Which organisms cause overwhelming post‑splenectomy infection (OPSI)?

Organisms Causing Overwhelming Post-Splenectomy Infection (OPSI)

Streptococcus pneumoniae is the dominant pathogen, accounting for approximately 50% of all OPSI cases, followed by Haemophilus influenzae type B and Neisseria meningitidis as the other major encapsulated bacteria responsible for this life-threatening complication. 1, 2, 3

Primary Causative Organisms

The three encapsulated bacteria that cause the vast majority of OPSI cases are:

• Streptococcus pneumoniae is responsible for over 50% of OPSI episodes and represents the single most important pathogen in asplenic patients 2, 3, 4, 5
• Haemophilus influenzae type B is the second most common cause, particularly significant in children 1, 2, 3
• Neisseria meningitidis is associated with serious infection and carries 40-70% mortality in asplenic patients 1, 6, 2

Additional Pathogens

Beyond the three primary encapsulated bacteria, several other organisms pose significant risk:

• Escherichia coli and other Gram-negative bacilli can cause OPSI 1, 7
• Capnocytophaga canimorsus from dog, cat, or other animal bites represents a specific risk requiring 5-day prophylaxis with co-amoxiclav 6, 5
• Malaria (Plasmodium species) causes severe disease in asplenic travelers to endemic areas 1, 3
• Babesiosis from tick bites can present with fever, fatigue, and hemolytic anemia 6

Critical Clinical Context

The encapsulated nature of the primary pathogens (S. pneumoniae, H. influenzae, N. meningitidis) is the key pathophysiologic feature—the spleen's role in clearing these organisms through opsonization and phagocytosis is irreplaceable, making asplenic patients uniquely vulnerable. 1, 8

Why These Organisms Dominate

• The polysaccharide capsule of these bacteria requires splenic macrophages for effective clearance 8
• Without splenic function, these encapsulated organisms can proliferate unchecked, leading to fulminant sepsis within hours 2, 8
• Current pneumococcal vaccines cover only 23 of approximately 90 known serotypes, and meningococcal vaccines cover 5 of 6 clinically relevant serogroups, leaving residual risk even with optimal vaccination 6, 3

Age-Related Risk Patterns

• Children under 5 years—especially infants—face infection rates exceeding 10%, compared to less than 1% in adults, with neonates having greater than 30% risk 1, 2, 3
• The highest risk period is the first year after splenectomy, but risk remains elevated for more than 10 years and is almost certainly lifelong 2, 3
• Cases of fulminant OPSI have been documented more than 20 years after splenectomy 1, 6

Common Pitfalls to Avoid

• Assuming vaccination eliminates risk: A documented case of fatal pneumococcal sepsis occurred despite complete and timely vaccination, as the isolated strain (serogroup 12F) was covered by the 23-valent vaccine but still caused OPSI 9
• Underestimating non-pneumococcal pathogens: While S. pneumoniae dominates, H. influenzae and N. meningitidis together account for a substantial proportion of cases and require separate vaccination strategies 6, 2
• Forgetting travel-related and zoonotic risks: Malaria, babesiosis, and Capnocytophaga infections are preventable with appropriate counseling and prophylaxis 6, 3, 5
• Overlooking functional hyposplenism: Patients with sickle cell disease, thalassemia major, celiac disease, inflammatory bowel disease, and lymphoproliferative disorders face identical organism-specific risks as surgical splenectomy patients 1, 2