis when culture fails.
- Biopsy: Warthin-Starry silver staining of a lymph node or skin lesion can show Bartonella bacilli in suspected cat-scratch disease.
- Treatment: therapy is tailored to the organism and severity, using streptomycin or gentamicin for tularaemia and amoxicillin-clavulanate for infected bites.
Where the exam tries to catch you
These organisms appear together in zoonosis questions, so the exposure detail usually decides the answer.
- Trap: expecting tularaemia to respond to penicillin Correct: Francisella is treated first with streptomycin or gentamicin, with doxycycline or ciprofloxacin as alternatives.
- Trap: assuming cat-scratch disease needs antibiotics in every patient Correct: it is usually self-limiting, and antibiotics are reserved for severe or systemic disease.
- Trap: dismissing a small dog bite as trivial Correct: Pasteurella multocida can cause rapid cellulitis, abscess and sepsis, so bites need prompt review.
Frequently asked questions
What is the commonest cause of cat-scratch disease?
Bartonella henselae is the commonest cause of cat-scratch disease, and it is carried by cats, especially kittens. Infection follows a scratch or bite and produces tender regional lymphadenopathy that usually resolves without specific treatment.
Why is Francisella tularensis difficult to culture?
Francisella tularensis is a tiny, fastidious organism that needs cysteine-enriched media and does not grow on routine culture media. Laboratories must be alerted to suspected tularaemia because the organism is a biosafety hazard and a Category A bioterrorism agent.
Which antibiotic is used first for tularaemia?
Streptomycin or gentamicin is the standard first-line treatment for tularaemia caused by Francisella tularensis. Doxycycline and ciprofloxacin are used as alternatives, and treatment should start early in severe or pneumonic disease.
Why is Pasteurella multocida important after animal bites?
Pasteurella multocida is part of the normal oral flora of cats and dogs, and it commonly causes rapid cellulitis and abscess after a bite. Infected bite wounds are usually treated with amoxicillin-clavulanate, because Pasteurella multocida is generally susceptible to penicillin-based agents per current guidance.
What is bacillary angiomatosis caused by Bartonella?
Bacillary angiomatosis is a vascular proliferative disease caused by Bartonella henselae, seen mainly in immunosuppressed people such as those with advanced HIV infection. It produces skin and visceral lesions that respond to prolonged antibiotic therapy, usually with erythromycin or doxycycline.