Capnocytophaga — SCE Dermatology MCQ

Correct answer: B — Asplenia or immunosuppression — Capnocytophaga infections are disproportionately severe in asplenic and immunocompromised patients

The correct answer is B, asplenia or immunosuppression, because Capnocytophaga canimorsus infections are disproportionately severe in patients who lack normal splenic or immune clearance of encapsulated and slow-growing organisms. The spleen is central to clearing poorly opsonised bacteria from the bloodstream, so splenectomised or hyposplenic patients (and those on immunosuppressants, with alcohol excess, or with liver disease) can progress rapidly from a minor dog bite to fulminant sepsis, meningitis, endocarditis or disseminated intravascular coagulation. This is why UK bite management guidance specifically flags immunocompromised and asplenic patients as needing a lower threshold for antibiotic prophylaxis and urgent review after any animal bite. The 3-month history of a non-healing ulcer with a fastidious Gram-negative rod is classic for Capnocytophaga, and the exam point being tested is recognising which host factor converts this usually indolent organism into a life-threatening pathogen. Why the other options are wrong: A. Smoking: Smoking impairs local tissue healing and mucosal immunity but has no established specific link to overwhelming Capnocytophaga sepsis; it is not a recognised risk stratifier in UK bite guidance. E. Young age: Severe Capnocytophaga disease is actually more common in older or comorbid adults, not the young, and paediatric cases are rare and usually mild. D. Obesity: Obesity affects wound healing and surgical risk generally but is not linked mechanistically to failure of clearance of encapsulated or slow-growing bacteria. C. Female sex: There is no sex-based predisposition described for Capnocytophaga canimorsus; reported severe cases occur across both sexes, reflecting host immune status rather than sex. Key point: Asplenia (surgical or functional) and immunosuppression are the key host factors that convert a Capnocytophaga canimorsus bite wound infection into life-threatening sepsis, so these patients need urgent assessment and low-threshold antibiotic prophylaxis after any dog or cat bite.

Reference: NICE guideline NG184, Human and animal bites: antimicrobial prescribing (2020), Recommendations section, nice.org.uk/guidance/ng184