ImmunologyTier 2Medical Sciences concept

Asplenia and encapsulated organism susceptibility

Core concept

The spleen does two things no other organ does
- Filters opsonised, poorly-opsonised and antibody-coated organisms from blood - the only site with slow open sinusoidal flow allowing macrophage contact
- Houses the marginal zone B cells that mount the T-INDEPENDENT antibody response to capsular polysaccharide
-> Encapsulated organisms are the entire problem
- Capsule resists phagocytosis unless opsonised; opsonisation needs anti-capsular antibody, and that antibody needs the spleen to be made
- Also produces tuftsin and properdin, and clears intra-erythrocytic parasites
*Streptococcus pneumoniae causes 50-90% of overwhelming post-splenectomy infection*
- Then Haemophilus influenzae type b, Neisseria meningitidis
- Capnocytophaga canimorsus - dog and cat bites; Babesia; severe falciparum malaria; Bordetella, E. coli, salmonella (in sickle cell)
Overwhelming post-splenectomy infection (OPSI)
- Non-specific prodrome -> fulminant sepsis, DIC and shock within HOURS
- Mortality 50-70%; lifetime risk ~5%, highest in the first 2 years but never falls to zero

Causes of hyposplenism (not just splenectomy)

Surgical - trauma, ITP, hereditary spherocytosis, staging laparotomy, iatrogenic at gastric or pancreatic surgery
Functional
- Sickle cell disease - autosplenectomy by early childhood
- Coeliac disease (commonly missed - up to a third have some hyposplenism)
- IBD, autoimmune disease (SLE, RA, sarcoid, amyloid), graft-versus-host disease
- Splenic irradiation, splenic artery embolisation
- Congenital asplenia/polysplenia (heterotaxy)
Film clue: Howell-Jolly bodies, target cells, acanthocytes, thrombocytosis, lymphocytosis

Vaccination - the sequence is examined

Give at least 2 weeks BEFORE elective splenectomy; if emergency, wait until >=7-14 days after surgery (and until acute illness has settled)
Pneumococcal: CONJUGATE (PCV) first, then 23vPPV at least 8 weeks later - conjugate-first gives a better response and avoids hyporesponsiveness
Meningococcal ACWY conjugate (2 doses) and MenB (2 doses), with boosters
Haemophilus influenzae type b conjugate
Annual influenza - reduces secondary bacterial pneumonia and pneumococcal sepsis
COVID-19 as per schedule
All are inactivated - no contraindication in asplenia

Antibiotic strategy

Daily prophylaxis: phenoxymethylpenicillin 250 mg bd (or amoxicillin; roxithromycin if penicillin-allergic)
- Lifelong in children under 5, for at least 3 years after splenectomy in adults, and lifelong in sickle cell, prior OPSI, ongoing immunosuppression or haematological malignancy
- Adherence is poor - the standby supply matters at least as much
Standby antibiotics - amoxicillin-clavulanate, to be taken immediately at the first sign of fever or rigors, then present to hospital urgently
Any fever is a medical emergency - blood cultures and IV ceftriaxone before results; do not wait for a source

The rest of the package

Medical alert bracelet and a written asplenia card
Travel advice: malaria chemoprophylaxis and rigorous bite avoidance (asplenia markedly worsens falciparum severity), babesiosis in endemic areas
Animal bites - immediate amoxicillin-clavulanate for Capnocytophaga
Australia: Spleen Australia registry provides reminders, education and 24-hour advice - enrol every patient
Look for and treat the underlying cause - test for coeliac disease in unexplained Howell-Jolly bodies

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